Micro-LED device with porous light limiting layer

Abstract

The invention discloses a micro-LED device with a porous light limiting layer and a preparation method of the micro-LED device. A multi-layer nitride semiconductor thin layer structure which is prepared on a substrate and contains special doping design realizes the basic function of a light emitting diode (LED), and a special doping layer in an epitaxial wafer is converted into a porous layer in combination with an electrochemical corrosion technology; and furthermore, the micro-LED chip shape design is combined, and a high-performance micro-LED device is realized. By adopting the porous layer structure, the refractive index difference between the active region and the surrounding medium is improved, the light limitation in the light-emitting active region is greatly enhanced, the light emission region is modulated, and the light emission loss influenced by the substrate is reduced.

Description

Technical Field

[0001] This invention relates to a micro-LED device with a porous light confinement layer, belonging to the field of semiconductor device technology. Background Technology

[0002] Gallium nitride (GaN), a typical representative of third-generation wide-bandgap semiconductor materials, is a direct bandgap semiconductor with a large bandgap (3.4 eV) and irreplaceable luminescence advantages in the blue-violet light band. Therefore, it is an ideal material for fabricating light-emitting diode (LED) devices and has already achieved widespread practical applications. With technological advancements, Micro-LEDs, as a cutting-edge development direction for LEDs, possess advantages such as high brightness and high pixel density, showing significant advantages in miniaturization and high-resolution displays. Micro-LEDs theoretically have higher external quantum efficiency; however, when the size is shrunk to the tens of micrometers level or below, its peak external quantum efficiency drops significantly. This phenomenon of efficiency decrease with size reduction is called the "efficiency-size effect." During the miniaturization of Micro-LED chips, the surface-to-volume ratio of the emitting mesa increases, and the surface area of ​​the sidewalls increases for the same emitting volume, leading to increased non-radiative recombination of charge carriers due to various adverse surface factors. In particular, the dry etching process inevitably used in the fabrication of Micro-LEDs to form the chip mesa introduces a large number of defects into the sidewalls, further increasing surface non-radiative recombination. Currently, the mainstream methods for reducing surface nonradiative recombination and improving the quantum efficiency of Micro-LEDs are sidewall repair and passivation. On the other hand, GaN is mostly obtained through heteroepitaxial growth, which generates a large number of defects at the interface between the epitaxial layer and the substrate, causing severe defect light absorption. However, LEDs typically emit light in all directions, and only the light emitted from the top surface is actually utilized. Light emitted from the back surface enters the substrate or penetrates the substrate to reach the encapsulation material, making it difficult to utilize and resulting in actual light loss. Conversely, if the light is confined to the quantum well region, the electron-photon coupling of the quantum well can be excited, improving the internal quantum efficiency.

[0003] To mitigate the efficiency degradation caused by sidewall defects in Micro-LEDs and improve overall luminous efficiency, it is necessary to minimize sidewall defect density, restrict light propagation to the back side, and further enhance electron-photon coupling to improve internal quantum efficiency. Therefore, improvements to conventional LED structures and shapes are required. This invention proposes a novel device epitaxial structure that strengthens the restriction of light propagation from the microdisk to the substrate by fabricating a porous layer structure, thereby enhancing electron-photon coupling and improving internal quantum efficiency. Furthermore, various chip geometries are designed to reduce sidewall defect density. Summary of the Invention

[0004] The purpose of this invention is to provide a micro-LED device with a porous light confinement layer.

[0005] The technical solution adopted in this invention is as follows: A micro-LED device with a porous light confinement layer, the structure of which comprises, from bottom to top: (1) Substrate layer; (2) Buffer layer; (3) First u-GaN layer (unintentionally doped layer); (4) n-GaN layer (relatively low concentration doped layer); (5) n+GaN layer (relatively high concentration doped layer); (6) Second u-GaN layer (unintentionally doped layer); (7) Quantum well active light-emitting layer; (8) p-AlGaN / GaN; The doping concentration of the n+GaN layer is higher than that of the n-GaN layer, and the n+GaN layer is processed into an n+GaN layer with a porous structure.

[0006] This invention replaces the conventional n-GaN layer in the LED structure with an n-GaN / n layer designed with a special doping concentration. + The GaN layer has a double-layer structure and meets the requirement of passing through electrochemical corrosion under special conditions, allowing only n to be removed. + The GaN layer is transformed into a uniform porous layer structure, and the n-GaN layer is used to conduct the external circuit of electrochemical corrosion. At the same time, the u-GaN-2 layer is added as a protective layer to protect the quantum well from damage during electrochemical corrosion.

