Algan-based nanorod array deep ultraviolet micro-led epitaxial wafer and preparation method thereof

Abstract

This invention relates to the field of semiconductor device technology, and more particularly to an AlGaN-based nanopillar array deep-ultraviolet MicroLED epitaxial wafer and its fabrication method. The epitaxial wafer includes a substrate, and an AlN buffer layer, an N-type AlGaN layer, and an AlGaN multiple quantum well layer sequentially stacked on the substrate along the epitaxial direction. The AlGaN multiple quantum well layer has a nanopillar array, comprising multiple nanopillars. Each nanopillar includes the AlGaN multiple quantum well layer, a P-type AlGaN electron blocking layer, a P-type AlGaN layer, and a P-type ohmic contact layer sequentially stacked along the epitaxial direction. The outer walls of the nanopillars are coated with a dielectric passivation layer, and the outer walls of the dielectric passivation layer are coated with a metal reflective layer. The epitaxial wafer of this invention has high light extraction efficiency, effectively reduces the difficulty of TM mode polarized light extraction, and reduces the absorption of the P-type GaN layer in the deep-ultraviolet band.

Description

Technical Field

[0001] This invention relates to the field of semiconductor device technology, and in particular to an AlGaN-based nanopillar array deep ultraviolet MicroLED epitaxial wafer and its fabrication method. Background Technology

[0002] With advancements in micro-nano fabrication technology, MicroLED (micrometer-scale light-emitting diodes, with mesa dimensions typically less than 100 μm) technology has developed rapidly. Compared to conventional large-size LEDs, MicroLEDs offer higher current density tolerance, better heat dissipation, and faster modulation speeds. In the deep ultraviolet band, MicroLEDs have unique application value in high-speed solar-blind ultraviolet communication, miniaturized point-source sterilization modules, and high-resolution ultraviolet lithography. However, existing deep ultraviolet MicroLED solutions all employ micrometer-scale mesa structures and do not involve nanoscale light extraction and enhancement structures.

[0003] One of the most critical bottlenecks restricting the performance improvement of AlGaN-based deep ultraviolet LEDs and Micro LEDs is their extremely low light extraction efficiency. The root causes include two main aspects: First, total internal reflection and material absorption losses. The refractive index of AlGaN is approximately 2.4–2.6, which differs significantly from air (refractive index 1.0), resulting in a critical angle of only about 23° for total internal reflection, preventing most photons from escaping the device surface. Furthermore, the band gap of the P-type GaN contact layer (approximately 3.4 eV) is much smaller than the energy of deep ultraviolet photons (>4.4 eV), leading to strong absorption of deep ultraviolet light and contributing significantly to light loss. Second, the difficulty in extracting TM polarized light. Unlike InGaN-based blue LEDs, which primarily emit transversely polarized (TE) mode light, when the Al content in the AlGaN alloy exceeds approximately 0.25%, the crystal field splitting hole (CH) band at the valence band top gradually shifts upward, causing the emission to shift from TE mode dominance to transversely magnetic (TM) mode dominance. The electric field vector of TM mode light is parallel to the c-axis, and its radiation direction is mainly parallel to the epitaxial layer plane, making it extremely difficult to emit from the top or bottom of the device. This unique polarization characteristic is one of the fundamental physical reasons why the light extraction efficiency of deep ultraviolet LEDs is much lower than that of blue LEDs.

[0004] To improve the light extraction efficiency of deep ultraviolet (DUV) LEDs, researchers have proposed various technical solutions. While surface roughening is simple and effective, its enhancement of TM mode light extraction is limited. Photonic crystal structures can enhance light extraction through Bragg diffraction, but their fabrication typically requires expensive electron beam lithography, resulting in high costs and limited large-area applications. Nanopillar arrays have attracted widespread attention due to their unique advantages, but current research largely remains at the theoretical simulation level or primarily focuses on the visible light band. It is particularly important to emphasize that the optical properties of materials in the deep ultraviolet band (200nm~280nm) are fundamentally different from those in the visible light band; conventional SiN... x The passivation layer has a transmittance of less than 10% at a wavelength of 275nm, making it unsuitable for the deep ultraviolet band; the conventional Ag reflective layer has a reflectance of less than 30% at a wavelength of 275nm, making it equally unsuitable. Summary of the Invention

[0005] The purpose of this invention is to overcome the shortcomings of the prior art. This invention provides an AlGaN-based nanopillar array deep ultraviolet MicroLED epitaxial wafer with high light extraction efficiency, reduced difficulty in extracting TM mode polarized light, and reduced absorption rate of P-type GaN layer in the deep ultraviolet band.

[0006] To address the aforementioned issues, this invention proposes an AlGaN-based nanopillar array deep ultraviolet MicroLED epitaxial wafer, comprising a substrate, and an AlN buffer layer, an N-type AlGaN layer, and an AlGaN multiple quantum well layer sequentially stacked on the substrate along the epitaxial direction. The AlGaN multiple quantum well layer has a nanopillar array, which includes multiple nanopillars. Each nanopillar includes the AlGaN multiple quantum well layer, a P-type AlGaN electron blocking layer, a P-type AlGaN layer and a P-type ohmic contact layer stacked sequentially along the epitaxial direction. The outer wall of the nanopillar is covered with a dielectric passivation layer, and the outer wall of the dielectric passivation layer is covered with a metal reflective layer.

