Spacer LED architecture for high efficiency micro-LED displays

Abstract

A method of forming an optical device includes the steps of: forming a mesa including an active layer configured to emit light from a first light-emitting surface of the mesa when subjected to an electric current, the mesa further including a second surface opposite to the light-emitting surface and substantially perpendicular sidewalls; forming spacers on the mesa sidewalls, the spacers being formed of a first electrically insulating optically transparent material and having an inner surface facing the mesa sidewalls and an outer surface opposite thereto; depositing a first layer of transparent conductive oxide on the light-emitting surface of the mesa, the transparent conductive oxide having an inner surface facing the second surface of the mesa and an outer surface opposite thereto; and depositing a layer of reflective conductive material on the transparent conductive oxide and on the outer surface of the spacers.

Description

Technical Field

[0001] This invention relates to light-emitting device arrays and methods for forming such arrays. Specifically, but not exclusively, this invention relates to light-emitting devices with optimized light extraction. Background Technology

[0002] As is well known, light-emitting diode (LED) devices provide efficient light sources for a wide variety of applications. Improvements in LED light generation efficiency and extraction, along with the production of smaller LEDs (with smaller luminous surface areas) and the integration of LED emitters of different wavelengths in arrays, have led to the provision of high-quality color arrays for a variety of applications, especially in display technology.

[0003] Several display technologies are being considered and used in micro-LED displays for a variety of applications, including augmented reality, merged reality, virtual reality, and direct-view displays such as smartwatches and mobile devices. Technologies such as digital micromirrors (DMD) and liquid crystal on silicon (LCoS) are based on reflective technology, where an external light source is used to generate red, green, and blue photons in a time-sequential pattern, and pixels either deflect light away from the optical element (DMD) or absorb light (LCoS) to adjust the pixel's brightness to form an image. Liquid crystal displays (LCDs) typically use backlighting, an LCD panel on an addressable backplane, and color filters to produce images. A backplane is needed to turn individual pixels on and off and adjust the brightness of individual pixels for each video frame. Light-emitting display technologies such as organic light-emitting diodes (OLEDs) or active-matrix OLEDs (AMOLEDs), and more recently, micro-LEDs, are increasingly being considered because they offer lower power consumption and higher image contrast for unrestricted micro-display applications. Micro-LEDs, in particular, offer higher efficiency and better reliability compared to micro-OLED and AMOLED displays.

[0004] The present invention described in this document relates to a method for manufacturing a high-efficiency micro-LED array that combines techniques to improve internal quantum efficiency (IQE) and light extraction efficiency (LEE) to improve efficiency and brightness quality factor.

[0005] Structures designed to improve light extraction efficiency are well-known in the LED industry, including the use of pseudo-parabolic MESAs to guide photons generated in multiple quantum wells (MQWs) to the emitting surface.

[0006] The techniques used to manufacture MESAs with this shape involve techniques such as reactive ion etching (RIE) or inductively coupled plasma etching (ICP). In these etching techniques, high-energy plasmas, including RF, high voltage (DC bias), and reactive gases (typically including free radicals), are used to selectively etch the semiconductor material. A photolithography process is used to define the features, employing a photosensitive material to define the areas that will undergo the etching process and the areas that will remain unetched. The precise shape of the MESA can be controlled by the contour of the photosensitive material used to define the pattern, as well as by the etching pressure, power, gas flow, and gas type.

[0007] This not only complicates the manufacturing process, but also, due to this etching process, the edges of the MESA may be damaged, which can affect the IQE of the micro-LED.

[0008] like Figure 1 As shown, increasing DC bias and plasma density causes more damage to the feature edges, leading to surface leakage paths formed by crystal damage, nitrogen vacancies, and dangling bonds. Dry etching generates numerous crystal defects due to high-energy ion bombardment of the surface. Dangling bonds are easily oxidized, and crystal damage creates many defect levels in the band structure. These defect levels act as carrier recombination centers on the surface, resulting in nonradiative recombination.

[0009] Surface recombination rate (non-radiative recombination rate) is faster than radiative recombination rate in bulk MQW, so small micro-LEDs are more susceptible to surface recombination, resulting in a decrease in IQE.

