A gallium nitride-based light-emitting diode epitaxial structure and preparation method thereof, and LED chip

Abstract

The present invention provides a gallium nitride-based light-emitting diode epitaxial structure, a preparation method thereof, and an LED chip. In the epitaxial structure, a multi-level electron-phonon scattering structure, a stepwise increase in the bandgap width of the quantum well layer in the multi-quantum well barrier layer, and an InGaN step structure are designed to suppress electron leakage in different situations, targeting electron ballistic transport, electron acceleration caused by the quantum Stark effect structure in the quantum well, and hot electron transition. This effectively improves the external quantum efficiency under high current.

Description

Technical Field

[0001] The present invention relates to the field of semiconductor technology, and in particular to a gallium nitride-based light-emitting diode epitaxial structure and a preparation method thereof, and an LED chip. Background Art

[0002] At present, GaN-based LED chips have a low current density (1A / cm 2 -10A / cm 2 The blue light band with a room temperature peak external quantum efficiency of nearly 90% has been achieved in the ampere per square centimeter (ampere per square centimeter). However, the problem of luminous efficiency degradation at high current density is still serious, that is, as the current density increases, the external quantum efficiency continues to decline; for example, at 200A / cm 2 Under the current density, the room temperature external quantum efficiency usually drops to about 65%. In actual working conditions, the chip junction temperature can reach about 150℃, 200A / cm 2 The external quantum efficiency under current density is further reduced to below 50%. However, there is an increasing demand for high current density LED devices. For example, the matrix adaptive car headlights with 10,000 pixels and integrated display require LEDs with a current density of 200A / cm 2 Achieve peak illumination close to 100 million nits at operating current density; LED chips used for special applications such as high-intensity projection can have a driving current density of up to 600A / cm 2 Such applications place increasingly higher demands on the external quantum efficiency of LEDs at high current densities.

[0003] At high current density (accompanied by higher drive voltage), the main physical mechanism for the decline in efficiency of GaN-based LEDs is electron leakage and Auger recombination. Currently, the following three methods are commonly used to solve this problem:

[0004] 1. Insert a p-type AlGaN electron blocking layer between the multi-quantum well barrier layer and the p-type GaN current spreading layer to slow down the leakage of electrons from the multi-quantum well barrier region to the p-type layer.

[0005] Although the p-type AlGaN electron blocking layer can alleviate electron leakage to a certain extent, it also hinders the injection of holes into the multi-quantum well barrier region. Therefore, in practical applications, its thickness and barrier height (Al component) cannot be too high; this weakens its effect in blocking electron leakage.

[0006] Second, increase the number of multi-quantum well barriers to reduce Auger recombination and carrier leakage.

[0007] As the number of multi-quantum well barriers increases, although the probability of carrier radiation recombination increases, the carrier concentration decreases, the Auger recombination probability decreases, and electron leakage is also alleviated, the number of multi-quantum well barriers in the current commercial GaN-based LED structure has reached 10 to 12 pairs. Continuing to increase the number of multi-quantum well barriers on this basis will lead to the degradation of the active area crystal quality and a more unbalanced distribution of electrons and holes.

[0008] 3. Insert an n-type AlGaN electron blocking layer between the multi-quantum well barrier and the n-type GaN current spreading layer.

[0009] This design not only introduces additional series resistance, reducing the photoelectric conversion efficiency of the LED, but also theoretically cannot effectively reduce the movement rate of hot electrons injected into the multi-quantum well barrier. Therefore, it has low feasibility and has not been used in engineering and production practice.

[0010] In summary, the three commonly used methods for alleviating the problem of reduced external quantum efficiency of LEDs at high current densities all have certain limitations. With the continuous emergence of new lighting and display application scenarios, people need to adopt more effective methods to improve the external quantum efficiency of LEDs at high current densities. Summary of the Invention

[0011] To overcome the above shortcomings, the present invention provides a gallium nitride-based light-emitting diode epitaxial structure and its preparation method, as well as an LED chip, to reduce the leakage of electrons into the p-type region outside the multi-quantum well barrier layer, thereby improving the external quantum efficiency of the LED under high current density.

[0012] The technical solution provided by the present invention is:

[0013] In one aspect, the present invention provides a gallium nitride-based light-emitting diode epitaxial structure, comprising: a buffer layer, an unintentionally doped GaN layer, an n-type GaN current spreading layer, a multi-level electron-phonon scattering structure, a multi-quantum well barrier layer, a p-type electron blocking layer, a p-type GaN current spreading layer, and a p-type ohmic contact layer, which are sequentially formed on the surface of a growth substrate;

[0014] The multi-level electron-phonon scattering structure is composed of a periodic stack of InGaN layers and GaN layers, with the number of periods being 2-5, and the conduction band energy level of the InGaN layer decreases step by step in the direction away from the substrate;

[0015] The multi-quantum well barrier layer is composed of a periodic stack of InGaN quantum well layers and GaN quantum barrier layers, and the band gap width of the InGaN quantum well layer increases step by step in a direction away from the substrate.