[0007] As an improvement to the above technical solution, the diameter of the nanopillar is 80nm~350nm, the center-to-center distance between two adjacent nanopillars is 150nm~700nm, and the height of the nanopillar is 100nm~500nm.

[0008] As an improvement to the above technical solution, the nanopillar array is arranged periodically according to a preset pattern, and the gap between two adjacent nanopillars is filled or completely filled with the metal reflective layer to form a mesa structure in the nanopillar array region.

[0009] As an improvement to the above technical solution, the AlGaN multi-quantum-well layer is etched to form an etched mesa structure by etching down to the N-type AlGaN layer. The projected shape of the etching mesa structure is circular with a diameter of 5μm to 100μm; or the projected shape of the etching mesa structure is square with a length of 5μm to 100μm. The center of the etched mesa structure coincides with the center of the mesa structure in the nanopillar array region.

[0010] As an improvement to the above technical solution, the dielectric passivation layer is an Al2O3 layer, a SiO2 layer, or a SiN layer. x One or more of the following: a dielectric passivation layer or an HfO2 layer, wherein the thickness of the dielectric passivation layer is 3 nm to 30 nm.

[0011] As an improvement to the above technical solution, the top of the metal reflective layer is lower than the top of the nanopillar; The metal reflective layer is an aluminum metal layer, an aluminum-based alloy layer, or a rhodium metal layer.

[0012] As an improvement to the above technical solution, a P-type reflective electrode is provided on the top of the nanopillar array. The P-type reflective electrode includes a Ni layer and an Au layer, and the Ni layer forms an ohmic contact with the P-type ohmic contact layer. The AlGaN multi-quantum-well layer emits light at wavelengths of 200 nm to 280 nm and comprises quantum well layers and barrier layers that are periodically and alternately stacked along the epitaxial direction.

[0013] Accordingly, the present invention also provides a method for preparing an AlGaN-based nanopillar array deep ultraviolet MicroLED epitaxial wafer, comprising the following steps: S1. Provide a substrate, and sequentially stack and grow an AlN buffer layer, an N-type AlGaN layer, an AlGaN multiple quantum well layer, a P-type AlGaN electron blocking layer, a P-type AlGaN layer and a P-type ohmic contact layer on the substrate. S2. A nanosphere array is disposed on the surface of the P-type ohmic contact layer. An etching mask is formed by reactive ion etching. The etching penetrates the P-type ohmic contact layer, the P-type AlGaN layer and the P-type AlGaN electron blocking layer and extends into the AlGaN multiple quantum well layer to form a nanopillar array. S3. Remove the etching mask and clean it, and deposit a dielectric passivation layer and a metal reflective layer on the surface of the nanopillars of the nanopillar array.

[0014] As an improvement to the above technical solution, step S1 further includes: etching the P-type ohmic contact layer down to the N-type AlGaN layer to form an etched mesa structure; Step S2 further includes: forming a monolayer nanosphere array on the surface of the P-type ohmic contact layer using a self-assembly technique, wherein the nanosphere array comprises multiple nanospheres, and reducing the size of the nanospheres by reactive ion etching to form an etching mask; After the nanopillar array is etched, the etching mask is removed and the surface is cleaned with an alkaline solution. The cleaning temperature of the alkaline solution is 50℃~90℃, and the treatment time is 1min~10min. The alkaline solution is tetramethylammonium hydroxide solution or potassium hydroxide solution. Step S3 further includes: performing back etching to make the top of the metal reflective layer lower than the top of the nanopillars, and preparing a P-type reflective electrode on the top of the nanopillar array, preparing an N-type electrode on the N-type AlGaN layer, and performing flip-chip packaging.

[0015] As an improvement to the above technical solution, the initial diameter of the nanosphere is 150nm~700nm, and the diameter of the nanosphere after etching is 80nm~350nm. The nanosphere is a silica nanosphere or a polystyrene nanosphere. The deposition temperature of the dielectric passivation layer is 150℃~300℃.

[0016] The implementation of this invention has the following beneficial effects: This invention provides an AlGaN-based nanopillar array deep ultraviolet MicroLED epitaxial wafer, wherein the AlGaN multi-quantum-well layer has a nanopillar array, and the nanopillar array comprises the AlGaN multi-quantum-well layer, the P-type AlGaN electron blocking layer, the P-type AlGaN layer and the P-type ohmic contact layer sequentially stacked along the epitaxial direction.

[0017] (1) This structure can generate strong photonic crystal diffraction and waveguide coupling effects, effectively redirecting the TM mode guided light that originally propagated in the epitaxial layer plane to the vertical direction for emission. This can improve the extraction efficiency of TM polarized light, and the extraction efficiency of TE polarized light can also be improved simultaneously.

[0018] (2) Reduce the absorption loss of the P-type ohmic contact layer. The nanopillar array penetrates the P-type AlGaN layer and the P-type ohmic contact layer, physically removing a large amount of P-GaN material that has a strong absorption effect on deep ultraviolet light, thereby reducing the absorption of the P-type AlGaN layer in the deep ultraviolet band.

[0019] (3) The outer walls of the nanopillars in the nanopillar array are coated with a dielectric passivation layer, which effectively suppresses nonradiative recombination on the sidewalls. The dielectric passivation layer covering the sidewalls of the nanopillars can effectively saturate the surface dangling bonds on the sidewalls, reducing the interface state density and the surface nonradiative recombination rate.

