Epitaxial structure and preparation method thereof

Abstract

The invention provides an epitaxial structure and a preparation method thereof, the epitaxial structure is deposited and grown by adopting an MOCVD (Metal Organic Chemical Vapor Deposition) method, and the epitaxial structure comprises a substrate, an N-type semiconductor layer, a QW-QB-QDs coupling layer and a P-type semiconductor layer which are sequentially stacked from bottom to top; wherein the QW-QB-QDs coupling layer comprises a QW layer, a QB layer and a QDs layer, the QW layer, the QB layer and the QDs layer grow at least one cycle from bottom to top, the QW layer is an InGaN carrier retention layer, the QB layer is a GaN-AlGaN-GaN tunneling barrier layer, and the QDs layer is an InGaN quantum dot layer; by arranging the QW-QB-QDs coupling structure, the problem of low efficiency of directly injecting carriers into the QDs is solved, the growth morphology and stress of the QDs can be adjusted, the crystal quality of the QDs is improved, and the luminous efficiency of the device is effectively improved.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of semiconductor devices, and particularly relates to an epitaxial structure and a preparation method thereof. BACKGROUND

[0002] III-V group GaN-based materials are widely concerned and applied in the fields of electricity and optics due to their excellent physical and chemical properties (large band gap, high breakdown field, high electron saturation mobility, etc.), such as the popular Micro and Mini LED on the market. However, in actual semiconductor structure design, most of them are grown by heteroepitaxy, which inevitably causes problems such as strong polarization electric field and poor material crystal quality, limiting the further improvement of device light-emitting efficiency. QDs (Quantum Dots, semiconductor crystals with nanoscale size) are widely concerned due to their strong quantum confinement effect, which can effectively weaken the polarization electric field and improve the carrier radiation recombination power. However, due to the difficulty in controlling the crystal quality, the size, size and density are randomly distributed, which seriously affects the carrier transport and recombination, greatly reducing the device light-emitting efficiency. Therefore, adjusting the growth morphology and stress of QDs, improving the crystal quality of QDs, and reducing the imbalance of carrier transport have become urgent problems to be solved. SUMMARY

[0003] The purpose of the present application is to provide an epitaxial structure and a preparation method thereof, which sets a QW-QB-QDs coupling structure, solves the problem of low carrier injection efficiency to QDs, and can adjust the growth morphology and stress of QDs, improve the crystal quality of QDs, and effectively improve the light-emitting efficiency of the device.

[0004] To achieve the above purpose, the solution of the present application is as follows: An epitaxial structure is provided, which comprises a substrate, an N-type semiconductor layer, a QW-QB-QDs coupling layer and a P-type semiconductor layer which are sequentially stacked from bottom to top. The QW-QB-QDs coupling layer comprises at least one period of QW layer, QB layer and QDs layer grown from bottom to top, the QW layer is an InGaN carrier accumulation layer, the QB layer is a GaN-AlGaN-GaN tunnel barrier layer, and the QDs layer is an InGaN quantum dot layer. The thickness of the QW layer is W, and the In content is X, the thickness of the QB layer is D, and X / 5≤D / W≤3X.

[0005] Optionally, the number of growth periods of the QW-QB-QDs coupling layer is 2-5, including the end point.

[0006] Optionally, a GaN cap layer is arranged on the QDs layer in each period of the QW-QB-QDs coupling layer.

[0007] Optionally, a low-temperature nucleation layer and a buffer layer are further arranged between the substrate and the N-type semiconductor layer, the buffer layer is an undoped GaN layer; a low-temperature N-type GaN layer and an SSL layer are further arranged between the N-type semiconductor layer and the QW-QB-QDs coupling layer, the SSL layer is a superlattice structure of InGaN / GaN grown for 5-10 periods, including end point values; a low-temperature P-type GaN layer is further arranged between the QW-QB-QDs coupling layer and the P-type semiconductor layer.

[0008] The application further provides a preparation method of the epitaxial structure, comprising: providing a substrate, and growing a N-type semiconductor layer, a QW-QB-QDs coupling layer and a P-type semiconductor layer on the substrate from bottom to top by using a MOCVD method; The QW-QB-QDs coupling layer comprises a QW layer, a QB layer and a QDs layer grown from bottom to top for at least one period, the QW layer is an InGaN carrier accumulation layer, the QB layer is a GaN-AlGaN-GaN tunnel barrier layer, and the QDs layer is an InGaN quantum dot layer, the QDs layer is grown by using a growth mode of interrupting Ga source and In source and alternately controlling high and low V / III ratios.

