Facebook Patent | Carrier confinement in leds by valence band engineering

Patent: Carrier confinement in leds by valence band engineering

Drawings: Click to check drawins

Publication Number: 20220209044

Publication Date: 20220630

Applicant: Facebook

Abstract

A micro-light emitting diode (micro-LED) includes a substrate, an n-type semiconductor layer on the substrate, a p-type semiconductor layer, and an active region between the n-type semiconductor layer and the p-type semiconductor layer and configured to emit red light. The active region includes a barrier layer characterized by a first lattice constant, and a quantum well layer next to the barrier layer. The quantum well layer is characterized by a second lattice constant greater than the first lattice constant and by an in-plane compressive strain. The active region has a lateral linear dimension equal to or less than about 10 .mu.m.

Claims

1. A micro-light emitting diode comprising: a substrate; an n-type semiconductor layer on the substrate; a p-type semiconductor layer; and an active region between the n-type semiconductor layer and the p-type semiconductor layer and configured to emit red light, the active region comprising: a barrier layer characterized by a first lattice constant; and a quantum well layer next to the barrier layer, the quantum well layer characterized by a second lattice constant greater than the first lattice constant and by an in-plane compressive strain, wherein the active region is characterized by a lateral linear dimension equal to or less than 10 .mu.m.

2. The micro-light emitting diode of claim 1, wherein the substrate is characterized by a third lattice constant that matches the first lattice constant.

3. The micro-light emitting diode of claim 1, wherein: the quantum well layer is characterized by an energy band structure including a heavy-hole band and a light-hole band in a valence band; and a top of the heavy-hole band is higher than a top of the light-hole band.

4. The micro-light emitting diode of claim 3, wherein a difference between the top of the heavy-hole band and the top of the light-hole band is greater than 0.075 eV at room temperature.

5. The micro-light emitting diode of claim 1, wherein: the substrate includes a GaAs substrate; the barrier layer includes a (Al.sub.xGa.sub.1-x).sub.0.5In.sub.0.5P layer; and the quantum well layer includes a Ga.sub.yIn.sub.1-yP or (AlxGai-x).sub.yIn.sub.1-yP layer, wherein y is less than 0.5.

6. The micro-light emitting diode of claim 1, wherein a difference between the second lattice constant and the first lattice constant is greater than 1% of the first lattice constant.

7. The micro-light emitting diode of claim 1, wherein the red light is characterized by a wavelength greater than 600 nm.

8. The micro-light emitting diode of claim 1, wherein the active region includes a plurality of quantum well layers interleaved with a plurality of barrier layers, the plurality of quantum well layers including the quantum well layer.

9. The micro-light emitting diode of claim 1, further comprising a passivation layer on sidewalls of the active region.

10. The micro-light emitting diode of claim 1, wherein a thickness of the quantum well layer is less than a thickness of the barrier layer.

11. A device comprising: a substrate; and an array of micro-light emitting diodes on the substrate, each micro-light emitting diode of the array of micro-light emitting diode comprising a mesa structure that comprises: an n-type semiconductor layer; a p-type semiconductor layer; and an active region between the n-type semiconductor layer and the p-type semiconductor layer and configured to emit red light, the active region comprising: a barrier layer characterized by a first lattice constant; and a quantum well layer next to the barrier layer, the quantum well layer characterized by a second lattice constant greater than the first lattice constant and by an in-plane compressive strain, wherein the active region is characterized by a lateral linear dimension equal to or less than 10 .mu.m.

12. The device of claim 11, wherein the substrate is characterized by a third lattice constant that matches the first lattice constant.

13. The device of claim 11, wherein: the quantum well layer is characterized by an energy band structure including a heavy-hole band and a light-hole band in a valence band; and a top of the heavy-hole band is higher than a top of the light-hole band.

14. The device of claim 13, wherein a difference between the top of the heavy-hole band and the top of the light-hole band is greater than 0.075 eV at room temperature.

15. The device of claim 11, wherein: the substrate includes a GaAs substrate; the barrier layer includes a (Al.sub.xGa.sub.1-x).sub.0.5In.sub.0.5P layer; and the quantum well layer includes a Ga.sub.yIn.sub.1-yP or (Al.sub.xGa.sub.1-x).sub.yIn.sub.1-yP layer, wherein y is less than 0.5.

16. The device of claim 11, wherein a difference between the second lattice constant and the first lattice constant is greater than 1% of the first lattice constant.

17. The device of claim 11, wherein the mesa structure is characterized by a vertical, conical, parabolic, inward-tilted, or outward-tilted shape.

18. A method comprising: growing an n-type semiconductor layer on a substrate; alternately growing a plurality of barrier layers and a plurality of quantum well layers on the n-type semiconductor layer to form a multi-quantum-well structure, wherein the plurality of quantum well layers is characterized by a first lattice constant greater than 101% of a second lattice constant of the plurality of barrier layers and by an in-plane compressive strain; growing a p-type semiconductor layer on the multi-quantum-well structure; and selectively etching the p-type semiconductor layer, the multi-quantum-well structure, and the n-type semiconductor layer to form an array of mesa structures, wherein each mesa structure of the array of mesa structures is characterized by a lateral linear dimension equal to or less than 10 .mu.m.

19. The method of claim 18, wherein: the substrate includes a GaAs substrate; the plurality of barrier layers includes a (Al.sub.xGa.sub.1-x).sub.0.5In.sub.0.5P layer; and the plurality of quantum well layers includes a Ga.sub.yIn.sub.1-yP or (Al.sub.zGa.sub.1-z).sub.yyP layer, wherein y is less than 0.5.

20. The method of claim 18, wherein: a quantum well layer of the plurality of quantum well layers is characterized by an energy band structure including a heavy-hole band and a light-hole band in a valence band; a top of the heavy-hole band is higher than a top of the light-hole band; and a difference between the top of the heavy-hole band and the top of the light-hole band is greater than 0.075 eV at room temperature.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of and priority to U.S. Provisional Application No. 63/130,537, filed Dec. 24, 2020, entitled “CARRIER CONFINEMENT IN LEDS BY VALENCE BAND ENGINEERING,” which is herein incorporated by reference in its entirety for all purposes.

BACKGROUND

[0002] Light emitting diodes (LEDs) convert electrical energy into optical energy, and offer many benefits over other light sources, such as reduced size, improved durability, and increased efficiency. LEDs can be used as light sources in many display systems, such as televisions, computer monitors, laptop computers, tablets, smartphones, projection systems, and wearable electronic devices. Micro-LEDs (“.mu.LEDs”) based on III-V semiconductors, such as alloys of AlN, GaN, InN, GaAs, GaInP, AlGaInP, other quaternary phosphide compositions, and the like, have begun to be developed for various display applications due to their small size, high packing density, higher resolution, and high brightness. For example, micro-LEDs that emit light of different colors (e.g., red, green, and blue) can be used to form the sub-pixels of a display system, such as a television or a near-eye display system.

SUMMARY

[0003] This disclosure relates generally to micro light emitting diodes (micro-LEDs). More specifically, this disclosure relates to improving the quantum efficiencies of micro-LEDs, such as small AlGaInP-based micro-LEDs. According to certain embodiments, a micro-LED may include a substrate, an n-type semiconductor layer on the substrate, a p-type semiconductor layer, and an active region between the n-type semiconductor layer and the p-type semiconductor layer and configured to emit red light. The active region may include a barrier layer characterized by a first lattice constant, and a quantum well layer next to the barrier layer. The quantum well layer may be characterized by a second lattice constant greater than the first lattice constant and by an in-plane compressive strain. The active region may be characterized by a lateral linear dimension equal to or less than about 10 .mu.m.

[0004] In some embodiments of the micro-LED, the substrate may be characterized by a third lattice constant that matches the first lattice constant. The quantum well layer may be characterized by an energy band structure including a heavy-hole band and a light-hole band in a valence band, and a top of the heavy-hole band may be higher than a top of the light-hole band. In some embodiments, a difference between the top of the heavy-hole band and the top of the light-hole band is greater than about 0.075 eV at room temperature. In some embodiments, the substrate may include a GaAs substrate, the barrier layer may include a (Al.sub.xGa.sub.1-x)0.5In.sub.0.5P layer, and the quantum well layer may include a Ga.sub.yIn.sub.1-yP or (Al.sub.xGa.sub.1-x).sub.yIn.sub.-yP layer, where y is less than about 0.5. In some embodiments, a difference between the second lattice constant and the first lattice constant may be greater than about 1% of the first lattice constant. The red light may be characterized by a wavelength greater than 600 nm. The active region may include a plurality of quantum well layers interleaved with a plurality of barrier layers, the plurality of quantum well layers including the quantum well layer. A thickness of the quantum well layer may be less than a thickness of the barrier layer. The micro-LED may also include a passivation layer on sidewalls of the active region.

[0005] According to some embodiments, a device may include a substrate and an array of micro-light emitting diodes on the substrate. Each micro-light emitting diode of the array of micro-light emitting diode may include a mesa structure. The mesa structure may include an n-type semiconductor layer, a p-type semiconductor layer, and an active region between the n-type semiconductor layer and the p-type semiconductor layer and configured to emit red light. The active region may include a barrier layer characterized by a first lattice constant, and a quantum well layer next to the barrier layer. The quantum well layer characterized by a second lattice constant greater than the first lattice constant and by an in-plane compressive strain. The active region may be characterized by a lateral linear dimension equal to or less than 10 .mu.m.

[0006] In some embodiments of the device, the substrate may be characterized by a third lattice constant that matches the first lattice constant. The quantum well layer may be characterized by an energy band structure including a heavy-hole band and a light-hole band in a valence band, and a top of the heavy-hole band may be higher than a top of the light-hole band. In some embodiments, a difference between the top of the heavy-hole band and the top of the light-hole band is greater than 0.075 eV at room temperature. In some embodiments, the substrate may include a GaAs substrate, the barrier layer may include a (Al.sub.xGa.sub.1-x)0.5In.sub.0.5P layer, and the quantum well layer may include a Ga.sub.yIn.sub.1-yP or (Al.sub.xGa.sub.1-x).sub.yIn.sub.1-yP layer, where y is less than about 0.5. In some embodiments, a difference between the second lattice constant and the first lattice constant may be greater than about 1% of the first lattice constant. The mesa structure may be characterized by a vertical, conical, parabolic, inward-tilted, or outward-tilted shape.

[0007] According to some embodiments, a method may include growing an n-type semiconductor layer on a substrate, alternately growing a plurality of barrier layers and a plurality of quantum well layers on the n-type semiconductor layer to form a multi-quantum-well structure, growing a p-type semiconductor layer on the multi-quantum-well structure, and selectively etching the p-type semiconductor layer, the multi-quantum-well structure, and the n-type semiconductor layer to form an array of mesa structures. The plurality of quantum well layers may be characterized by a first lattice constant greater than 101% of a second lattice constant of the plurality of barrier layers and by an in-plane compressive strain. Each mesa structure of the array of mesa structures may be characterized by a lateral linear dimension equal to or less than 10 .mu.m. In some embodiments, the substrate may include a GaAs substrate, the barrier layer may include a (Al.sub.xGa.sub.1-x).sub.0.5In.sub.0.5P layer, and the quantum well layer may include a Ga.sub.yIn.sub.1-yP or (Al.sub.xGa.sub.1-x).sub.yIn.sub.1-yP layer, where y is less than about 0.5. In some embodiments, a quantum well layer of the plurality of quantum well layers may be characterized by an energy band structure including a heavy-hole band and a light-hole band in a valence band, a top of the heavy-hole band is higher than a top of the light-hole band, and a difference between the top of the heavy-hole band and the top of the light-hole band is greater than 0.075 eV at room temperature.

[0008] This summary is neither intended to identify key or essential features of the claimed subject matter, nor is it intended to be used in isolation to determine the scope of the claimed subject matter. The subject matter should be understood by reference to appropriate portions of the entire specification of this disclosure, any or all drawings, and each claim. The foregoing, together with other features and examples, will be described in more detail below in the following specification, claims, and accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] Illustrative embodiments are described in detail below with reference to the following figures.

[0010] FIG. 1 is a simplified block diagram of an example of an artificial reality system environment including a near-eye display according to certain embodiments.

[0011] FIG. 2 is a perspective view of an example of a near-eye display in the form of a head-mounted display (HMD) device for implementing some of the examples disclosed herein.

[0012] FIG. 3 is a perspective view of an example of a near-eye display in the form of a pair of glasses for implementing some of the examples disclosed herein.

[0013] FIG. 4 illustrates an example of an optical see-through augmented reality system including a waveguide display according to certain embodiments.

[0014] FIG. 5A illustrates an example of a near-eye display device including a waveguide display according to certain embodiments.

[0015] FIG. 5B illustrates an example of a near-eye display device including a waveguide display according to certain embodiments.

[0016] FIG. 6 illustrates an example of an image source assembly in an augmented reality system according to certain embodiments.

[0017] FIG. 7A illustrates an example of a light emitting diode (LED) having a vertical mesa structure according to certain embodiments.

[0018] FIG. 7B is a cross-sectional view of an example of an LED having a parabolic mesa structure according to certain embodiments.

[0019] FIG. 8 illustrates the relationship between the optical emission power and the current density of a light emitting diode.

[0020] FIG. 9A illustrates an example of a micro-LED with a mesa structure.

[0021] FIG. 9B illustrates a simplified energy band structure of the active region of the example of micro-LED shown in FIG. 9A.

[0022] FIG. 10 illustrates surface recombination velocities of various III-V semiconductor materials.

[0023] FIG. 11A illustrates an energy band diagram of an example of a direct gap semiconductor material.

[0024] FIG. 11B illustrates an example of an ideal energy band diagram of a direct gap semiconductor laser.

[0025] FIG. 12A illustrates an energy band diagram of an example of a direct gap semiconductor material.

[0026] FIG. 12B illustrates an energy band diagram of an example of a direct gap semiconductor material under a compressive strain.

[0027] FIG. 12C illustrates an energy band diagram of an example of a direct gap semiconductor material under a tensile strain.

[0028] FIG. 13 illustrates bandgap energy levels, corresponding wavelengths, and lattice constants of semiconductor materials having different compositions.

[0029] FIG. 14 illustrates an example of a micro-LED with compressive strain introduced in the quantum well layers to improve the quantum efficiency according to certain embodiments.

[0030] FIG. 15A illustrates energy bands of an example of a thick quantum well.

[0031] FIG. 15B illustrates energy bands of an example of a thin quantum well.

[0032] FIG. 16A shows bandgap energy of examples of Ga.sub.xIn.sub.1-xP layers.

[0033] FIG. 16B shows measured relative external quantum efficiencies of micro-LEDs with different quantum well layers at a same current density.

[0034] FIG. 17 includes a flowchart illustrating an example of a method of manufacturing micro-LEDs with reduced surface recombination according to certain embodiments.

[0035] FIG. 18A illustrates an example of a method of die-to-wafer bonding for arrays of LEDs according to certain embodiments.

[0036] FIG. 18B illustrates an example of a method of wafer-to-wafer bonding for arrays of LEDs according to certain embodiments.

[0037] FIGS. 19A-19D illustrate an example of a method of hybrid bonding for arrays of LEDs according to certain embodiments.

[0038] FIG. 20 illustrates an example of an LED array with secondary optical components fabricated thereon according to certain embodiments.

[0039] FIG. 21 is a simplified block diagram of an electronic system of an example of a near-eye display according to certain embodiments.

[0040] The figures depict embodiments of the present disclosure for purposes of illustration only. One skilled in the art will readily recognize from the following description that alternative embodiments of the structures and methods illustrated may be employed without departing from the principles, or benefits touted, of this disclosure.

[0041] In the appended figures, similar components and/or features may have the same reference label. Further, various components of the same type may be distinguished by following the reference label by a dash and a second label that distinguishes among the similar components. If only the first reference label is used in the specification, the description is applicable to any one of the similar components having the same first reference label irrespective of the second reference label.

DETAILED DESCRIPTION

[0042] This disclosure relates generally to micro-light emitting diodes (micro-LEDs). More specifically, and without limitation, disclosed herein are techniques for improving the quantum efficiency of micro-LEDs with small physical dimensions, such as small AlGaInP-based red micro-LEDs, by reducing the non-radiative recombination at the mesa sidewalls. According to certain embodiments, the energy band of the light emitting layers of micro-LEDs is engineered to reduce the mobility of holes in the light emitting layers, thereby reducing the diffusion constant of holes and/or the surface recombination velocity at the mesa sidewalls of the micro-LEDs to confine the injected holes to the center regions of the light emitting layering. Various inventive embodiments are described herein, including devices, systems, structures, methods, materials, processes, and the like.

[0043] In semiconductor light emitting diodes (LEDs), photons are usually generated at a certain internal quantum efficiency (IQE) through the recombination of electrons and holes within an active region or light emitting region. The active region may include one or more semiconductor layers that may form one or more quantum wells. The IQE may indicate the proportion of the radiative electron-hole recombination that emits photons in the active region. The generated light may be extracted from the LEDs in a particular direction or within a particular solid angle. The ratio between the number of emitted photons extracted from an LED and the number of electrons or holes injected into the LED is generally referred to as the external quantum efficiency (EQE), which may describe how efficiently the LED converts injected electrons/holes to photons that are extracted from the LED. For LEDs, and in particular, micro-LEDs with reduced physical dimensions, the internal and external quantum efficiencies can be very low.

[0044] The quantum efficiency of LEDs may depend on the relative rates of competitive radiative (light producing) recombination and non-radiative (lossy) recombination that occur in the active region of the LEDs. Non-radiative recombination processes in the active region include Shockley-Read-Hall (SRH) recombination at defect sites, and electron-electron-hole (eeh) and/or electron-hole-hole (ehh) Auger recombination that involves three carriers. In micro-LEDs that include an array of mesa structures to form an array of micro-LEDs, because the lateral size (e.g., diameter or side) of the mesa structure of each micro-LED may be comparable to the minority carrier diffusion length, a larger proportion of the active region may be within a distance less than the minority carrier diffusion length from the mesa sidewalls. At the mesa sidewalls, the defect density of the active region may be very high due to the abrupt ending of the lattice structure, chemical contamination, and/or structural damages (e.g., due to dry etch).