[0010] The widespread observation that damage occurs during MESA etching results in reduced efficiency as the size of microLEDs shrinks, such as... Figure 2 As shown. External quantum efficiency (EQE) is a product of IQE (the ratio of the number of photons produced to the number of electrons). The mechanism driving this trend is the ratio of the perimeter to the area of ​​the microLED. As the size of the microLED decreases, the area of ​​the sidewalls increases relative to the area of ​​the MQW, thus the surface leakage paths at the edges of the microLED lead to an increase in nonradiative recombination.

[0011] Micro-LED displays and head-mounted displays for augmented reality will be available at 1A / cm. 2 Up to 10A / cm 2 It operates at a current density of [insert current density here]. This could mean that the efficiency of a small LED is reduced to 1 / 20th that of a large LED.

[0012] like Figure 3As shown, repairing damage caused by MESA etching can significantly improve the efficiency of micro-LEDs. By implementing an optimized damage repair scheme, EQE can typically be improved by 10 times. The peak EQE increases after damage repair and occurs at lower current densities, resulting in a 10-fold efficiency improvement under typical operating conditions. However, this approach is incompatible with preserving the MESA shape (optimized for high LEE) because the repair process removes semiconductor material damaged by MESA etching, such as… Figure 4 As shown. Summary of the Invention

[0013] To mitigate at least some of the aforementioned problems, a method for forming one or more optical devices is provided according to the appended claims. Furthermore, an optical device according to the appended claims is provided.

[0014] In a first aspect of the invention, a method for forming an optical device is provided, the method comprising the steps of: forming a mesa including an active layer configured to emit light from a first light-emitting surface of the mesa when subjected to an electric current, the mesa further including a second surface opposite to the light-emitting surface and substantially perpendicular sidewalls; forming spacers on the mesa sidewalls, the spacers being formed of a first electrically insulating optically transparent material and having an inner surface facing the mesa sidewalls and an outer surface opposite thereto; depositing a first layer of transparent conductive material on the light-emitting surface of the mesa, the transparent conductive oxide having an inner surface facing the second surface of the mesa and an outer surface opposite thereto; and depositing a layer of reflective conductive material on the transparent conductive oxide and on the outer surface of the spacers.

[0015] Advantageously, spacers and transparent conductive materials are used as optical components to enhance light extraction from the active layer of the mesa, while reflective conductive materials are used as the outermost mirror layer to further enhance light extraction.

[0016] Preferably, the outer surface of the first layer of transparent conductive material is generally convex.

[0017] Preferably, a second layer of transparent conductive material is formed on the light-emitting surface of the platform.

[0018] Preferably, the transparent conductive material is a transparent conductive oxide. More preferably, the transparent conductive material is indium tin oxide.

[0019] Preferably, the outer surface of these spacers is at an angle relative to the inner surface.

[0020] Preferably, the outer surfaces of these spacers have a pseudo-parabolic profile. The parabolic shape serves to direct the emitted photons toward the light-emitting surface of the device, such that the photons are incident on the surface at an angle of incidence below the critical angle, thereby allowing for efficient extraction of photons into the air.

[0021] Preferably, the profile of the outer surface of these spacers approximates a Bézier curve with two control points and a Bézier coefficient of 0.5. This has been found to provide maximum light extraction.

[0022] Preferably, these spacers are formed of silicon nitride, silicon oxide, or tin oxide.

[0023] Preferably, the light-emitting structure has roughened sidewalls. This has been found to improve brightness uniformity and further enhance light extraction.

[0024] Preferably, the method further includes the step of depositing a second electrically insulating optically transparent material on the outer surface of each spacer, the second electrically insulating optically transparent material having a different refractive index than the first electrically insulating optically transparent material. This allows the use of materials with graded refractive indices, so that emitted photons can be better directed to the emitting surface.

[0025] Preferably, the refractive index of the first material is greater than that of the second material.

[0026] Preferably, the active layer of the mesa is located between the n-doped n-cladding layer and the p-doped p-cladding layer.

[0027] Preferably, a first electrical contact is formed between the p-cladding layer and the first transparent conductive oxide layer and the reflective conductive material, and a second electrical contact is formed between the n-cladding layer and the second transparent conductive oxide layer.

[0028] In a second aspect of the invention, an optical device manufactured according to the above-described method steps is provided.