[0009] Optionally, when the QDs layer is grown, In source, Ga source and N source are introduced, 0.5-2.5nm InGaN is grown, then the In source and Ga source are interrupted for a time t', so that the adsorbed atoms migrate on the surface and reach the most suitable position, after the system approaches a thermodynamic equilibrium state, the low V / III ratio for a time t1 and the high V / III ratio for a time t2 are alternately controlled, and the growth of the QDs layer is completed, the V / III ratio is the molar ratio of N source and Ga source.

[0010] Optionally, when the QDs layer is grown, t' is 10-30s, t1 and t2 are both 3-10s, t1 / t2 is 0.3-3; 2000

[0011] Optionally, after the growth of the QDs layer is completed, a GaN cap layer is grown on the QDs layer, and the growth of one period of QW-QB-QDs coupling layer is completed.

[0012] Optionally, when growing the QW layer and the QB layer, firstly, the In source, the Ga source and the N source are introduced, and an InGaN carrier storage layer is grown, the growth thickness of the InGaN carrier storage layer is W, and the In component content is X; then, the H2 and the Al source are introduced, and a GaN-AlGaN-GaN tunnel barrier layer is grown, the thickness of the QB layer is D, wherein X / 5≤D / W≤3X.

[0013] Optionally, when the QB layer is grown, the Al introduction flow rate is gradually increased first and then gradually reduced, and the H2 introduction flow rate is gradually reduced first and then gradually increased.

[0014] After the above scheme is adopted, the application has the following beneficial effects: 1. The active region of the application is provided with a QW-QB-QDs coupling structure, wherein the QW layer is used as a carrier storage layer, can bind the carriers to the greatest extent, and can inject the carriers into the QDs layer through the tunneling effect of the QB layer, thereby improving the injection efficiency of the carriers, solving the problem of low injection efficiency of the carriers directly into the QDs layer, and effectively improving the light emitting efficiency of the device.

[0015] 2. The QB layer of the application adopts a GaN-AlGaN-GaN structure, can protect the crystal quality of the QW layer, can improve the conduction band barrier height of the AlGaN by using the band step characteristics, can reduce the electron overflow, and further improve the light emitting efficiency. In addition, the growth process of the GaN is treated with H2, and the AlGaN is gradually treated with Al and H2, which can inhibit the dislocation extension, reduce the interface defects, and is beneficial to improving the crystal quality of the material.

[0016] 3. The QDs layer of the application adopts a growth mode of interrupting the Ga source and the In source in the growth process and controlling the high and low V / III ratio alternately, the introduction of the Ga source and the In source in the growth interruption can allow the atoms to have sufficient time to migrate on the surface, reach the most suitable position in energy, and approach the thermodynamic equilibrium state, so as to release the stress and reduce the QCSE (quantum confinement Stark effect). At the same time, the low V / III ratio is controlled first to promote the three-dimensional growth and form high-density nucleation points, and then the V / III ratio is pulled high to promote the two-dimensional growth, change the size, size and density of the QDs layer, reshape the morphology, and improve the crystal quality of the QDs layer; thus, the cycle of controlling the high and low V / III ratio alternately can enhance the quantum confinement effect and improve the light emitting efficiency of the device.

[0017] 4. The QW-QB-QDs coupling layer can also be constructed into a multi-period structure, which further expands the above beneficial effects. BRIEF DESCRIPTION OF DRAWINGS

[0018] Figure 1 It is an epitaxial structure diagram of the application.

[0019] Figure 2Schematic diagram of the conduction band of the QW-QB-QDs coupling layer of the present invention.

[0020] Figure 3 Schematic diagram of the growth of the QW-QB-QDs coupling layer of the present invention.

[0021] Figure 4 It is a flow chart of the preparation method of the present invention.