[0045] Therefore, the non-radiative recombination rate may be high at the mesa sidewalls. As such, a larger proportion of the injected carriers may diffuse to regions near the mesa sidewalls and may be subjected to a higher SRH recombination rate. This may cause the peak efficiency of the LED to decrease and/or cause the peak efficiency operating current to increase. Increasing the current injection may cause the efficiencies of the micro-LEDs to drop due to the higher eeh or ehh Auger recombination rate at a higher current density. As the physical size of LEDs is further reduced, efficiency losses due to surface recombination near the etched mesa sidewalls that include surface imperfections may become much more significant.

[0046] Compared with, for example, GaN-based material systems, AlGaAs, InGaAlAsP, and AlGaInP materials may have high surface recombination velocities and minority carrier diffusion lengths. Thus, InGaAlAsP, AlGaAs, and AlGaInP-based red or near-infrared light-emitting devices (e.g. LEDs/VCSELs) may suffer from higher surface losses, especially for devices with active regions having lateral sizes less than about 50 .mu.m, less than about 20 .mu.m, less than about 10 .mu.m, less than about 5 .mu.m, less than about 3 .mu.m, or less than about 2 .mu.m. For example, carriers in AlGaInP materials can have high diffusivity (mobility), and AlGaInP materials may have an order of magnitude higher surface recombination velocity than III-nitride materials. Thus, the internal and external quantum efficiencies of red micro-LEDs may drop even more significantly as the device size reduces, due to the enhanced surface losses.

[0047] In general, it may be desirable to reduce the strains (e.g., compressive or tensile strain) in the epitaxial layers. The strains may increase the defect density and thus the non-radiative recombination in the active region. The strains may also cause other effects, such as polarization states and internal electrical fields induced by the piezo- and spontaneous polarization.

[0048] According to certain embodiments, to improve the light-emitting efficiency of a micro-LED, the light-emitting layers of the micro-LED may be made to have a higher in-plane compressive strain at the light-emitting region, which may cause a split of the heavy-hole (HH) sub-band and the light-hole (LH) sub-band in the valence band, with the HH sub-band being higher than the LH sub-band. As a result, most holes would be heavy holes that have a higher effective mass and a lower mobility, and thus are more likely to recombine before diffusing to the mesa sidewall region. Therefore, the diffusion of holes to the mesa sidewall area and thus the radiative recombination of holes with electrons at the mesa sidewalls may be reduced. In some embodiments, the in-plane compressive strain may be introduced in the active region by changing the composition of the active region, such as the In to Ga (or Al) ratio in GaInP quantum wells and/or barrier layers of the active region of AlGaInP-based red micro-LEDS.

[0049] The in-plane compressive strain in the active region may reduce the bandgap of the active region and thus may cause the wavelength of the emitted light to shift to a longer wavelength. To compensate for the wavelength shift, the quantum well thickness may be reduced, which may further separate the HH sub-band and the LH sub-band in the valence band and increase the confinement of the carriers in the quantum wells. As such, the diffusion of the holes to the mesa sidewall regions may be further reduced. The compressive strain and the quantum confinement may enable the splitting of the HH sub-band and the LH sub-band at room temperature.

[0050] In contrast, in semiconductor lasers, the valence band structure may be engineered to reduce the effective mass (and thus increase the mobility) of carriers (e.g., holes) in the valence band, in order to reduce the asymmetry between the effective mass of the conduction band and the effective mass of the valence band, intervalence band optical absorption, the current and carrier density for lasing, Auger recombination, and the like. Thus, an in-plane tensile strain, rather than the compressive strain described above for micro-LEDs, may need to be introduced in the active region to separate the HH sub-band and the LH sub-band in the opposite way (with the top of the LH sub-band higher than the top of the HH sub-band) to increase the relative population of light holes.

[0051] The micro-LEDs described herein may be used in conjunction with various technologies, such as an artificial reality system. An artificial reality system, such as a head-mounted display (HMD) or heads-up display (HUD) system, generally includes a display configured to present artificial images that depict objects in a virtual environment. The display may present virtual objects or combine images of real objects with virtual objects, as in virtual reality (VR), augmented reality (AR), or mixed reality (MR) applications. For example, in an AR system, a user may view both displayed images of virtual objects (e.g., computer-generated images (CGIs)) and the surrounding environment by, for example, seeing through transparent display glasses or lenses (often referred to as optical see-through) or viewing displayed images of the surrounding environment captured by a camera (often referred to as video see-through). In some AR systems, the artificial images may be presented to users using an LED-based display subsystem.

[0052] As used herein, the term “light emitting diode (LED)” refers to a light source that includes at least an n-type semiconductor layer, a p-type semiconductor layer, and a light-emitting region (i.e., active region) between the n-type semiconductor layer and the p-type semiconductor layer. The light-emitting region may include one or more semiconductor layers that form one or more heterostructures, such as quantum wells. In some embodiments, the light-emitting region may include multiple semiconductor layers that form one or more multiple-quantum-wells (MQWs), each including multiple (e.g., about 2 to 8) quantum wells.

[0053] As used herein, the term “micro-LED” or “.mu.LED” refers to an LED that has a chip where a lateral linear dimension (e.g., the diameter or a side) of the active region of the chip is less than about 200 nm, such as less than 100 nm, less than 50 nm, less than 20 nm, less than 10 nm, or smaller. For example, the linear dimension of a micro-LED may be as small as 6 nm, 5 nm, 4 nm, 2 nm, or smaller. Some micro-LEDs may have active regions (e.g., mesas) with a linear dimension (e.g., length or diameter) comparable to the minority carrier diffusion length. However, the disclosure herein is not limited to micro-LEDs, and may also be applied to mini-LEDs. As used herein, the lateral linear size of a micro-LED may refer to the lateral linear dimension of the active region or the mesa structure of the micro-LED, such as the diameter or side of the mesa structure or the active region.

[0054] As used herein, the term “bonding” may refer to various methods for physically and/or electrically connecting two or more devices and/or wafers, such as adhesive bonding, metal-to-metal bonding, metal oxide bonding, wafer-to-wafer bonding, die-to-wafer bonding, hybrid bonding, soldering, under-bump metallization, and the like. For example, adhesive bonding may use a curable adhesive (e.g., an epoxy) to physically bond two or more devices and/or wafers through adhesion. Metal-to-metal bonding may include, for example, wire bonding or flip chip bonding using soldering interfaces (e.g., pads or balls), conductive adhesive, or welded joints between metals. Metal oxide bonding may form a metal and oxide pattern on each surface, bond the oxide sections together, and then bond the metal sections together to create a conductive path. Wafer-to-wafer bonding may bond two wafers (e.g., silicon wafers or other semiconductor wafers) without any intermediate layers and is based on chemical bonds between the surfaces of the two wafers. Wafer-to-wafer bonding may include wafer cleaning and other preprocessing, aligning and pre-bonding at room temperature, and annealing at elevated temperatures, such as about 250.degree. C. or higher. Die-to-wafer bonding may use bumps on one wafer to align features of a pre-formed chip with drivers of a wafer. Hybrid bonding may include, for example, wafer cleaning, high-precision alignment of contacts of one wafer with contacts of another wafer, dielectric bonding of dielectric materials within the wafers at room temperature, and metal bonding of the contacts by annealing at, for example, 250-300.degree. C. or higher. As used herein, the term “bump” may refer generically to a metal interconnect used or formed during bonding.

[0055] In the following description, for the purposes of explanation, specific details are set forth in order to provide a thorough understanding of examples of the disclosure. However, it will be apparent that various examples may be practiced without these specific details. For example, devices, systems, structures, assemblies, methods, and other components may be shown as components in block diagram form in order not to obscure the examples in unnecessary detail. In other instances, well-known devices, processes, systems, structures, and techniques may be shown without necessary detail in order to avoid obscuring the examples. The figures and description are not intended to be restrictive. The terms and expressions that have been employed in this disclosure are used as terms of description and not of limitation, and there is no intention in the use of such terms and expressions of excluding any equivalents of the features shown and described or portions thereof. The word “example” is used herein to mean “serving as an example, instance, or illustration.” Any embodiment or design described herein as “example” is not necessarily to be construed as preferred or advantageous over other embodiments or designs.

[0056] FIG. 1 is a simplified block diagram of an example of an artificial reality system environment 100 including a near-eye display 120 in accordance with certain embodiments. Artificial reality system environment 100 shown in FIG. 1 may include near-eye display 120, an optional external imaging device 150, and an optional input/output interface 140, each of which may be coupled to an optional console 110. While FIG. 1 shows an example of artificial reality system environment 100 including one near-eye display 120, one external imaging device 150, and one input/output interface 140, any number of these components may be included in artificial reality system environment 100, or any of the components may be omitted. For example, there may be multiple near-eye displays 120 monitored by one or more external imaging devices 150 in communication with console 110. In some configurations, artificial reality system environment 100 may not include external imaging device 150, optional input/output interface 140, and optional console 110. In alternative configurations, different or additional components may be included in artificial reality system environment 100.

[0057] Near-eye display 120 may be a head-mounted display that presents content to a user. Examples of content presented by near-eye display 120 include one or more of images, videos, audio, or any combination thereof. In some embodiments, audio may be presented via an external device (e.g., speakers and/or headphones) that receives audio information from near-eye display 120, console 110, or both, and presents audio data based on the audio information. Near-eye display 120 may include one or more rigid bodies, which may be rigidly or non-rigidly coupled to each other. A rigid coupling between rigid bodies may cause the coupled rigid bodies to act as a single rigid entity. A non-rigid coupling between rigid bodies may allow the rigid bodies to move relative to each other. In various embodiments, near-eye display 120 may be implemented in any suitable form-factor, including a pair of glasses. Some embodiments of near-eye display 120 are further described below with respect to FIGS. 2 and 3. Additionally, in various embodiments, the functionality described herein may be used in a headset that combines images of an environment external to near-eye display 120 and artificial reality content (e.g., computer-generated images). Therefore, near-eye display 120 may augment images of a physical, real-world environment external to near-eye display 120 with generated content (e.g., images, video, sound, etc.) to present an augmented reality to a user.

[0058] In various embodiments, near-eye display 120 may include one or more of display electronics 122, display optics 124, and an eye-tracking unit 130. In some embodiments, near-eye display 120 may also include one or more locators 126, one or more position sensors 128, and an inertial measurement unit (IMU) 132. Near-eye display 120 may omit any of eye-tracking unit 130, locators 126, position sensors 128, and IMU 132, or include additional elements in various embodiments. Additionally, in some embodiments, near-eye display 120 may include elements combining the function of various elements described in conjunction with FIG. 1.

[0059] Display electronics 122 may display or facilitate the display of images to the user according to data received from, for example, console 110. In various embodiments, display electronics 122 may include one or more display panels, such as a liquid crystal display (LCD), an organic light emitting diode (OLED) display, an inorganic light emitting diode (ILED) display, a micro light emitting diode (.mu.LED) display, an active-matrix OLED display (AMOLED), a transparent OLED display (TOLED), or some other display. For example, in one implementation of near-eye display 120, display electronics 122 may include a front TOLED panel, a rear display panel, and an optical component (e.g., an attenuator, polarizer, or diffractive or spectral film) between the front and rear display panels. Display electronics 122 may include pixels to emit light of a predominant color such as red, green, blue, white, or yellow. In some implementations, display electronics 122 may display a three-dimensional (3D) image through stereoscopic effects produced by two-dimensional panels to create a subjective perception of image depth. For example, display electronics 122 may include a left display and a right display positioned in front of a user’s left eye and right eye, respectively. The left and right displays may present copies of an image shifted horizontally relative to each other to create a stereoscopic effect (i.e., a perception of image depth by a user viewing the image).

[0060] In certain embodiments, display optics 124 may display image content optically (e.g., using optical waveguides and couplers) or magnify image light received from display electronics 122, correct optical errors associated with the image light, and present the corrected image light to a user of near-eye display 120. In various embodiments, display optics 124 may include one or more optical elements, such as, for example, a substrate, optical waveguides, an aperture, a Fresnel lens, a convex lens, a concave lens, a filter, input/output couplers, or any other suitable optical elements that may affect image light emitted from display electronics 122. Display optics 124 may include a combination of different optical elements as well as mechanical couplings to maintain relative spacing and orientation of the optical elements in the combination. One or more optical elements in display optics 124 may have an optical coating, such as an anti-reflective coating, a reflective coating, a filtering coating, or a combination of different optical coatings.

[0061] Magnification of the image light by display optics 124 may allow display electronics 122 to be physically smaller, weigh less, and consume less power than larger displays. Additionally, magnification may increase a field of view of the displayed content. The amount of magnification of image light by display optics 124 may be changed by adjusting, adding, or removing optical elements from display optics 124. In some embodiments, display optics 124 may project displayed images to one or more image planes that may be further away from the user’s eyes than near-eye display 120.

[0062] Display optics 124 may also be designed to correct one or more types of optical errors, such as two-dimensional optical errors, three-dimensional optical errors, or any combination thereof. Two-dimensional errors may include optical aberrations that occur in two dimensions.

[0063] Example types of two-dimensional errors may include barrel distortion, pincushion distortion, longitudinal chromatic aberration, and transverse chromatic aberration. Three-dimensional errors may include optical errors that occur in three dimensions. Example types of three-dimensional errors may include spherical aberration, comatic aberration, field curvature, and astigmatism.

[0064] Locators 126 may be objects located in specific positions on near-eye display 120 relative to one another and relative to a reference point on near-eye display 120. In some implementations, console 110 may identify locators 126 in images captured by external imaging device 150 to determine the artificial reality headset’s position, orientation, or both. A locator 126 may be an LED, a corner cube reflector, a reflective marker, a type of light source that contrasts with an environment in which near-eye display 120 operates, or any combination thereof. In embodiments where locators 126 are active components (e.g., LEDs or other types of light emitting devices), locators 126 may emit light in the visible band (e.g., about 380 nm to 750 nm), in the infrared (IR) band (e.g., about 750 nm to 1 mm), in the ultraviolet band (e.g., about 10 nm to about 380 nm), in another portion of the electromagnetic spectrum, or in any combination of portions of the electromagnetic spectrum.

[0065] External imaging device 150 may include one or more cameras, one or more video cameras, any other device capable of capturing images including one or more of locators 126, or any combination thereof. Additionally, external imaging device 150 may include one or more filters (e.g., to increase signal to noise ratio). External imaging device 150 may be configured to detect light emitted or reflected from locators 126 in a field of view of external imaging device 150. In embodiments where locators 126 include passive elements (e.g., retroreflectors), external imaging device 150 may include a light source that illuminates some or all of locators 126, which may retro-reflect the light to the light source in external imaging device 150. Slow calibration data may be communicated from external imaging device 150 to console 110, and external imaging device 150 may receive one or more calibration parameters from console 110 to adjust one or more imaging parameters (e.g., focal length, focus, frame rate, sensor temperature, shutter speed, aperture, etc.).

[0066] Position sensors 128 may generate one or more measurement signals in response to motion of near-eye display 120. Examples of position sensors 128 may include accelerometers, gyroscopes, magnetometers, other motion-detecting or error-correcting sensors, or any combination thereof. For example, in some embodiments, position sensors 128 may include multiple accelerometers to measure translational motion (e.g., forward/back, up/down, or left/right) and multiple gyroscopes to measure rotational motion (e.g., pitch, yaw, or roll). In some embodiments, various position sensors may be oriented orthogonally to each other.

[0067] IMU 132 may be an electronic device that generates fast calibration data based on measurement signals received from one or more of position sensors 128. Position sensors 128 may be located external to IMU 132, internal to IMU 132, or any combination thereof. Based on the one or more measurement signals from one or more position sensors 128, IMU 132 may generate fast calibration data indicating an estimated position of near-eye display 120 relative to an initial position of near-eye display 120. For example, IMU 132 may integrate measurement signals received from accelerometers over time to estimate a velocity vector and integrate the velocity vector over time to determine an estimated position of a reference point on near-eye display 120. Alternatively, IMU 132 may provide the sampled measurement signals to console 110, which may determine the fast calibration data. While the reference point may generally be defined as a point in space, in various embodiments, the reference point may also be defined as a point within near-eye display 120 (e.g., a center of IMU 132).

[0068] Eye-tracking unit 130 may include one or more eye-tracking systems. Eye tracking may refer to determining an eye’s position, including orientation and location of the eye, relative to near-eye display 120. An eye-tracking system may include an imaging system to image one or more eyes and may optionally include a light emitter, which may generate light that is directed to an eye such that light reflected by the eye may be captured by the imaging system. For example, eye-tracking unit 130 may include a non-coherent or coherent light source (e.g., a laser diode) emitting light in the visible spectrum or infrared spectrum, and a camera capturing the light reflected by the user’s eye. As another example, eye-tracking unit 130 may capture reflected radio waves emitted by a miniature radar unit. Eye-tracking unit 130 may use low-power light emitters that emit light at frequencies and intensities that would not injure the eye or cause physical discomfort. Eye-tracking unit 130 may be arranged to increase contrast in images of an eye captured by eye-tracking unit 130 while reducing the overall power consumed by eye-tracking unit 130 (e.g., reducing power consumed by a light emitter and an imaging system included in eye-tracking unit 130). For example, in some implementations, eye-tracking unit 130 may consume less than 100 milliwatts of power.

[0069] Near-eye display 120 may use the orientation of the eye to, e.g., determine an inter-pupillary distance (IPD) of the user, determine gaze direction, introduce depth cues (e.g., blur image outside of the user’s main line of sight), collect heuristics on the user interaction in the VR media (e.g., time spent on any particular subject, object, or frame as a function of exposed stimuli), some other functions that are based in part on the orientation of at least one of the user’s eyes, or any combination thereof. Because the orientation may be determined for both eyes of the user, eye-tracking unit 130 may be able to determine where the user is looking. For example, determining a direction of a user’s gaze may include determining a point of convergence based on the determined orientations of the user’s left and right eyes. A point of convergence may be the point where the two foveal axes of the user’s eyes intersect. The direction of the user’s gaze may be the direction of a line passing through the point of convergence and the mid-point between the pupils of the user’s eyes.