[0007] Preferably, the substrate layer is Si, sapphire, or SiC.

[0008] Preferably, the thickness of the first u-GaN layer is between 500 nm and 2 m.

[0009] Preferably, the thickness of the n-GaN layer is between 1 m and 3 m.

[0010] Preferably, the doping concentration of the n+GaN layer is 5 to 20 times that of the n-GaN layer, and the thickness is 500 nm to 3 m.

[0011] Preferably, the thickness of the second u-GaN layer is between 50 nm and 200 nm.

[0012] Preferably, the active light-emitting layer of the quantum well is InGaN / GaN, AlGaN / GaN, or AlGaN / AlN.

[0013] Preferably, the micro-LED device further includes a p-type electrode and an n-type electrode.

[0014] This invention also discloses a method for fabricating the above-mentioned micro-LED device with a porous light confinement layer, the steps of which include: (1) The micro-LED substrate is etched into a micro disk with a geometric shape, and etched down to the n-GaN layer; (2) Electrochemical corrosion transforms the n+GaN layer into a porous layer; (3) A passivation layer is deposited on the substrate surface by vapor deposition; (4) Evaporation of p-type and n-type electrodes.

[0015] Preferably, in the electrochemical corrosion process of step (2), the micro disk sample is used as the anode, the Pt electrode is used as the cathode, the electrochemical solution is a weak acid solution, selected from oxalic acid, phosphoric acid or a mixture of the two, and the voltage range is 10~30V.

[0016] To obtain high-quality GaN epitaxial layers, the n-GaN grown before the quantum well in conventional LEDs is typically quite thick. One key aspect of this invention is the design of multiple doped layers to achieve a porous layer structure, through the use of n... + The GaN layer is transformed into a porous layer. This porous layer, incorporated into the microcavity, exhibits a significantly low refractive index, creating a substantial refractive index difference with the upper active region structure. This enhances the light confinement capability of the structure above the quantum well, restricting the path of light propagation towards the substrate. Porous structures, such as… Figure 2 As shown. Furthermore, based on this, the present invention designed and fabricated an electrically injected micro-LED device with a special geometric structure, such as... Figure 3 As shown, the flat sidewalls not only provide a smoother and more complete sidewall crystal structure, but also reduce the number of light reflections, thereby reducing the impact of sidewall defects.

[0017] The mechanism of this invention primarily involves increasing the refractive index difference between the active region and the surrounding medium. Because the porous layer is filled with air, its refractive index is lower than that of GaN, significantly reducing absorption and loss of the light field propagating to the substrate, thereby enhancing the light confinement of the microdisk. By changing the experimental parameters of electrochemical etching, including voltage and time, different porosities can be achieved, thus adjusting the refractive index of the porous layer. This invention better confines the light field in the microdisk upwards towards the active region, thereby exciting the electron-photon coupling of the quantum well, strengthening the photon-electron coupling strength in the microdisk, and achieving an improved internal quantum efficiency. This results in a micro-LED device with superior performance, achieving a maximum front-side light emission enhancement of up to 9 times. These features differ from previously reported micro-LED devices. Attached Figure Description

[0018] Figure 1 This is a schematic diagram of the LED epitaxial multilayer structure designed in this invention.

[0019] Figure 2 This is a SEM image of the porous square disk structure prepared according to the present invention.

[0020] Figure 3 These are SEM images and illuminated images of the porous disc, square disc, and hexagonal disc micro-LED devices prepared according to the present invention.

[0021] Figure 4 The comparison of the electro-injection emission spectrum of the porous micro-LED device prepared by this invention with that of conventional micro-LED devices shows that the intensity is increased by up to 9 times. Detailed Implementation

[0022] The present invention will be further described below with reference to the embodiments, but the description of the embodiments does not limit the scope of protection of the present invention in any way.

[0023] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used herein in the specification of this invention is for the purpose of describing particular embodiments only and is not intended to limit the invention. Furthermore, while this document provides examples of parameters containing specific values, it should be understood that the parameters need not be exactly equal to the corresponding values, but can approximate the corresponding values ​​within acceptable error tolerances or design constraints. Directional terms mentioned in the embodiments, such as “up,” “down,” “front,” “back,” “left,” “right,” etc., are only for reference to the accompanying drawings. Therefore, the directional terms used are for illustrative purposes and not for limiting the scope of protection of this invention.