[0022] Description of labels: 1. Substrate; 2. Buffer layer; 3. N-type semiconductor layer; 4. Low-temperature N-type GaN layer; 5. SSL layer; 6. QW-QB-QDs coupling layer; 7. Low-temperature P-type GaN layer; 8. P-type semiconductor layer. DETAILED DESCRIPTION

[0023] The following will be combined with the drawings in the embodiments of the present application to clearly and completely describe the technical solutions in the embodiments of the present application. Obviously, the embodiments described are only part of the embodiments of the present application, not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of this application, and the range values ​​described in the present invention include both endpoints.

[0024] like Figure 1 As shown, the present application provides an epitaxial structure, including a substrate 1, an N-type semiconductor layer 3, a QW-QB-QDs coupling layer 6 and a P-type semiconductor layer 8 stacked in sequence from bottom to top; wherein the QW-QB-QDs coupling layer 6 includes a QW layer, a QB layer and a QDs layer grown from bottom to top for at least one period, the QDs layer is an InGaN quantum dot layer, and the QW layer is an InGaN carrier storage layer. As a carrier storage layer, it can confine carriers to the greatest extent, and at the same time, through the tunneling effect of the QB layer, carriers can be injected into the QDs layer, thereby improving the carrier injection efficiency, solving the problem of low efficiency of directly injecting carriers into the QDs layer, and effectively improving the luminous efficiency of the device; the QB layer is a GaN-AlGaN-GaN tunneling barrier layer, which can protect the crystal quality of the QW layer while utilizing its band step characteristics to increase the conduction band barrier height of its AlGaN, reduce electron overflow, and further improve the luminous efficiency.

[0025] Optional, such as Figure 2As shown, the QW layer has a thickness of W, a potential well depth of H, and an In composition content of X; the QB layer has a thickness of D. Increasing the potential well depth H and the thickness W of the QW layer can enhance the carrier confinement capability, thereby increasing the carrier injection into the QDs layer; meanwhile, reducing the tunneling barrier thickness D of the QB layer can increase the tunneling probability. However, the increase of W and H can exacerbate the QCSE (quantum confinement Stark effect), and the In composition content X is positively correlated with H (X∝H). Further, to balance the carrier confinement strength and the inhibition of QCSE, the constraint condition X / 5≤D / W≤3X needs to be met. In addition, the value of H needs to be indirectly determined by precisely regulating X in combination with band gap testing.

[0026] Optionally, the QW-QB-QDs coupling layer 6 can also be constructed as a multi-period structure to further expand the above beneficial effects. In a preferred embodiment, the QW-QB-QDs coupling layer 6 is constructed as 2-5 periods.

[0027] Preferably, as shown in the figure, a GaN cap layer (Cap) can be provided on the QDs layer in each period of the QW-QB-QDs coupling layer 6, which can protect the QDs layer from being damaged and complete the growth of a single period. Figure 3

[0028] Optionally, the N-type semiconductor layer 3 is an N-type doped GaN layer, which serves to provide an N-type contact layer, has a growth thickness of 1-2 nm, and a doping concentration of 1-5×10 19 cm -3 ; the P-type semiconductor layer 8 is a P-type doped GaN, which serves to provide a P-type contact, has a doping concentration of 5-10×10 19 cm -3 .

[0029] The substrate 1 and the N-type semiconductor layer 3 are further provided with a low-temperature nucleation layer (not shown in the figure) and a buffer layer 2. The low-temperature nucleation layer is an undoped GaN layer, which has a thickness of 15-25 nm. The buffer layer 2 is an undoped GaN layer, which has a thickness of 2.0-2.5 μm.

[0030] The N-type semiconductor layer 3 and the QW-QB-QDs coupling layer 6 are further provided with a low-temperature N-type GaN layer 4 and an SSL layer 5. The low-temperature N-type GaN layer 4 can form V-pits by taking advantage of the growth difference of Ga on different polar planes at low temperature. The SSL layer 5 is a superlattice structure of InGaN / GaN grown for 5-10 periods, which can regulate the spatial distribution of V-pits and release stress.

[0031] ​The low-temperature P-type GaN layer 7 is arranged between the QW-QB-QDs coupling layer 6 and the P-type semiconductor layer 8, and has a doping concentration of 1-5×10 19 cm -3 .