[0016] On the other hand, the present invention provides a method for preparing a gallium nitride-based light-emitting diode epitaxial structure, comprising the following steps: sequentially growing a buffer layer, an unintentionally doped GaN layer, an n-type GaN current spreading layer, a multi-level electron-phonon scattering structure, a multi-quantum well barrier layer and a p-type electron blocking layer, a p-type GaN current spreading layer, and a p-type ohmic contact layer on the surface of a growth substrate;

[0017] The multi-level electron-phonon scattering structure is composed of a periodic stack of InGaN layers and GaN layers, with the number of periods being 2-5, and the conduction band energy level of the InGaN layer decreases step by step in the direction away from the substrate;

[0018] The multi-quantum well barrier layer is composed of a periodic stack of InGaN quantum well layers and GaN quantum barrier layers, and the band gap width of the quantum well layer increases step by step in a direction away from the substrate.

[0019] In another aspect, the present invention provides an LED chip, which includes the above-mentioned gallium nitride-based light-emitting diode epitaxial structure.

[0020] The gallium nitride-based light-emitting diode epitaxial structure and its preparation method, as well as the LED chip provided by the present invention, have at least the following technical effects:

[0021] 1) Due to the increased current density of the LED, the electron movement speed increases. In addition, there is a large conduction band energy level difference between the n-type GaN current expansion layer and the multi-quantum well barrier layer. Therefore, the electrons in the n-type GaN current expansion layer will gain greater kinetic energy when entering the multi-quantum well barrier region, and some of them will become high-energy electrons and undergo ballistic transport, leaking out of the multi-quantum well barrier layer. The present invention designs a multi-level electron-phonon scattering structure between the n-type GaN current expansion layer and the multi-quantum well barrier layer. The energy of the electrons entering the multi-quantum well barrier layer is relaxed through the InGaN layer with gradually decreasing conduction band energy levels in the multi-level electron-phonon scattering structure. That is, before entering the multi-quantum well barrier layer, the high-energy electrons first exchange energy with the lattice in the multi-level electron-phonon scattering structure, undergoing electron-phonon scattering, which reduces the kinetic energy of the electrons, suppresses ballistic transport leakage, and increases the probability of the multi-quantum well barrier layer capturing the injected electrons.

[0022] 2) For electrons that have been injected into the multi-quantum well barrier layer, since the band gap width of the quantum well layer in the present invention is designed to be a step-by-step increasing structure, it can partially offset the band tilt (built-in electric field) caused by the quantum Stark effect of the InGaN quantum well, slow down the accelerated movement of electrons in the multi-quantum well barrier layer toward the p-type GaN current extension layer, achieve more radiative recombination in the quantum well, and reduce the probability of electrons escaping from the multi-quantum well barrier layer at an accelerated rate.

[0023] 3) Electrons at the bottom of the multi-quantum well barrier layer (i.e., the rear end of the multi-quantum well barrier layer, near the p-type electron blocking layer) may escape from the multi-quantum well barrier region by crossing the last quantum barrier and the p-type electron blocking layer through the hot electron transition mechanism. The InGaN step structure in the present invention is located in the last period of the multi-quantum well barrier layer. If electrons directly jump from the bottom of the quantum well to the table of the InGaN step structure through thermionic emission, some of the electrons can undergo multi-level electron-phonon scattering through the InGaN step structure during the fall back process, relaxing the electron energy. This allows some of the hot electrons that may have originally crossed the p-type electron blocking layer of the quantum barrier layer to fall back to the low energy level of the quantum well layer, reducing the probability of hot electron escape. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 Schematic diagram of the epitaxial structure of a gallium nitride-based light-emitting diode in one embodiment of the present invention;

[0025] Figure 2 Schematic diagram of electrons passing through a multi-level electron-phonon scattering structure in one embodiment of the present invention;

[0026] Figure 3 A schematic diagram of part of the conduction band energy levels of a gallium nitride-based light-emitting diode epitaxial structure according to another embodiment of the present invention;

[0027] Figure 4 Schematic diagram of the conduction band energy levels of electrons passing through an InGaN step structure in another embodiment of the present invention;

[0028] Figure 5 FIG1 is a flow chart for preparing an LED epitaxial structure of a gallium nitride-based light-emitting diode in one embodiment of the present invention.

[0029] Reference numerals:

[0030] 1-growth substrate, 2-buffer layer, 3-unintentionally doped GaN layer, 4-n-type GaN current spreading layer, 5-multi-level electron-phonon scattering structure, 6-multi-quantum well barrier layer, 61-InGaN step structure, 7-p-type electron blocking layer, 8-p-type GaN current spreading layer, 9-p-type ohmic contact layer. DETAILED DESCRIPTION

[0031] In order to more clearly illustrate the implementation cases of the present invention or the technical solutions in the prior art, the specific embodiments of the present invention will be described below with reference to the accompanying drawings. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings and other embodiments can be obtained based on these drawings without inventive work.

