Preparation method of high-conductivity AlN / GaN heterojunction

Abstract

The invention discloses a high-conductivity AlN / GaN heterojunction which sequentially comprises a sapphire substrate, an AlN buffer layer, a GaN channel layer and an AlN barrier layer from bottom to top. The total polarization intensity in an AlN material of the GaN channel layer is far greater than the total polarization intensity in a GaN material of the AlN barrier layer, net positive charges appear at an interface, the positive charges attract free electrons, a triangular potential well is formed on one side of GaN near the interface, the electrons are limited in the potential well by quantum, and high-density two-dimensional electron gas is formed. Polycrystal AlN is converted into single crystal AlN through magnetron sputtering in combination with a thermal annealing technology, so that the characteristic of small stress of the single crystal AlN is reserved, and two-dimensional electron gas can be formed at the interface of the AlN / GaN heterojunction.

Description

Technical Field

[0001] The present invention belongs to the technical field of AlN / GaN heterojunctions, and in particular relates to a method for preparing a high-conductivity AlN / GaN heterojunction. Background Art

[0002] Group III nitride semiconductor materials, such as GaN, InN, AlN, and their alloys, constitute the core components of wide bandgap semiconductor materials. Among them, AlN and GaN materials have the advantages of large bandgap, high electron mobility, good chemical stability and temperature stability, and high breakdown voltage, and are therefore widely used in the new generation of electronic devices. Due to the huge thermal mismatch and lattice mismatch between GaN and AlN, a large number of dislocations exist in the epitaxial layer, and cracks will occur to release stress when the external stress is large. These problems will seriously affect the crystal quality of the AlN epitaxial layer. AlN / GaN heterojunctions can be prepared by metal-organic chemical vapor deposition (MOCVD). Using MOCVD. Based on the two-step growth method of MOCVD, LT-MOCVD can avoid the degradation of the performance of other functional layers in the heterostructure during the epitaxial growth process of AlN / GaN heterostructures. In addition, due to the scalability of MOCVD, it can be adapted to large-scale production.

[0003] However, take the above-mentioned "method for preparing AlN / GaN heterojunction by MOCVD" as an example. Due to the mutual diffusion of AlN and GaN at high growth temperatures, the AlN layer obtained by MOCVD actually has a higher Ga content, resulting in a lower purity of the AlN barrier layer. In addition, due to the large stress problem caused by the lattice mismatch between the different materials used in the epitaxial growth of AlN on the GaN surface by MOCVD, the GaN channel layer will also experience corresponding stretching during the MOCVD epitaxy of AlN, resulting in the emergence of stress in the GaN channel layer. In addition, the surface morphology of the AlN barrier layer obtained by MOCVD epitaxy is poor, with a step-like structure. This technology leads to large stress and uneven surface morphology in the heterojunction film, which affects the continuity of the two-dimensional electron gas at the interface and reduces the density and mobility of the two-dimensional electron gas. Summary of the Invention

[0004] In order to overcome the shortcomings of the above-mentioned prior art, the purpose of the present invention is to provide a method for preparing a high-conductivity AlN / GaN heterojunction. This method converts polycrystalline AlN into single-crystalline AlN through magnetron sputtering combined with thermal annealing technology, which not only retains its low stress characteristics but also enables the formation of a two-dimensional electron gas at the interface of the AlN / GaN heterojunction.

[0005] In order to achieve the above object, the technical solution adopted by the present invention is:

[0006] A high-conductivity AlN / GaN heterojunction, comprising, from bottom to top, a sapphire substrate, an AlN buffer layer, a GaN channel layer, and an AlN barrier layer;

[0007] The total polarization intensity within the AlN material of the GaN channel layer is much greater than the total polarization intensity within the GaN material of the AlN barrier layer. A net positive charge appears at the interface, which attracts free electrons and forms a triangular potential well on the GaN side near the interface. The electrons are quantum confined in this potential well, forming a high-density two-dimensional electron gas.

[0008] The sapphire substrate has a thickness of approximately 200-1000 μm, providing mechanical support and a lattice matching foundation. The AlN buffer layer has a thickness of 20-50 nm, which is used to alleviate the lattice mismatch between the substrate and the functional layer. The GaN channel layer has a thickness of 800-3000 nm, which is the main body of the heterojunction. The AlN barrier layer has a thickness of approximately 20-50 nm, which acts as a barrier layer to enhance carrier confinement.

[0009] In the two-dimensional planar structure of the heterojunction, the horizontal length of the heterojunction is 100-2000nm;

[0010] The spontaneous polarization intensity of the AlN material in the GaN channel layer is three times that of the GaN material in the AlN barrier layer; the high density is 10 13 cm -2 .