[0032] The application further provides a preparation method of the epitaxial structure, which comprises the following steps: sequentially growing, from bottom to top, a low-temperature nucleation layer, a buffer layer 2, an N-type semiconductor layer 3, a low-temperature N-type GaN layer 4, an SSL layer 5, a QW-QB-QDs coupling layer 6, a low-temperature P-type GaN layer 7 and a P-type semiconductor layer 8 on the substrate 1 by a metal organic chemical vapor deposition (MOCVD) method. The QW-QB-QDs coupling layer 6 comprises at least one period of QW layers, QB layers and QDs layers grown from bottom to top, wherein the QW layers are InGaN carrier storage layers, the QB layers are GaN-AlGaN-GaN tunnel barrier layers, and the QDs layers are InGaN quantum dot layers.

[0033] Optionally, the substrate 1 includes but is not limited to a sapphire substrate, and in this embodiment, the sapphire substrate is taken as an example, trimethyl / ethyl gallium TMGa / TEGa, trimethyl aluminum TMAl and ammonia NH3 are taken as Ga source, Al source and N source, N2 is taken as a carrier gas, and silane SiH4 and cyclopentadienyl magnesium CP2Mg are taken as N-type and P-type doping sources respectively.

[0034] Please refer to Figure 4 , the preparation method comprises the following steps: S1, growing a low-temperature nucleation layer on the substrate 1, comprising: The sapphire substrate 1 is placed in an MOCVD reaction chamber, high-purity hydrogen is introduced at a high temperature of 1100℃ for 5-10 min, then the temperature is lowered to 800-900℃, the TEGa source and the N source are introduced, and a 15-25 nm thick and undoped GaN low-temperature nucleation layer is grown.

[0035] S2, growing a buffer layer 2 on the low-temperature nucleation layer, comprising: After the low-temperature nucleation layer is grown, the temperature is continuously raised to 1000-1100℃, the TEGa source is closed, the TMGa source is introduced, and a 2.0-2.5 μm thick and undoped GaN buffer layer 2 is grown. The purpose of growing the buffer layer 2 is to reduce the lattice mismatch between the sapphire substrate 1 and the subsequently grown material by growing a high-quality GaN layer.

[0036] S3, growing an N-type semiconductor layer 3 on the buffer layer 2, comprising: After the buffer layer 2 is grown, silane is introduced, and a 1-2 μm thick and Si-doped N-type semiconductor layer 3, specifically a GaN layer, is grown, and has a doping concentration of 1-5×10 19 cm -3The purpose of growing the N-type semiconductor layer 3 is to provide an N-type contact.

[0037] S4, growing a low-temperature N-type GaN layer 4 on the N-type semiconductor layer 3, comprising: After the N-type semiconductor layer 3 is grown, the temperature is lowered to 700-800°C, and the low-temperature N-type GaN layer 4 is grown. By taking advantage of the growth difference of Ga on different polar surfaces at low temperature, V-pits can be formed.

[0038] S5, growing an SSL layer 5 on the low-temperature N-type GaN layer 4, comprising: After the low-temperature N-type GaN layer 4 is grown, the temperature is raised to 800-850°C, and In source and Ga source are introduced. The InGaN / GaN superlattice structure is alternately grown for 5-10 periods, which is the SSL layer 5. The SSL layer 5 can adjust the spatial distribution of V-pits and release stress.

[0039] S6, growing a QW-QB-QDs coupling layer 6 on the SSL layer 5, comprising: After the SSL layer 5 is grown, In source, TEGa source and N source are introduced, and the temperature is lowered to 700-800°C to grow the InGaN carrier storage layer, i.e. the QW layer, which has a thickness of W, a potential well depth of H, and an In content of X. Then the temperature is raised to 800-850°C, and H2 and Al are introduced to grow the GaN-AlGaN-GaN tunnel barrier layer, i.e. the QB layer, which has an overall thickness of D. Since the QW layer acts as a carrier storage layer, it can bind carriers. Through the tunneling effect of the QB layer, carriers can be injected into the QDs layer, thereby increasing W and H, both of which can increase the injection of carriers into the QDs layer. At the same time, reducing the thickness D of the tunnel barrier of the QB layer can increase the tunneling probability. However, the increase of W and H will exacerbate QCSE (quantum confinement Stark effect), and the In content X is positively correlated with H (X∝H). Therefore, to balance the binding strength of carriers and the inhibition of QCSE, the constraint condition X / 5≤D / W≤3X needs to be met. In addition, the value of H needs to be indirectly determined by accurately adjusting X and combining with bandgap testing.