[0020] (4) The outer wall of the dielectric passivation layer is covered with a metal reflective layer, which enhances directional light emission and improves heat dissipation. The metal reflective layer can effectively block photons from leaking to the P side and reflect them to the substrate side for emission. At the same time, it has good thermal conductivity, which helps to quickly dissipate the heat generated by the MicroLED when it is operating at high current density and alleviate the heat accumulation effect. Attached Figure Description

[0021] Figure 1 This is a schematic diagram of the structure of an AlGaN-based nanopillar array deep ultraviolet MicroLED epitaxial wafer according to an embodiment of the present invention; Figure 2 yes Figure 1 Enlarged diagram of point A in the diagram; Figure 3 This is a schematic diagram of the structure of an AlGaN-based nanopillar array deep ultraviolet MicroLED epitaxial wafer after the deposition of the P-type ohmic contact layer, according to an embodiment of the present invention. Figure 4 This is a schematic diagram of the structure of an AlGaN-based nanopillar array deep ultraviolet MicroLED epitaxial wafer after forming a nanosphere array on the surface of the P-type ohmic contact layer, according to an embodiment of the present invention. Figure 5 This is a schematic diagram of the structure of an AlGaN-based nanopillar array deep ultraviolet MicroLED epitaxial wafer after forming nanopillars, according to an embodiment of the present invention. Figure 6 This is a schematic diagram of the structure of an AlGaN-based nanopillar array deep ultraviolet MicroLED epitaxial wafer according to an embodiment of the present invention, after a dielectric passivation layer is formed on the outer wall of the nanopillars; Figure 7 yes Figure 6 Enlarged diagram of point B in the diagram; Figure 8 This is a schematic diagram of the structure of an AlGaN-based nanopillar array deep ultraviolet MicroLED epitaxial wafer after a metal reflective layer is deposited on the outer wall of the dielectric passivation layer, according to an embodiment of the present invention. Figure 9 yes Figure 8 Enlarged diagram of point C in the diagram; Figure 10 This is a schematic diagram of the structure of an AlGaN-based nanopillar array deep ultraviolet MicroLED epitaxial wafer according to an embodiment of the present invention, after the gap between two adjacent nanopillars is completely filled with a metal reflective layer. Figure 11 This is a schematic diagram of the structure of an AlGaN-based nanopillar array deep ultraviolet MicroLED epitaxial wafer after the P-type ohmic contact layer forms a P-type reflective electrode, according to an embodiment of the present invention. Figure 12This is a top view of an AlGaN-based nanopillar array deep ultraviolet MicroLED epitaxial wafer according to an embodiment of the present invention, wherein the nanopillar array is arranged in a hexagonal close-packed periodic pattern. Figure 13 This is a top view of an AlGaN-based nanopillar array deep ultraviolet MicroLED epitaxial wafer according to an embodiment of the present invention, wherein the nanopillar array is arranged in a square periodic pattern. Detailed Implementation

[0022] To make the objectives, technical solutions, and advantages of the present invention clearer, the present invention will be further described in detail below with reference to the accompanying drawings.

[0023] See Figure 1 and Figure 2 As shown, this embodiment of the invention provides an AlGaN-based nanopillar array deep ultraviolet MicroLED epitaxial wafer, including a substrate 100, and an AlN buffer layer 200, an N-type AlGaN layer 300, and an AlGaN multiple quantum well layer 400 sequentially stacked on the substrate 100 along the epitaxial direction.

[0024] In this embodiment of the invention, a novel nanopillar array structure is designed on an AlGaN multi-quantum-well layer 400 to achieve high light extraction efficiency, low difficulty in extracting TM-mode polarized light, and reduced absorption of the P-type AlGaN layer in the deep ultraviolet band.

[0025] Specifically, the AlGaN multiple quantum well layer 400 has a nanopillar array 800, which includes a plurality of nanopillars 801. Each nanopillar 801 includes the AlGaN multiple quantum well layer 400, a P-type AlGaN electron blocking layer 500, a P-type AlGaN layer 600 and a P-type ohmic contact layer 700 stacked sequentially along the epitaxial direction. The outer wall of the nanopillar 801 is covered with a dielectric passivation layer 810, and the outer wall of the dielectric passivation layer 810 is covered with a metal reflective layer 820.

[0026] An array of nanopillars 800 is embedded in the AlGaN multi-quantum-well layer 400. This structure generates strong photonic crystal diffraction and waveguide coupling effects, effectively redirecting the TM-mode guided light, which originally propagates within the epitaxial layer plane, to a vertical emission direction. This improves the extraction efficiency of TM-polarized light, and simultaneously enhances the extraction efficiency of TE-polarized light.

[0027] Moreover, it can reduce the absorption loss of the P-type ohmic contact layer 700. The nanopillar array 800 penetrates the P-type ohmic contact layer 700, the P-type AlGaN layer 600 and the P-type AlGaN electron blocking layer 500, physically removing a large amount of P-AlGaN material that has a strong absorption effect on deep ultraviolet light, thereby reducing the absorption of the P-type AlGaN layer in the deep ultraviolet band.

[0028] The outer walls of the nanopillars 801 in the nanopillar array 800 are coated with a dielectric passivation layer 810, which effectively suppresses nonradiative recombination on the sidewalls. The dielectric passivation layer 810 covering the sidewalls of the nanopillars 801 can effectively saturate the surface dangling bonds on the sidewalls, reducing the interface state density and the surface nonradiative recombination rate.

