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

Abstract

The 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-stage electron-phonon scattering structure is designed for the situations of electron acceleration and hot electron transition caused by electron ballistic transport and a quantum stark effect structure in a quantum well, the forbidden band width of a quantum well layer in a multi-quantum well barrier layer is gradually increased, and an InGaN step structure is used for inhibiting electron leakage under different situations, so that the quantum efficiency is improved. And the external quantum efficiency under large current is effectively improved.

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, a preparation method thereof, and an LED chip. Background Art

[0002] At present, GaN-based LED chips have achieved a room-temperature peak external quantum efficiency of nearly 90% in the blue light band at a small to medium current density (1 A / cm 2 -10 A / cm 2 ). However, the problem of decreasing luminous efficiency at high current density is still serious, that is, as the current density increases, the external quantum efficiency continues to decrease; for example, at a current density of 200 A / cm 2 , the room-temperature external quantum efficiency usually drops to about 65%. In actual working conditions, the chip junction temperature can reach about 150 °C, and the external quantum efficiency at a current density of 200 A / cm 2 further decreases to less than 50%. However, the application demand for LED devices at high current density is increasing. For example, a display and illumination integrated matrix adaptive automotive headlamp with tens of thousands of pixels requires the LED to achieve a peak illuminance of nearly 100 million nits (nit) at a working current density of 200 A / cm 2 ; the drive current density of LED chips for special applications such as high-intensity projection can be as high as 600 A / cm 2 . Such applications put forward higher and higher requirements for the external quantum efficiency of LEDs at high current density.

[0003] At high current density (accompanied by a higher drive voltage), the main physical mechanisms for the efficiency decrease of GaN-based LEDs are electron leakage and Auger recombination. Currently, the following three methods are usually used to solve this problem: First, 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 electron leakage from the multi-quantum well barrier region to the p-type layer.

[0004] Although the p-type AlGaN electron blocking layer can alleviate electron leakage to a certain extent, it will also hinder the injection of holes into the multi-quantum well barrier region. Therefore, in practical applications, its thickness and barrier height (Al composition) cannot be too high; this weakens its effect of blocking electron leakage.

[0005] Second, increase the number of multi-quantum well barriers to reduce Auger recombination and carrier leakage.

[0006] With the increase in the number of multiple quantum well barriers, although the radiative recombination probability of carriers increases, the carrier concentration decreases, the Auger recombination probability decreases, and electron leakage is alleviated, in the currently commercial GaN-based LED structures, the number of multiple quantum well barriers has reached 10 to 12 pairs. Continuing to increase the number of multiple quantum well barriers on this basis will lead to deterioration of the crystal quality in the active region and more imbalance in the distribution of electrons and holes.

[0007] III. Insert an n-type AlGaN electron blocking layer between the multiple quantum well barriers and the n-type GaN current spreading layer.

[0008] This design not only introduces an additional series resistance and reduces the photoelectric conversion efficiency of the LED, but also theoretically cannot effectively reduce the movement rate of hot electrons injected into the multiple quantum well barriers. Therefore, the feasibility is relatively low and it has not been applied in engineering and production practices.

[0009] In summary, the above three commonly used methods for alleviating the problem of reduced external quantum efficiency of LEDs under high current density all have certain limitations. With the continuous emergence of new application scenarios for lighting and display, people need to adopt more effective methods to improve the external quantum efficiency of LEDs under high current density. Summary of the Invention

[0010] In order to overcome the above deficiencies, the present invention provides a gallium nitride-based light-emitting diode epitaxial structure, a preparation method thereof, and an LED chip, which reduce the leakage of electrons to the p-type region outside the multiple quantum well barrier layer, thereby improving the external quantum efficiency of the LED under high current density.