[0070] Input/output interface 140 may be a device that allows a user to send action requests to console 110. An action request may be a request to perform a particular action. For example, an action request may be to start or to end an application or to perform a particular action within the application. Input/output interface 140 may include one or more input devices. Example input devices may include a keyboard, a mouse, a game controller, a glove, a button, a touch screen, or any other suitable device for receiving action requests and communicating the received action requests to console 110. An action request received by the input/output interface 140 may be communicated to console 110, which may perform an action corresponding to the requested action. In some embodiments, input/output interface 140 may provide haptic feedback to the user in accordance with instructions received from console 110. For example, input/output interface 140 may provide haptic feedback when an action request is received, or when console 110 has performed a requested action and communicates instructions to input/output interface 140. In some embodiments, external imaging device 150 may be used to track input/output interface 140, such as tracking the location or position of a controller (which may include, for example, an IR light source) or a hand of the user to determine the motion of the user. In some embodiments, near-eye display 120 may include one or more imaging devices to track input/output interface 140, such as tracking the location or position of a controller or a hand of the user to determine the motion of the user.

[0071] Console 110 may provide content to near-eye display 120 for presentation to the user in accordance with information received from one or more of external imaging device 150, near-eye display 120, and input/output interface 140. In the example shown in FIG. 1, console 110 may include an application store 112, a headset tracking module 114, an artificial reality engine 116, and an eye-tracking module 118. Some embodiments of console 110 may include different or additional modules than those described in conjunction with FIG. 1. Functions further described below may be distributed among components of console 110 in a different manner than is described here.

[0072] In some embodiments, console 110 may include a processor and a non-transitory computer-readable storage medium storing instructions executable by the processor. The processor may include multiple processing units executing instructions in parallel. The non-transitory computer-readable storage medium may be any memory, such as a hard disk drive, a removable memory, or a solid-state drive (e.g., flash memory or dynamic random access memory (DRAM)). In various embodiments, the modules of console 110 described in conjunction with FIG. 1 may be encoded as instructions in the non-transitory computer-readable storage medium that, when executed by the processor, cause the processor to perform the functions further described below.

[0073] Application store 112 may store one or more applications for execution by console 110. An application may include a group of instructions that, when executed by a processor, generates content for presentation to the user. Content generated by an application may be in response to inputs received from the user via movement of the user’s eyes or inputs received from the input/output interface 140. Examples of the applications may include gaming applications, conferencing applications, video playback application, or other suitable applications.

[0074] Headset tracking module 114 may track movements of near-eye display 120 using slow calibration information from external imaging device 150. For example, headset tracking module 114 may determine positions of a reference point of near-eye display 120 using observed locators from the slow calibration information and a model of near-eye display 120. Headset tracking module 114 may also determine positions of a reference point of near-eye display 120 using position information from the fast calibration information. Additionally, in some embodiments, headset tracking module 114 may use portions of the fast calibration information, the slow calibration information, or any combination thereof, to predict a future location of near-eye display 120. Headset tracking module 114 may provide the estimated or predicted future position of near-eye display 120 to artificial reality engine 116.

[0075] Artificial reality engine 116 may execute applications within artificial reality system environment 100 and receive position information of near-eye display 120, acceleration information of near-eye display 120, velocity information of near-eye display 120, predicted future positions of near-eye display 120, or any combination thereof from headset tracking module 114. Artificial reality engine 116 may also receive estimated eye position and orientation information from eye-tracking module 118. Based on the received information, artificial reality engine 116 may determine content to provide to near-eye display 120 for presentation to the user. For example, if the received information indicates that the user has looked to the left, artificial reality engine 116 may generate content for near-eye display 120 that mirrors the user’s eye movement in a virtual environment. Additionally, artificial reality engine 116 may perform an action within an application executing on console 110 in response to an action request received from input/output interface 140, and provide feedback to the user indicating that the action has been performed. The feedback may be visual or audible feedback via near-eye display 120 or haptic feedback via input/output interface 140.

[0076] Eye-tracking module 118 may receive eye-tracking data from eye-tracking unit 130 and determine the position of the user’s eye based on the eye tracking data. The position of the eye may include an eye’s orientation, location, or both relative to near-eye display 120 or any element thereof. Because the eye’s axes of rotation change as a function of the eye’s location in its socket, determining the eye’s location in its socket may allow eye-tracking module 118 to more accurately determine the eye’s orientation.

[0077] FIG. 2 is a perspective view of an example of a near-eye display in the form of an HMD device 200 for implementing some of the examples disclosed herein. HMD device 200 may be a part of, e.g., a VR system, an AR system, an MR system, or any combination thereof. HMD device 200 may include a body 220 and a head strap 230. FIG. 2 shows a bottom side 223, a front side 225, and a left side 227 of body 220 in the perspective view. Head strap 230 may have an adjustable or extendible length. There may be a sufficient space between body 220 and head strap 230 of HMD device 200 for allowing a user to mount HIVID device 200 onto the user’s head. In various embodiments, HMD device 200 may include additional, fewer, or different components. For example, in some embodiments, HMD device 200 may include eyeglass temples and temple tips as shown in, for example, FIG. 3 below, rather than head strap 230.

[0078] HMD device 200 may present to a user media including virtual and/or augmented views of a physical, real-world environment with computer-generated elements. Examples of the media presented by HMD device 200 may include images (e.g., two-dimensional (2D) or three-dimensional (3D) images), videos (e.g., 2D or 3D videos), audio, or any combination thereof.

[0079] The images and videos may be presented to each eye of the user by one or more display assemblies (not shown in FIG. 2) enclosed in body 220 of HMD device 200. In various embodiments, the one or more display assemblies may include a single electronic display panel or multiple electronic display panels (e.g., one display panel for each eye of the user). Examples of the electronic display panel(s) may include, for example, an LCD, an OLED display, an ILED display, a .mu.LED display, an AMOLED, a TOLED, some other display, or any combination thereof. HMD device 200 may include two eye box regions.

[0080] In some implementations, HMD device 200 may include various sensors (not shown), such as depth sensors, motion sensors, position sensors, and eye tracking sensors. Some of these sensors may use a structured light pattern for sensing. In some implementations, HMD device 200 may include an input/output interface for communicating with a console. In some implementations, HMD device 200 may include a virtual reality engine (not shown) that can execute applications within HMD device 200 and receive depth information, position information, acceleration information, velocity information, predicted future positions, or any combination thereof of HMD device 200 from the various sensors. In some implementations, the information received by the virtual reality engine may be used for producing a signal (e.g., display instructions) to the one or more display assemblies. In some implementations, HMD device 200 may include locators (not shown, such as locators 126) located in fixed positions on body 220 relative to one another and relative to a reference point. Each of the locators may emit light that is detectable by an external imaging device.

[0081] FIG. 3 is a perspective view of an example of a near-eye display 300 in the form of a pair of glasses for implementing some of the examples disclosed herein. Near-eye display 300 may be a specific implementation of near-eye display 120 of FIG. 1, and may be configured to operate as a virtual reality display, an augmented reality display, and/or a mixed reality display. Near-eye display 300 may include a frame 305 and a display 310. Display 310 may be configured to present content to a user. In some embodiments, display 310 may include display electronics and/or display optics. For example, as described above with respect to near-eye display 120 of FIG. 1, display 310 may include an LCD display panel, an LED display panel, or an optical display panel (e.g., a waveguide display assembly).

[0082] Near-eye display 300 may further include various sensors 350a, 350b, 350c, 350d, and 350e on or within frame 305. In some embodiments, sensors 350a-350e may include one or more depth sensors, motion sensors, position sensors, inertial sensors, or ambient light sensors. In some embodiments, sensors 350a-350e may include one or more image sensors configured to generate image data representing different fields of views in different directions. In some embodiments, sensors 350a-350e may be used as input devices to control or influence the displayed content of near-eye display 300, and/or to provide an interactive VR/AR/MR experience to a user of near-eye display 300. In some embodiments, sensors 350a-350e may also be used for stereoscopic imaging.

[0083] In some embodiments, near-eye display 300 may further include one or more illuminators 330 to project light into the physical environment. The projected light may be associated with different frequency bands (e.g., visible light, infra-red light, ultra-violet light, etc.), and may serve various purposes. For example, illuminator(s) 330 may project light in a dark environment (or in an environment with low intensity of infra-red light, ultra-violet light, etc.) to assist sensors 350a-350e in capturing images of different objects within the dark environment. In some embodiments, illuminator(s) 330 may be used to project certain light patterns onto the objects within the environment. In some embodiments, illuminator(s) 330 may be used as locators, such as locators 126 described above with respect to FIG. 1.

[0084] In some embodiments, near-eye display 300 may also include a high-resolution camera 340. Camera 340 may capture images of the physical environment in the field of view. The captured images may be processed, for example, by a virtual reality engine (e.g., artificial reality engine 116 of FIG. 1) to add virtual objects to the captured images or modify physical objects in the captured images, and the processed images may be displayed to the user by display 310 for AR or MR applications.

[0085] FIG. 4 illustrates an example of an optical see-through augmented reality system 400 including a waveguide display according to certain embodiments. Augmented reality system 400 may include a projector 410 and a combiner 415. Projector 410 may include a light source or image source 412 and projector optics 414. In some embodiments, light source or image source 412 may include one or more micro-LED devices described above. In some embodiments, image source 412 may include a plurality of pixels that displays virtual objects, such as an LCD display panel or an LED display panel. In some embodiments, image source 412 may include a light source that generates coherent or partially coherent light. For example, image source 412 may include a laser diode, a vertical cavity surface emitting laser, an LED, and/or a micro-LED described above. In some embodiments, image source 412 may include a plurality of light sources (e.g., an array of micro-LEDs described above), each emitting a monochromatic image light corresponding to a primary color (e.g., red, green, or blue). In some embodiments, image source 412 may include three two-dimensional arrays of micro-LEDs, where each two-dimensional array of micro-LEDs may include micro-LEDs configured to emit light of a primary color (e.g., red, green, or blue). In some embodiments, image source 412 may include an optical pattern generator, such as a spatial light modulator. Projector optics 414 may include one or more optical components that can condition the light from image source 412, such as expanding, collimating, scanning, or projecting light from image source 412 to combiner 415. The one or more optical components may include, for example, one or more lenses, liquid lenses, mirrors, apertures, and/or gratings. For example, in some embodiments, image source 412 may include one or more one-dimensional arrays or elongated two-dimensional arrays of micro-LEDs, and projector optics 414 may include one or more one-dimensional scanners (e.g., micro-mirrors or prisms) configured to scan the one-dimensional arrays or elongated two-dimensional arrays of micro-LEDs to generate image frames. In some embodiments, projector optics 414 may include a liquid lens (e.g., a liquid crystal lens) with a plurality of electrodes that allows scanning of the light from image source 412.

[0086] Combiner 415 may include an input coupler 430 for coupling light from projector 410 into a substrate 420 of combiner 415. Combiner 415 may transmit at least 50% of light in a first wavelength range and reflect at least 25% of light in a second wavelength range. For example, the first wavelength range may be visible light from about 400 nm to about 650 nm, and the second wavelength range may be in the infrared band, for example, from about 800 nm to about 1000 nm. Input coupler 430 may include a volume holographic grating, a diffractive optical element (DOE) (e.g., a surface-relief grating), a slanted surface of substrate 420, or a refractive coupler (e.g., a wedge or a prism). For example, input coupler 430 may include a reflective volume Bragg grating or a transmissive volume Bragg grating. Input coupler 430 may have a coupling efficiency of greater than 30%, 50%, 75%, 90%, or higher for visible light. Light coupled into substrate 420 may propagate within substrate 420 through, for example, total internal reflection (TIR). Substrate 420 may be in the form of a lens of a pair of eyeglasses. Substrate 420 may have a flat or a curved surface, and may include one or more types of dielectric materials, such as glass, quartz, plastic, polymer, poly(methyl methacrylate) (PMMA), crystal, or ceramic. A thickness of the substrate may range from, for example, less than about 1 mm to about 10 mm or more. Substrate 420 may be transparent to visible light.

[0087] Substrate 420 may include or may be coupled to a plurality of output couplers 440, each configured to extract at least a portion of the light guided by and propagating within substrate 420 from substrate 420, and direct extracted light 460 to an eyebox 495 where an eye 490 of the user of augmented reality system 400 may be located when augmented reality system 400 is in use. The plurality of output couplers 440 may replicate the exit pupil to increase the size of eyebox 495 such that the displayed image is visible in a larger area. As input coupler 430, output couplers 440 may include grating couplers (e.g., volume holographic gratings or surface-relief gratings), other diffraction optical elements (DOEs), prisms, etc. For example, output couplers 440 may include reflective volume Bragg gratings or transmissive volume Bragg gratings. Output couplers 440 may have different coupling (e.g., diffraction) efficiencies at different locations. Substrate 420 may also allow light 450 from the environment in front of combiner 415 to pass through with little or no loss. Output couplers 440 may also allow light 450 to pass through with little loss. For example, in some implementations, output couplers 440 may have a very low diffraction efficiency for light 450 such that light 450 may be refracted or otherwise pass through output couplers 440 with little loss, and thus may have a higher intensity than extracted light 460. In some implementations, output couplers 440 may have a high diffraction efficiency for light 450 and may diffract light 450 in certain desired directions (i.e., diffraction angles) with little loss. As a result, the user may be able to view combined images of the environment in front of combiner 415 and images of virtual objects projected by projector 410.

[0088] FIG. 5A illustrates an example of a near-eye display (NED) device 500 including a waveguide display 530 according to certain embodiments. NED device 500 may be an example of near-eye display 120, augmented reality system 400, or another type of display device. NED device 500 may include a light source 510, projection optics 520, and waveguide display 530. Light source 510 may include multiple panels of light emitters for different colors, such as a panel of red light emitters 512, a panel of green light emitters 514, and a panel of blue light emitters 516. The red light emitters 512 are organized into an array; the green light emitters 514 are organized into an array; and the blue light emitters 516 are organized into an array. The dimensions and pitches of light emitters in light source 510 may be small. For example, each light emitter may have a diameter less than 2 .mu.m (e.g., about 1.2 .mu.m) and the pitch may be less than 2 .mu.m (e.g., about 1.5 .mu.m). As such, the number of light emitters in each red light emitters 512, green light emitters 514, and blue light emitters 516 can be equal to or greater than the number of pixels in a display image, such as 960.times.720, 1280.times.720, 1440.times.1080, 1920.times.1080, 2160.times.1080, or 2560.times.1080 pixels. Thus, a display image may be generated simultaneously by light source 510. A scanning element may not be used in NED device 500.

[0089] Before reaching waveguide display 530, the light emitted by light source 510 may be conditioned by projection optics 520, which may include a lens array. Projection optics 520 may collimate or focus the light emitted by light source 510 to waveguide display 530, which may include a coupler 532 for coupling the light emitted by light source 510 into waveguide display 530. The light coupled into waveguide display 530 may propagate within waveguide display 530 through, for example, total internal reflection as described above with respect to FIG. 4. Coupler 532 may also couple portions of the light propagating within waveguide display 530 out of waveguide display 530 and towards user’s eye 590.

[0090] FIG. 5B illustrates an example of a near-eye display (NED) device 550 including a waveguide display 580 according to certain embodiments. In some embodiments, NED device 550 may use a scanning mirror 570 to project light from a light source 540 to an image field where a user’s eye 590 may be located. NED device 550 may be an example of near-eye display 120, augmented reality system 400, or another type of display device. Light source 540 may include one or more rows or one or more columns of light emitters of different colors, such as multiple rows of red light emitters 542, multiple rows of green light emitters 544, and multiple rows of blue light emitters 546. For example, red light emitters 542, green light emitters 544, and blue light emitters 546 may each include N rows, each row including, for example, 2560 light emitters (pixels). The red light emitters 542 are organized into an array; the green light emitters 544 are organized into an array; and the blue light emitters 546 are organized into an array. In some embodiments, light source 540 may include a single line of light emitters for each color. In some embodiments, light source 540 may include multiple columns of light emitters for each of red, green, and blue colors, where each column may include, for example, 1080 light emitters. In some embodiments, the dimensions and/or pitches of the light emitters in light source 540 may be relatively large (e.g., about 3-5 .mu.m) and thus light source 540 may not include sufficient light emitters for simultaneously generating a full display image. For example, the number of light emitters for a single color may be fewer than the number of pixels (e.g., 2560.times.1080 pixels) in a display image. The light emitted by light source 540 may be a set of collimated or diverging beams of light.

[0091] Before reaching scanning mirror 570, the light emitted by light source 540 may be conditioned by various optical devices, such as collimating lenses or a freeform optical element 560. Freeform optical element 560 may include, for example, a multi-facet prism or another light folding element that may direct the light emitted by light source 540 towards scanning mirror 570, such as changing the propagation direction of the light emitted by light source 540 by, for example, about 90.degree. or larger. In some embodiments, freeform optical element 560 may be rotatable to scan the light. Scanning mirror 570 and/or freeform optical element 560 may reflect and project the light emitted by light source 540 to waveguide display 580, which may include a coupler 582 for coupling the light emitted by light source 540 into waveguide display 580. The light coupled into waveguide display 580 may propagate within waveguide display 580 through, for example, total internal reflection as described above with respect to FIG. 4. Coupler 582 may also couple portions of the light propagating within waveguide display 580 out of waveguide display 580 and towards user’s eye 590.

[0092] Scanning mirror 570 may include a microelectromechanical system (MEMS) mirror or any other suitable mirrors. Scanning mirror 570 may rotate to scan in one or two dimensions. As scanning mirror 570 rotates, the light emitted by light source 540 may be directed to a different area of waveguide display 580 such that a full display image may be projected onto waveguide display 580 and directed to user’s eye 590 by waveguide display 580 in each scanning cycle. For example, in embodiments where light source 540 includes light emitters for all pixels in one or more rows or columns, scanning mirror 570 may be rotated in the column or row direction (e.g., x or y direction) to scan an image. In embodiments where light source 540 includes light emitters for some but not all pixels in one or more rows or columns, scanning mirror 570 may be rotated in both the row and column directions (e.g., both x and y directions) to project a display image (e.g., using a raster-type scanning pattern).

[0093] NED device 550 may operate in predefined display periods. A display period (e.g., display cycle) may refer to a duration of time in which a full image is scanned or projected. For example, a display period may be a reciprocal of the desired frame rate. In NED device 550 that includes scanning mirror 570, the display period may also be referred to as a scanning period or scanning cycle. The light generation by light source 540 may be synchronized with the rotation of scanning mirror 570. For example, each scanning cycle may include multiple scanning steps, where light source 540 may generate a different light pattern in each respective scanning step.