[0029] The outer wall of the dielectric passivation layer 810 is covered with a metal reflective layer 820, which enhances directional light emission and improves heat dissipation. The metal reflective layer 820 can effectively block photons from leaking to the P-side and reflect them to the substrate side for emission. At the same time, it has good thermal conductivity, which helps to quickly dissipate the heat generated by the MicroLED when it operates at high current density and alleviates the heat accumulation effect.

[0030] In some embodiments, the diameter of the nanopillar 801 is 80nm~350nm, the center-to-center distance between two adjacent nanopillars 801 is 150nm~700nm, and the height of the nanopillar 801 is 100nm~500nm.

[0031] The diameter of the nanopillars 801 can be 80nm, 100nm, 150nm, 170nm, 190nm, 210nm, 230nm, 250nm, 290nm, 300nm, 320nm, or 350nm, but is not limited to these values. The center-to-center distance between two adjacent nanopillars 801 can be 150nm, 200nm, 230nm, 280nm, 350nm, 400nm, 420nm, 490nm, 510nm, 600nm, 630nm, 690nm, or 700nm, but is not limited to these values. The height of the nanopillars 801 can be 100nm, 130nm, 170nm, 200nm, 250nm, 300nm, 370nm, 400nm, or 500nm, but is not limited to these values.

[0032] Preferably, the diameter of the nanopillar 801 is 100nm~300nm, the center-to-center distance between two adjacent nanopillars 801 is 200nm~600nm, and the height of the nanopillar 801 is 130nm~400nm.

[0033] This embodiment adjusts the period and fill ratio of the nanopillar array 800 by controlling the diameter of the nanopillars 801 and the center-to-center distance between two adjacent nanopillars 801. Photons that would otherwise exceed the escape angle are diffracted and coupled into a radiation mode that can be emitted, thereby widening the escape angle of light. Combined with the height of the nanopillars 801, this ensures that light emitted from the quantum well forms constructive interference upon reflection at the top and bottom of the nanopillars 801, maximizing light radiation towards the top.

[0034] When the diameter of the nanopillar 801 is less than 80 nm, the proportion of light emitted from the damaged region caused by the etching of the nanopillar 801 increases, the probability of defect states trapping charge carriers increases significantly, and the internal quantum efficiency decreases. When the diameter of the nanopillar 801 is greater than 350 nm, the optical field mode degenerates from the nanoresonator to the planar waveguide, the TM polarized light is re-confined to the horizontal direction, and the top outgoing light flux decreases.

[0035] When the spacing between the 801 nanopillars is less than 150 nm, the small spacing induces a strong near-field coupling effect, causing the deep ultraviolet TM polarized light to be locked in the horizontal direction, resulting in attenuation of the vertically emitted light flux, failure of the periodic diffraction grating effect, and a decrease in light extraction efficiency due to photon localization. When the spacing between the 801 nanopillars is greater than 700 nm, the lateral resistance of the 600-layer p-type AlGaN increases, and the effective light-emitting area shrinks.

[0036] When the height of the 801 nanopillars is less than 100 nm, the AlGaN multi-quantum-well layer 400 cannot be completely contained. The quantum wells are etched through, resulting in a loss of luminescent area, failure of electron blocking, and an increase in carrier leakage rate. When the height of the 801 nanopillars is greater than 500 nm, the probability of photons emitted from the bottom quantum wells being absorbed before reaching the top increases, and the density of non-radiative recombination centers increases.

[0037] Preferred, see Figure 12 and Figure 13 As shown, the nanopillar array 800 is arranged periodically according to a preset pattern; see also Figures 8 to 10 As shown, the gap between two adjacent nanopillars 801 is partially or completely filled with the metal reflective layer 820 to form a mesa structure 830 in the nanopillar array region. More preferably, the gap between two adjacent nanopillars 801 is completely filled with the metal reflective layer 820 to avoid internal total internal reflection and improve TM light extraction efficiency.

[0038] The nanopillar array 800 can be arranged in a hexagonal close-packed periodic pattern or a square periodic pattern. More preferably, it is arranged in a hexagonal close-packed periodic pattern. The hexagonal close-packed periodic pattern enhances photon localization and Bragg diffraction by maximizing the fill density. This arrangement has a higher extraction efficiency for deep ultraviolet TM polarized light than the square arrangement. The hexagonal close-packed honeycomb conductive network reduces the lateral resistance of the p-type AlGaN layer 600, and the current distribution uniformity is improved compared to the square arrangement. The mesa structure 830 of the nanopillar array region is cylindrical. The cylindrical symmetry eliminates azimuth dependence, allowing TM polarized light to be efficiently coupled to the vertical mode in any direction. The continuously changing radius of curvature avoids electric field concentration at the corners, resulting in a higher breakdown voltage and extended operating life compared to the square pillar structure.

[0039] Better, see Figure 12 and Figure 13As shown, the AlGaN multi-quantum-well layer 400 is etched down to the N-type AlGaN layer 300 to form an etched mesa structure 401. The projected shape of the etching mesa structure 401 is circular or square; When the projected shape of the etched mesa structure 401 is circular, its diameter is 5μm~100μm; When the projected shape of the etching mesa structure 401 is square, its length is 5μm~100μm; The center of the etched mesa structure 401 coincides with the center of the nanopillar array region mesa structure 830.