[0032] A first embodiment of the present invention provides a gallium nitride-based light-emitting diode epitaxial structure, comprising: a buffer layer, an unintentionally doped GaN layer, an n-type GaN current spreading layer, a multi-level electron-phonon scattering structure, a multi-quantum well barrier layer, a p-type electron blocking layer, a p-type GaN current spreading layer, and a p-type ohmic contact layer, all formed in sequence on the surface of a growth substrate; the multi-level electron-phonon scattering structure is composed of a periodic stack of InGaN layers and GaN layers, with the number of periods being 2-5, and the conduction band energy level of the InGaN layer gradually decreasing in a direction away from the substrate; the multi-quantum well barrier layer is composed of a periodic stack of InGaN quantum well layers and GaN quantum barrier layers, and the bandgap width of the InGaN quantum well layer gradually increasing in a direction away from the substrate.

[0033] like Figure 1 As shown, the growth substrate 1 used in this embodiment includes but is not limited to a silicon substrate, a sapphire substrate, a GaN substrate, a silicon carbide substrate, etc. For example, a Si(111) substrate is used in one example. The Si(111) substrate is placed in a MOCVD reaction chamber, heated to 1100°C, and H2 is introduced for 5 minutes of high-temperature surface treatment to remove the oxide layer and form clear atomic steps before subsequent growth. The buffer layer 2 between the growth substrate and the unintentionally doped GaN layer plays the role of buffering lattice differences and stress regulation. In one example, it is AlN, AlN and AlN stacked in sequence. 0.40 Ga 0.60 N, Al 0.20 Ga 0.80 N three-layer structure, grown at 1000℃-1100℃, with thickness of 200nm AlN, 120nm Al 0.40 Ga 0.60 N, 150nm Al 0.20 Ga 0.80 During the preparation process, trimethylaluminum (TMAl) and ammonia (NH3) were first introduced into the reaction chamber to grow an AlN layer with a thickness of 200nm. Then, the flow rate of ammonia was increased and trimethylgallium (TMGa) was introduced to grow an AlN layer with a thickness of 120nm. 0.40 Ga 0.60 Finally, the flow rate of ammonia was further increased and the flow rate of trimethylaluminum was reduced to reduce the Al content of the epitaxial layer and grow an Al layer with a thickness of 150nm. 0.20 Ga 0.80 N layer. The Al content gradually decreases with the distance from the substrate, which is beneficial to reduce the threading dislocation density of the subsequent unintentionally doped GaN layer and buffer layer, and improve the crystal quality. The unintentionally doped GaN layer 3 is grown at 1050℃ and has a thickness of 800nm. The n-type GaN current spreading layer 4 is a Si-doped GaN layer with a Si doping concentration of 1×10 19 / cm 3During the growth process, silane (SiH4) was used as a dopant, grown at 1050°C and 200 torr, with a thickness of 2000nm. When a forward voltage is applied to the LED, electrons in the n-type GaN current spreading layer 4, under the influence of the electric field, enter the multi-quantum well barrier layer, undergoing radiative recombination with holes, generating photons that are emitted to the outside world. However, at high current densities (i.e., high electron velocity), a considerable portion of the electrons have excessive kinetic energy and may be ballistically transported across the multi-quantum well barrier layer and the p-type electron blocking layer, resulting in electron leakage. This significantly reduces the external quantum efficiency in traditional LED epitaxial structures. Therefore, the present invention inserts a multi-level electron-phonon scattering structure 5 between the n-type GaN current spreading layer 4 and the multi-quantum well barrier layer to reduce hot electron ballistic transport leakage.