[0094] In each scanning cycle, as scanning mirror 570 rotates, a display image may be projected onto waveguide display 580 and user’s eye 590. The actual color value and light intensity (e.g., brightness) of a given pixel location of the display image may be an average of the light beams of the three colors (e.g., red, green, and blue) illuminating the pixel location during the scanning period. After completing a scanning period, scanning mirror 570 may revert back to the initial position to project light for the first few rows of the next display image or may rotate in a reverse direction or scan pattern to project light for the next display image, where a new set of driving signals may be fed to light source 540. The same process may be repeated as scanning mirror 570 rotates in each scanning cycle. As such, different images may be projected to user’s eye 590 in different scanning cycles.

[0095] FIG. 6 illustrates an example of an image source assembly 610 in a near-eye display system 600 according to certain embodiments. Image source assembly 610 may include, for example, a display panel 640 that may generate display images to be projected to the user’s eyes, and a projector 650 that may project the display images generated by display panel 640 to a waveguide display as described above with respect to FIGS. 4-5B. Display panel 640 may include a light source 642 and a driver circuit 644 for light source 642. Light source 642 may include, for example, light source 510 or 540. Projector 650 may include, for example, freeform optical element 560, scanning mirror 570, and/or projection optics 520 described above. Near-eye display system 600 may also include a controller 620 that synchronously controls light source 642 and projector 650 (e.g., scanning mirror 570). Image source assembly 610 may generate and output an image light to a waveguide display (not shown in FIG. 6), such as waveguide display 530 or 580. As described above, the waveguide display may receive the image light at one or more input-coupling elements, and guide the received image light to one or more output-coupling elements. The input and output coupling elements may include, for example, a diffraction grating, a holographic grating, a prism, or any combination thereof. The input-coupling element may be chosen such that total internal reflection occurs with the waveguide display. The output-coupling element may couple portions of the total internally reflected image light out of the waveguide display.

[0096] As described above, light source 642 may include a plurality of light emitters arranged in an array or a matrix. Each light emitter may emit monochromatic light, such as red light, blue light, green light, infra-red light, and the like. While RGB colors are often discussed in this disclosure, embodiments described herein are not limited to using red, green, and blue as primary colors. Other colors can also be used as the primary colors of near-eye display system 600. In some embodiments, a display panel in accordance with an embodiment may use more than three primary colors. Each pixel in light source 642 may include three subpixels that include a red micro-LED, a green micro-LED, and a blue micro-LED. A semiconductor LED generally includes an active light-emitting layer within multiple layers of semiconductor materials. The multiple layers of semiconductor materials may include different compound materials or a same base material with different dopants and/or different doping densities. For example, the multiple layers of semiconductor materials may include an n-type material layer, an active region that may include hetero-structures (e.g., one or more quantum wells), and a p-type material layer. The multiple layers of semiconductor materials may be grown on a surface of a substrate having a certain orientation. In some embodiments, to increase light extraction efficiency, a mesa that includes at least some of the layers of semiconductor materials may be formed.

[0097] Controller 620 may control the image rendering operations of image source assembly 610, such as the operations of light source 642 and/or projector 650. For example, controller 620 may determine instructions for image source assembly 610 to render one or more display images.

[0098] The instructions may include display instructions and scanning instructions. In some embodiments, the display instructions may include an image file (e.g., a bitmap file). The display instructions may be received from, for example, a console, such as console 110 described above with respect to FIG. 1. The scanning instructions may be used by image source assembly 610 to generate image light. The scanning instructions may specify, for example, a type of a source of image light (e.g., monochromatic or polychromatic), a scanning rate, an orientation of a scanning apparatus, one or more illumination parameters, or any combination thereof. Controller 620 may include a combination of hardware, software, and/or firmware not shown here so as not to obscure other aspects of the present disclosure.

[0099] In some embodiments, controller 620 may be a graphics processing unit (GPU) of a display device. In other embodiments, controller 620 may be other kinds of processors. The operations performed by controller 620 may include taking content for display and dividing the content into discrete sections. Controller 620 may provide to light source 642 scanning instructions that include an address corresponding to an individual source element of light source 642 and/or an electrical bias applied to the individual source element. Controller 620 may instruct light source 642 to sequentially present the discrete sections using light emitters corresponding to one or more rows of pixels in an image ultimately displayed to the user. Controller 620 may also instruct projector 650 to perform different adjustments of the light. For example, controller 620 may control projector 650 to scan the discrete sections to different areas of a coupling element of the waveguide display (e.g., waveguide display 580) as described above with respect to FIG. 5B. As such, at the exit pupil of the waveguide display, each discrete portion is presented in a different respective location. While each discrete section is presented at a different respective time, the presentation and scanning of the discrete sections occur fast enough such that a user’s eye may integrate the different sections into a single image or series of images.

[0100] Image processor 630 may be a general-purpose processor and/or one or more application-specific circuits that are dedicated to performing the features described herein. In one embodiment, a general-purpose processor may be coupled to a memory to execute software instructions that cause the processor to perform certain processes described herein. In another embodiment, image processor 630 may be one or more circuits that are dedicated to performing certain features. While image processor 630 in FIG. 6 is shown as a stand-alone unit that is separate from controller 620 and driver circuit 644, image processor 630 may be a sub-unit of controller 620 or driver circuit 644 in other embodiments. In other words, in those embodiments, controller 620 or driver circuit 644 may perform various image processing functions of image processor 630. Image processor 630 may also be referred to as an image processing circuit.

[0101] In the example shown in FIG. 6, light source 642 may be driven by driver circuit 644, based on data or instructions (e.g., display and scanning instructions) sent from controller 620 or image processor 630. In one embodiment, driver circuit 644 may include a circuit panel that connects to and mechanically holds various light emitters of light source 642. Light source 642 may emit light in accordance with one or more illumination parameters that are set by the controller 620 and potentially adjusted by image processor 630 and driver circuit 644. An illumination parameter may be used by light source 642 to generate light. An illumination parameter may include, for example, source wavelength, pulse rate, pulse amplitude, beam type (continuous or pulsed), other parameter(s) that may affect the emitted light, or any combination thereof. In some embodiments, the source light generated by light source 642 may include multiple beams of red light, green light, and blue light, or any combination thereof.

[0102] Projector 650 may perform a set of optical functions, such as focusing, combining, conditioning, or scanning the image light generated by light source 642. In some embodiments, projector 650 may include a combining assembly, a light conditioning assembly, or a scanning mirror assembly. Projector 650 may include one or more optical components that optically adjust and potentially re-direct the light from light source 642. One example of the adjustment of light may include conditioning the light, such as expanding, collimating, correcting for one or more optical errors (e.g., field curvature, chromatic aberration, etc.), some other adjustments of the light, or any combination thereof. The optical components of projector 650 may include, for example, lenses, mirrors, apertures, gratings, or any combination thereof.

[0103] Projector 650 may redirect image light via its one or more reflective and/or refractive portions so that the image light is projected at certain orientations toward the waveguide display. The location where the image light is redirected toward the waveguide display may depend on specific orientations of the one or more reflective and/or refractive portions. In some embodiments, projector 650 includes a single scanning mirror that scans in at least two dimensions. In other embodiments, projector 650 may include a plurality of scanning mirrors that each scan in directions orthogonal to each other. Projector 650 may perform a raster scan (horizontally or vertically), a bi-resonant scan, or any combination thereof. In some embodiments, projector 650 may perform a controlled vibration along the horizontal and/or vertical directions with a specific frequency of oscillation to scan along two dimensions and generate a two-dimensional projected image of the media presented to user’s eyes. In other embodiments, projector 650 may include a lens or prism that may serve similar or the same function as one or more scanning mirrors. In some embodiments, image source assembly 610 may not include a projector, where the light emitted by light source 642 may be directly incident on the waveguide display.

[0104] The overall efficiency of a photonic integrated circuit or a waveguide-based display (e.g., in augmented reality system 400 or NED device 500 or 550) may be a product of the efficiency of individual components and may also depend on how the components are connected. For example, the overall efficiency .gamma..sub.tot of the waveguide-based display in augmented reality system 400 may depend on the light-emitting efficiency of image source 412, the light coupling efficiency from image source 412 into combiner 415 by projector optics 414 and input coupler 430, and the output coupling efficiency of output coupler 440, and thus may be determined as:

.gamma..sub.tot=.gamma..sub.EQE.times..gamma..sub.in.times..gamma..sub.o- ut (1)

where .gamma..sub.NQE is the external quantum efficiency of image source 412, .gamma..sub.in is the in-coupling efficiency of light from image source 412 into the waveguide (e.g., substrate 420), and .gamma..sub.out is the outcoupling efficiency of light from the waveguide towards the user’s eye by output coupler 440. Thus, the overall efficiency .gamma..sub.tot of the waveguide-based display can be improved by improving one or more of .gamma..sub.EQE, .gamma..sub.in, and .gamma..sub.out.

[0105] The optical coupler (e.g., input coupler 430 or coupler 532) that couples the emitted light from a light source to a waveguide may include, for example, a grating, a lens, a micro-lens, a prism. In some embodiments, light from a small light source (e.g., a micro-LED) can be directly (e.g., end-to-end) coupled from the light source to a waveguide, without using an optical coupler. In some embodiments, the optical coupler (e.g., a lens or a parabolic-shaped reflector) may be manufactured on the light source.

[0106] The light sources, image sources, or other displays described above may include one or more LEDs. For example, each pixel in a display may include three subpixels that include a red micro-LED, a green micro-LED, and a blue micro-LED. A semiconductor light emitting diode generally includes an active light-emitting layer within multiple layers of semiconductor materials. The multiple layers of semiconductor materials may include different compound materials or a same base material with different dopants and/or different doping densities. For example, the multiple layers of semiconductor materials may generally include an n-type material layer, an active layer that may include hetero-structures (e.g., one or more quantum wells), and a p-type material layer. The multiple layers of semiconductor materials may be grown on a surface of a substrate having a certain orientation.

[0107] Photons can be generated in a semiconductor LED (e.g., a micro-LED) at a certain internal quantum efficiency through the recombination of electrons and holes within the active layer (e.g., including one or more semiconductor layers). The generated light may then be extracted from the LEDs in a particular direction or within a particular solid angle. The ratio between the number of emitted photons extracted from the LED and the number of electrons passing through the LED is referred to as the external quantum efficiency, which describes how efficiently the LED converts injected electrons to photons that are extracted from the device. The external quantum efficiency may be proportional to the injection efficiency, the internal quantum efficiency, and the extraction efficiency. The injection efficiency refers to the proportion of electrons passing through the device that are injected into the active region. The extraction efficiency is the proportion of photons generated in the active region that escape from the device. For LEDs, and in particular, micro-LEDs with reduced physical dimensions, improving the internal and external quantum efficiency can be challenging. In some embodiments, to increase the light extraction efficiency, a mesa that includes at least some of the layers of semiconductor materials may be formed.

[0108] FIG. 7A illustrates an example of an LED 700 having a vertical mesa structure. LED 700 may be a light emitter in light source 510, 540, or 642. LED 700 may be a micro-LED made of inorganic materials, such as multiple layers of semiconductor materials. The layered semiconductor light emitting device may include multiple layers of III-V semiconductor materials. A III-V semiconductor material may include one or more Group III elements, such as aluminum (Al), gallium (Ga), or indium (In), in combination with a Group V element, such as nitrogen (N), phosphorus (P), arsenic (As), or antimony (Sb). When the Group V element of the III-V semiconductor material includes nitrogen, the III-V semiconductor material is referred to as a III-nitride material. The layered semiconductor light emitting device may be manufactured by growing multiple epitaxial layers on a substrate using techniques such as vapor-phase epitaxy

[0109] (VPE), liquid-phase epitaxy (LPE), molecular beam epitaxy (MBE), or metalorganic chemical vapor deposition (MOCVD). For example, the layers of the semiconductor materials may be grown layer-by-layer on a substrate with a certain crystal lattice orientation (e.g., polar, nonpolar, or semi-polar orientation), such as a GaN, GaAs, or GaP substrate, or a substrate including, but not limited to, sapphire, silicon carbide, silicon, zinc oxide, boron nitride, lithium aluminate, lithium niobate, germanium, aluminum nitride, lithium gallate, partially substituted spinels, or quaternary tetragonal oxides sharing the beta-LiAO.sub.2 structure, where the substrate may be cut in a specific direction to expose a specific plane as the growth surface.

[0110] In the example shown in FIG. 7A, LED 700 may include a substrate 710, which may include, for example, a sapphire substrate or a GaN substrate. A semiconductor layer 720 may be grown on substrate 710. Semiconductor layer 720 may include a III-V material, such as GaN, and may be p-doped (e.g., with Mg, Ca, Zn, or Be) or n-doped (e.g., with Si or Ge). One or more active layers 730 may be grown on semiconductor layer 720 to form an active region. Active layer 730 may include III-V materials, such as one or more InGaN layers, one or more AlGaInP layers, and/or one or more GaN layers, which may form one or more heterostructures, such as one or more quantum wells or MQWs. A semiconductor layer 740 may be grown on active layer 730. Semiconductor layer 740 may include a III-V material, such as GaN, and may be p-doped (e.g., with Mg, Ca, Zn, or Be) or n-doped (e.g., with Si or Ge). One of semiconductor layer 720 and semiconductor layer 740 may be a p-type layer and the other one may be an n-type layer. Semiconductor layer 720 and semiconductor layer 740 sandwich active layer 730 to form the light-emitting region. For example, LED 700 may include a layer of InGaN situated between a layer of p-type GaN doped with magnesium and a layer of n-type GaN doped with silicon or oxygen. In some embodiments, LED 700 may include a layer of AlGaInP situated between a layer of p-type AlGaInP doped with zinc or magnesium and a layer of n-type AlGaInP doped with selenium, silicon, or tellurium. It is generally desirable to reduce the strains in the semiconductor layers because the strains may cause defects in the crystal lattice, introduce polarization states, and induce internal electric fields by the piezo- and spontaneous polarization.

[0111] In some embodiments, an electron-blocking layer (EBL) (not shown in FIG. 7A) may be grown to form a layer between active layer 730 and at least one of semiconductor layer 720 or semiconductor layer 740. The EBL may reduce the electron leakage current and improve the efficiency of the LED. In some embodiments, a heavily-doped semiconductor layer 750, such as a P.sup.+ or P.sup.++ semiconductor layer, may be formed on semiconductor layer 740 and act as a contact layer for forming an ohmic contact and reducing the contact impedance of the device. In some embodiments, a conductive layer 760 may be formed on heavily-doped semiconductor layer 750. Conductive layer 760 may include, for example, an indium tin oxide (ITO) or Al/Ni/Au film. In one example, conductive layer 760 may include a transparent ITO layer.

[0112] To make contact with semiconductor layer 720 (e.g., an n-GaN layer) and to more efficiently extract light emitted by active layer 730 from LED 700, the semiconductor material layers (including heavily-doped semiconductor layer 750, semiconductor layer 740, active layer 730, and semiconductor layer 720) may be etched to expose semiconductor layer 720 and to form a mesa structure that includes layers 720-760. The mesa structure may confine the carriers within the device. Etching the mesa structure may lead to the formation of mesa sidewalls 732 that may be orthogonal to the growth planes. A passivation layer 770 may be formed on mesa sidewalls 732 of the mesa structure. Passivation layer 770 may include an oxide layer, such as a SiO2 layer, and may act as a reflector to reflect emitted light out of LED 700. A contact layer 780, which may include a metal layer, such as Al, Au, Ni, Ti, or any combination thereof, may be formed on semiconductor layer 720 and may act as an electrode of LED 700. In addition, another contact layer 790, such as an Al/Ni/Au metal layer, may be formed on conductive layer 760 and may act as another electrode of LED 700.

[0113] When a voltage signal is applied to contact layers 780 and 790, electrons and holes may recombine in active layer 730, where the recombination of electrons and holes may cause photon emission. The wavelength and energy of the emitted photons may depend on the energy bandgap between the valence band and the conduction band in active layer 730. For example, InGaN active layers may emit green or blue light, AlGaN active layers may emit blue to ultraviolet light, while AlGaInP active layers may emit red, orange, yellow, or green light. The emitted photons may be reflected by passivation layer 770 and may exit LED 700 from the top (e.g., conductive layer 760 and contact layer 790) or bottom (e.g., substrate 710).

[0114] In some embodiments, LED 700 may include one or more other components, such as a lens, on the light emission surface, such as substrate 710, to focus or collimate the emitted light or couple the emitted light into a waveguide. In some embodiments, an LED may include a mesa of another shape, such as planar, conical, semi-parabolic, or parabolic, and a base area of the mesa may be circular, rectangular, hexagonal, or triangular. For example, the LED may include a mesa of a curved shape (e.g., paraboloid shape) and/or a non-curved shape (e.g., conical shape). The mesa may be truncated or non-truncated.

[0115] FIG. 7B is a cross-sectional view of an example of an LED 705 having a parabolic mesa structure. Similar to LED 700, LED 705 may include multiple layers of semiconductor materials, such as multiple layers of III-V semiconductor materials. The semiconductor material layers may be epitaxially grown on a substrate 715, such as a GaN substrate or a sapphire substrate. For example, a semiconductor layer 725 may be grown on substrate 715. Semiconductor layer 725 may include a III-V material, such as GaN, and may be p-doped (e.g., with Mg, Ca, Zn, or Be) or n-doped (e.g., with Si or Ge). One or more active layer 735 may be grown on semiconductor layer 725. Active layer 735 may include III-V materials, such as one or more InGaN layers, one or more AlGaInP layers, and/or one or more GaN layers, which may form one or more heterostructures, such as one or more quantum wells. A semiconductor layer 745 may be grown on active layer 735. Semiconductor layer 745 may include a III-V material, such as GaN, and may be p-doped (e.g., with Mg, Ca, Zn, or Be) or n-doped (e.g., with Si or Ge).

[0116] One of semiconductor layer 725 and semiconductor layer 745 may be a p-type layer and the other one may be an n-type layer.

[0117] To make contact with semiconductor layer 725 (e.g., an n-type GaN layer) and to more efficiently extract light emitted by active layer 735 from LED 705, the semiconductor layers may be etched to expose semiconductor layer 725 and to form a mesa structure that includes layers 725-745. The mesa structure may confine carriers within the injection area of the device. Etching the mesa structure may lead to the formation of mesa side walls (also referred to herein as facets) that may be non-parallel with, or in some cases, orthogonal, to the growth planes associated with crystalline growth of layers 725-745.