[0040] The diameter or side length of the projected shape of the etched mesa structure 401 can be 5μm, 10μm, 20μm, 40μm, 50μm, 70μm, 80μm, or 100μm, but is not limited to these.

[0041] In some embodiments, the dielectric passivation layer 810 is an Al2O3 layer, a SiO2 layer, or a SiN layer. x The dielectric passivation layer 810 is one or more of a layer or an HfO2 layer, and its thickness is 3 nm to 30 nm. Preferably, the dielectric passivation layer 810 is an Al2O3 layer. A dense dielectric passivation layer 810 (Al2O3 layer) is prepared using atomic layer deposition (ALD) technology and conformally covers the sidewalls of the nanopillars 801. The Al2O3 layer prepared by ALD can effectively saturate the surface dangling bonds of the sidewalls, significantly reducing the interface state density and the surface nonradiative recombination rate. The Al2O3 layer has a transmittance of more than 90% at a wavelength of 275 nm, and its refractive index forms a gradient transition with the refractive index of AlGaN, reducing interface reflection loss. It does not cause additional absorption loss for the propagation of deep ultraviolet light.

[0042] The step coverage of the dielectric passivation layer 810 is greater than 95%. It should be noted that step coverage measures the uniformity of the dielectric passivation layer 810 prepared by atomic layer deposition (ALD) on the surface of a high aspect ratio nanostructure. Specifically, it refers to the ratio of the actual deposited passivation layer thickness to the target nominal thickness in the "stepped" regions with different morphologies, such as the top, vertical sidewalls, and bottom corners of the nanopillars 801. This ratio must be greater than 95% across the entire region. If the coverage in the bottom region of the sidewalls is insufficient, resulting in a sudden drop in local film thickness or even pinholes, the surface defects introduced by the etching cannot be completely passivated, and the non-radiative recombination rate will increase significantly.

[0043] The thickness of the dielectric passivation layer 810 can be 3nm, 7nm, 10nm, 15nm, 20nm, 25nm, or 30nm, but is not limited to these.

[0044] When the thickness of the dielectric passivation layer 810 is less than 3 nm, pinholes appear in the dielectric passivation layer 810, the leakage current density increases, the sub-tunneling effect causes carrier leakage, and the internal quantum efficiency decreases. When the thickness of the dielectric passivation layer 810 is greater than 30 nm, stress accumulation leads to film peeling, the adhesion of the dielectric passivation layer 810 decreases, and the risk of peeling increases.

[0045] In some embodiments, see Figure 9 As shown, the top of the metal reflective layer 820 is lower than the top of the nanopillar 801; specifically, the top of the metal reflective layer 820 is 10nm~50nm lower than the top of the nanopillar 801.

[0046] The height difference between the top of the metal reflective layer 820 and the top of the nanopillar 801 can be 10nm, 15nm, 20nm, 25nm, 30nm, 40nm, 45nm, or 50nm, but is not limited to these values.

[0047] A height difference of 10nm to 50nm ensures that the upper edge of the metal reflective layer 820 is precisely aligned with the quantum well's light-emitting center. Horizontally propagating TM-polarized light is efficiently captured by the aluminum layer and reflected vertically. The reserved top space of the nanopillars 801 forms an optical isolation region, preventing the metal from absorbing the vertically emitted light.

[0048] When the height difference is less than 10 nm, the electromagnetic field at the metal edge strongly couples with the emitted light from the top, inducing surface plasmon resonance absorption. The quantum tunneling effect leads to a surge in leakage current, causing a sharp drop in device reliability. When the height difference is greater than 50 nm, the quantum well position is below the metal layer coverage area, and the escape rate of horizontally propagating TM light increases.

[0049] The metal reflective layer 820 is an aluminum metal layer, an aluminum-based alloy layer, or a rhodium metal layer. Preferably, the metal reflective layer 820 is an aluminum metal layer, which enhances directional light emission and improves heat dissipation within the nanopillar gaps after the deposited passivation layer 810.

[0050] Aluminum exhibits high reflectivity in the deep ultraviolet (DUV) band, significantly higher than that of silver and gold in this band, making it the preferred metallic reflective material for this region. The passivation layer Al₂O₃ has a transmittance exceeding 90% at 275 nm, while aluminum metal itself has a reflectivity exceeding 90% at the same wavelength. The aluminum reflective layer effectively blocks photons from leaking to the P-side, reflecting them back to the substrate side for emission. Simultaneously, aluminum's excellent thermal conductivity helps to quickly dissipate the heat generated by the MicroLED during high current density operation, mitigating heat buildup.

[0051] Preferred, see Figure 11As shown, a P-type reflective electrode 710 is formed on the top of the nanopillar array 800. The P-type reflective electrode 710 includes a Ni layer and an Au layer, and the Ni layer forms an ohmic contact with the P-type ohmic contact layer. The AlGaN multi-quantum-well layer 400 emits light at wavelengths of 200 nm to 280 nm and comprises quantum well layers and barrier layers that are periodically and alternately stacked along the epitaxial direction.

[0052] The emission wavelength of the AlGaN multi-quantum-well layer 400 can be 200nm, 210nm, 230nm, 250nm, 260nm, 270nm, or 280nm, but is not limited to these.

[0053] The substrate 100 is an AlN substrate, a sapphire substrate, a Si substrate, or a SiC substrate.