[0029] Further aspects of the invention will become apparent from the description and appended claims. Attached Figure Description

[0030] Specific embodiments will now be described by way of example only, with reference to the accompanying drawings, in which:

[0031] Figure 1 The study demonstrates the crystal damage to InGaN materials caused by increasing plasma power and DC bias.

[0032] Figure 2 The relationship between external quantum efficiency (EQE) and current density is shown as the microLED size decreases from A1 (256 μm) to A9 (1 μm).

[0033] Figure 3 The EQE of the microLED is shown with and without MESA damage reduction and repair.

[0034] Figure 4 The cross-sections of the etched MESA are shown before (a) and after (b) the damage repair process.

[0035] Figures 5 to 10 The various stages of the monolithic manufacturing process for optical devices are illustrated.

[0036] Figures 11 to 13 An optical device according to one aspect of the present invention is shown.

[0037] Figure 12 An example using two different spacer materials is shown.

[0038] Figure 13 An embodiment with roughened MESA sidewalls is shown.

[0039] Figure 14 The light extraction efficiency (LEE) is shown as a function of the radius of curvature R and the Bessel coefficient B.

[0040] Figure 15 The coupling efficiency of the F / 2 projection lens varies with the radius of curvature R and the Bessel coefficient B.

[0041] Figures 16A to 16D Examples of optical devices with square, circular, triangular, and pentagonal cross sections are shown respectively. Detailed Implementation

[0042] Figure 5 (a) illustrates a preliminary stage of the fabrication process in which an epitaxial silicon wafer having a substrate 100, an n-cladment 110, an active layer 120, and a p-cladment 130 is fabricated. In one embodiment, the active layer comprises one or more quantum wells that emit light when a current is applied to the active layer 120. In another embodiment, the n-cladment 110 and the p-cladment 130 are formed of n-doped gallium nitride and p-doped gallium nitride, respectively. In a particular embodiment, an electron blocking layer is located between the p-cladment 130 and the active layer 120. In yet another embodiment, one or more buffer layers are included.

[0043] Although described as being grown on a silicon wafer, those skilled in the art will understand that any suitable substrate can be used. In this embodiment, a sapphire substrate is used. In a further embodiment, additional or alternative intermediate layers are used to account for the lattice mismatch between the substrate and subsequently grown layers (such as an aluminum nitride buffer layer). Similarly, alternative or additional etching techniques can be utilized, provided they produce the described MESA array.

[0044] exist Figure 5 (b) shows the stage in which multiple openings are formed in the p-cladding 130, n-cladding 110, and active layer 120, one opening per sub-pixel, using photolithography followed by reactive ion etching (RIE) or inductively coupled plasma (ICP) etching processes. This produces a MESA array with generally tilted sidewalls, where each mesa represents a single light-emitting structure 150. In an embodiment, the etching is tuned to provide pseudo-parabolic sidewalls.

[0045] As a result of the etching process, the MESA sidewalls contain damaged crystal structures that lead to surface leakage paths. To repair these damaged crystal structures, a repair process is applied to remove the damaged material, revealing a high-quality crystal structure with reduced dangling bonds and nitrogen vacancies. In this embodiment, this is achieved via potassium hydroxide wet etching. In an alternative embodiment, the repair process includes wet etching using tetramethylammonium hydroxide. Thus, the opening sidewall profile changes from inclined to vertical or is shaped to be vertical—see [link to documentation]. Figure 4 .

[0046] Alternatively, the surface roughness of the sidewalls can be adjusted by performing further dry etching or by using a photoresist with a suitable resist profile. Advantageously, it has been found that substantially vertical but roughened sidewalls improve brightness uniformity and enhance light extraction from optics.

[0047] exist Figure 6 In stage (a), a conformal coating of silicon dioxide is deposited, and the resulting film is etched back using RIE etching to form a uniform pseudo-parabolic spacer 200. In alternative embodiments, one of silicon nitride, titanium oxide, or any other dielectric material is used as the spacer material. Those skilled in the art will recognize that any suitable high-refractive-index nonconductive material can be used. The purpose of the spacer is to serve as an optical component to enhance light extraction from the active layer 120. Figure 6 As can be seen in (a), the portion of n-cladding 110 exposed by etching is also coated with spacer material.