[0034] The multi-level electron-phonon scattering structure is composed of a periodic stack of InGaN and GaN layers. The thickness of the single InGaN layer is 2nm-10nm, the thickness of the single GaN layer is 2nm-10nm, the number of periods is 2-5, and the conduction band energy level of the InGaN layer decreases step by step in the direction away from the substrate; and the conduction band energy level gradient difference ΔE1 of the InGaN layer in adjacent periods is <ΔE1< ,in, is the longitudinal optical mode phonon energy. The multi-layer scattering structure designed here is conducive to increasing the probability of electron-phonon scattering of hot electrons. By emitting LO phonons for energy relaxation, the hot electrons interact with the lattice before entering the multi-quantum well barrier layer, relax some of the energy, and thus reduce electron ballistic transport leakage. In this embodiment, the multi-level electron-phonon scattering structure 5 is composed of three periods of InGaN layers and GaN layers stacked in sequence. In the first period, the thickness of the InGaN layer is 8nm, the bandgap is 3.20eV, and the GaN thickness is 4nm; in the second period, the thickness of the InGaN layer is 6nm, the bandgap is 3.05eV, and the GaN thickness is 4nm; in the third period, the thickness of the InGaN layer is 4nm, the bandgap is 2.90eV, and the GaN thickness is 4nm. The difference in the bandgap width of the InGaN layers in adjacent periods is 0.15 eV. In the direction toward the multi-quantum well barrier layer, the bandgap width of the InGaN layer gradually decreases. It should be noted here that because the conduction band energy level difference of InGaN is much larger than its valence band energy level difference, the difference in the band gap width is mainly reflected in its conduction band energy level difference. Therefore, it can also be described here as, in the direction away from the growth substrate, the conduction band energy level of the InGaN layer gradually decreases, and the conduction band energy level gradient difference ΔE1 of the adjacent period InGaN layer is 0.15eV. According to the research in the literature Phys. Status Solidi B 247, No. 1, 189–193 (2010) / DOI 10.1002 / pssb.200945144, at this time, the longitudinal optical mode (LO) phonon energy in InGaN is 88meV, then the 0.088eV<0.15eV<0.176eV designed in the present invention satisfies <ΔE1< .like Figure 2 As shown in the energy level structure diagram (from right to left is the growth direction of the epitaxial structure), the initial kinetic energy of the electrons in the n-type GaN current expansion layer 4 under the action of the forward voltage is En, and the energy level difference between the n-type GaN current expansion layer and the multi-quantum well layer is 3ΔEc. When the electrons are injected from the n-type current expansion layer into the multi-quantum well layer, they will gain an additional kinetic energy of 3ΔEc, and the total kinetic energy will increase to (En+3ΔEc). After the kinetic energy of the electrons increases, the probability of ballistic transport and leakage from the multi-quantum well barrier layer also increases accordingly. Under high current density (higher initial electron velocity and initial kinetic energy), this electron leakage phenomenon will be more serious. However, in the process of electrons passing through the multi-level electron-phonon scattering structure 5 composed of 3 cycles, electron-phonon scattering may occur between the high-energy electrons and the lattice, that is, after each electron-phonon scattering cycle, the electrons relax. By reducing the energy of the multi-quantum well barrier region, electrons that might otherwise leak out of the multi-quantum well barrier region due to ballistic transport can now undergo localized radiative recombination within the quantum well. Furthermore, the multi-level electron-phonon scattering structure 5 can buffer the lattice compressive stress exerted on the multi-quantum well by the n-type GaN current expansion layer 4, facilitating the high-quality growth of the multi-quantum well barrier layer. During the fabrication process, the conduction band energy level of the InGaN layer can be adjusted by adjusting the flow rate of trimethylindium (TMIn). Specifically, by adjusting the In content in the InGaN layer, the conduction band energy level of the InGaN layer in different periods is altered, forming a designed conduction band energy level gradient.

[0035] The multi-quantum well barrier layer 6 is formed on the surface of the multi-level electron-phonon scattering structure 5. The number of periods is usually greater than or equal to 5 and less than or equal to 11. The thickness of a single InGaN quantum well layer is between 2.5nm and 5.5nm, and the thickness of a single GaN quantum barrier layer is between 4nm and 15nm. Electrons and holes emit light of a specific wavelength after radiative recombination in the quantum well. In practical applications, a person skilled in the art can design the bandgap width of the quantum well according to needs to obtain a specific emission wavelength, such as blue light, green light, red light, etc. In the multi-quantum well barrier layer of light in the traditional epitaxial structure, the emission wavelength of each quantum well is consistent. In this embodiment, in order to improve the external quantum efficiency under high current, different quantum wells in the multi-quantum well barrier layer are formed with a gradient difference in emission wavelength. The gradient difference in emission wavelength Δλ is 1nm-10nm. The wavelength of the entire multi-quantum well barrier layer changes very little, and the emission color of the overall LED is almost unaffected. In one embodiment, the multi-quantum well barrier layer 6 is composed of nine periods of InGaN quantum well layers and GaN quantum barrier layers stacked in sequence. The thickness of a single InGaN quantum well layer is 3nm, and the emission wavelength is in the blue light band. The thickness of a single GaN quantum barrier layer is 4nm. The thin thickness can enhance the resonant tunneling effect of carriers and promote the radiative recombination of carriers injected into the quantum well. In this embodiment, during the preparation process, the silicon doping concentration in the GaN quantum barrier layer is 3×10 17 cm -3, using N2 as the carrier gas, silane is introduced for Si doping. For the InGaN quantum well layer, the growth temperature is adjusted to change its emission wavelength. For example, the growth temperature of the InGaN multiple quantum well is increased from 780°C to 789°C. The growth temperature of all GaN quantum barriers remains unchanged at 900°C. The bandgap widths of the nine grown InGaN quantum well layers increase step by step in the direction away from the substrate. According to the formula λ=1240 / Eg (output wavelength λ in nm, bandgap width Eg in eV), the emission wavelengths of the nine InGaN quantum well layers decrease step by step accordingly. The emission wavelength of the InGaN quantum well layer in the first cycle close to the growth substrate is 450nm. The emission wavelength of the InGaN quantum well layer in each subsequent cycle decreases by 1.5nm until the emission wavelength of the InGaN quantum well layer in the last cycle decreases to 440nm. This design of gradually decreasing quantum well wavelength in the multi-quantum well barrier layer can partially offset the band tilt (built-in electric field) caused by the quantum Stark effect structure of the InGaN quantum well, so that the electrons in the multi-quantum well barrier layer slow down their accelerated movement toward the p-type GaN current extension layer during their movement in the multi-quantum well barrier layer, thereby retaining the electrons in the multi-quantum well barrier layer and achieving more radiative recombination with holes in the quantum well, thereby preventing the electrons from escaping from the multi-quantum well barrier layer at an accelerated rate.