[0011] The technical solution provided by the present invention is as follows: On the one hand, the present invention provides a gallium nitride-based light-emitting diode epitaxial structure, including: a buffer layer, an unintentionally doped GaN layer, an n-type GaN current spreading layer, a multi-stage electron-phonon scattering structure, a multiple 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-stage electron-phonon scattering structure is composed of a periodic stack of InGaN layers and GaN layers, the number of periods is 2 - 5, and the conduction band energy level of the InGaN layer gradually decreases in the direction away from the substrate; The multiple 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 increases in the direction away from the substrate.

[0012] 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-stage 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 on the surface of a growth substrate; The multi-stage electron-phonon scattering structure is composed of a periodic stack of InGaN layers and GaN layers, the number of periods is 2-5, and the conduction band energy level of the InGaN layer gradually decreases in the 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 band gap width of the quantum well layer gradually increases in the direction away from the substrate.

[0013] On yet another aspect, the present invention provides an LED chip, and the LED chip includes the above-mentioned gallium nitride-based light-emitting diode epitaxial structure.

[0014] The gallium nitride-based light-emitting diode epitaxial structure, the preparation method thereof, and the LED chip provided by the present invention at least have the following technical effects: 1) Since the increase in the current density of the LED leads to an increase in the electron movement speed, and there is a large conduction band energy level difference between the n-type GaN current spreading layer and the multi-quantum well barrier layer, the electrons in the n-type GaN current spreading layer will obtain a large kinetic energy when entering the multi-quantum well barrier region, so that some of them become high-energy electrons and undergo ballistic transport, leaking out of the multi-quantum well barrier layer. The present invention designs a multi-stage electron-phonon scattering structure between the n-type GaN current spreading layer and the multi-quantum well barrier layer. Through the InGaN layer with a gradually decreasing conduction band energy level in the multi-stage electron-phonon scattering structure, the energy of the electrons entering the multi-quantum well barrier layer is relaxed, that is, before the high-energy electrons enter the multi-quantum well barrier layer, they first exchange energy with the lattice in the multi-stage electron-phonon scattering structure, undergo electron-phonon scattering, reduce the kinetic energy of the electrons, inhibit ballistic transport leakage, and increase the capture probability of the injected electrons by the multi-quantum well barrier layer.

[0015] 2) For the 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 gradually 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 the electrons in the multi-quantum well barrier layer towards the p-type GaN current spreading layer direction, and more radiative recombination occurs in the quantum well, reducing the probability of electrons accelerating and escaping from the multi-quantum well barrier layer.

[0016] 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 barrier layer) may escape from the multi-quantum well barrier region by crossing the last quantum barrier and the p-type electron barrier 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 transition from the bottom of the quantum well to the table of the InGaN step structure by hot electron emission, some electrons can undergo multi-level electron-phonon scattering through the InGaN step structure during the fall back process, relax the electron energy, and make some hot electrons that may have crossed the p-type electron barrier layer of the quantum barrier layer fall back to the low energy level of the quantum well layer, reducing the probability of hot electron escape. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 A schematic diagram of the epitaxial structure of a gallium nitride-based light-emitting diode in one embodiment of the present invention; Figure 2 A schematic diagram of electrons passing through a multi-level electron-phonon scattering structure in one embodiment of the present invention; Figure 3 A schematic diagram of a portion of the conduction band energy levels of a gallium nitride-based light-emitting diode epitaxial structure according to another embodiment of the present invention; Figure 4 A schematic diagram of the conduction band energy levels of electrons passing through an InGaN step structure in another embodiment of the present invention; Figure 5 The present invention is a flow chart of the preparation of the LED epitaxial structure of a gallium nitride-based light-emitting diode in one embodiment of the present invention.

[0018] Reference numerals: 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

[0019] In order to more clearly illustrate the implementation cases of the present invention or the technical solutions in the prior art, the specific implementation methods of the present invention will be described below with reference to the accompanying drawings. Obviously, the accompanying drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other accompanying drawings and other implementation methods can be obtained based on these drawings without creative work.

[0020] The first embodiment of the present invention is 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-stage 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-stage 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 decreases in the 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 increases in the direction away from the substrate.