[0118] As shown in FIG. 7B, LED 705 may have a mesa structure that includes a flat top. A dielectric layer 775 (e.g., SiO.sub.2 or SiNx) may be formed on the facets of the mesa structure. In some embodiments, dielectric layer 775 may include multiple layers of dielectric materials. In some embodiments, a metal layer 795 may be formed on dielectric layer 775. Metal layer 795 may include one or more metal or metal alloy materials, such as aluminum (Al), silver (Ag), gold (Au), platinum (Pt), titanium (Ti), copper (Cu), or any combination thereof. Dielectric layer 775 and metal layer 795 may form a mesa reflector that can reflect light emitted by active layer 735 toward substrate 715. In some embodiments, the mesa reflector may be parabolic-shaped to act as a parabolic reflector that may at least partially collimate the emitted light.

[0119] Electrical contact 765 and electrical contact 785 may be formed on semiconductor layer 745 and semiconductor layer 725, respectively, to act as electrodes. Electrical contact 765 and electrical contact 785 may each include a conductive material, such as Al, Au, Pt, Ag, Ni, Ti, Cu, or any combination thereof (e.g., Ag/Pt/Au or Al/Ni/Au), and may act as the electrodes of LED 705. In the example shown in FIG. 7B, electrical contact 785 may be an n-contact, and electrical contact 765 may be a p-contact. Electrical contact 765 and semiconductor layer 745 (e.g., a p-type semiconductor layer) may form a back reflector for reflecting light emitted by active layer 735 back toward substrate 715. In some embodiments, electrical contact 765 and metal layer 795 include same material(s) and can be formed using the same processes. In some embodiments, an additional conductive layer (not shown) may be included as an intermediate conductive layer between the electrical contacts 765 and 785 and the semiconductor layers.

[0120] When a voltage signal is applied across electrical contacts 765 and 785, electrons and holes may recombine in active layer 735. The recombination of electrons and holes may cause photon emission, thus producing light. The wavelength and energy of the emitted photons may depend on the energy bandgap between the valence band and the conduction band in active layer 735. For example, InGaN active layers may emit green or blue light, while AlGaInP active layers may emit red, orange, yellow, or green light. The emitted photons may propagate in many different directions, and may be reflected by the mesa reflector and/or the back reflector and may exit LED 705, for example, from the bottom side (e.g., substrate 715) shown in FIG. 7B. One or more other secondary optical components, such as a lens or a grating, may be formed on the light emission surface, such as substrate 715, to focus or collimate the emitted light and/or couple the emitted light into a waveguide.

[0121] When the mesa structure is formed (e.g., etched), the facets of the mesa structure, such as mesa sidewalls 732, may include some imperfections, such as unsatisfied bonds, chemical contamination, and structural damages (e.g., when dry-etched), that may decrease the internal quantum efficiency of the LED. For example, at the facets, the atomic lattice structure of the semiconductor layers may end abruptly, where some atoms of the semiconductor materials may lack neighbors to which bonds may be attached. This results in “dangling bonds,” which may be characterized by unpaired valence electrons. These dangling bonds create energy levels that otherwise would not exist within the bandgap of the semiconductor material, causing non-radiative electron-hole recombination at or near the facets of the mesa structure. Thus, these imperfections may become the recombination centers where electrons and holes may be confined until they combine non-radiatively.

[0122] As described above, the internal quantum efficiency may indicate the proportion of the radiative electron-hole recombination in the active region that emits photons. The internal quantum efficiency of LEDs may depend on the relative rates of competitive radiative (light producing) recombination and non-radiative (lossy) recombination that occur in the active region of the LEDs. Non-radiative recombination processes in the active region may include Shockley-Read-Hall (SRH) recombination at defect sites and eeh/ehh Auger recombination that involves three carriers. The internal quantum efficiency of an LED may be approximately determined by:

IQE = BN 2 AN + BN 3 + CN 3 , ( 2 ) ##EQU00001##

where A, B and C are the rates of SRH recombination, bimolecular (radiative) recombination, and Auger recombination, respectively, and N is the charge-carrier density (i.e., charge-carrier concentration) in the active region.

[0123] FIG. 8 illustrates the relationship between the optical emission power and the current density of a light emitting diode. As illustrated by a curve 810 in FIG. 8, the optical emission power of a micro-LED device may be low when the current density (and thus the charge carrier density N) is low, where the low external quantum efficiency may be caused by the relatively high non-radiative SRH recombination when the charge carrier density N is low according to equation (2). As the current density (and thus the charge carrier density N) increases, the optical emission power may increase as shown by a curve 820 in FIG. 8, because the radiative recombination may increase at a higher rate (.varies.N.sup.2) than the non-radiative SRH recombination (.varies.N) when the charge carrier density N is high according to equation (2). As the current density increases further, the optical emission power may increase at a slower rate as shown by a curve 830 in FIG. 8 and thus the external quantum efficiency may drop because, for example, the non-radiative Auger recombination may increase at a higher rate (.varies.N.sup.3) than the radiative recombination (.varies.N.sup.2) when the charge carrier density N is sufficiently high according to equation (2).

[0124] Auger recombination may be a major cause of efficiency droop and may be direct or indirect. For example, direct Auger recombination occurs when an electron and a hole recombine, but instead of producing light, either an electron is raised higher into the conduction band or a hole is pushed deeper into the valence band. Auger recombination may be reduced to mitigate the efficiency droop by lowering the charge-carrier density N in the active region for a given injection current density J, which may be written as:

J=qd.sub.eff (AN+BN.sup.2+CN.sup.3) (3)

where d.sub.eff is the effective thickness of the active region. Thus, according to equation (3), the effect of the Auger recombination may be reduced and thus the IQE of the LED may be improved by reducing the charge-carrier density N for a given injection current density, which may be achieved by increasing the effective thickness of the active region d.sub.eff. The effective thickness of the active region may be increased by, for example, growing multiple quantum wells (MQWs). Alternatively, an active region including a single thick double heterostructure (DH) may be used to increase the effective thickness of the active region.

[0125] One factor affecting the effective thickness of the active region is the presence of internal fields E.sub.qw (e.g., strain-induced internal field) in the quantum wells. Internal fields E.sub.qw may localize charge carriers and reduce the overlap integral between carrier wave functions, which may reduce the radiative efficiency of LEDs. Some LEDs including heterostructures (e.g., quantum wells) may have a strong internal strain-induced piezoelectric field in the carrier transport direction. The strain-induced internal field may cause the electron and hole energy levels to shift (thus changing the bandgap) and cause the electrons and holes to shift to opposite sides of a quantum well, thereby decreasing the spatial electron-hole overlap and reducing the radiative recombination efficiency and thus the internal quantum efficiency of the LED.

[0126] While the Auger recombination due to a high current density (and high charge carrier density) may be an intrinsic process depending on material properties, non-radiative SRH recombination depends on the characteristics and the quality of material, such as the defect density in the active region. As described above with respect to FIGS. 7A and 7B, LEDs may be fabricated by etching mesa structures into the active emitting layers to confine carriers within the mesa structures of the individual LEDs and to expose the n-type material beneath the active emitting layers for electrical contact. Etching the mesa structures may lead to the formation of mesa sidewalls that may be approximately orthogonal to the growth plane. Due to the etching, the active region in proximity to the exposed sidewalls may have a higher density of defects, such as dislocations, dangling bonds, pores, grain boundaries, vacancies, inclusion of precipitates, and the like. The defects may introduce energy states having deep or shallow energy levels in the bandgap. Carriers may be trapped by these energy states until they combine non-radiatively. Therefore, the active region in proximity to the exposed sidewalls may have a higher rate of SRH recombination than the bulk region that is far from the sidewalls.

[0127] Parameters that may affect the impact of the non-radiative surface recombination on the LED efficiency may include, for example, the surface recombination velocity (SRV) S, the carrier diffusion coefficient (diffusivity) D, and the carrier lifetime .tau.. The high recombination rate in the vicinity of the sidewalls due to the high defect density may depend on the number of excess carriers (in particular, the minority carriers) in the region. The high recombination rate may deplete the carriers in the region. The depletion of the carriers in the region may cause carriers to diffuse to the region from surrounding regions with higher carrier concentrations. Thus, the amount of surface recombination may depend on the surface recombination velocity S at which carriers move to the regions near the sidewalls. The carrier lifetime .tau. is the average time that a carrier can spend in an excited state after the electron-hole generation but before the carrier recombines with another carrier. The carrier lifetime ti generally depends on the carrier concentration and the recombination rate in the active region. The carrier diffusion coefficient D of the material and the carrier lifetime .tau. may determine the carrier diffusion length L= {square root over (D.tau.)}, which is the average distance a carrier can travel from the time of generation until it recombines with another carrier. The carrier diffusion length L characterizes the width of the region that is adjacent to a sidewall surface of the active region and where the contribution of surface recombination to the carrier losses is significant. Charge carriers injected or diffused into the regions that are within a minority carrier diffusion length from the sidewall surfaces may be subject to the higher SRH recombination rate.

[0128] A higher current density (e.g., in units of amps/cm.sup.2) may associate with a lower surface recombination velocity as the surface defects may be more and more saturated at higher carrier densities. Thus, the surface recombination velocity may be reduced by increasing the current density. In addition, the diffusion length of a given material may vary with the current density at which the device is operated. However, LEDs generally may not be able operate at very high current densities. Increasing the current injection may also cause the efficiencies of the micro-LEDs to drop due to the higher Auger recombination rate and the lower conversion efficiency at higher temperatures caused by self-heating at the higher current density.

[0129] For traditional, broad area LEDs used in lighting and backlighting applications (e.g., with a lateral device area about 0.1 mm.sup.2 to about 1 mm.sup.2), the sidewalls are at the far ends of the devices. The devices can be designed such that little or no current is injected into regions within a minority carrier diffusion length from the mesa sidewalls, and thus the sidewall surface area to volume ratio and the overall rate of SRH recombination may be low. However, in micro-LEDs, as the size of the LED is reduced to a value comparable to or having a same order of magnitude as the minority carrier diffusion length, the increased surface area to volume ratio may lead to a high carrier surface recombination rate, because a greater proportion of the total active region may fall within the minority carrier diffusion length from the LED sidewalls. Therefore, more injected carriers may be subjected to the higher SRH recombination rate. This can cause the leakage current of the LED to increase and the efficiency of the LED to decrease as the size of the LED decreases, and/or cause the peak efficiency operating current to increase as the size of the LED decreases. For example, for a first LED with a 100 .mu.m.times.100 .mu.m.times.2 .mu.m mesa, the side-wall surface area to volume ratio may be about 0.04. However, for a second LED with a 5 .mu.m.times.5 nm.times.2 .mu.m mesa, the side wall surface area to volume ratio may be about 0.8, which is about 20 times higher than the first LED. Thus, with a similar surface defect density, the SRH recombination coefficient of the second LED may be about 20 times higher as well. Therefore, the efficiency of the second LED may be significantly lower than the first LED.

[0130] FIG. 9A illustrates an example of a micro-LED 900 with a mesa structure 905 where surface recombination may occur. Micro-LED 900 may be an example of LED 700 or 705. Micro-LED 900 may include an n-type semiconductor layer 920 epitaxially grown on a substrate 910 that may be similar to substrate 710 or 715. In one example, substrate 910 may include a GaN substrate or a sapphire substrate, and n-type semiconductor layer 920 may include a layer of GaN doped with, for example, Si or Ge. In another example, substrate 910 may include a GaAs substrate. In the illustrated example, n-type semiconductor layer 920 may be partially etched during a mesa formation process after the epitaxially layers are grown, where mesa structure 905 may include a portion 930 of n-type semiconductor layer 920. One or more epitaxial layers, such as GaN barrier layers and InGaN quantum well layers, or AlGaInP barrier layers and GaInP quantum well layers, may be grown on n-type semiconductor layer 920 to form active layers 940 that includes quantum wells. A p-type semiconductor layer 950 may be grown on active layers 940. P-type semiconductor layer 950 may be doped with, for example, Mg, Ca, Zn, or Be. The layer stack may then be etched to form individual mesa structures 905 that each include a p region, an active region that includes active layers 940, and an n region. Mesa structure 905 may have a lateral linear dimension less than about 100 .mu.m, less than about 50 .mu.m, less than about 20 .mu.m, less than about 10 .mu.m, less than about 5 .mu.m, less than about 3 .mu.m, less than about 2 .mu.m, or smaller. P-contacts 960 and n-contacts 970 may be formed on the p regions and the exposed n regions of n-type semiconductor layer 920. Each p-contact 960 may include, for example, a metal layer (e.g., Al, Au, Ni, Ti, or any combination thereof), or an indium tin oxide (ITO) or Al/Ni/Au film. Each n-contact 970 may also include a layer of a metal material, such as Al, Au, Ni, or Ti.

[0131] Even though not shown in FIG. 9, a passivation layer, such as an oxide layer (e.g., a SiO.sub.2 layer) or another dielectric layer, may be formed on sidewalls of mesa structure 905. The passivation layer may have a lower refractive index than the active region and may function as a reflector (e.g., due to total internal reflection) to reflect certain emitted light out of micro-LED 900 as described above. A metal layer may be formed on the passivation layer to form a sidewall metal reflector. Even though FIG. 9 shows a vertical mesa structure 905, micro-LED 900 may include different mesa shapes, such as a conical, parabolic, inward-tilted, or outward-tilted mesa shape.

[0132] When a voltage signal is applied to p-contact 960 and n-contact 970, holes and electrons may be injected into active layers 940 from p-type semiconductor layer 950 and portion 930 of n-type semiconductor layer 920, respectively. The electrons and holes may recombine in the quantum wells of active layers 940, where the recombination of electrons and holes may cause photon emission. The emitted photons may be reflected by the passivation layer and may exit micro-LED 900 from the bottom (e.g., n-type semiconductor layer 920 side) or the top (e.g., p-contact 960 side). At the sidewalls of the mesa structure, active layers 940 may have a higher density of defects, such as dislocations, dangling bonds, pores, grain boundaries, vacancies, inclusion of precipitates, and the like, due to the abrupt ending of the lattice structure and the etching. Thus, holes and electrons injected into the quantum wells of active layers 940 may recombine at the defect sites, without generating photons. Thus, the internal quantum efficiency and the external quantum efficiency of micro-LED 900 may be low at least due to the losses caused by the non-radiative surface recombination.

[0133] FIG. 9B illustrates a simplified energy band structure of the active layers in the active region of the example of micro-LED 900 shown in FIG. 9A. A curve 980 shows the conduction band of the active region and a curve 980 shows the valence band of the active region. The active region of micro-LED 900 may include multiple quantum well layers each sandwiched by two barrier layers. In the example shown in FIG. 9B, the conduction band and the valence band of a barrier layer are shown by a level 982 and a level 992, respectively, and the conduction band and the valence band of a quantum well layer are shown by a level 984 and a level 994, respectively. As illustrated, the quantum well layer may have a lower bandgap between the conduction band and the valence band than the barrier layer. Thus, carriers (electrons and holes) injected into the active region may be confined by the energy barriers to the quantum well layers, where the electrons and holes may recombine to emit light.

[0134] AlGaInP material may have a high surface recombination velocity and minority carrier diffusion length than some other light emission materials, such as III-nitride materials. For example, red AlGaInP LEDs may generally operate at a reduced carrier concentration (e.g., about 10.sup.17 to 10.sup.18 cm.sup.-3), and thus may have a relatively long carrier lifetime r. The carrier diffusivity D in the active region in the undoped quantum wells of red AlGaInP LEDs may also be rather large. As a result, the carrier diffusion length L= {square root over (D.times..tau.)} can be, for example, about 10-25 .mu.m or longer in some devices. In addition, the surface recombination velocity of AlGaInP material may be an order of magnitude higher than the surface recombination velocities of III-nitride materials. Thus, compared with LED made of III-nitride materials (e.g., blue and green LEDs made of GaN), the internal and external quantum efficiencies of AlGaInP-based red LEDs can drop even more significantly as the device size decreases.

[0135] FIG. 10 illustrates surface recombination velocities of various III-V semiconductor materials. Bars 1010 in FIG. 10 show the ranges of reported SRV values of the III-V semiconductor materials, whereas symbols 1020 on bars 1010 indicate the common or average SRVs. A box 1030 shows a general trend of the variation of the surface recombination velocity with the change of the material bandgap. As illustrated in FIG. 10, the average SRV is high in GaAs (e.g., about 10.sup.6 cm/s) compared to InP (e.g., about 10.sup.5 cm/s) or GaN (e.g., less than about 0.5.times.10.sup.5 cm/s). The surface recombination velocity of AlGaInP material (e.g., about 10.sup.6 cm/s) may be at least an order of magnitude higher than the surface recombination velocity of III-nitride materials (e.g., <10.sup.5 cm/s). In addition, in Al-containing alloys, such as AlGaInP, SRVs may scale appreciably with the Al fraction. For example, the SRV may increase from about 10.sup.5 cm/s for (Al.sub.0.1Ga.sub.0.9).sub.0.5In.sub.0.5P to about 10.sup.6 cm/s for Al.sub.0.51In.sub.0.49P.

[0136] In addition, nitride LEDs can operate at non-equilibrium carrier concentrations much higher than phosphide LEDs, which may result in considerably shorter carrier lifetime in nitride LEDs. Therefore, the carrier diffusion lengths in the active regions of III-nitride LEDs may be considerably shorter than the carrier diffusion lengths in phosphide LEDs. As such, phosphide LEDs, such as AlGaInP-based red micro-LEDs, may have both higher SRVs and longer carrier diffusion lengths, and thus may have much higher surface recombination and efficiency reduction than III-nitride LEDs.

[0137] Because the minority carrier (lateral) diffusion length in the active material of red micro-LED devices is much higher than, for example, the minority carrier (lateral) diffusion lengths in GaN-based material systems, red/NIR light-emitting devices (e.g. LEDs/VCSELs) based on AlGaInP, AlGaAs, or other material systems may suffer from high surface loss, especially for devices with lateral sizes less than about 50 .mu.m, such as less than about 20 .mu.m, less than about 10 .mu.m, less than about 5 .mu.m, less than about 3 .mu.m, or less than about 2 .mu.m, leading to much lower quantum efficiencies.