[0036] The p-type electron blocking layer 7 is a Mg-doped AlGaN layer. In one embodiment, the layer is a 20 nm AlGaN layer grown at 900°C. 0.25 Ga 0.65 During the growth of the N layer, N2 was used as the carrier gas to introduce TMAl, TMGa, and NH3. Cp2Mg was used as the dopant, and the Mg doping concentration was 2.0×10 19 / cm 3 Because the composition contains Al, a certain barrier height is formed, which can prevent electrons escaping from the multi-quantum well barrier layer from moving further to the p-type GaN current expansion layer.

[0037] The p-type GaN current spreading layer 8 is a Mg-doped GaN layer with a Mg doping concentration of 1×10 18 / cm 3 , with a thickness of 80nm. Using H2 and N2 as carrier gases, after applying voltage to the epitaxial structure, the p-type GaN current extension layer 8 can provide holes. Under the action of the electric field, the holes enter the multi-quantum well barrier layer and undergo radiation recombination with electrons to generate photons and emit them to the outside world.

[0038] The p-type ohmic contact layer 9 is a highly doped GaN layer, which is also a Mg-doped GaN layer. However, the Mg doping concentration is greater than the doping concentration in the p-type GaN current spreading layer (for example, in one embodiment, the Mg doping concentration in the p-type GaN current spreading layer 8 is 1×10 18 / cm 3 The Mg doping concentration in the p-type ohmic contact layer 9 is 3×10 20 / cm 3 The Mg doping concentration in the p-type ohmic contact layer 9 is 300 times that of the p-type GaN current spreading layer 8. During the growth process, H2 or N2 is used as a carrier gas, bismuth magnesium (Cp2Mg) is used as a dopant, the epitaxial growth temperature is 900°C to 1000°C, and the thickness is 10 nm.

[0039] The above embodiment is further improved to obtain the present embodiment. In this embodiment, the conduction band energy level structure of the gallium nitride-based light-emitting diode epitaxial structure from the n-type GaN current spreading layer 4 to the p-type GaN current spreading layer 8 is shown in FIG. Figure 3 As shown (from right to left is the growth direction of the growth substrate), not only is a multi-level electron-phonon scattering structure 5 formed between the n-type GaN current expansion layer 4 and the multi-quantum well barrier layer 6, the band gap width of the quantum wells in the multi-quantum well barrier layer increases step by step to form a wavelength gradient difference Δλ, but also an InGaN step structure 61 is formed in the multi-quantum well barrier layer 6. The InGaN step structure is located between the quantum well layer and the quantum barrier layer of the last period in the multi-quantum well barrier layer and is composed of multiple InGaN layers with step-by-step increase in conduction band energy levels. The energy level gradient difference ΔE2 between adjacent InGaN layers is <ΔE2< When electrons transition upward from the bottom of the last quantum well to the multilayered terraces of the InGaN step structure (including the last quantum barrier) via thermal electron emission, they can utilize the InGaN step structure to undergo electron-phonon scattering and energy relaxation. This reduces the electron's energy as it descends from the InGaN step back to the bottom of the last quantum well, thereby reducing the probability of leakage through the p-type electron barrier layer 7 via thermal electron emission. The InGaN step structure can be an InGaN layer with one to three conduction band energy levels increasing in a stepwise manner.

[0040] In one example, the InGaN step structure 61 is a two-layer InGaN layer, wherein the bandgap widths of the first InGaN layer and the second InGaN layer are 3.02eV and 3.22eV respectively, and the thickness of both is 3nm. The difference in the bandgap widths between the two is 0.2eV. Since the conduction band energy level difference of InGaN is much larger than its valence band energy level difference, the difference in the bandgap width is mainly reflected in its conduction band energy level difference, which is equivalent to the conduction band energy level difference of the two InGaN layers in the InGaN step structure being 0.2eV. Here, the triple conduction band energy level difference between the last InGaN quantum well layer, the first InGaN layer, the second InGaN layer, and the GaN quantum barrier layer is greater than the longitudinal optical mode phonon energy of 88meV, which is conducive to the hot electrons that jump here to relax their energy through electron-phonon scattering, and fall back into the multi-quantum well barrier layer for radiative recombination. As shown in FIG. Figure 4As shown in the schematic diagram of the conduction band energy levels of the InGaN step structure (from right to left in the growth direction of the growth substrate), when electrons transition upward from the bottom of the last quantum well to the multi-layered terraces of the InGaN step structure 61 (including the last quantum barrier) via thermal electron emission, some electrons fall back to the next terrace. During this fall, the hot electrons may undergo multi-level electron-phonon scattering and energy relaxation, thereby reducing their energy and the probability of leakage through the p-type electron blocking layer 7 via thermal electron emission.