[0021] As Figure 1 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, in one instance, a Si(111) substrate is used. The Si(111) substrate is placed in an 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, and then subsequent growth is carried out. The buffer layer 2 is between the growth substrate and the unintentionally doped GaN layer, and plays a role in buffering lattice differences and stress regulation. In one instance, it is a three-layer structure of AlN, Al 0.40 Ga 0.60 N, Al 0.20 Ga 0.80 N, which is grown at 1000 °C - 1100 °C, and the thicknesses are 200 nm of AlN, 120 nm of Al 0.40 Ga 0.60 N, and 150 nm of Al 0.20 Ga 0.80 N. During the preparation process, first, trimethylaluminum (TMAl) and ammonia gas (NH 3 3) are introduced into the reaction chamber to grow an AlN layer with a thickness of 200 nm. Then, the flow rate of ammonia gas is increased, and trimethylgallium (TMGa) is introduced to grow an Al 0.40 Ga 0.60 N layer with a thickness of 120 nm. Finally, the flow rate of ammonia gas is further increased, and the flow rate of trimethylaluminum is reduced to reduce the Al component content of the epitaxial layer, and an Al 0.20 Ga 0.80 N layer with a thickness of 150 nm is grown. The Al content gradually decreases as the distance from the substrate increases, which is beneficial to reducing the threading dislocation density between the subsequently grown unintentionally doped GaN layer and the buffer layer and improving the crystal quality. The unintentionally doped GaN layer 3 is grown at 1050 °C and has a thickness of 800 nm. The n-type GaN current spreading layer 4 is a GaN layer doped with Si, and the Si doping concentration is 1×10 19 18 / cm3 , during the growth process, silane (SiH 4 ) is used as a dopant and grown under the conditions of 1050 °C and 200 torr, with a thickness of 2000 nm. After applying a forward voltage to the LED, electrons in the n-type GaN current spreading layer 4 enter the multi-quantum well barrier layer under the action of an electric field and undergo radiative recombination with holes, generating photons and emitting them to the outside. However, at a high current density (i.e., a relatively high electron movement speed), a considerable part of the electrons have too much kinetic energy, and they may cross the multi-quantum well barrier layer and the p-type electron blocking layer in a ballistic transport manner, resulting in electron leakage, which significantly reduces the external quantum efficiency in the traditional LED epitaxial structure. Therefore, in the present invention, a multi-stage electron-phonon scattering structure 5 is inserted between the n-type GaN current spreading layer 4 and the multi-quantum well barrier layer to reduce the leakage of hot electron ballistic transport.