[0024] Unless otherwise specified, all substances or instruments used in the following examples can be obtained from conventional commercial sources.

[0025] Example 1 The present invention discloses a method for fabricating a micro-LED mesa with a porous light confinement layer, comprising the following steps: like Figure 1 The micro-LED device with a porous light confinement layer has the following structure from bottom to top: (1) Si substrate layer, thickness 0.5 mm; (2) AlN / AlGaN buffer layer, 500 nm thick; (3) u-GaN-1 layer (unintentionally doped layer, thickness 1200 nm). (4) n-GaN layer (relatively low doping concentration layer, 1500 nm, 1×10 18 / cm 3 ); (5) n+GaN layer (relatively high concentration doped layer 1500 nm, 8×1018 / cm 3 ); (6) u-GaN-2 layer (unintentionally doped layer, 100 nm thick); (7) InGaN multi-quantum-well active light-emitting layer, 200 nm thick; (8) p-AlGaN / GaN, thickness 120 nm; Epitaxial wafers utilize metal-organic chemical vapor deposition (MOCVD) equipment to epitaxially grow AlGaN materials and other intermediate nitride intercalation layer materials on a substrate material. The typical structure is as follows: a buffer layer is grown on a substrate material such as sapphire, silicon carbide or silicon, and then n-type layers, light-emitting layers and p-type layers with different doping concentrations are grown sequentially on the buffer layer.

[0026] The preparation process is as follows: 1) The shape of the microdisk is defined as square by photolithography, with a side length ranging from 1 to 100 μm. The sample prepared the disk as 4 μm. 2) Etch down to the n-GaN layer (etching depth is less than the total thickness of the n-GaN layer and above); 3) Electrochemical corrosion of n + The GaN layer is transformed into a porous layer. The microdisk sample is used as the anode, the Pt electrode is used as the cathode, and the electrochemical solution is a weak acid solution that completely submerges the device. It can be oxalic acid, phosphoric acid, or a mixture of both. The voltage is 16V, and the corrosion stops when the current is less than 1mA. The microdisk sample is then removed and cleaned. 4) The prepared square micro-LED mesa with a porous structure was observed using a scanning electron microscope (SEM). The resulting SEM images are shown below. Figure 2 As shown.

[0027] Example 2 The present invention discloses a method for fabricating a micro-LED device with a porous light confinement layer, comprising the following steps: like Figure 1 The micro-LED device with a porous light confinement layer has the following structure from bottom to top: (1) Sapphire substrate, 0.4 mm thick; (2) GaN buffer layer, 25 nm thick; (3) u-GaN-1 layer (unintentionally doped layer, thickness 1200 nm). (4) n-GaN layer (relatively low doping concentration layer, 1500 nm, 1×10 18 / cm 3 ); (5) n+GaN layer (relatively high concentration doped layer 1500 nm, 8×10 18 / cm 3 ); (6) u-GaN-2 layer (unintentionally doped layer, 100 nm thick); (7) InGaN multi-quantum-well active light-emitting layer, 200 nm thick; (8) p-AlGaN / GaN, thickness 120 nm; (9) SiO2 passivation layer, 200 nm thick; (10) p-type electrode, wherein the p-type electrode is a Ni / Au metal electrode fabricated on a p-GaN layer; (11) n-type electrode, wherein the n-type electrode is a Ti / Al metal electrode fabricated on an n-GaN layer; Epitaxial wafers utilize metal-organic chemical vapor deposition (MOCVD) equipment to epitaxially grow AlGaN materials and other intermediate nitride intercalation layer materials on a substrate material. The typical structure is as follows: a buffer layer is grown on a substrate material such as sapphire, silicon carbide or silicon, and then n-type layers, light-emitting layers and p-type layers with different doping concentrations are grown sequentially on the buffer layer.