[0041] During the growth of GaN-based light-emitting diode epitaxial structures, such as Figure 5 As shown, first, MOCVD growth equipment is used and Si (111) substrate is selected as the growth substrate layer. In the MOCVD system, the growth substrate is subjected to high-temperature surface treatment to remove the oxide layer. Then, for different structures, the required precursor sources, gas sources and carrier gases are introduced at specific temperatures, including Ga source, Al source, In source, Mg, Si source and N source and N2, H2, Ar, etc. In the following embodiments, the Ga source is trimethylgallium (TMG), the Al source is trimethylaluminum (TMAl), the In source is trimethylindium (TMIn), the Mg source is bis(cyclopentadienyl)magnesium (Cp2Mg), the Si source is silane (SiH4), the N source is ammonia (NH3), and the carrier gas is N2 and H2. A buffer layer, an unintentionally doped GaN layer, an n-type GaN current expansion layer, a multi-level electron-phonon scattering structure, a multi-quantum well barrier layer and a p-type electron blocking layer, a p-type GaN current expansion layer, and a p-type ohmic contact layer are grown in sequence on the surface of the growth substrate. In this embodiment, an InGaN step structure is further formed in the multi-quantum well barrier layer. The InGaN step structure is located between the quantum well layer and the quantum barrier layer of the last period in the multi-quantum well barrier layer and consists of multiple InGaN layers with gradually increasing conduction band energy levels.

[0042] The specific steps include:

[0043] 1. Place the Si(111) substrate in an MOCVD reaction chamber, heat it to 1100°C, and introduce H2 for 5 minutes of high-temperature surface treatment to remove the oxide layer and form clear atomic steps.

[0044] 2. Set the reaction chamber temperature at 1000-1100°C. First, introduce trimethylaluminum (TMAl) and ammonia (NH3) into the reaction chamber. Using H2 as the carrier gas, grow a 200nm thick AlN layer. Then, increase the flow of ammonia and introduce trimethylgallium (TMGa) to grow a 120nm thick AlN layer. 0.40 Ga 0.60 N layer, increase the flow of ammonia again and reduce the flow of trimethylaluminum (TMAl) to grow Al with a thickness of 150nm 0.20 Ga 0.80N layer. The buffer layer 2 is composed of AlN layer, Al 0.40 Ga 0.60 N layer and Al 0.20 Ga 0.80 It is composed of N layers.

[0045] 3. Grow an 800nm ​​unintentionally doped GaN layer 3 at 1050°C.

[0046] 4. Silane (SiH4) is used as the dopant, and the Si doping concentration is 1×10 19 / cm 3 , under the conditions of 1050° C. and 200 torr, a GaN layer with a thickness of 2000 nm is grown as the n-type GaN current spreading layer 4 .

[0047] 5. Three cycles of stacked InGaN and GaN layers are grown to form a multi-level electron-phonon scattering structure 5. During the fabrication process, the conduction band energy level of the InGaN layer is adjusted by adjusting the flow rate of trimethylindium (TMIn). This means that the conduction band energy level of the InGaN layer in different cycles is varied by adjusting the In content in the InGaN layer, forming a conduction band energy level gradient. In the first cycle, the InGaN layer has an 8nm thickness, a 3.20eV bandgap, and a GaN thickness of 4nm. In the second cycle, the InGaN layer has a 6nm thickness, a 3.05eV bandgap, and a GaN thickness of 4nm. In the third cycle, the InGaN layer has a 4nm thickness, a 2.90eV bandgap, and a GaN thickness of 4nm. It should be noted here that the conduction band energy level difference of InGaN is much larger than its valence band energy level difference, that is, the difference in bandgap width is mainly reflected in its conduction band energy level difference. Therefore, it can also be described here that the conduction band energy level of the InGaN layer gradually decreases in the direction toward the multi-quantum well barrier layer, and the conduction band energy level gradient difference ΔE1 of the InGaN layer in adjacent periods is 0.15eV.