[0022] The multi-stage electron-phonon scattering structure is composed of a periodic stack of InGaN layers and GaN layers. The thickness of a single InGaN layer is 2 nm - 10 nm, the thickness of a single GaN layer is 2 nm - 10 nm, the number of periods is 2 - 5, and the conduction band energy level of the InGaN layer gradually decreases in the direction away from the substrate; and the gradient difference ΔE 1 of the conduction band energy levels of the InGaN layers in adjacent periods is <ΔE 1 < , where is the longitudinal optical phonon energy. The designed multi-layer scattering structure here is beneficial to increasing the probability of hot electrons undergoing electron-phonon scattering, relaxing the energy by emitting LO phonons, interacting with the lattice before the hot electrons enter the multi-quantum well barrier layer, and relaxing part of the energy, thereby reducing the electron ballistic transport leakage. In this embodiment, the multi-stage electron-phonon scattering structure 5 is composed of 3 periods of sequentially stacked InGaN layers and GaN layers. In the first period, the thickness of the InGaN layer is 8 nm, the bandgap width is 3.20 eV, and the GaN thickness is 4 nm; in the second period, the thickness of the InGaN layer is 6 nm, the bandgap width is 3.05 eV, and the GaN thickness is 4 nm; in the third period, the thickness of the InGaN layer is 4 nm, the bandgap width is 2.90 eV, and the GaN thickness is 4 nm. The difference in the bandgap widths of the InGaN layers in adjacent periods is 0.15 eV. In the direction towards the multi-quantum well barrier layer, the bandgap width of the InGaN layer gradually decreases. It should be noted here that since the difference in the conduction band energy levels of InGaN is much larger than the difference in its valence band energy levels, that is, the difference in the bandgap widths is mainly reflected in the difference in its conduction band energy levels. Therefore, it can also be described here that in the direction away from the growth substrate, the conduction band energy level of the InGaN layer gradually decreases, and the gradient difference ΔE 1is 0.15 eV. According to the research in the literature Phys. Status Solidi B 247, No. 1, 189–193 (2010) / DOI 10.1002 / pssb.200945144, the longitudinal optical (LO) phonon energy in InGaN at this time is 88 meV, so the designed 0.088 eV < 0.15 eV < 0.176 eV in the present invention satisfies <ΔE 1 < . As Figure 2 . As shown in the energy level structure diagram (the growth direction of the epitaxial structure is from right to left), the initial kinetic energy of electrons in the n-type GaN current spreading layer 4 under the action of a forward voltage is En, and the energy level difference between the n-type GaN current spreading layer and the multi-quantum well layer is 3ΔEc. Then, when electrons are injected from the n-type current spreading layer into the multi-quantum well layer, they will obtain an additional kinetic energy of 3ΔEc, and the total kinetic energy increases to (En + 3ΔEc). After the electron kinetic energy increases, the probability of ballistic transport leakage from the multi-quantum well barrier layer also increases accordingly. At high current densities (higher electron initial rates and initial kinetic energies), this electron leakage phenomenon will be more serious. However, during the process of electrons passing through the multi-stage electron-phonon scattering structure 5 composed of 3 cycles, high-energy electrons and the lattice may undergo electron-phonon scattering, that is, every time an electron-phonon scattering cycle passes, the electrons relax the energy, so that electrons that might have leaked out of the multi-quantum well barrier region due to ballistic transport can also be localized in the quantum well to undergo radiative recombination. At the same time, the multi-stage electron-phonon scattering structure 5 can also buffer the lattice compressive stress exerted on the multi-quantum well from the n-type GaN current spreading layer 4, which is beneficial to the high-quality growth of the multi-quantum well barrier layer. During the preparation process, the conduction band energy level of the InGaN layer can be achieved by adjusting the flow rate of trimethylindium (TMIn), that is, by adjusting the In content in the InGaN layer to change the conduction band energy level of the InGaN layer in different cycles, forming the designed conduction band energy level gradient difference.

[0023] The multi-quantum well barrier layer 6 is formed on the surface of the multi-stage electron-phonon scattering structure 5, and 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 2.5 nm and 5.5 nm, and the thickness of a single GaN quantum barrier layer is between 4 nm and 15 nm. After electrons and holes radiatively recombine in the quantum well, light of a specific wavelength is emitted. In practical applications, those skilled in the art can design the bandgap width of the quantum well according to requirements 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 wavelengths of each quantum well are the same. In this embodiment, in order to improve the external quantum efficiency under high current, in the multi-quantum well barrier layer, different quantum wells have a gradient difference in emission wavelength. The emission wavelength gradient difference Δλ is 1 nm to 10 nm, and the wavelength change of the entire multi-quantum well barrier layer is very small, and the emission color of the overall LED is hardly affected. In one example, the multi-quantum well barrier layer 6 is composed of 9 periods of sequentially stacked InGaN quantum well layers and GaN quantum barrier layers. The thickness of a single InGaN quantum well layer is 3 nm, and the emission wavelength is in the blue light band. The thickness of a single GaN quantum barrier layer is 4 nm, and the thickness is relatively thin, which can enhance the resonant tunneling effect of carriers and promote the injection of carriers into the quantum well for radiative recombination. In this embodiment, during the preparation process, the silicon doping concentration in the GaN quantum barrier layer is 3×10 17 cm -3 , using N 2 as the carrier gas, and 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 multi-quantum well increases from 780 °C to 789 °C in sequence. While the growth temperature of all GaN quantum barriers remains unchanged, all being 900 °C. The bandgap widths of the 9 grown InGaN quantum well layers increase step by step in the direction away from the substrate. According to the formula λ = 1240 / Eg (the output wavelength λ is in nm, and the bandgap width Eg is in eV), the emission wavelengths of the 9 InGaN quantum well layers decrease correspondingly step by step. The emission wavelength of the InGaN quantum well layer in the first period near the growth substrate is 450 nm, and the emission wavelength of each subsequent period of InGaN quantum well layer decreases by 1.5 nm in sequence until the emission wavelength of the last period of InGaN quantum well layer decreases to 440 nm. This design of the gradually decreasing wavelength of the quantum wells 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 during the movement of electrons in the multi-quantum well barrier layer, the accelerated movement of electrons in the multi-quantum well barrier layer towards the p-type GaN current spreading layer direction is slowed down, thereby keeping the electrons in the multi-quantum well barrier layer and realizing more radiative recombination with holes in the quantum well, and avoiding the accelerated escape of electrons from the multi-quantum well barrier layer.