[0040] In addition, when growing AlGaN for the QB layer, the flow rate of Al is first gradually increased and then gradually decreased, and the flow rate of H2 is opposite to that of Al, i.e. first gradually decreased and then gradually increased. This setting can inhibit dislocation extension and reduce interface defects, which is conducive to improving the crystal quality of the material.

[0041] Then the QDs layer is grown. Specifically, the growth mode of interrupting Ga source, In source and controlling high and low V / III ratio is adopted, such as Figure 3As shown, first, the temperature is lowered to 600-700℃, Ga source, In source and N source are introduced, and 0.5-2.5nm InGaN layer is grown. The InGaN layer is a template layer before the growth of QDs, which is equivalent to a base layer, facilitating the growth of subsequent QDs. Then, the Ga source and In source are interrupted, and the interruption time is t'. The adsorbed atoms are allowed to migrate on the surface, reach the most suitable position in terms of energy, and approach the thermodynamic equilibrium state, release stress, and reduce QCSE. However, the QDs layer in this state is in a disordered distribution, and the size, size and density of the QDs layer are not controlled. Therefore, the growth mode of alternating high and low V / III (N source / Ga source) flux ratio is used to reshape the morphology and improve the crystal quality of the QDs layer. The V / III ratio is the molar ratio of N source and Ga source. Specifically, a lower V / III ratio, which can be 500-2000 (including 2000), is used to promote three-dimensional growth, form high-density nucleation points, and grow for a time t1. Then, the V / III ratio is increased to 2000-8000 (not including 2000) to promote two-dimensional growth and change the size, size and density of the QDs layer. The growth time is t2. By repeating the above steps for multiple cycles, the quantum confinement effect can be enhanced, and the device light-emitting efficiency can be improved.

[0042] Specifically, t' is 10-30s, t1 and t2 are both 3-10s, and by adjusting the ratio of t1 / t2, a QDs layer with uniform size and high density can be obtained. The ratio of t1 / t2 is preferably 0.3-3. In addition, the number of alternating cycles of high and low V / III ratio is preferably 3-10.

[0043] After the growth of the QDs layer, a layer of GaN cap is grown to ensure that the QDs layer is not damaged, and a single cycle of growth is completed. Then, the above steps can be repeated to grow multiple cycles of QW-QB-QDs coupled layers, which can increase the beneficial effects of QW-QB-QDs coupled layers. The number of growth cycles is preferably 2-5.

[0044] S7, growing a low-temperature P-type GaN layer 7 on the QW-QB-QDs coupled layer 6, comprising: After the growth of the QW-QB-QDs coupled layer 6, the temperature is increased to 700-800℃, and the P-type doping concentration is controlled to be 1-5×10 19 cm -3 . The low-temperature P-type GaN layer 7 is grown.

[0045] S8, growing a P-type semiconductor layer 8 on the low-temperature P-type GaN layer 7, comprising: After the growth of the low-temperature P-type GaN layer 7, a P-type doping source, cyclopentadienyl magnesium (CP2Mg), is introduced, and annealing is performed at 850-900℃ for 20-30 minutes in a N2 atmosphere. The doping concentration is 5-10×10 19 cm-3 P-type semiconductor layer 8, the whole reaction growth pressure is 100-300 torr, the purpose of growing P-type semiconductor layer 8 is to provide P-type contact.

[0046] It is worth noting that the thickness of the substrate 1, buffer layer 2, N-type semiconductor layer 3, low-temperature N-type GaN layer 4, SSL layer 5, QW-QB-QDs coupling layer 6, low-temperature P-type GaN layer 7, P-type semiconductor layer 8 shown in the drawings of the present application is only an example, and does not represent its true thickness. And the true ratio between the substrate 1, buffer layer 2, N-type semiconductor layer 3, low-temperature N-type GaN layer 4, SSL layer 5, QW-QB-QDs coupling layer 6, low-temperature P-type GaN layer 7, P-type semiconductor layer 8 is not as shown in the drawings, only for reference.

[0047] The various embodiments in the specification are described in a progressive manner, and each embodiment focuses on the difference from other embodiments. The same or similar parts between the various embodiments can be referred to each other.