[0048] exist Figure 6 (b) shows the stage in which a first transparent conductive material 250 is deposited on the exposed p-cladding of each mesa via a known process, thereby forming a separate p-contact with each light-emitting structure 150. In this embodiment, the first transparent conductive material is a transparent conductive oxide 250, such as indium tin oxide (ITO), but those skilled in the art will understand that any suitable transparent conductive material can be used.

[0049] exist Figure 7In the stage shown in (a), the first transparent conductive oxide 250 is shaped to produce a convex lens on each light-emitting structure 150. In an embodiment, this is achieved by patterning a photoresist material onto the surface of the first transparent conductive oxide 250, reflowing the photoresist with heat or a suitable solvent to form the photoresist into hemispherical droplets, and applying etching (such as reactive ion etching) to provide a convex profile to the transparent conductive oxide 250 due to the difference in etch selectivity (i.e., etch rate) between the oxide 250 and the photoresist.

[0050] exist Figure 7 (b) In the stage shown, reflective conductive material 300 is deposited over the entire structure, and a chemical mechanical polishing process is applied to ensure a flat outermost surface. In this embodiment, the reflective conductive material 300 is aluminum, but those skilled in the art will recognize that any suitable material can be used. In this embodiment, the interface between the spacer 200 and the reflective conductive material 300 has a surface roughness Ra < 50 nm, most preferably Ra < 10 nm, to prevent light diffusion, which would otherwise reduce light extraction efficiency.

[0051] exist Figure 8 In the stage shown in (a), to electrically isolate each light-emitting structure 150 from its adjacent structures, a series of channels are etched in the reflective conductive material 300 between each mesa in a known manner. A layer of silicon dioxide 350 is applied to the surface of the reflective conductive material 300 to fill the channels. Although silicon dioxide is preferred, those skilled in the art will recognize that any electrically insulating material can be used.

[0052] exist Figure 8 (b) shows the stage in which a window is fabricated by extending through the silicon dioxide layer 350 down to the underlying reflective conductive material 300. The window is then filled with bonding metal 360, thereby allowing electrical connection through the reflective conductive material 300 to the p-contact of the first transparent conductive oxide 250.

[0053] exist Figure 9 In stage (a), a complementary metal-oxide-semiconductor (CMOS) backplane wafer 400 is fabricated, having a top layer composed of alternating metal regions 410 and oxide regions 420. This structure is formed using known methods. The metal regions 410 are partially aligned with bonding metal 360, and the wafer is held together via processes known to those skilled in the art. Figure 9 As shown in (b), the overlying substrate 100 is then removed by a known method (such as wet or dry etching).

[0054] according to Figure 10 A second layer of transparent conductive oxide 500 is applied to the newly exposed n-cladding layer 110. In this embodiment, indium tin oxide is used as the transparent conductive oxide.

[0055] To further improve light extraction efficiency, the refractive index of the transparent conductive oxide 500 can be altered by changing its porosity. One known method for changing the porosity of transparent conductive oxides (such as ITO) is oblique deposition using electron beam evaporation. By changing the angle of the deposition surface relative to the vapor fludeposition, the amount of shadow cast by the initially deposited material can be controlled, thereby controlling the porosity of the initial layer. Further explanation of ITO oblique deposition can be found at least in the following literature: “Light-Extraction Enhancement of GaInN Light Emitting Diodes by Graded-Refractive-Index IndiumTin Oxide Anti-Reflection Contact,” Jong Kyu Kim et al., Advanced Materials, 0000, 00, 1-5.