[0048] 6. The multi-quantum well barrier layer 6 is composed of 9 periods of InGaN quantum well layers and GaN quantum barrier layers stacked in sequence. The thickness of a single InGaN quantum well layer is 3nm, and the emission wavelength is in the blue light band. The thickness of a single GaN quantum barrier layer is 4nm. The thin thickness can enhance the resonant tunneling effect of carriers and promote the radiative recombination of carriers injected into the quantum well. In this embodiment, during the preparation process, the silicon doping concentration in the GaN quantum barrier layer is 3×10 17 cm -3Silane was introduced with N2 as the carrier gas for Si doping. For the InGaN quantum well layer, the growth temperature was adjusted to change its emission wavelength. For example, the growth temperature of the InGaN multiple quantum well was increased from 780°C to 789°C. The growth temperature of all GaN quantum barriers remained unchanged at 900°C. The bandgap widths of the nine InGaN quantum well layers finally obtained gradually increased in the direction toward the p-type electron barrier layer. According to the formula λ=1240 / Eg (output wavelength λ in nm, bandgap width Eg in eV), the emission wavelengths of the nine InGaN quantum well layers decreased accordingly. The emission wavelength of the InGaN quantum well layer in the first cycle close to the growth substrate was 450nm. The emission wavelength of the InGaN quantum well layer in each subsequent cycle decreased by 1.5nm until the emission wavelength of the InGaN quantum well layer in the last cycle decreased to 440nm.

[0049] At the same time, an InGaN step structure 61 is formed between the quantum well layer and the quantum barrier layer of the last period in the multi-quantum well barrier layer. The InGaN step structure 61 is two InGaN layers, wherein the band gap widths of the first InGaN layer and the second InGaN layer are 3.02eV and 3.22eV respectively, and the thickness is 3nm.

[0050] 10. The p-type electron blocking layer 7 is a Mg-doped AlGaN layer. In one embodiment, it is a 20 nm AlGaN layer grown at 900°C. 0.25 Ga 0.65 During the growth of the N layer, N2 was used as the carrier gas to introduce TMAl, TMGa, and NH3. Cp2Mg was used as the dopant, and the Mg doping concentration was 2.0×10 19 / cm 3 Because the composition contains Al, a certain barrier height is formed, which can prevent the electrons escaping from the multi-quantum well barrier layer from moving further to the side of the p-type GaN current expansion layer.

[0051] 11. The p-type GaN current spreading layer 8 is a Mg-doped GaN layer with a Mg doping concentration of 1×10 18 / cm 3 , with a thickness of 80nm. After voltage is applied to the epitaxial structure, holes can be provided in the p-type GaN current expansion layer 8. Under the action of the electric field, the holes enter the multi-quantum well barrier layer and undergo radiation recombination with electrons to generate photons and emit them to the outside world.

[0052] 12. Using H2 or N2 as carrier gas, Cp2Mg as dopant, and epitaxial growth temperature of 900℃-1000℃, the Mg doping concentration is 3×10 20 / cm 3A GaN layer with a thickness of 10 nm serves as the p-type ohmic contact layer 9 .

[0053] Another embodiment of the present invention provides an LED chip comprising the aforementioned gallium nitride-based light-emitting diode epitaxial structure. The multi-level electron-phonon scattering structure within the epitaxial structure of the LED chip relaxes the energy of electrons during injection into the multi-quantum well barrier layer, reducing the electrons' kinetic energy and the probability of ballistic transport leakage. Furthermore, the bandgap width of the quantum well layers within the multi-quantum well barrier layer increases stepwise, partially offsetting the band tilt (built-in electric field) caused by the quantum Stark effect of the InGaN quantum wells. This slows the acceleration of electrons in the multi-quantum well barrier layer toward the p-type GaN current extension layer, allowing for better localized radiative recombination within the quantum wells and preventing accelerated electron escape from the multi-quantum well barrier layer. Furthermore, the InGaN step structure in the last period of the multi-quantum well barrier layer can target electrons that undergo hot electron transitions, and can promote multi-level electron-phonon scattering in the process of electrons falling from the InGaN table to the bottom of the last quantum well, thereby relaxing the electron energy and allowing some of the hot electrons that may have originally passed through the multi-quantum well barrier layer and the p-type electron blocking layer to fall back into the multi-quantum well barrier layer.

[0054] The LED chip fabrication process involves pixelating the gallium nitride-based light-emitting diode epitaxial structure using a combination of ICP dry and wet etching techniques, at least to isolate the multi-quantum well barrier layers between different LED chips so that each LED chip can be driven independently. Furthermore, a first electrode and a second electrode are prepared to connect an external power source to the LED chip. The first electrode is conductively connected to the n-type GaN current spreading layer in the LED chip, and the second electrode is conductively connected to the p-type GaN current spreading layer in the LED chip. The first and second electrodes can be located on the same side of the LED chip (forming a flip-chip or face-up structure) or on different sides of the LED chip (forming a vertical structure). Regardless of the design of the first and second electrodes, the first electrode is conductively connected to the n-type GaN current spreading layer in the LED chip, and the second electrode is conductively connected to the p-type GaN current spreading layer in the LED chip. Thus, external current is conducted into the LED chip through the first and second electrodes. In addition, in other embodiments, a bonding layer, a reflective layer, a passivation layer, a connecting metal layer, a transparent conductive layer and other structures are formed in the LED chip to enhance the luminous effect of the chip. This embodiment does not impose any restrictions on this, as long as the LED chip can emit light normally.