[0054] See Figures 3 to 11 As shown, correspondingly, this embodiment of the invention also provides a method for fabricating an AlGaN-based nanopillar array deep ultraviolet MicroLED epitaxial wafer, comprising the following steps: S1, see also Figure 3 As shown, a substrate 100 is provided, on which an AlN buffer layer 200, an N-type AlGaN layer 300, an AlGaN multiple quantum well layer 400, a P-type AlGaN electron blocking layer 500, a P-type AlGaN layer 600 and a P-type ohmic contact layer 700 are sequentially stacked and grown. S2, see also Figure 4 and Figure 5 As shown, a nanosphere array 802 is disposed on the surface of the P-type ohmic contact layer 700. An etching mask is formed by reactive ion etching, which etches through the P-type ohmic contact layer 700, the P-type AlGaN layer 600 and the P-type AlGaN electron blocking layer 500, and extends into the AlGaN multiple quantum well layer 400 to form a nanopillar array 800. S3, see also Figures 6 to 11 As shown, the etching mask is removed and cleaned, and a dielectric passivation layer 810 and a metal reflective layer 820 are deposited on the surface of the nanopillars 801 of the nanopillar array 800.

[0055] Figures 6 to 7 As shown, a dielectric passivation layer 810 is deposited on the surface of the nanopillars 801 of the nanopillar array 800. Figures 8 to 11 As shown, a dielectric passivation layer 810 is deposited on the surface of the nanopillars 801 of the nanopillar array 800. Wherein, Figures 8-9 In one embodiment, a metal reflective layer 820 is deposited on the outer wall of the dielectric passivation layer 810, and the gap between two adjacent nanopillars 801 is filled with the metal reflective layer 820. Figures 10-11In another embodiment, the metal reflective layer 820 is deposited on the outer wall of the dielectric passivation layer 810, and the gap between two adjacent nanopillars 801 is completely filled with the metal reflective layer 820.

[0056] Preferably, step S1 further includes: etching the P-type ohmic contact layer 700 down to the N-type AlGaN layer 300 to form an etched mesa structure 401; Step S2 further includes: forming a monolayer nanosphere array 802 on the surface of the P-type ohmic contact layer 700 through self-assembly technology, and reducing the size of the nanospheres through reactive ion etching to form an etching mask; After the nanopillar array 800 is etched, the etching mask is removed and the array is cleaned with an alkaline solution to remove damage. The cleaning temperature of the alkaline solution is 50℃~90℃, and the treatment time is 1min~10min. The alkaline solution is tetramethylammonium hydroxide solution or potassium hydroxide solution. Step S3 further includes: performing back etching to make the top of the metal reflective layer 820 lower than the top of the nanopillars 801, and preparing a P-type reflective electrode 710 on the top of the nanopillar array 800, preparing an N-type electrode 410 on the N-type AlGaN layer 300, and performing flip-chip packaging.

[0057] The MicroLED in this embodiment of the invention has a flip-chip structure, emitting light from the back side of the substrate 100, which further avoids the absorption loss of photons passing through the P-type layer.

[0058] Preferably, the initial diameter of the nanospheres is 150 nm to 700 nm, and the diameter of the nanospheres after etching is 80 nm to 350 nm. The nanospheres are silica nanospheres or polystyrene nanospheres. The initial diameter of the nanospheres can be 150 nm, 200 nm, 300 nm, 500 nm, 600 nm, or 700 nm, but is not limited to these. The diameter of the nanospheres after etching can be 80 nm, 120 nm, 150 nm, 200 nm, 250 nm, 300 nm, or 350 nm, but is not limited to these.

[0059] The deposition temperature of the dielectric passivation layer 810 is 150℃~300℃; the deposition temperature can be 150℃, 170℃, 190℃, 220℃, 250℃, 280℃, 300℃, but is not limited to this.

[0060] The following specific examples will provide further details.