[0056] Figure 11 An optical device formed from a single light-emitting structure 150 and surrounding material is shown. As shown, the light-emitting structure 150 has an active layer 120 configured to emit light when exposed to an electric current. The active layer 120 is sandwiched between an n-cladding layer 110 (such as n-doped gallium nitride) and a p-cladding layer 130 (such as p-doped gallium nitride). In an embodiment, the active layer 120 comprises multiple quantum wells. In a further embodiment, alternative layer structures with alternative and / or additional layers are used. Those skilled in the art will understand that any number of potential light-emitting structures can be used, as long as they operate as described. In a particular embodiment, the light-emitting structure includes an electron-blocking layer located between the p-cladding layer 130 and the active layer 120. In a further embodiment, the light-emitting structure 150 includes one or more buffer layers. The light-emitting structure 150 has a top light-emitting surface 155 and substantially vertical sidewalls. Figure 13 An embodiment with roughened sidewalls is shown, which has been found to improve brightness uniformity and enhance light extraction, especially when there is a significant difference in refractive index between the spacer material and the material of the light-emitting structure 150. As shown, a p-contact is provided in the form of a first transparent conductive oxide 250 in the form of a convex lens to contact the p-cladding 130. Thus, the first transparent conductive oxide 250 forms a first electrical contact with the light-emitting structure 150, and a second common electrical contact is formed with the n-contact layer of each light-emitting structure 150 via a second transparent conductive oxide layer 500.

[0057] Contacting the sidewalls of the light-emitting structure is a corresponding pseudo-parabolic spacer 200 formed of silicon dioxide and having a refractive index n1. In alternative embodiments, the spacer is formed of silicon nitride or titanium oxide. Although the spacers in the illustrated embodiment have a pseudo-parabolic profile, these sides can have any suitable profile described by a series of Bézier curves having two control points and a coefficient B (where B is one of 0.1, 0.5, 0.2, and 0.05). In a preferred embodiment, the Bézier coefficient is 0.5, resulting in spacers with approximately straight sides angled outwards from the tableside sidewalls.

[0058] Figure 12 An embodiment is depicted in which the spacer 200 is formed by an inner portion 200a and an outer portion 200b having refractive indices n1 and n2, respectively. In a preferred embodiment, n1>n2 can be achieved by using silicon nitride as the inner spacer material and aluminum oxide as the second spacer material. In a further embodiment, a decreasing refractive index (i.e., n1>n2>n2) can be used on the sidewalls away from the light-emitting structure 150. N Additional spacer layer. Although in the illustration Figure 12 The spacers are depicted as two separate spacers, but the spacers can actually be formed as a continuous layer, as shown in the cross-sectional view depicted in Figure 16, where the light-emitting structure 150 has any preferred cross-section depending on its application.

[0059] Although not shown, reflective conductive material 300 is coated on the outer surfaces of spacer 200 and transparent conductive oxide 250 to form an electrical contact with n-cladding 110.

[0060] Similarly, although not shown, the light-emitting surface 155 of the light-emitting structure 150 is covered by a second layer of transparent conductive oxide 500. In an embodiment, light extraction features are disposed above each underlying light-emitting structure 150 in the form of a convex lens. In a particular embodiment, the light extraction features are patterned in the transparent conductive oxide itself. In an alternative embodiment, it is provided by a separate layer formed of a suitable transparent material, such as resin.

[0061] In use, current is applied to the light-emitting structure through a common electrode formed of a second transparent conductive oxide 500 and a p-contact provided by a first transparent conductive oxide 250, with the reflective conductive material 300 further serving as a current diffusion layer. Light emitted by the active layer 120 is directed to the light-emitting surface 155 by: directly, i) via reflection and / or refraction at the spacer 200, ii) via reflection at the interface between the reflective conductive material 300 covering the spacer 200 and the first transparent conductive oxide 250 (which itself acts as a convex lens), or iii) via multiple reflections within a structure comprising the above combinations. Therefore, the spacer 200, the first transparent conductive oxide 250, and the reflective conductive material 300 are arranged to increase the proportion of light incident on the light-emitting surface 155 within the critical angle range to allow light transmission.

[0062] The light extraction and coupling efficiency were studied based on optical simulations as a function of the radius of curvature of the convex lens provided by the first transparent conductive oxide 250 and the MESA depth.

[0063] Figure 14 The relationship between light extraction efficiency and the radius of curvature and MESA depth of the convex lens provided by the first transparent conductive oxide 250 is shown, assuming that the LED spacing of the light-emitting structure 150 is 3 micrometers, with silicon nitride spacers, and indium tin oxide as the first transparent conductive material 250. Silicon nitride is used in particular because of its high refractive index (2.05 at a wavelength of 450 nm) and its common use in the semiconductor industry.

[0064] Figure 14 The optimal light extraction was demonstrated when the MESA depth was between 1 micrometer and 1.3 micrometers and the radius of curvature of the convex lens provided by the first transparent conductive oxide 250 was greater than 1.5 micrometers.