[0055] It should be noted that the above embodiments can be freely combined as needed. The above description is only a preferred embodiment of the present invention. It should be pointed out that those skilled in the art can make several improvements and modifications without departing from the principles of the present invention, and such improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A gallium nitride-based light-emitting diode epitaxial structure, characterized in that: LED chips used for high current density include: a buffer layer, an unintentionally doped GaN layer, an n-type GaN current spreading layer, a multi-level electron-phonon scattering structure, a multi-quantum well barrier layer, a p-type electron blocking layer, a p-type GaN current spreading layer, and a p-type ohmic contact layer, which are sequentially formed on the surface of a growth substrate; The multi-level electron-phonon scattering structure is composed of a periodic stack of InGaN layers and GaN layers, with the number of periods being 3-5; in the direction toward the multi-quantum well barrier layer, the conduction band energy level of each InGaN layer decreases step by step; The multi-quantum well barrier layer is composed of a periodic stack of InGaN quantum well layers and GaN quantum barrier layers; in the direction toward the p-type electron barrier layer, the band gap width of each InGaN quantum well layer increases step by step; An InGaN step structure is also formed in the multi-quantum well barrier layer. The InGaN step structure is located between the quantum well layer and the quantum barrier layer of the last period close to the p-type electron blocking layer, and is composed of multiple InGaN layers whose conduction band energy levels gradually increase in the direction toward the p-type electron blocking layer.

2. The gallium nitride-based light-emitting diode epitaxial structure according to claim 1, wherein: In the multi-level electron-phonon scattering structure, the thickness of the single-layer InGaN layer is 2nm-10nm, and the thickness of the single-layer GaN layer is 2nm-10nm; the conduction band energy level gradient difference ΔE1 of the InGaN layer in adjacent periods satisfies: ,in, is the longitudinal optical mode phonon energy.

3. The gallium nitride-based light-emitting diode epitaxial structure according to claim 1, wherein: In the multi-quantum well barrier layer, the light emission wavelength gradient difference Δλ of the InGaN quantum well layers in adjacent periods is 1 nm-10 nm.

4. The gallium nitride-based light-emitting diode epitaxial structure according to claim 1, wherein: In the InGaN step structure, the conduction band energy level gradient difference ΔE2 between adjacent InGaN layers satisfies: ,in, is the longitudinal optical mode phonon energy.

5. A method for preparing a gallium nitride-based light-emitting diode epitaxial structure, characterized in that: The method is applied to LED chips with high current density, comprising the following steps: sequentially growing a buffer layer, an unintentionally doped GaN layer, an n-type GaN current spreading layer, a multi-level electron-phonon scattering structure, a multi-quantum well barrier layer and a p-type electron blocking layer, a p-type GaN current spreading layer, and a p-type ohmic contact layer on the surface of a growth substrate; The multi-level electron-phonon scattering structure is composed of a periodic stack of InGaN layers and GaN layers, with the number of periods being 3-5; in the direction toward the multi-quantum well barrier layer, the conduction band energy level of each InGaN layer decreases step by step; The multi-quantum well barrier layer is composed of a periodic stack of InGaN quantum well layers and GaN quantum barrier layers; in the direction toward the p-type electron barrier layer, the band gap width of each InGaN quantum well layer increases step by step in the direction away from the substrate; An InGaN step structure is also formed in the multi-quantum well barrier layer. The InGaN step structure is located between the quantum well layer and the quantum barrier layer of the last period close to the p-type electron blocking layer, and is composed of multiple InGaN layers whose conduction band energy levels gradually increase in the direction toward the p-type electron blocking layer.

6. The method for preparing a gallium nitride-based light-emitting diode epitaxial structure according to claim 5, wherein: In the multi-level electron-phonon scattering structure, the thickness of the single-layer InGaN layer is 2nm-10nm, and the thickness of the single-layer GaN layer is 2nm-10nm; the conduction band energy level gradient difference ΔE1 of the InGaN layer in adjacent periods satisfies: ,in, is the longitudinal optical mode phonon energy.

7. The method for preparing a gallium nitride-based light-emitting diode epitaxial structure according to claim 5, wherein: In the multi-quantum well barrier layer, the light emission wavelength gradient difference Δλ of the InGaN quantum well layers in adjacent periods is 1 nm-10 nm.

8. The method for preparing a gallium nitride-based light-emitting diode epitaxial structure according to claim 5, wherein: In the InGaN step structure, the energy level gradient difference ΔE2 between adjacent InGaN layers satisfies: ,in, is the longitudinal optical mode phonon energy.

9. An LED chip, characterized in that: The LED chip includes the gallium nitride-based light-emitting diode epitaxial structure according to any one of claims 1 to 4.

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