[0138] The non-radiative surface recombination may be reduced by passivating the surface with a suitable dielectric material, such as SiO.sub.2, SiN.sub.x, or Al.sub.2O.sub.3. The SRV may be reduced by etching away highly defective surface material using a chemical treatment. Alternatively or in addition, surface recombination may be reduced by decreasing the lateral carrier mobility. In one example, the lateral carrier mobility may be decreased by using ion implantation to disrupt the semiconductor lattice outside of a central portion of the micro-LED. The ion implantation may reduce the number of carriers that can reach the sidewall surface of the micro-LED, and therefore may reduce the amount of surface recombination. Bombarding the semiconductor material with high-energy ions may have two effects. First, the lattice of the semiconductor material may become less electrically conductive, such that the current may not spread through the entire structure in all directions, but may instead be funneled vertically through the central region. Second, the diffusivity is reduced in the bombarded region, such that the carriers do not move as far laterally. Thus, both the diffusivity D and the electron diffusion length L may be reduced by the ion implantation.

[0139] Alternatively or additionally, the lateral carrier mobility may be decreased by using quantum well intermixing to change the composition of areas of the semiconductor layer outside of the central portion of the micro-LED. Quantum well intermixing may increase the bandgap in an outer region of the semiconductor layer by implanting ions in the outer region of the semiconductor layer and subsequently annealing the outer region of the semiconductor layer to intermix the ions with atoms within the outer region of the semiconductor layer. The quantum well intermixing may reduce the number of carriers that can reach the sidewall surface of the micro-LED, and therefore can reduce the amount of surface recombination.

[0140] Despite these efforts to reduce surface recombination, when the micro-LED mesa size reduces, the efficiency of the micro-LED generally decreases and the peak efficiency operating current density generally increases, due to the non-radiative surface recombination. The overall electrical-to-optical power conversion efficiency (referred to as the wall-plug efficiency) of small devices may be very low.

[0141] In semiconductor lasers, the valence band structure may be engineered to reduce the effective mass (and thus increase the mobility) of carriers (e.g., holes) in the valence band, in order to reduce the asymmetry between the conduction band mass and the valence band mass, the intervalence band optical absorption, the current and carrier density for lasing, the Auger recombination, and the like. In-plane tensile strain may be introduced in the active region of semiconductor lasers to separate the heavy-hole (HH) sub-band and the light-hole (LH) sub-band (with the top of the LH sub-band higher than the top of the HH sub-band) to increase the relative population of light holes and thus the effective mass of the holes in the valence band.

[0142] FIG. 11A includes a simplified energy band diagram 1100 illustrating energy bands of an example of a direct bandgap semiconductor material, such as InP, GaAs, GaN, ternary compounds (e.g., GaInP, AlInP, AlGaAs, and InGaAs), and quaternary compounds (e.g., AlGaInP, InGaAsP, and InAlAsP). The horizonal axis of energy band diagram 1100 represents the wave vector k, while the vertical axis of energy band diagram 1100 represents energy E.

[0143] Thus, curves in energy band diagram 1100 may be referred to as E-k curves. A curve 1110 shows the conduction band and a curve 1120 shows the valence band, where each point on curves 1110 and 1120 corresponds to a possible state (wave function) that may exist in the semiconductor material. Curves 1110 and 1120 may generally have a parabolic shape. Direct bandgap semiconductor materials have a valence band maximum corresponding to the conduction band minimum when the wave vector k is null. Therefore, an electronic transition from the conduction band’s lowest energy level to the valence band’s highest energy level is possible without any change of the wave vector value.

[0144] The effective mass of carriers may be determined based on the curvatures of curve 1110 and curve 1120. In general, the curvature of the curve (e.g., a parabola) is inversely proportional to the effective mass. As illustrated in FIG. 11A, curve 1110 may have a large curvature, and thus the effective mass of the electrons in the conduction band may be small. On the other hand, curve 1120 may have a small curvature, and thus the effective mass of the holes in the valence band may be large. As such, there may be an asymmetry between the light conduction mass and the heavy valence band mass. Due to the heavy valence band mass and the effective mass asymmetry, the hole quasi-Fermi level Fv may be above the top of the valence band, and the electron quasi-Fermi level Fc may be above the bottom of the conduction band. Therefore, the hole occupation probability at the top of the valence band may be low. As such, a large number of carriers may be needed for lasing.

[0145] FIG. 11B illustrates an example of an ideal energy band diagram 1102 of a direct gap semiconductor laser. In ideal energy band diagram 1102 for semiconductor lasers, a conduction band curve 1112 and a valence band curve 1122 have similar and large curvatures and thus have similar and low effective masses. The hole quasi-Fermi level F.sub.V may be at the top of the valence band, and the electron quasi-Fermi level F.sub.C may be at the bottom of the conduction band. Thus, the lasing condition can be fulfilled with a low carrier injection. The reduction in the carrier injection may reduce the Auger recombination described above and thus may improve the quantum efficiency. The desired large curvature of valence band curve 1122 and thus the low effective mass of the holes in the valence band in semiconductor lasers may be achieved by engineering the band structures through tensile strain in the light emitting region.

[0146] In contrast, according to certain embodiments, to improve the light-emitting efficiency of a micro-LED, the light-emitting layers of the micro-LED may be made to have a higher in-plane compressive strain (rather than a tensile strain) at the light-emitting region, which may cause a split of the HH sub-band and the LH sub-band in the valence band, with the HH sub-band being higher than the LH sub-band. As a result, most holes may be heavy holes that have a higher mass and a lower mobility, and thus are more likely to recombine before diffusing to the mesa sidewall regions. Therefore, the diffusion of holes to the mesa sidewall regions and hence the non-radiative recombination of holes with electrons at the mesa sidewall regions may be reduced.

[0147] FIG. 12A illustrates an energy band diagram 1200 of an example of a direct gap semiconductor material, such as InP, GaAs, GaN, ternary compounds (e.g., AlGaAs and

[0148] InGaAs), and quaternary compounds (e.g., InGaAsP and InAlAsP). In semiconductor materials, holes may experience different magnitudes of spin-orbit interaction due to the different symmetry of their spatial distribution. The different spin-orbit interaction may cause different properties of the holes, such as the effective mass that indicates how easy a hole may be moved. In FIG. 12A, a curve 1210 in energy band diagram 1200 shows the E-k curve of the conduction band. In the valence band, a curve 1220 shows the E-k curve of the heavy-hole band, whereas a curve 1230 shows the E-k curve of the light-hole band. In the example shown in FIG. 12A, the semiconductor material may have no internal strains, and the top of the heavy-hole band and the top of the light hole band may overlap.

[0149] FIG. 12B illustrates an energy band diagram 1202 of an example of a direct gap semiconductor material under a compressive strain. As illustrated, when the semiconductor material is under a stress, there may be a splitting of the degenerate heavy-hole valence band and light-hole valence band, causing an energy separation between the light-hole band and the heavy-hole band. When the stress is a compressive strain, the top of the heavy-hole band and the top of the light-hole band may separate, with the top of the heavy-hole band higher than the top of the light-hole band. Therefore, more holes in the valence band may be heavy holes that have a higher effective mass and thus a lower mobility.

[0150] FIG. 12C illustrates an energy band diagram 1204 of an example of a direct gap semiconductor material under a tensile strain. As illustrated, when the semiconductor material is under the tensile strain, there may be a splitting of the degenerate heavy-hole band and light-hole band, causing an energy separation between the light-hole band and the heavy-hole band. The top of the heavy-hole band and the top of the light-hole band may separate, with the top of the light-hole band higher than the top of the heavy-hole band. Therefore, more holes in the valence band may be light holes that have a lower effective mass and thus a higher mobility.

[0151] According to certain embodiments, in-plane compressive strain may be introduced in the active region to increase the proportion of heavy holes and the effective mass of the holes, thereby reducing the mobility of the holes and the diffusion of the holes to the mesa sidewall regions that may cause non-radiative recombination at the mesa sidewall regions. The in-plane compressive strain may be introduced in the quantum well layers by changing the difference between the lattice constants of the barrier layers and the quantum well layers. When the lattice constant of the quantum well layers is the same as the lattice constant of the barrier layers, there may be little or no strain in the quantum well layers. When the lattice constant of the quantum well layers is lower than that of the barrier layers, the barrier layers may pull the crystal lattices of the quantum well layers and thus there may be tensile strain in the quantum well layers. When the lattice constant of the quantum well layers is greater than that of the barrier layers, the barrier layers may compress the crystal lattices of the quantum well layers and thus there may be compressive strain in the quantum well layers. Thus, to introduce compressive strain in the quantum well layers, the lattice constant of the quantum well layers may need to be larger than that of the barrier layers. According to certain embodiments, the lattice constant of the quantum well layers may be increased by changing the composition of the semiconductor material in the quantum well layers, or the lattice constant of the barrier layers may be decreased by changing the composition of the semiconductor material in the barrier layers, during the epitaxial growth of the semiconductor layers.

[0152] FIG. 13 illustrates bandgap energy levels, corresponding wavelengths, and lattice constants of semiconductor materials having different compositions of Al, Ga, In, and P. In FIG. 13, the horizontal axis corresponds to the lattice constants of the different semiconductor materials, the primary vertical axis corresponds to the energy bandgaps of the semiconductor materials, and the secondary vertical axis shows the wavelengths corresponding to the energy bandgaps. Area 1310 delineated by solid lines corresponds to direct gap semiconductor materials, while area 1320 delineated by dashed lines corresponds to indirect gap semiconductor materials.

[0153] A line 1330 in FIG. 13 shows semiconductor materials having lattice constants matching the lattice constant of GaAs (e.g., about 5.65 .ANG.), which may be used as the substrate for growing the epitaxial layers of red micro-LEDs. As illustrated, (Al.sub.xGa.sub.1-x).sub.0.5In.sub.0.5P, which may be used as the barrier layers of red micro-LEDs, may have a lattice constant matching that of

[0154] GaAs, and thus can be epitaxial grown on a GaAs substrate with little or no defects or strains. Thus, when materials of the quantum well layers have lattice constants greater than about 5.65 .ANG., the quantum well layers may experience compressive strain due to the difference between the lattice constants of the quantum well layers and the barrier layers. For example, for AlGaInP-based red micro-LEDs grown on GaAs substrates, compressive strain may be introduced in the quantum well layers by using certain compositions of AlGaInP or GaInP semiconductor materials as the quantum well layers, such as Ga.sub.xIn.sub.1-zP materials with x less than about 0.5 or (Al.sub.xGa.sub.1-x)In.sub.1-yP materials with y less than about 0.5.

[0155] FIG. 14 illustrates an example of a micro-LED 1400 with compressive strain introduced in quantum well layers 1440 to improve the quantum efficiency according to certain embodiments. Micro-LED 1400 may include an n-type semiconductor layer 1420 epitaxially grown on a substrate 1410 that may be similar to substrate 710 or 715. In the illustrated example, substrate 1410 may include a GaAs substrate, and n-type semiconductor layer 1420 may include AlInP or AlGaInP doped with, for example, selenium, silicon, tellurium, or germanium. A plurality of barrier layers 1430 and a plurality of quantum well layers 1440 may be alternately grown on n-type semiconductor layer 1420 to form multiple quantum wells. Barrier layers 1430 may include, for example, (Al.sub.xGa.sub.1-x).sub.0.5In.sub.0.5P, which may match GaAs in the lattice constant as described above. Quantum well layers 1440 may include, for example, Ga.sub.xIn.sub.1-xP with x less than about 0.5 or (Al.sub.xGa.sub.1-x).sub.yIn.sub.1-yP with y less than about 0.5. As such, quantum well layers 1440 may have a larger lattice constant than barrier layers 1430, and thus may experience in-plane compressive strain applied by barrier layers 1430 that have a smaller lattice constant. For example, the lattice constant of quantum well layers 1440 may be greater than about 101% or greater than about 102% of the lattice constant of barrier layers 1430. Each quantum well layer 1440 may be characterized by an energy band structure including a heavy-hole band and a light-hole band in the valence band, where a top of the heavy-hole band may be higher than a top of the light-hole band by about 0.075 eV (or about 3 kT at room temperature) or higher. As described above, in some embodiments, the compressive strain may be applied to the quantum well layers by reducing the lattice constants of the barrier layers. A p-type semiconductor layer 1450 may be grown on the multiple quantum wells. P-type semiconductor layer 1450 may include, for example, p-doped AlInP or AlGaInP. P-type semiconductor layer 1450 may be doped with, for example, Mg, Ca, Zn, or Be. Micro-LED 1400 may be configured to emit red light, such as light with a wavelength greater than about 600 nm.

[0156] The epitaxial layer stack including p-type semiconductor layer 1450, barrier layers 1430, quantum well layers 1440, and n-type semiconductor layer 1420 may be etched to form individual mesa structures of a micro-LED array. N-type semiconductor layer 1420 may be partially etched during the mesa formation process. Each mesa structure may have a lateral linear dimension less than about 50 .mu.m, less than about 20 .mu.m, less than about 10 .mu.m, less than about 5 .mu.m, less than about 3 .mu.m, less than about 2 .mu.m, or smaller. P-contacts 1460 and n-contacts 1470 may be formed on p-type semiconductor layer 1450 and the exposed regions on n-type semiconductor layer 1420. P-contact 1460 may include, for example, a metal layer (e.g., Al, Au, Ni, Ti, or any combination thereof), or an indium tin oxide (ITO) or Al/Ni/Au film. N-contacts 1470 may also include a layer of a metal material, such as Al, Au, Ni, or Ti.

[0157] Even though not shown in FIG. 14, a passivation layer, such as an oxide layer (e.g., a SiO.sub.2 layer) or another dielectric layer (e.g., SiN), may be formed on sidewalls 1442 of mesa structure 1405. The passivation layer may have a lower refractive index than the active region and may function as a reflector (e.g., due to total internal reflection) to reflect certain emitted light out of micro-LED 1400 as described above. A metal layer may be formed on the passivation layer to form a sidewall metal reflector. Even though FIG. 14 shows a vertical mesa structure 1405, micro-LED 1400 may include different mesa shapes, such as a conical, parabolic, inward-tilted, or outward-tilted mesa shape.

[0158] When a voltage signal is applied to p-contact 1460 and n-contact 1470, holes and electrons may be injected into quantum well layers 1440 from p-type semiconductor layer 1450 and n-type semiconductor layer 1420, respectively. The electrons and holes may recombine in quantum well layers 1440, where the recombination of electrons and holes may cause photon emission. Due to the compressive strain in quantum well layers 1440, the mobility of the holes may be low as described above and thus fewer injected holes may move to the sidewall regions of the mesa structure to non-radiatively recombine with electrons. The emitted photons may be reflected by the passivation layer or metal reflectors and may exit micro-LED 1400 from the bottom (e.g., n-type semiconductor layer 1420 side) or the top (e.g., p-contact 1460 side).

[0159] In some implementations, a quantum well material with a larger lattice constant to introduce in-plane compressive strains in the quantum wells may have a lower bandgap as shown in FIG. 13. Thus, the wavelength of the emitted light may shift to a longer wavelength due to the reduced bandgap of the quantum wells. To compensate for the wavelength shift, the quantum well thickness may be reduced, which may further separate the HH sub-band and the LH sub-band in the valence band and increase the confinement of the carriers in the quantum wells. As such, the diffusion of the holes to the mesa sidewall regions may be further reduced. The compressive strain and/or the quantum confinement may enable the sufficient splitting of the HH sub-band and the LH sub-band at room temperature.

[0160] FIG. 15A illustrates energy bands of an example of a thick quantum well 1500. A curve 1510 and a curve 1520 show the conduction band and the valence band of quantum well 1500, respectively. As illustrated, due to the large thickness, carriers may spread throughout the thickness of the quantum well and may have a lower density within the quantum well.

[0161] FIG. 15B illustrates energy bands of an example of a thin quantum well 1505. Thin quantum well 1505 may be an example of a quantum well of micro-LED 1400. A curve 1530 and a curve 1540 show the conduction band and the valence band of quantum well 1500, respectively. Thin quantum well 1505 may have a thickness less than the thickness of each adjacent barrier layer, such as less than a half of the thickness of the adjacent barrier layers. The quantum well layer having a larger lattice constant and a lower thickness between two adjacent barrier layers having a higher thickness and a smaller lattice constant may have a higher compressive strain throughout the entire thickness of the quantum well layer. Due to the lower thickness and the confinement of carriers by the barrier layers, the carriers may have a higher density in quantum well 1505. The effective bandgap may be increased to emit light with a shorter wavelength. In addition, the HH sub-band and the LH sub-band in the valence band may be further separated, such that more holes in the valence band may be heavy holes, thereby further increasing the effective mass and lowering the mobility of the holes to reduce the diffusion of holes to the mesa sidewall regions. Therefore, the quantum efficiency may be further improved.

[0162] FIG. 16A shows bandgap energy of examples of Ga.sub.xIn.sub.1-xP layers having different compositions, different lattice constants, different strains, and different levels of quantum confinement. A point 1610 shows the bandgap energy of a first Ga.sub.xIn.sub.1-xP layer with a first x value such that the first Ga.sub.xIn.sub.1-xP layer has the same lattice constant as a GaAs substrate and thus has no in-plane strain. The first Ga.sub.xIn.sub.1-xP layer may have a higher thickness, and thus the carriers may spread throughout the thickness of the first Ga.sub.xIn.sub.1-xP layer and may have a lower density and a lower quantum confinement. The bandgap energy of the first Ga.sub.xIn.sub.1-xP layer may be lower than a desired level (e.g., about 2.0 eV) for achieving a desired wavelength of the emitted light. A point 1615 shows the bandgap energy of a second Ga.sub.xIn.sub.1-xP layer that also has the first x value, and thus may have the same lattice constant as the GaAs substrate and may have no in-plane strain. The second Ga.sub.xIn.sub.1-xP layer may have a lower thickness than the first Ga.sub.xIn.sub.1-xP layer and the effective bandgap energy of the second Ga.sub.xIn.sub.1-xP layer may be increased to the desired level to achieve the desired emission wavelength, due to the quantum confinement of carriers in the second Ga.sub.xIn.sub.1-xP layer.

[0163] A point 1620 shows the bandgap energy of a third Ga.sub.xIn.sub.1-xP layer that has a second x value greater than the first x value, and thus may have a lower lattice constant than the GaAs substrate and may experience a tensile strain. Due to the higher Ga concentration in third Ga.sub.xIn.sub.1-xP layer, the third Ga.sub.xIn.sub.1-xP layer may have a bandgap energy at the desired level. The third Ga.sub.xIn.sub.1-xP layer may have a higher thickness and thus the carriers may spread throughout the thickness of the third Ga.sub.xIn.sub.1-xP layer and may have a lower density and lower quantum confinement. As such, the bandgap energy of the third Ga.sub.xIn.sub.1-xP layer may not be increased due to the lower quantum confinement.