[0065] Therefore, the micro-LED array device of the present invention is particularly suitable for virtual and augmented reality systems, wherein the array device is coupled to a projection lens system to form a virtual image perceived by the eye. Typically, the projection has an F-number between 1.5 and 4. In this disclosure, we employed a projection lens with an F-number of 2 (F / 2) and performed ray tracing simulation. An F / 2 projection lens has an acceptance angle of approximately + / -14 degrees, so light emitted outside this angle range will not couple into the imaging optical path and thus become undesirable stray light within the system.

[0066] Figure 15 The coupling efficiency of this system (F / 2) is shown, where maximum coupling efficiency is achieved when the MESA depth is approximately 1.2 micrometers, the radius of curvature of the convex lens provided by the first transparent conductive oxide 250 is 1.1 micrometers, and the LED spacing (i.e., the interval between adjacent light-emitting structures 150) is 3 micrometers.

Claims

1. A method for forming an optical device, the method comprising the following steps: A mesa is formed, the mesa including an active layer configured to emit light from a first light-emitting surface of the mesa when subjected to an electric current, the mesa further including a second light-emitting surface opposite to the first light-emitting surface and vertical sidewalls, wherein the vertical sidewalls of the mesa are roughened. Spacers are formed on these tabletop sidewalls. These spacers are formed of a first electrically insulating optically transparent material and have an inner surface facing these tabletop sidewalls and an outer surface facing away from them. The outer surface of the spacer is a straight side and is angled outward from these tabletop sidewalls. A first layer of transparent conductive material is deposited on the second light-emitting surface of the platform. The first layer of transparent conductive material has an inner surface facing the second light-emitting surface of the platform and an outer surface facing the opposite surface. The outer surface of the first layer of transparent conductive material is generally convex and the radius of curvature of the outer surface of the first layer of transparent conductive material is between 1 and 1.5 micrometers. A second layer of transparent conductive material is formed on the first light-emitting surface of the platform. as well as A layer of reflective conductive material is deposited on the first layer of transparent conductive material and on the outer surface of these spacers.

2. The method as described in claim 1, wherein, The first layer of transparent conductive material and the second layer of transparent conductive material are transparent conductive oxides.

3. The method as described in claim 2, wherein, The transparent conductive oxide is indium tin oxide.

4. The method of claim 1, wherein, These spacers are formed of at least one of silicon nitride, silicon oxide, or tin oxide.

5. The method of claim 1, further comprising the step of depositing a second electrically insulating optically transparent material on the outer surface of each spacer, the second electrically insulating optically transparent material having a different refractive index than the first electrically insulating optically transparent material.

6. The method of claim 5, wherein, The refractive index of the first electrically insulating optically transparent material is greater than that of the second electrically insulating optically transparent material.

7. The method of claim 1, wherein, The active layer of this mesa is located between the n-doped n-cladding and the p-doped p-cladding.

8. The method of claim 7, wherein, A first electrical contact is formed between the p-cladding layer and the first transparent conductive material and the reflective conductive material, and a second electrical contact is formed between the n-cladding layer and the second transparent conductive material.

9. An optical device array manufactured by the method according to any one of claims 1 to 8.

10. An optical device, comprising: A light-emitting structure having a first light-emitting surface, a second light-emitting surface opposite to the first light-emitting surface, and a vertical sidewall, the light-emitting structure further including an active layer configured to emit light when a current is applied to the device, wherein the vertical sidewall is roughened. An electrically insulating optically transparent spacer material having an inner surface facing the sidewalls of the light-emitting structure and an opposite outer surface, wherein the outer surface of the spacer is a straight side surface and angled outward away from these sidewalls; A first layer of transparent conductive material has an inner surface facing the second light-emitting surface of the light-emitting structure and an outer surface facing the opposite surface. The outer surface of the first layer of transparent conductive material is generally convex, and the radius of curvature of the outer surface is between 1 and 1.5 micrometers. A reflective conductive material is disposed on the outer surface of the spacer layer and the first layer of transparent conductive material; A second layer of transparent conductive material on the first luminescent surface; The spacer material, the transparent conductive material, and the reflective conductive material are configured to enhance light extraction from the active layer.

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