[0061] Example 1 This embodiment provides an AlGaN-based nanopillar array deep ultraviolet MicroLED epitaxial wafer, which is prepared by the following method: Provide a substrate, which is a sapphire substrate; An AlN buffer layer with a thickness of 2 μm is deposited on the substrate; An N-type AlGaN layer is deposited on the AlN buffer layer. The N-type AlGaN layer is a Si-doped AlGaN layer with a Si doping concentration of 2 × 10⁻⁶. 18 / cm 3 The Al component content is 0.55%, and the thickness is 2μm; An AlGaN multiple quantum well layer is deposited on the N-type AlGaN layer. The AlGaN multiple quantum well layer includes quantum well layers and barrier layers periodically stacked along the epitaxial direction, with a period number of 5 and an emission wavelength of 275 nm. The quantum well layer is an AlGaN layer with an Al content of 0.45% and a thickness of 3 nm. The barrier layer is an AlGaN layer with an Al content of 0.55% and a thickness of 12 nm. A p-type AlGaN electron blocking layer is deposited on the AlGaN multi-quantum-well layer, with an Al composition of 0.65% and a Mg doping concentration of 5 × 10⁻⁶. 17 / cm 3 The thickness is 20nm; A P-type AlGaN layer is deposited on the P-type AlGaN electron blocking layer, with an Al composition of 0.40 and a Mg doping concentration of 5 × 10⁻⁶. 17 / cm 3 The thickness is 50nm; A P-type ohmic contact layer is deposited on the P-type AlGaN layer, with a Mg doping concentration of 1×10⁻⁶. 20 / cm 3 The thickness is 15nm; A hexagonal close-packed periodic array of nanospheres is formed on the P-type ohmic contact layer by spin-coating self-assembly. Subsequently, the size of the nanospheres is reduced by reactive ion etching to form an etching mask. An inductively coupled plasma dry etching process is used to penetrate the P-type ohmic contact layer, the P-type AlGaN layer and the P-type AlGaN electron blocking layer and extend into the AlGaN multiple quantum well layer to form a hexagonal close-packed periodic cylindrical nanopillar array. The nanopillar array comprises multiple nanopillars, each with a diameter of 150 nm, a center-to-center spacing of 300 nm between adjacent nanopillars, and a height of 150 nm. Remove the nanosphere mask and wash with potassium hydroxide solution at 50℃~90℃ for 10 min; A 10 nm thick dielectric passivation layer (Al2O3 layer) is coated on the sidewalls and bottom of the gaps of the nanopillar array using ALD technology. A metal reflective layer (aluminum metal reflective layer) is deposited on the outer wall of the dielectric passivation layer, and the gap between two adjacent nanopillars is filled with the metal reflective layer, with its top 20 nm below the nanopillars. On top of the nanopillar array, a P-type reflective electrode with a thickness of 155 nm is deposited. The P-type reflective electrode includes a Ni layer and an Au layer. The Ni layer forms an ohmic contact with the P-type ohmic contact layer. The thickness ratio of the Ni layer to the Au layer is 1:30. An N-type electrode is formed on the N-type AlGaN layer.

[0062] Example 2 The difference between this embodiment and Embodiment 1 is that: During the deposition of the metal reflective layer, the gap between two adjacent nanopillars is completely filled with the metal reflective layer (aluminum metal reflective layer) to form a mesa structure of the nanopillar array region.

[0063] Example 3 The difference between this embodiment and Embodiment 2 is as follows: The nanopillar array is arranged in a square periodic pattern, with a diameter of 120 nm, a center-to-center spacing of 250 nm, and a height of 130 nm. A dielectric passivation layer (Al2O3 layer) with a thickness of 8 nm is coated on the sidewalls and bottom of the gaps of the nanopillar array using ALD technology. During the deposition of the metal reflective layer, the gap between two adjacent nanopillars is completely filled with the metal reflective layer (aluminum metal reflective layer), with its top 20 nm below the nanopillars.

[0064] Example 4 The difference between this embodiment and Embodiment 2 is as follows: The nanopillars have a diameter of 200 nm, a center-to-center spacing of 400 nm, and a height of 350 nm. A 15 nm thick dielectric passivation layer (Al2O3 layer) is coated on the sidewalls and bottom of the gaps of the nanopillar array using ALD technology. During the deposition of the metal reflective layer, the gap between two adjacent nanopillars is completely filled with the metal reflective layer (rhodium metal reflective layer), with its top 30 nm below the nanopillars.

[0065] Example 5 The difference between this embodiment and Embodiment 2 is as follows: The diameter of the nanopillars is 400 nm, and the center-to-center distance between two adjacent nanopillars is 800 nm.

[0066] Comparative Example 1 The difference between this comparative example and Example 2 is as follows: An N-type electrode is deposited in an N-type AlGaN layer. This device does not undergo nanopillar array etching and has no nanopillar array structure.

[0067] The technical tests conducted on Examples 1 to 5, as well as Comparative Example 1, yielded the following results: Detection method: 3D-FDTD Light Extraction Efficiency Simulation and Boundary Condition Analysis: Material optical parameter settings: The refractive index and extinction coefficient of each AlGaN layer are obtained by linear difference of Al composition through Vegard's law. The refractive index of GaN at 275nm is 2.65+0.45i (including absorption), the refractive index of AlN at 275nm is 2.20, and the refractive index of Al2O3 passivation layer is 1.67 (at 275nm).

[0068] Simulation Software and Methods: Commercial 3D-FDTD software was used, and the simulation region employed perfectly matched layer (PML) absorbing boundary conditions. The light source was set as a random dipole light source array located at the center of the MQW active layer, with an emission wavelength of 275 nm, simulating both TE and TM polarization cases.

[0069] Based on Comparative Example 1, the extraction efficiency of TM polarized light and TE polarized light in each embodiment was calculated, and the results are shown in Table 1: Table 1 Polarization light extraction efficiency

[0070] Referring to the above detection results, the nanopillar array effectively redirects the TM-mode guided light, which originally propagated within the epitaxial layer plane, to a vertical emission direction. This improves the extraction efficiency of TM-polarized light, and simultaneously enhances the extraction efficiency of TE-polarized light. The outer walls of the nanopillars in the nanopillar array are coated with a dielectric passivation layer, effectively suppressing sidewall nonradiative recombination. The dielectric passivation layer covering the sidewalls of the nanopillars effectively saturates the surface dangling bonds, reducing the interface state density and surface nonradiative recombination rate. The outer walls of the dielectric passivation layer are coated with a metallic reflective layer, enhancing directional light emission and improving heat dissipation.

[0071] The above description represents the preferred embodiments of the present invention. It should be noted that those skilled in the art can make various improvements and modifications without departing from the principles of the present invention, and these improvements and modifications are also considered to be within the scope of protection of the present invention.