[0024] The p-type electron blocking layer 7 is a Mg-doped AlGaN layer. In one example, it is a 20-nm Al 0.25 Ga 0.65 N layer grown at 900 °C. During the growth process, using N 2 as the carrier gas, TMAl, TMGa, and NH 3 are introduced. Using bis(cyclopentadienyl)magnesium (Cp 2 Mg) as the dopant, the Mg doping concentration is 2.0×10 19 / cm 3 . Since the composition contains Al, a certain barrier height is formed, which can prevent the electrons escaping from the multi-quantum well barrier layer from further moving towards the p-type GaN current spreading layer side.

[0025] 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 , a thickness of 80 nm. Using H 2 and N 2 as the carrier gas, after applying a voltage to the epitaxial structure, the p-type GaN current spreading layer 8 can provide holes. Under the action of an electric field, the holes enter the multi-quantum well barrier layer and recombine radiatively with electrons, generating photons and emitting them to the outside.

[0026] The p-type ohmic contact layer 9 is highly doped and is also a Mg-doped GaN layer, but the Mg doping concentration is greater than that in the p-type GaN current spreading layer (in one example, the Mg doping concentration in the p-type GaN current spreading layer 8 is 1×10 18 / cm 3 , and the Mg doping concentration in the p-type ohmic contact layer 9 is 3×10 20 / cm 3 , and 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, using H 2 or N 2 as the carrier gas, using bis(cyclopentadienyl)magnesium (Cp 2 Mg) as the dopant, the epitaxial growth temperature is 900 °C to 1000 °C, and the thickness is 10 nm.

[0027] This embodiment is obtained by further improving the above embodiments. In this embodiment, the conduction band energy level structure diagram between the n-type GaN current spreading layer 4 and the p-type GaN current spreading layer 8 of the gallium nitride-based light-emitting diode epitaxial structure is as shown in Figure 3As shown (the growth direction of the growth substrate is from right to left), not only is a multi-level electron-phonon scattering structure 5 formed between the n-type GaN current spreading layer 4 and the multi-quantum well barrier layer 6, and the bandgap widths of the quantum wells in the multi-quantum well barrier layer increase 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 gradually increasing conduction band energy levels. The energy level gradient difference ΔE between adjacent InGaN layers 2 is <ΔE 2 < , if electrons jump upward from the bottom of the last quantum well to the multi-level mesa (including the last quantum barrier) of the InGaN step structure in the form of hot electron emission, the electrons can achieve electron-phonon scattering and energy relaxation by means of this InGaN step structure, and reduce the electron energy during the process of falling back from the InGaN step to the bottom of the last quantum well, thereby reducing the probability of their leakage through the p-type electron blocking layer 7 by hot electron emission. The InGaN step structure can be 1 - 3 InGaN layers with gradually increasing conduction band energy levels.