[0048] The above description of the disclosed embodiments enables a person skilled in the art to implement or use the present application. Various modifications to the embodiments will be apparent to those skilled in the art, and the general principles defined herein can be implemented in other embodiments without departing from the spirit or scope of the present application. Therefore, the present application will not be limited to the embodiments shown herein, but will conform to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. An epitaxial structure, characterized by: The substrate, the N-type semiconductor layer, the QW-QB-QDs coupling layer and the P-type semiconductor layer are sequentially stacked from bottom to top. The QW-QB-QDs coupling layer comprises at least one period of QW layer, QB layer and QDs layer grown from bottom to top, the QW layer is an InGaN carrier storage layer, the QB layer is a GaN-AlGaN-GaN tunnel barrier layer, and the QDs layer is an InGaN quantum dot layer. The thickness of the QW layer is W, and the In content of the QW layer is X, the thickness of the QB layer is D, and X / 5≤D / W≤3X.

2. An epitaxial structure as claimed in claim 1, characterized in that: The number of growth periods of the QW-QB-QDs coupling layer is 2-5, including the end point.

3. An epitaxial structure as claimed in claim 1, characterized in that: A GaN cap layer is arranged on the QDs layer in each period of the QW-QB-QDs coupling layer.

4. An epitaxial structure as claimed in claim 1, characterized in that: A low-temperature nucleation layer and a buffer layer are further arranged between the substrate and the N-type semiconductor layer, the buffer layer is an undoped GaN layer, a low-temperature N-type GaN layer and an SSL layer are further arranged between the N-type semiconductor layer and the QW-QB-QDs coupling layer, the SSL layer is a superlattice structure of InGaN / GaN with 5-10 growth periods, including the end point, and a low-temperature P-type GaN layer is further arranged between the QW-QB-QDs coupling layer and the P-type semiconductor layer.

5. A method of producing an epitaxial structure as claimed in any one of claims 1-4, characterized in that The substrate, the N-type semiconductor layer, the QW-QB-QDs coupling layer and the P-type semiconductor layer are sequentially grown from bottom to top on the substrate by MOCVD method. The QW-QB-QDs coupling layer comprises at least one period of QW layer, QB layer and QDs layer grown from bottom to top, the QW layer is an InGaN carrier storage layer, the QB layer is a GaN-AlGaN-GaN tunnel barrier layer, and the QDs layer is an InGaN quantum dot layer, the QDs layer is grown by interrupting Ga source and In source and alternating high and low V / III ratio in growth. During the growth of the QDs layer, In source, Ga source and N source are introduced, 0.5-2.5nm InGaN is grown, then the In source and Ga source are interrupted for t' time, so that the adsorbed atoms migrate on the surface and reach the most suitable position, after reaching the thermodynamic equilibrium state, low V / III ratio for t1 time and high V / III ratio for t2 time are controlled in turn, and the growth of the QDs layer is completed, the V / III ratio is the molar ratio of N source and Ga source.

6. A method of producing an epitaxial structure as claimed in claim 5, characterized in that: During the growth of the QDs layer, t' is 10-30s, including the end point, t1 and t2 are both 3-10s, including the end point, t1 / t2 is 0.3-3, including the end point, 2000≤high V / III ratio≤8000, 500≤low V / III ratio≤2000, and the alternating period number of high and low V / III ratio is 3-10, including the end point.

7. A method of producing an epitaxial structure as claimed in claim 6, characterized in that: After the growth of the QDs layer, a GaN cap layer is grown on the QDs layer to complete the growth of one period of QW-QB-QDs coupling layer.

8. A method of producing an epitaxial structure as claimed in claim 5, characterized in that: ​ 9. A method of producing an epitaxial structure as claimed in claim 5, characterized in that: In the growth of the QW layer and the QB layer, firstly, the In source, the Ga source and the N source are introduced, the InGaN carrier storage layer is grown, the growth thickness is W, and the In component content is X; then, the H2 and the Al source are introduced, the GaN-AlGaN-GaN tunnel barrier layer is grown, and the thickness of the QB layer is D, wherein, X / 5≤D / W≤3X.

10. A method of producing an epitaxial structure as claimed in claim 9, characterized in that: In the growth of the QB layer, the introduction flow of Al is gradually increased first and then gradually decreased, and the introduction flow of H2 is gradually decreased first and then gradually increased.

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