[0164] A point 1630 shows the bandgap energy of a fourth Ga.sub.xIn.sub.1-xP layer that has a third x value lower than the first x value, and thus may have a higher lattice constant than the GaAs substrate and may experience a compressive strain. Due to the higher In concentration, the bandgap energy of the fourth Ga.sub.xIn.sub.1-xP layer may be lower than the desired level. The fourth Ga.sub.xIn.sub.1-xP layer may have a higher thickness and thus the carriers may spread throughout the thickness of the fourth Ga.sub.xIn.sub.1-xP layer and may have a lower density and a lower quantum confinement. As such, the bandgap energy of the fourth Ga.sub.xIn.sub.1-xP layer may not be increased due to the lower quantum confinement. A point 1635 shows the bandgap energy of a fifth Ga.sub.xIn.sub.1-xP layer that also has the third x value (i.e., the same In concentration as the fourth Ga.sub.xIn.sub.1-xP layer), and thus may have a higher lattice constant than the GaAs substrate and may experience a compressive strain. The fifth Ga.sub.xIn.sub.1-xP layer may have a lower thickness than the fourth Ga.sub.xIn.sub.1-xP layer, and thus the effective bandgap energy of the fifth Ga.sub.xIn.sub.1-xP layer may be increased to the desired level to achieve the desired emission wavelength, due to the quantum confinement of carriers in the fifth Ga.sub.xIn.sub.1-xP quantum well layer.

[0165] FIG. 16B shows measured relative external quantum efficiencies of micro-LEDs with different quantum well layers at a same current density. In the illustrated example, the current density for the measurements is about 500 A/cm.sup.2. A point 1612 in FIG. 16B shows the measured EQE of a micro-LED with the second Ga.sub.xIn.sub.1-xP layer (indicated by point 1615) described above as the quantum well layer, where the actual measured EQE is normalized to 100% and used as a reference point for comparing the micro-LEDs with different quantum well layers. As described above, the second Ga.sub.xIn.sub.1-xP layer may have no or little in-plane strain. A point 1622 shows the normalized measured EQE of a micro-LED with the third Ga.sub.xIn.sub.1-xP layer (indicated by point 1620) described above as the quantum well layer. As described above, the third Ga.sub.xIn.sub.1-xP layer may have a tensile strain. Point 1622 shows that the EQE of the micro-LED with the tensile strain in the quantum well layer is only about 60% of the reference point. A point 1632 shows the normalized measured EQE of a micro-LED with the fifth Ga.sub.xIn.sub.1-xP layer (indicated by point 1635) described above as the quantum well layer. As described above, the fifth Ga.sub.xIn.sub.1-xP layer may have a compressive strain. Point 1632 shows that the EQE of the micro-LED with the compressive strain in the quantum well layer may be about 112% of the reference point. FIG. 16B shows that using a quantum well layer having a compressive in-plane strain may increase the EQE of a micro-LED.

[0166] FIG. 17 includes a flowchart 1700 illustrating an example of a method of manufacturing micro-LEDs with reduced surface recombination according to certain embodiments. At block 1710, an n-type semiconductor layer may be epitaxially grown on a substrate, such as, for example, a GaAs substrate. The n-type semiconductor layer may include, for example, an AlInP or AlGaInP layer doped with selenium, silicon, tellurium, or germanium. The n-type semiconductor layer may be epitaxially grown on the substrate using techniques such as VPE, LPE, MBE, or MOCVD described above.

[0167] At block 1720, a plurality of quantum well layers and a plurality of barrier layers may be alternately grown on the n-type semiconductor layer to form an MQW structure. The quantum well layers and the barrier layers may be epitaxially grown on n-type semiconductor layer using techniques such as VPE, LPE, MBE, or MOCVD. The plurality of quantum well layers may be characterized by a first lattice constant greater than about 101% or greater than about 102% of a second lattice constant of the plurality of barrier layers, and may have an in-plane compressive strain caused by the lattice constant mismatch. In one example, the barrier layers may include (Al.sub.xGa.sub.1-x).sub.0.5In.sub.0.5P, and the quantum well layers may include Ga.sub.xIn.sub.1-xP with x less than about 0.5 or (Al.sub.xGa.sub.1-x).sub.yIn.sub.1-yP with y less than about 0.5. The MQW structure may be configured to emit red light, such as light with a wavelength greater than about 600 nm. Each quantum well layer of the plurality of quantum well layers may be characterized by an energy band structure including a heavy-hole band and a light-hole band in the valence band, where a top of the heavy-hole band may be higher than a top of the light-hole band by about 0.075 eV (or 3 kT at room temperature) or higher.

[0168] In some embodiments, the thickness of each quantum well layer may be smaller than the thickness of each barrier layer, such as smaller than a half of the thickness of each barrier layer. Therefore, the quantum well layer may have a higher compressive strain throughout the entire thickness of the quantum well layer, and the carriers may have a higher density in the quantum well. As such, the effective bandgap may be increased to emit light with a shorter wavelength. In addition, the HH sub-band and the LH sub-band in the valence band may be further separated, such that a larger portion of the holes in the valence band may be heavy holes, thereby increasing the effective mass and lowering the mobility of the holes to reduce the diffusion of holes to the mesa sidewall regions.

[0169] At block 1730, a p-type semiconductor layer may be epitaxially grown on the MQW structure. The p-type semiconductor layer may include, for example, an AlInP or AlGaInP layer doped with Mg, Ca, Zn, or Be.

[0170] At block 1740, the layer stack that includes the n-type semiconductor layer, the MQW structure, and the p-type semiconductor layer may be etched to form individual mesa structures for individual micro-LEDs as shown in, for example, FIGS. 7A, 7B, 9A, and 14. The mesa structure may have, for example, a vertical, conical, parabolic, inward-tilted, or outward-tilted mesa shape. At the sidewalls of each mesa structure, the MQW structure may have a high defect density due to the etching.

[0171] Optionally, at block 1750, a passivation layer and/or a reflective layer may be deposited on the sidewalls of the mesa structures as described above with respect to, for example, FIGS. 7A, 7B, 9A, and 14. The passivation layer may include a dielectric layer, such as SiO.sub.2 or SiN. In some embodiments, a metal layer may be formed on the passivation layer to form a mirror at the sidewalls of the mesa structure as described above with respect to, for example, FIGS. 7A, 7B, 9A, and 14. P-contacts and n-contacts may be formed on the p-type semiconductor layer and the n-type semiconductor layer, respectively to inject carriers into the MQW structure.

[0172] One or two-dimensional arrays of the LEDs described above may be manufactured on a wafer to form light sources (e.g., light source 642). Driver circuits (e.g., driver circuit 644) may be fabricated, for example, on a silicon wafer using CMOS processes. The LEDs and the driver circuits on wafers may be diced and then bonded together, or may be bonded on the wafer level and then diced. Various bonding techniques can be used for bonding the LEDs and the driver circuits, such as adhesive bonding, metal-to-metal bonding, metal oxide bonding, wafer-to-wafer bonding, die-to-wafer bonding, hybrid bonding, and the like.

[0173] FIG. 18A illustrates an example of a method of die-to-wafer bonding for arrays of LEDs according to certain embodiments. In the example shown in FIG. 18A, an LED array 1801 may include a plurality of LEDs 1807 on a carrier substrate 1805. Carrier substrate 1805 may include various materials, such as GaAs, InP, GaN, MN, sapphire, SiC, Si, or the like. LEDs 1807 may be fabricated by, for example, growing various epitaxial layers, forming mesa structures, and forming electrical contacts or electrodes, before performing the bonding. The epitaxial layers may include various materials, such as GaN, InGaN, AlGaInP, (AlGaIn)AsP, (AlGaIn)AsN, (Eu:InGa)N, (AlGaIn)N, or the like, and may include an n-type layer, a p-type layer, and an active layer that includes one or more heterostructures, such as one or more quantum wells or MQWs. The electrical contacts may include various conductive materials, such as a metal or a metal alloy.

[0174] A wafer 1803 may include a base layer 1809 having passive or active integrated circuits (e.g., driver circuits 1811) fabricated thereon. Base layer 1809 may include, for example, a silicon wafer. Driver circuits 1811 may be used to control the operations of LEDs 1807. For example, the driver circuit for each LED 1807 may include a 2T1C pixel structure that has two transistors and one capacitor. Wafer 1803 may also include a bonding layer 1813. Bonding layer 1813 may include various materials, such as a metal, an oxide, a dielectric, CuSn, AuTi, and the like. In some embodiments, a patterned layer 1815 may be formed on a surface of bonding layer 1813, where patterned layer 1815 may include a metallic grid made of a conductive material, such as Cu, Ag, Au, Al, or the like.

[0175] LED array 1801 may be bonded to wafer 1803 via bonding layer 1813 or patterned layer 1815. For example, patterned layer 1815 may include metal pads or bumps made of various materials, such as CuSn, AuSn, or nanoporous Au, that may be used to align LEDs 1807 of LED array 1801 with corresponding driver circuits 1811 on wafer 1803. In one example, LED array 1801 may be brought toward wafer 1803 until LEDs 1807 come into contact with respective metal pads or bumps corresponding to driver circuits 1811. Some or all of LEDs 1807 may be aligned with driver circuits 1811, and may then be bonded to wafer 1803 via patterned layer 1815 by various bonding techniques, such as metal-to-metal bonding. After LEDs 1807 have been bonded to wafer 1803, carrier substrate 1805 may be removed from LEDs 1807.

[0176] FIG. 18B illustrates an example of a method of wafer-to-wafer bonding for arrays of LEDs according to certain embodiments. As shown in FIG. 18B, a first wafer 1802 may include a substrate 1804, a first semiconductor layer 1806, active layers 1808, and a second semiconductor layer 1810. Substrate 1804 may include various materials, such as GaAs, InP, GaN, MN, sapphire, SiC, Si, or the like. First semiconductor layer 1806, active layers 1808, and second semiconductor layer 1810 may include various semiconductor materials, such as GaN, InGaN, (AlGaIn)P, (AlGaIn)AsP, (AlGaIn)AsN, (AlGaIn)Pas, (Eu:InGa)N, (AlGaIn)N, or the like. In some embodiments, first semiconductor layer 1806 may be an n-type layer, and second semiconductor layer 1810 may be a p-type layer. For example, first semiconductor layer 1806 may be an n-doped GaN layer (e.g., doped with Si or Ge), and second semiconductor layer 1810 may be a p-doped GaN layer (e.g., doped with Mg, Ca, Zn, or Be). Active layers 1808 may include, for example, one or more GaN layers, one or more InGaN layers, one or more AlGaInP layers, and the like, which may form one or more heterostructures, such as one or more quantum wells or MQWs.

[0177] In some embodiments, first wafer 1802 may also include a bonding layer. Bonding layer 1812 may include various materials, such as a metal, an oxide, a dielectric, CuSn, AuTi, or the like. In one example, bonding layer 1812 may include p-contacts and/or n-contacts (not shown). In some embodiments, other layers may also be included on first wafer 1802, such as a buffer layer between substrate 1804 and first semiconductor layer 1806. The buffer layer may include various materials, such as polycrystalline GaN or MN. In some embodiments, a contact layer may be between second semiconductor layer 1810 and bonding layer 1812. The contact layer may include any suitable material for providing an electrical contact to second semiconductor layer 1810 and/or first semiconductor layer 1806.

[0178] First wafer 1802 may be bonded to wafer 1803 that includes driver circuits 1811 and bonding layer 1813 as described above, via bonding layer 1813 and/or bonding layer 1812. Bonding layer 1812 and bonding layer 1813 may be made of the same material or different materials. Bonding layer 1813 and bonding layer 1812 may be substantially flat. First wafer 1802 may be bonded to wafer 1803 by various methods, such as metal-to-metal bonding, eutectic bonding, metal oxide bonding, anodic bonding, thermo-compression bonding, ultraviolet (UV) bonding, and/or fusion bonding.

[0179] As shown in FIG. 18B, first wafer 1802 may be bonded to wafer 1803 with the p-side (e.g., second semiconductor layer 1810) of first wafer 1802 facing down (i.e., toward wafer 1803). After bonding, substrate 1804 may be removed from first wafer 1802, and first wafer 1802 may then be processed from the n-side. The processing may include, for example, the formation of certain mesa shapes for individual LEDs, as well as the formation of optical components corresponding to the individual LEDs.

[0180] FIGS. 19A-19D illustrate an example of a method of hybrid bonding for arrays of LEDs according to certain embodiments. The hybrid bonding may generally include wafer cleaning and activation, high-precision alignment of contacts of one wafer with contacts of another wafer, dielectric bonding of dielectric materials at the surfaces of the wafers at room temperature, and metal bonding of the contacts by annealing at elevated temperatures. FIG. 19A shows a substrate 1910 with passive or active circuits 1920 manufactured thereon. As described above with respect to FIGS. 18A-18B, substrate 1910 may include, for example, a silicon wafer. Circuits 1920 may include driver circuits for the arrays of LEDs. A bonding layer may include dielectric regions 1940 and contact pads 1930 connected to circuits 1920 through electrical interconnects 1922. Contact pads 1930 may include, for example, Cu, Ag, Au, Al, W, Mo, Ni, Ti, Pt, Pd, or the like. Dielectric materials in dielectric regions 1940 may include SiCN, SiO.sub.2, SiN, Al.sub.2O.sub.3, HfO.sub.2, ZrO.sub.2, Ta.sub.2O.sub.5, or the like. The bonding layer may be planarized and polished using, for example, chemical mechanical polishing, where the planarization or polishing may cause dishing (a bowl like profile) in the contact pads. The surfaces of the bonding layers may be cleaned and activated by, for example, an ion (e.g., plasma) or fast atom (e.g., Ar) beam 1905. The activated surface may be atomically clean and may be reactive for formation of direct bonds between wafers when they are brought into contact, for example, at room temperature.

[0181] FIG. 19B illustrates a wafer 1950 including an array of micro-LEDs 1970 fabricated thereon as described above with respect to, for example, FIGS. 7A, 7B, 18A, and 18B. Wafer 1950 may be a carrier wafer and may include, for example, GaAs, InP, GaN, AlN, sapphire, SiC, Si, or the like. Micro-LEDs 1970 may include an n-type layer, an active region, and a p-type layer epitaxially grown on wafer 1950. The epitaxial layers may include various III-V semiconductor materials described above, and may be processed from the p-type layer side to etch mesa structures in the epitaxial layers, such as substantially vertical structures, parabolic structures, conical structures, or the like. Passivation layers and/or reflection layers may be formed on the sidewalls of the mesa structures. P-contacts 1980 and n-contacts 1982 may be formed in a dielectric material layer 1960 deposited on the mesa structures and may make electrical contacts with the p-type layer and the n-type layers, respectively. Dielectric materials in dielectric material layer 1960 may include, for example, SiCN, SiO.sub.2, SiN, Al.sub.2O.sub.3, HfO.sub.2, ZrO.sub.2, Ta.sub.2O.sub.5, or the like. P-contacts 1980 and n-contacts 1982 may include, for example, Cu, Ag, Au, Al, W, Mo, Ni, Ti, Pt, Pd, or the like. The top surfaces of p-contacts 1980, n-contacts 1982, and dielectric material layer 1960 may form a bonding layer. The bonding layer may be planarized and polished using, for example, chemical mechanical polishing, where the polishing may cause dishing in p-contacts 1980 and n-contacts 1982. The bonding layer may then be cleaned and activated by, for example, an ion (e.g., plasma) or fast atom (e.g., Ar) beam 1915. The activated surface may be atomically clean and reactive for formation of direct bonds between wafers when they are brought into contact, for example, at room temperature.

[0182] FIG. 19C illustrates a room temperature bonding process for bonding the dielectric materials in the bonding layers. For example, after the bonding layer that includes dielectric regions 1940 and contact pads 1930 and the bonding layer that includes p-contacts 1980, n-contacts 1982, and dielectric material layer 1960 are surface activated, wafer 1950 and micro-LEDs 1970 may be turned upside down and brought into contact with substrate 1910 and the circuits formed thereon. In some embodiments, compression pressure 1925 may be applied to substrate 1910 and wafer 1950 such that the bonding layers are pressed against each other. Due to the surface activation and the dishing in the contacts, dielectric regions 1940 and dielectric material layer 1960 may be in direct contact because of the surface attractive force, and may react and form chemical bonds between them because the surface atoms may have dangling bonds and may be in unstable energy states after the activation. Thus, the dielectric materials in dielectric regions 1940 and dielectric material layer 1960 may be bonded together with or without heat treatment or pressure.

[0183] FIG. 19D illustrates an annealing process for bonding the contacts in the bonding layers after bonding the dielectric materials in the bonding layers. For example, contact pads 1930 and p-contacts 1980 or n-contacts 1982 may be bonded together by annealing at, for example, about 210-400.degree. C. or higher. During the annealing process, heat 1935 may cause the contacts to expand more than the dielectric materials (due to different coefficients of thermal expansion), and thus may close the dishing gaps between the contacts such that contact pads 1930 and p-contacts 1980 or n-contacts 1982 may be in contact and may form direct metallic bonds at the activated surfaces.

[0184] In some embodiments where the two bonded wafers include materials having different coefficients of thermal expansion (CTEs), the dielectric materials bonded at room temperature may help to reduce or prevent misalignment of the contact pads caused by the different thermal expansions. In some embodiments, to further reduce or avoid the misalignment of the contact pads at a high temperature during annealing, trenches may be formed between micro-LEDs, between groups of micro-LEDs, through part or all of the substrate, or the like, before bonding.

[0185] After the micro-LEDs are bonded to the driver circuits, the substrate on which the micro-LEDs are fabricated may be thinned or removed, and various secondary optical components may be fabricated on the light-emitting surfaces of the micro-LEDs to, for example, extract, collimate, and redirect the light emitted from the active regions of the micro-LEDs. In one example, micro-lenses may be formed on the micro-LEDs, where each micro-lens may correspond to a respective micro-LED and may help to improve the light extraction efficiency and collimate the light emitted by the micro-LED. In some embodiments, the secondary optical components may be fabricated in the substrate or the n-type layer of the micro-LEDs. In some embodiments, the secondary optical components may be fabricated in a dielectric layer deposited on the n-type side of the micro-LEDs. Examples of the secondary optical components may include a lens, a grating, an antireflection (AR) coating, a prism, a photonic crystal, or the like.