[0028] The preparation process is as follows: 1) The shape of the microdisk is defined by photolithography as circular, square, or hexagonal, with a diameter or side length ranging from 1 to 100 μm. In the example, the microdisk is prepared with a diameter and a square with a variable length of 50 μm, and a hexagonal with a side length of 25 μm. 2) Etch down to the n-GaN layer (etching depth is less than the total thickness of the n-GaN layer and above); 3) Electrochemical corrosion of n + The GaN layer is transformed into a porous layer. The micro disk sample is the anode, the Pt electrode is the cathode, and the electrochemical solution is a weak acid solution, which can be oxalic acid, phosphoric acid, or a mixture of the two. The voltage is 20V, and the corrosion stops when the current is less than 1mA. The micro disk sample is then removed and cleaned. 4) A 200 nm thick SiO2 passivation layer was deposited by electron beam evaporation; 5) Define n-type and p-type ohmic contact windows using photolithography; 6) Cr / Au (50nm / 200nm) p-electrode and n-electrode are deposited by electron beam evaporation; 7) The fabricated micro-LED device with a porous structure was observed by SEM, and the resulting SEM images are as follows. Figure 3 As shown in (a)-(c), the optical microscope images after illumination are as follows: Figure 3 As shown in (d); 8) The fabricated micro-LED device with a porous structure, after electro-injection luminescence testing, showed significantly enhanced front-side luminescence compared to conventional micro-LED devices without a porous structure (the same layered structure, but without etching to create a porous layer). Figure 4 As shown (the test device uses a circular microdisk), the maximum enhancement is up to 9 times. Moreover, from... Figure 4 Looking at the surface, the emission spectrum of the porous micro-LED device changes and becomes sharper. This indicates that the porous structure enhances the electron-photon coupling of the quantum well. The stronger the coupling, the more the radiation process tends to resonant radiation.

[0029] The above embodiments are preferred embodiments of the present invention, but the embodiments of the present invention are not limited to the above embodiments. Any changes, modifications, substitutions, combinations, or simplifications made without departing from the spirit and principle of the present invention should be considered equivalent substitutions and are included within the protection scope of the present invention. For better illustration of the embodiments, some parts in the drawings may be omitted or scaled, and do not represent the actual dimensions of the devices.

Claims

1. A micro-LED device with a porous light confinement layer, characterized in that, Its structure, from bottom to top, includes: (1) Substrate layer; (2) Buffer layer; (3) First u-GaN layer; (4) n-GaN layer; (5) n + GaN layer; (6) Second u-GaN layer; (7) Quantum well active light-emitting layer; (8) p-AlGaN / GaN; Where n + The doping concentration of the GaN layer is higher than that of the n-GaN layer, where n + The GaN layer is processed into an n with a porous structure. + GaN layer.

2. The micro-LED device according to claim 1, characterized in that, The substrate layer is Si, sapphire, or SiC.

3. The micro-LED device according to claim 2, characterized in that, The thickness of the first u-GaN layer is 500 nm to 2 μm.

4. The micro-LED device according to claim 1, characterized in that, The n-GaN layer has a thickness of 1 μm to 3 μm and a doping concentration of 5 × 10⁻⁶. 17 / cm 3 ~2×10 18 / cm 3 .

5. The micro-LED device according to any one of claims 1-4, characterized in that, The n + The GaN layer has a doping concentration 5 to 20 times that of the n-GaN layer, and a thickness of 500 nm to 3 μm.

6. The micro-LED device according to claim 5, characterized in that, The thickness of the second u-GaN layer is 50 nm to 200 nm.

7. The micro-LED device according to claim 6, characterized in that, The active light-emitting layer of the quantum well is InGaN / GaN, AlGaN / GaN, or AlGaN / AlN.

8. The micro-LED device according to any one of claims 1-7, characterized in that, The micro-LED device also includes p-type electrodes and n-type electrodes.

9. A method for fabricating a micro-LED device with a porous light confinement layer according to any one of claims 1-8, characterized in that... The steps include: (1) The micro-LED substrate is etched into a micro disk with a geometric shape, and etched down to the n-GaN layer; (2) Electrochemical corrosion will n + The GaN layer is transformed into a porous layer; (3) A passivation layer is deposited on the substrate surface by vapor deposition; (4) Evaporation of p-type and n-type electrodes.

10. The preparation method according to claim 9, characterized in that, In step (2), during the electrochemical corrosion process, the micro disk sample is used as the anode, the Pt electrode is used as the cathode, and the electrochemical solution is a weak acid solution, selected from oxalic acid, phosphoric acid or a mixture of the two, with a voltage range of 10~30V.

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