[0028] In one example, the InGaN step structure 61 is two InGaN layers, where the bandgap widths of the first InGaN layer and the second InGaN layer are 3.02 eV and 3.22 eV respectively, and the thickness of both is 3 nm. The bandgap width difference between the two is 0.2 eV. Since the conduction band energy level difference of InGaN is much larger than its valence band energy level difference, that is, the bandgap width difference is mainly reflected in its conduction band energy level difference, which is equivalent to the conduction band energy level difference between the two InGaN layers in the InGaN step structure being 0.2 eV. Here, the triple conduction band energy level differences between the last InGaN quantum well layer, the first InGaN layer, the second InGaN layer, and the GaN quantum barrier layer are all greater than the longitudinal optical mode phonon energy of 88 meV, which is beneficial for the hot electrons that have jumped here to relax energy through electron-phonon scattering and fall back to the multi-quantum well barrier layer for radiative recombination. As Figure 4 shown in the conduction band energy level schematic diagram of the InGaN step structure (the growth direction of the growth substrate is from right to left), when electrons jump upward from the bottom of the last quantum well to the multi-level mesa (including the last quantum barrier) of the InGaN step structure 61 in the form of hot electron emission, some electrons will fall back to the next-level mesa again. During this falling-back process, the hot electrons may undergo multi-level electron-phonon scattering and energy relaxation, thereby reducing the electron energy and reducing the probability of their leakage through the p-type electron blocking layer 7 by hot electron emission.

[0029] During the growth process of the gallium nitride-based light-emitting diode epitaxial structure, such as Figure 5As shown, first, use the MOCVD growth equipment and select the Si(111) substrate as the growth substrate layer. In the MOCVD system, perform a high-temperature surface treatment on the growth substrate to remove the oxide layer. Then, for different structures, at specific temperatures, introduce the required precursor sources, gas sources, and carrier gases, including Ga source, Al source, In source, Mg, Si source, N source, and N 2 , H 2 , 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 (Cp 2 Mg), the Si source is silane (SiH 4 ), the N source is ammonia (NH 3 ), and the carrier gases are N 2 and H 2 . Grow a buffer layer, an unintentionally doped GaN layer, an n-type GaN current spreading layer, a multi-stage 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 on the surface of the growth substrate in sequence. In this embodiment, 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 in the multi-quantum well barrier layer and is composed of multiple InGaN layers with gradually increasing conduction band energy levels.

[0030] The specific steps include: 1. Place the Si(111) substrate into the MOCVD reaction chamber, heat it up to 1100 °C, and introduce H2 for 5 minutes of high-temperature surface treatment to remove the oxide layer and form clear atomic steps.

[0031] 2. Set the reaction chamber temperature at 1000 °C - 1100 °C. First, introduce trimethylaluminum (TMAl) and ammonia (NH 3 ) into the reaction chamber. Under the condition that H 2 is used as the carrier gas, grow an AlN layer with a thickness of 200 nm; then, increase the flow rate of ammonia and introduce trimethylgallium (TMGa) to grow an Al 0.40 Ga 0.60 N layer with a thickness of 120 nm. Again, increase the flow rate of ammonia and decrease the flow rate of trimethylaluminum (TMAl) to grow an Al 0.20 Ga 0.80 N layer with a thickness of 150 nm. The buffer layer 2 is composed of an AlN layer, an Al 0.40 Ga 0.60 N layer, and an Al 0.20 Ga 0.80 N layer together.

[0032] 3. Grow an 800-nm unintentionally doped GaN layer 3 at 1050 °C.

[0033] 4. Using silane (SiH 4 ), with a Si doping concentration of 1×10 19 / cm 3 , a GaN layer with a thickness of 2000 nm is grown as the n-type GaN current spreading layer 4 under the conditions of 1050 °C and 200 torr.