[0186] FIG. 20 illustrates an example of an LED array 2000 with secondary optical components fabricated thereon according to certain embodiments. LED array 2000 may be made by bonding an LED chip or wafer with a silicon wafer including electrical circuits fabricated thereon, using any suitable bonding techniques described above with respect to, for example, FIGS. 18A-19D. In the example shown in FIG. 20, LED array 2000 may be bonded using a wafer-to-wafer hybrid bonding technique as described above with respect to FIG. 19A-19D. LED array 2000 may include a substrate 2010, which may be, for example, a silicon wafer. Integrated circuits 2020, such as LED driver circuits, may be fabricated on substrate 2010. Integrated circuits 2020 may be connected to p-contacts 2074 and n-contacts 2072 of micro-LEDs 2070 through interconnects 2022 and contact pads 2030, where contact pads 2030 may form metallic bonds with p-contacts 2074 and n-contacts 2072. Dielectric layer 2040 on substrate 2010 may be bonded to dielectric layer 2060 through fusion bonding.

[0187] The substrate (not shown) of the LED chip or wafer may be thinned or may be removed to expose the n-type layer 2050 of micro-LEDs 2070. Various secondary optical components, such as a spherical micro-lens 2082, a grating 2084, a micro-lens 2086, an antireflection layer 2088, and the like, may be formed in or on top of n-type layer 2050. For example, spherical micro-lens arrays may be etched in the semiconductor materials of micro-LEDs 2070 using a gray-scale mask and a photoresist with a linear response to exposure light, or using an etch mask formed by thermal reflowing of a patterned photoresist layer. The secondary optical components may also be etched in a dielectric layer deposited on n-type layer 2050 using similar photolithographic techniques or other techniques. For example, micro-lens arrays may be formed in a polymer layer through thermal reflowing of the polymer layer that is patterned using a binary mask. The micro-lens arrays in the polymer layer may be used as the secondary optical components or may be used as the etch mask for transferring the profiles of the micro-lens arrays into a dielectric layer or a semiconductor layer. The dielectric layer may include, for example, SiCN, SiO.sub.2, SiN, Al.sub.2O.sub.3, HfO.sub.2, ZrO.sub.2, Ta.sub.2O.sub.5, or the like. In some embodiments, a micro-LED 2070 may have multiple corresponding secondary optical components, such as a micro-lens and an anti-reflection coating, a micro-lens etched in the semiconductor material and a micro-lens etched in a dielectric material layer, a micro-lens and a grating, a spherical lens and an aspherical lens, and the like. Three different secondary optical components are illustrated in FIG. 20 to show some examples of secondary optical components that can be formed on micro-LEDs 2070, which does not necessary imply that different secondary optical components are used simultaneously for every LED array.

[0188] Embodiments disclosed herein may be used to implement components of an artificial reality system or may be implemented in conjunction with an artificial reality system. Artificial reality is a form of reality that has been adjusted in some manner before presentation to a user, which may include, for example, a virtual reality, an augmented reality, a mixed reality, a hybrid reality, or some combination and/or derivatives thereof. Artificial reality content may include completely generated content or generated content combined with captured (e.g., real-world) content. The artificial reality content may include video, audio, haptic feedback, or some combination thereof, and any of which may be presented in a single channel or in multiple channels (such as stereo video that produces a three-dimensional effect to the viewer). Additionally, in some embodiments, artificial reality may also be associated with applications, products, accessories, services, or some combination thereof, that are used to, for example, create content in an artificial reality and/or are otherwise used in (e.g., perform activities in) an artificial reality. The artificial reality system that provides the artificial reality content may be implemented on various platforms, including an HMD connected to a host computer system, a standalone HMD, a mobile device or computing system, or any other hardware platform capable of providing artificial reality content to one or more viewers.

[0189] FIG. 21 is a simplified block diagram of an example of an electronic system 2100 of an example near-eye display (e.g., HMD device) for implementing some of the examples disclosed herein. Electronic system 2100 may be used as the electronic system of an HMD device or other near-eye displays described above. In this example, electronic system 2100 may include one or more processor(s) 2110 and a memory 2120. Processor(s) 2110 may be configured to execute instructions for performing operations at a number of components, and can be, for example, a general-purpose processor or microprocessor suitable for implementation within a portable electronic device. Processor(s) 2110 may be communicatively coupled with a plurality of components within electronic system 2100. To realize this communicative coupling, processor(s) 2110 may communicate with the other illustrated components across a bus 2140. Bus 2140 may be any subsystem adapted to transfer data within electronic system 2100. Bus 2140 may include a plurality of computer buses and additional circuitry to transfer data.

[0190] Memory 2120 may be coupled to processor(s) 2110. In some embodiments, memory 2120 may offer both short-term and long-term storage and may be divided into several units. Memory 2120 may be volatile, such as static random access memory (SRAM) and/or dynamic random access memory (DRAM) and/or non-volatile, such as read-only memory (ROM), flash memory, and the like. Furthermore, memory 2120 may include removable storage devices, such as secure digital (SD) cards. Memory 2120 may provide storage of computer-readable instructions, data structures, program modules, and other data for electronic system 2100. In some embodiments, memory 2120 may be distributed into different hardware modules. A set of instructions and/or code might be stored on memory 2120. The instructions might take the form of executable code that may be executable by electronic system 2100, and/or might take the form of source and/or installable code, which, upon compilation and/or installation on electronic system 2100 (e.g., using any of a variety of generally available compilers, installation programs, compression/decompression utilities, etc.), may take the form of executable code.

[0191] In some embodiments, memory 2120 may store a plurality of application modules 2122 through 2124, which may include any number of applications. Examples of applications may include gaming applications, conferencing applications, video playback applications, or other suitable applications. The applications may include a depth sensing function or eye tracking function. Application modules 2122-2124 may include particular instructions to be executed by processor(s) 2110. In some embodiments, certain applications or parts of application modules 2122-2124 may be executable by other hardware modules 2180. In certain embodiments, memory 2120 may additionally include secure memory, which may include additional security controls to prevent copying or other unauthorized access to secure information.

[0192] In some embodiments, memory 2120 may include an operating system 2125 loaded therein. Operating system 2125 may be operable to initiate the execution of the instructions provided by application modules 2122-2124 and/or manage other hardware modules 2180 as well as interfaces with a wireless communication subsystem 2130 which may include one or more wireless transceivers. Operating system 2125 may be adapted to perform other operations across the components of electronic system 2100 including threading, resource management, data storage control and other similar functionality.

[0193] Wireless communication subsystem 2130 may include, for example, an infrared communication device, a wireless communication device and/or chipset (such as a Bluetooth.RTM. device, an IEEE 802.11 device, a Wi-Fi device, a WiMax device, cellular communication facilities, etc.), and/or similar communication interfaces. Electronic system 2100 may include one or more antennas 2134 for wireless communication as part of wireless communication subsystem 2130 or as a separate component coupled to any portion of the system. Depending on desired functionality, wireless communication subsystem 2130 may include separate transceivers to communicate with base transceiver stations and other wireless devices and access points, which may include communicating with different data networks and/or network types, such as wireless wide-area networks (WWANs), wireless local area networks (WLANs), or wireless personal area networks (WPANs). A WWAN may be, for example, a WiMax (IEEE 802.16) network. A WLAN may be, for example, an IEEE 802.11xnetwork. A WPAN may be, for example, a Bluetooth network, an IEEE 802.15x, or some other types of network. The techniques described herein may also be used for any combination of WWAN, WLAN, and/or WPAN. Wireless communications subsystem 2130 may permit data to be exchanged with a network, other computer systems, and/or any other devices described herein. Wireless communication subsystem 2130 may include a means for transmitting or receiving data, such as identifiers of HMD devices, position data, a geographic map, a heat map, photos, or videos, using antenna(s) 2134 and wireless link(s) 2132. Wireless communication subsystem 2130, processor(s) 2110, and memory 2120 may together comprise at least a part of one or more of a means for performing some functions disclosed herein.

[0194] Embodiments of electronic system 2100 may also include one or more sensors 2190. Sensor(s) 2190 may include, for example, an image sensor, an accelerometer, a pressure sensor, a temperature sensor, a proximity sensor, a magnetometer, a gyroscope, an inertial sensor (e.g., a module that combines an accelerometer and a gyroscope), an ambient light sensor, or any other similar module operable to provide sensory output and/or receive sensory input, such as a depth sensor or a position sensor. For example, in some implementations, sensor(s) 2190 may include one or more inertial measurement units (IMUs) and/or one or more position sensors. An IMU may generate calibration data indicating an estimated position of the HMD device relative to an initial position of the HMD device, based on measurement signals received from one or more of the position sensors. A position sensor may generate one or more measurement signals in response to motion of the HMD device. Examples of the position sensors may include, but are not limited to, one or more accelerometers, one or more gyroscopes, one or more magnetometers, another suitable type of sensor that detects motion, a type of sensor used for error correction of the IMU, or any combination thereof. The position sensors may be located external to the IMU, internal to the IMU, or any combination thereof. At least some sensors may use a structured light pattern for sensing.

[0195] Electronic system 2100 may include a display module 2160. Display module 2160 may be a near-eye display, and may graphically present information, such as images, videos, and various instructions, from electronic system 2100 to a user. Such information may be derived from one or more application modules 2122-2124, virtual reality engine 2126, one or more other hardware modules 2180, a combination thereof, or any other suitable means for resolving graphical content for the user (e.g., by operating system 2125). Display module 2160 may use LCD technology, LED technology (including, for example, OLED, ILED, .mu.-LED, AMOLED, TOLED, etc.), light-emitting polymer display (LPD) technology, or some other display technology.

[0196] Electronic system 2100 may include a user input/output module 2170. User input/output module 2170 may allow a user to send action requests to electronic system 2100. An action request may be a request to perform a particular action. For example, an action request may be to start or end an application or to perform a particular action within the application. User input/output module 2170 may include one or more input devices. Example input devices may include a touchscreen, a touch pad, microphone(s), button(s), dial(s), switch(es), a keyboard, a mouse, a game controller, or any other suitable device for receiving action requests and communicating the received action requests to electronic system 2100. In some embodiments, user input/output module 2170 may provide haptic feedback to the user in accordance with instructions received from electronic system 2100. For example, the haptic feedback may be provided when an action request is received or has been performed.

[0197] Electronic system 2100 may include a camera 2150 that may be used to take photos or videos of a user, for example, for tracking the user’s eye position. Camera 2150 may also be used to take photos or videos of the environment, for example, for VR, AR, or MR applications. Camera 2150 may include, for example, a complementary metal-oxide-semiconductor (CMOS) image sensor with a few millions or tens of millions of pixels. In some implementations, camera 2150 may include two or more cameras that may be used to capture 3-D images.

[0198] In some embodiments, electronic system 2100 may include a plurality of other hardware modules 2180. Each of other hardware modules 2180 may be a physical module within electronic system 2100. While each of other hardware modules 2180 may be permanently configured as a structure, some of other hardware modules 2180 may be temporarily configured to perform specific functions or temporarily activated. Examples of other hardware modules 2180 may include, for example, an audio output and/or input module (e.g., a microphone or speaker), a near field communication (NFC) module, a rechargeable battery, a battery management system, a wired/wireless battery charging system, etc. In some embodiments, one or more functions of other hardware modules 2180 may be implemented in software.

[0199] In some embodiments, memory 2120 of electronic system 2100 may also store a virtual reality engine 2126. Virtual reality engine 2126 may execute applications within electronic system 2100 and receive position information, acceleration information, velocity information, predicted future positions, or any combination thereof of the HMD device from the various sensors. In some embodiments, the information received by virtual reality engine 2126 may be used for producing a signal (e.g., display instructions) to display module 2160. For example, if the received information indicates that the user has looked to the left, virtual reality engine 2126 may generate content for the HMD device that mirrors the user’s movement in a virtual environment. Additionally, virtual reality engine 2126 may perform an action within an application in response to an action request received from user input/output module 2170 and provide feedback to the user. The provided feedback may be visual, audible, or haptic feedback. In some implementations, processor(s) 2110 may include one or more GPUs that may execute virtual reality engine 2126.

[0200] In various implementations, the above-described hardware and modules may be implemented on a single device or on multiple devices that can communicate with one another using wired or wireless connections. For example, in some implementations, some components or modules, such as GPUs, virtual reality engine 2126, and applications (e.g., tracking application), may be implemented on a console separate from the head-mounted display device.

[0201] In some implementations, one console may be connected to or support more than one HMD.

[0202] In alternative configurations, different and/or additional components may be included in electronic system 2100. Similarly, functionality of one or more of the components can be distributed among the components in a manner different from the manner described above. For example, in some embodiments, electronic system 2100 may be modified to include other system environments, such as an AR system environment and/or an MR environment.

[0203] The methods, systems, and devices discussed above are examples. Various embodiments may omit, substitute, or add various procedures or components as appropriate. For instance, in alternative configurations, the methods described may be performed in an order different from that described, and/or various stages may be added, omitted, and/or combined. Also, features described with respect to certain embodiments may be combined in various other embodiments. Different aspects and elements of the embodiments may be combined in a similar manner. Also, technology evolves and, thus, many of the elements are examples that do not limit the scope of the disclosure to those specific examples.

[0204] Specific details are given in the description to provide a thorough understanding of the embodiments. However, embodiments may be practiced without these specific details. For example, well-known circuits, processes, systems, structures, and techniques have been shown without unnecessary detail in order to avoid obscuring the embodiments. This description provides example embodiments only, and is not intended to limit the scope, applicability, or configuration of the invention. Rather, the preceding description of the embodiments will provide those skilled in the art with an enabling description for implementing various embodiments. Various changes may be made in the function and arrangement of elements without departing from the spirit and scope of the present disclosure.

[0205] Also, some embodiments were described as processes depicted as flow diagrams or block diagrams. Although each may describe the operations as a sequential process, many of the operations may be performed in parallel or concurrently. In addition, the order of the operations may be rearranged. A process may have additional steps not included in the figure. Furthermore, embodiments of the methods may be implemented by hardware, software, firmware, middleware, microcode, hardware description languages, or any combination thereof. When implemented in software, firmware, middleware, or microcode, the program code or code segments to perform the associated tasks may be stored in a computer-readable medium such as a storage medium. Processors may perform the associated tasks.

[0206] It will be apparent to those skilled in the art that substantial variations may be made in accordance with specific requirements. For example, customized or special-purpose hardware might also be used, and/or particular elements might be implemented in hardware, software (including portable software, such as applets, etc.), or both. Further, connection to other computing devices such as network input/output devices may be employed.

[0207] With reference to the appended figures, components that can include memory can include non-transitory machine-readable media. The term “machine-readable medium” and “computer-readable medium” may refer to any storage medium that participates in providing data that causes a machine to operate in a specific fashion. In embodiments provided hereinabove, various machine-readable media might be involved in providing instructions/code to processing units and/or other device(s) for execution. Additionally or alternatively, the machine-readable media might be used to store and/or carry such instructions/code. In many implementations, a computer-readable medium is a physical and/or tangible storage medium. Such a medium may take many forms, including, but not limited to, non-volatile media, volatile media, and transmission media. Common forms of computer-readable media include, for example, magnetic and/or optical media such as compact disk (CD) or digital versatile disk (DVD), punch cards, paper tape, any other physical medium with patterns of holes, a RAM, a programmable read-only memory (PROM), an erasable programmable read-only memory (EPROM), a FLASH-EPROM, any other memory chip or cartridge, a carrier wave as described hereinafter, or any other medium from which a computer can read instructions and/or code. A computer program product may include code and/or machine-executable instructions that may represent a procedure, a function, a subprogram, a program, a routine, an application (App), a subroutine, a module, a software package, a class, or any combination of instructions, data structures, or program statements.

[0208] Those of skill in the art will appreciate that information and signals used to communicate the messages described herein may be represented using any of a variety of different technologies and techniques. For example, data, instructions, commands, information, signals, bits, symbols, and chips that may be referenced throughout the above description may be represented by voltages, currents, electromagnetic waves, magnetic fields or particles, optical fields or particles, or any combination thereof.

[0209] Terms, “and” and “or” as used herein, may include a variety of meanings that are also expected to depend at least in part upon the context in which such terms are used. Typically, “or” if used to associate a list, such as A, B, or C, is intended to mean A, B, and C, here used in the inclusive sense, as well as A, B, or C, here used in the exclusive sense. In addition, the term “one or more” as used herein may be used to describe any feature, structure, or characteristic in the singular or may be used to describe some combination of features, structures, or characteristics. However, it should be noted that this is merely an illustrative example and claimed subject matter is not limited to this example. Furthermore, the term “at least one of” if used to associate a list, such as A, B, or C, can be interpreted to mean any combination of A, B, and/or C, such as A, AB, AC, BC, AA, ABC, AAB, AABBCCC, etc.

[0210] Further, while certain embodiments have been described using a particular combination of hardware and software, it should be recognized that other combinations of hardware and software are also possible. Certain embodiments may be implemented only in hardware, or only in software, or using combinations thereof. In one example, software may be implemented with a computer program product containing computer program code or instructions executable by one or more processors for performing any or all of the steps, operations, or processes described in this disclosure, where the computer program may be stored on a non-transitory computer readable medium. The various processes described herein can be implemented on the same processor or different processors in any combination.

[0211] Where devices, systems, components or modules are described as being configured to perform certain operations or functions, such configuration can be accomplished, for example, by designing electronic circuits to perform the operation, by programming programmable electronic circuits (such as microprocessors) to perform the operation such as by executing computer instructions or code, or processors or cores programmed to execute code or instructions stored on a non-transitory memory medium, or any combination thereof. Processes can communicate using a variety of techniques, including, but not limited to, conventional techniques for inter-process communications, and different pairs of processes may use different techniques, or the same pair of processes may use different techniques at different times.

[0212] The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense. It will, however, be evident that additions, subtractions, deletions, and other modifications and changes may be made thereunto without departing from the broader spirit and scope as set forth in the claims. Thus, although specific embodiments have been described, these are not intended to be limiting. Various modifications and equivalents are within the scope of the following claims.