Claims

1. An AlGaN-based nanopillar array deep ultraviolet MicroLED epitaxial wafer, characterized in that, It includes a substrate, and an AlN buffer layer, an N-type AlGaN layer and an AlGaN multiple quantum well layer sequentially stacked on the substrate along the epitaxial direction; The AlGaN multiple quantum well layer has a nanopillar array, which includes multiple nanopillars. Each nanopillar includes the AlGaN multiple quantum well layer, a P-type AlGaN electron blocking layer, a P-type AlGaN layer and a P-type ohmic contact layer stacked sequentially along the epitaxial direction. The outer wall of the nanopillar is covered with a dielectric passivation layer, and the outer wall of the dielectric passivation layer is covered with a metal reflective layer.

2. The AlGaN-based nanopillar array deep ultraviolet MicroLED epitaxial wafer as described in claim 1, characterized in that, The diameter of the nanopillar is 80nm~350nm, the center-to-center distance between two adjacent nanopillars is 150nm~700nm, and the height of the nanopillar is 100nm~500nm.

3. The AlGaN-based nanopillar array deep ultraviolet MicroLED epitaxial wafer as described in claim 2, characterized in that, The nanopillar array is arranged periodically according to a preset pattern, and the gap between two adjacent nanopillars is filled or completely filled with the metal reflective layer to form a mesa structure in the nanopillar array region.

4. The AlGaN-based nanopillar array deep ultraviolet MicroLED epitaxial wafer as described in claim 3, characterized in that, The AlGaN multi-quantum-well layer is etched to form an etched mesa structure by etching down to the N-type AlGaN layer. The projected shape of the etched mesa structure is circular, with a diameter of 5μm to 100μm; Alternatively, the projected shape of the etched mesa structure may be square, with a length of 5μm to 100μm; The center of the etched mesa structure coincides with the center of the mesa structure in the nanopillar array region.

5. The AlGaN-based nanopillar array deep ultraviolet MicroLED epitaxial wafer as described in claim 1, characterized in that, The dielectric passivation layer is an Al2O3 layer, a SiO2 layer, or a SiN layer. x One or more of the following: a dielectric passivation layer or an HfO2 layer, wherein the thickness of the dielectric passivation layer is 3 nm to 30 nm.

6. The AlGaN-based nanopillar array deep ultraviolet MicroLED epitaxial wafer as described in claim 1, characterized in that, The top of the metal reflective layer is lower than the top of the nanopillar; The metal reflective layer is an aluminum metal layer, an aluminum-based alloy layer, or a rhodium metal layer.

7. The AlGaN-based nanopillar array deep ultraviolet MicroLED epitaxial wafer according to any one of claims 1 to 6, characterized in that, The top of the nanopillar array is provided with a P-type reflective electrode, which includes a Ni layer and an Au layer, and the Ni layer forms an ohmic contact with the P-type ohmic contact layer. The AlGaN multi-quantum-well layer emits light at wavelengths of 200 nm to 280 nm and comprises quantum well layers and barrier layers that are periodically and alternately stacked along the epitaxial direction.

8. A method for fabricating an AlGaN-based nanopillar array deep ultraviolet MicroLED epitaxial wafer as described in any one of claims 1 to 7, characterized in that, Includes the following steps: S1. Provide a substrate, and sequentially stack and grow an AlN buffer layer, an N-type AlGaN layer, an AlGaN multiple quantum well layer, a P-type AlGaN electron blocking layer, a P-type AlGaN layer and a P-type ohmic contact layer on the substrate. S2. A nanosphere array is disposed on the surface of the P-type ohmic contact layer. An etching mask is formed by reactive ion etching. The etching penetrates the P-type ohmic contact layer, the P-type AlGaN layer and the P-type AlGaN electron blocking layer and extends into the AlGaN multiple quantum well layer to form a nanopillar array. S3. Remove the etching mask and clean it, and deposit a dielectric passivation layer and a metal reflective layer on the surface of the nanopillars of the nanopillar array.

9. The method for fabricating an AlGaN-based nanopillar array deep ultraviolet MicroLED epitaxial wafer as described in claim 8, characterized in that, Step S1 further includes: etching the P-type ohmic contact layer down to the N-type AlGaN layer to form an etched mesa structure; Step S2 further includes: forming a monolayer nanosphere array on the surface of the P-type ohmic contact layer using a self-assembly technique, wherein the nanosphere array comprises multiple nanospheres, and reducing the size of the nanospheres by reactive ion etching to form an etching mask; After the nanopillar array is etched, the etching mask is removed and the surface is cleaned with an alkaline solution. The cleaning temperature of the alkaline solution is 50℃~90℃, and the treatment time is 1min~10min. The alkaline solution is tetramethylammonium hydroxide solution or potassium hydroxide solution. Step S3 further includes: performing back etching to make the top of the metal reflective layer lower than the top of the nanopillars, and preparing a P-type reflective electrode on the top of the nanopillar array, preparing an N-type electrode on the N-type AlGaN layer, and performing flip-chip packaging.

10. The method for fabricating an AlGaN-based nanopillar array deep ultraviolet MicroLED epitaxial wafer as described in claim 9, characterized in that, The initial diameter of the nanospheres is 150nm~700nm, and the diameter of the nanospheres after etching is 80nm~350nm. The nanospheres are silica nanospheres or polystyrene nanospheres. The deposition temperature of the dielectric passivation layer is 150℃~300℃.

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