[0034] 5. Taking the sequentially stacked InGaN layer and GaN layer as one period, 3 periods are grown to form a multi-level electron-phonon scattering structure 5. During the preparation process, the conduction band energy level of the InGaN layer is adjusted by regulating the flow rate of trimethylindium (TMIn), that is, the conduction band energy level of the InGaN layer in different periods is changed by adjusting the In content in the InGaN layer to form a conduction band energy level gradient difference. In the first period, the thickness of the InGaN layer is 8 nm, the bandgap width is 3.20 eV, and the thickness of the GaN layer is 4 nm; in the second period, the thickness of the InGaN layer is 6 nm, the bandgap width is 3.05 eV, and the thickness of the GaN layer is 4 nm; in the third period, the thickness of the InGaN layer is 4 nm, the bandgap width is 2.90 eV, and the thickness of the GaN layer is 4 nm. 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 bandgap width difference is mainly reflected in its conduction band energy level difference. Therefore, it can also be described here that in the direction towards the multi-quantum well barrier layer, the conduction band energy level of the InGaN layer gradually decreases, and the conduction band energy level gradient difference ΔE 1 of adjacent periods of InGaN layers is 0.15 eV.

[0035] 6. The multi-quantum well barrier layer 6 is composed of 9 periods of sequentially stacked InGaN quantum well layers and GaN quantum barrier layers. The thickness of a single InGaN quantum well layer is 3 nm, and the emission wavelength is in the blue light band. The thickness of a single GaN quantum barrier layer is 4 nm, which is relatively thin and can enhance the resonant tunneling effect of carriers and promote the injection of carriers into the quantum well for radiative recombination. In this embodiment, during the preparation process, the silicon doping concentration in the GaN quantum barrier layer is 3×10 17 cm -3 , with N 2As 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 increases from 780 °C to 789 °C successively. While the growth temperature of all GaN quantum barriers remains unchanged at 900 °C. The band gaps of the finally obtained 9 InGaN quantum well layers increase step by step in the direction towards the p-type electron blocking layer. According to the formula λ = 1240 / Eg (the unit of the output wavelength λ is nm, and the unit of the band gap Eg is eV), the emission wavelengths of the 9 InGaN quantum well layers decrease correspondingly step by step. The emission wavelength of the InGaN quantum well layer in the first period near the growth substrate is 450 nm, and the emission wavelength of the InGaN quantum well layer in each subsequent period decreases by 1.5 nm successively until the emission wavelength of the InGaN quantum well layer in the last period decreases to 440 nm.

[0036] Meanwhile, an InGaN step structure 61 is also formed between the quantum well layer and the quantum barrier layer in the last period of the multiple quantum well barrier layer. The InGaN step structure 61 is composed of two InGaN layers. The band gaps of the first InGaN layer and the second InGaN layer are 3.02 eV and 3.22 eV respectively, and the thickness of both is 3 nm.

[0037] 10. The p-type electron blocking layer 7 is an AlGaN layer doped with Mg. In one example, it is a 20-nm Al 0.25 Ga 0.65 N layer grown at 900 °C. During the growth process, N 2 is used as the carrier gas, and TMAl, TMGa, and NH 3 are introduced. Bis(cyclopentadienyl)magnesium (Cp 2 Mg) is used as the dopant, and the Mg doping concentration is 2.0×10 19 / cm 3 . Since the composition contains Al, a certain barrier height is formed, which can prevent the electrons escaping from the multiple quantum well barrier layer from further moving towards the p-type GaN current spreading layer side.

[0038] 11. The p-type GaN current spreading layer 8 is a GaN layer doped with Mg, and the Mg doping concentration is 1×10 18 / cm 3 . The thickness is 80 nm. After applying a voltage to the epitaxial structure, the p-type GaN current spreading layer 8 can provide holes. Under the action of the electric field, the holes enter the multiple quantum well barrier layer and recombine with electrons radiatively to generate photons and emit them to the outside.

[0039] 12. Using H 2 or N 2 as the carrier gas, and bis(cyclopentadienyl)magnesium (Cp 2Mg) as a dopant, the epitaxial growth temperature is 900℃-1000℃, and the Mg doping concentration is 3×10 20 / cm 3 A GaN layer with a thickness of 10 nm serves as the p-type ohmic contact layer 9 .

[0040] Another embodiment of the present invention is an LED chip, which includes the above-mentioned gallium nitride-based light-emitting diode epitaxial structure. Due to the multi-level electron-phonon scattering structure in the epitaxial structure of the LED chip, the energy of electrons can be relaxed during the injection process into the multi-quantum well barrier layer, thereby reducing the kinetic energy of the electrons and the probability of ballistic transport leakage; at the same time, the bandgap width of the quantum well layer in the multi-quantum well barrier layer increases step by step, which can partially offset the energy band tilt (built-in electric field) caused by the quantum Stark effect structure 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 expansion layer, better localize the radiation recombination in the quantum well, and prevent the accelerated escape of electrons 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 causing a portion of 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.

[0041] In the process of preparing LED chips, the preparation process includes pixelating the epitaxial structure of gallium nitride-based light-emitting diodes by using a combination of ICP dry etching and wet etching technology, so that at least the multi-quantum well barrier layers between different LED chips are independent of each other, so that each LED chip can be driven independently. In addition, it also includes the preparation of a first electrode and a second electrode, so as to connect an external power source to the LED chip. The first electrode is conductively connected to the n-type GaN current expansion layer in the LED chip, and the second electrode is conductively connected to the p-type GaN current expansion layer in the LED chip. The first electrode and the second electrode can be located on the same side of the LED chip (forming a flip-chip structure or a face-up structure), or on different sides of the LED chip (forming a vertical structure); no matter how the positions of the first electrode and the second electrode are designed, it can be determined that the first electrode is conductively connected to the n-type GaN current expansion layer in the LED chip, and the second electrode is conductively connected to the p-type GaN current expansion layer in the LED chip, so that the external current is conducted to the LED chip through the first electrode and the second electrode. 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 light-emitting effect of the chip. This embodiment does not limit this as long as the LED chip can emit light normally.

[0042] It should be noted that the above embodiments can be freely combined as needed. The above are only the preferred embodiments of the present invention. It should be pointed out that for those of ordinary skill in the art, without departing from the principle of the present invention, several improvements and refinements can be made, and these improvements and refinements should also be regarded as the protection scope of the present invention.

Claims

1. A gallium nitride-based light-emitting diode epitaxial structure, characterized in that: 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 and a p-type electron blocking layer, a p-type GaN current spreading layer, and a p-type ohmic contact layer are sequentially formed on the surface of the 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; 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 bandgap width of each InGaN quantum well layer increases step by step.

2. The gallium nitride-based light-emitting diode epitaxial structure according to claim 1, characterized in that: 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: <ΔE1< ,in, is the phonon energy of the longitudinal optical mode.

3. The gallium nitride-based light-emitting diode epitaxial structure according to claim 1, characterized in that: 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 any one of claims 1 to 3, characterized in that: 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.

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

6. A method for preparing a gallium nitride-based light-emitting diode epitaxial structure, characterized in that: The method comprises the following steps: sequentially growing 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 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; 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 periodic stacking of InGaN quantum well layers and GaN quantum barrier layers; in the direction toward the p-type electron barrier layer, the bandgap width of each InGaN quantum well layer increases step by step in the direction away from the substrate.

7. The method for preparing a gallium nitride-based light-emitting diode epitaxial structure according to claim 6, characterized in that: 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: <ΔE1< ,in, is the phonon energy of the longitudinal optical mode.

8. The method for preparing a gallium nitride-based light-emitting diode epitaxial structure according to claim 6, characterized in that: 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.

9. The method for preparing a gallium nitride-based light-emitting diode epitaxial structure according to any one of claims 6 to 8, characterized in that: 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.

10. The method for preparing a gallium nitride-based light-emitting diode epitaxial structure according to claim 9, characterized in that: In the InGaN step structure, the energy level gradient difference ΔE2 of adjacent InGaN layers satisfies: <ΔE2< ,in, is the phonon energy of the longitudinal optical mode.

11. An LED chip, characterized in that: The LED chip comprises the gallium nitride-based light-emitting diode epitaxial structure as described in any one of claims 1 to 5.

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