[0011] A method for preparing a high-conductivity AlN / GaN heterojunction comprises the following steps:

[0012] Step 1) Substrate cleaning:

[0013] The sapphire substrate is first ultrasonically cleaned in HF acid or HCl acid, then ultrasonically cleaned in acetone solution, then ultrasonically cleaned in anhydrous ethanol, then ultrasonically cleaned in deionized water, and finally blown dry with nitrogen gas;

[0014] Step 2) Magnetron sputtering AlN buffer layer:

[0015] The cleaned substrate is placed in a magnetron sputtering reaction chamber, the chamber is evacuated, the substrate is heated, N2 and Ar2 are introduced, and AlN film is sputtered under sputtering power conditions;

[0016] Step 3) MOCVD epitaxial GaN layer:

[0017] A sapphire substrate with an AlN film sputtered on it was placed in an MOCVD reaction chamber, and trimethylgallium (TMGa) and ammonia (NH3) were introduced as Ga and N sources, respectively, with H2 as the carrier gas.

[0018] Step 4) Cleaning the epitaxial wafer:

[0019] First, the GaN channel layer of the epitaxial wafer is ultrasonically cleaned with acetone to remove organic matter on the epitaxial wafer. Then, it is heated and cleaned in a constant temperature water bath. The sample is then ultrasonically cleaned in isopropyl alcohol. Finally, the residual isopropyl alcohol and dust on the surface of the sample are rinsed with deionized water and blown dry with nitrogen.

[0020] Step 5) Magnetron sputtering AlN on the epitaxial GaN channel layer:

[0021] The cleaned GaN epitaxial wafer obtained in step 4) is placed in a magnetron sputtering reaction chamber, the chamber is evacuated, the GaN epitaxial wafer is heated, N2 and Ar2 are introduced, and an AlN film is sputtered under sputtering power conditions;

[0022] Step 6) Thermal annealing of the AlN barrier layer:

[0023] The sample obtained in step 5) is placed in a tube furnace in a normal pressure N2 environment to achieve thermal annealing of the AlN barrier layer on the GaN epitaxial layer, completing the preparation of a high-performance AlN / GaN heterojunction with low stress and low dislocation based on the thermal annealing technology of magnetron sputtering AlN.

[0024] In the step 1), ultrasonic cleaning is performed in HF acid or HCl acid for 5-10 minutes, ultrasonic cleaning is performed in acetone solution for 5-10 minutes, ultrasonic cleaning is performed in anhydrous ethanol for 5-10 minutes, and ultrasonic cleaning is performed in deionized water for 5-10 minutes.

[0025] In step 2), the chamber is evacuated to a vacuum degree of 5×10 -6 After 300-4000 ℃, the substrate is heated to 400-700℃ and N2 and Ar2 are introduced to make the Ar:N ratio 1:1-1:2. A 20-50nm AlN film is sputtered at a sputtering power of 100-250W.

[0026] In the step 3), the temperature of the reaction chamber is 800-1200° C., the flow rate of ammonia is 2500-3000 sccm, the flow rate of gallium source is 150-180 sccm, and the epitaxial GaN layer has a thickness of 800-3000 nm.

[0027] In the step 4), ultrasonic cleaning is performed for 5-10 minutes, the temperature is kept at 50-70° C., and the heating time is 5-10 minutes.

[0028] In step 5), the vacuum degree reaches 5×10 -6 Torr, heat the GaN epitaxial wafer to 400-700℃ and introduce N2 and Ar2 to make the Ar:N ratio 1:2, and sputter 20-50nm AlN film under the condition of sputtering power 100-250W.

[0029] In step 6), the annealing temperature is set to 800-1500° C., the heating rate is set to 10-25° C. / min, and the holding time is set to 5-10 min.

[0030] The AlN buffer layer deposited by magnetron sputtering on a sapphire substrate can effectively reduce the dislocation density of the GaN layer. Depositing an AlN barrier layer by magnetron sputtering on a GaN channel layer grown by MOCVD can reduce the stress in the material due to its low deposition temperature and the fact that it does not cause distortion of the GaN layer's lattice constant during the process. Thermal annealing of the AlN barrier layer can promote the recrystallization of AlN and release the internal stress of samples with poor crystallization quality, providing energy to the material and allowing it to rearrange its crystals. After thermal annealing, a high-density two-dimensional electron gas forms at the interface between the GaN layer and the AlN barrier layer due to their unified polarization electric field, and exhibits high electron mobility due to the low dislocation density at the surface.

[0031] In addition, AlN obtained by magnetron sputtering does not require the high-temperature film formation conditions of the MOCVD process, and will not cause lattice constant distortion on the GaN surface; and because the result of magnetron sputtering is polycrystalline AlN, it has less stress than single-crystalline AlN, and maintains the low stress characteristics after thermal annealing, so it has higher quality.

[0032] The high-quality AlN / GaN heterojunction is used to prepare high-power, high-current power electronic devices.

[0033] Beneficial effects of the present invention:

[0034] The present invention uses a thermal annealing treatment of magnetron sputtered AlN to directly deposit a layer of polycrystalline AlN on the GaN surface by magnetron sputtering. Unlike MOCVD deposition of AlN, which requires the introduction of a transition layer pseudocrystal between the substrate and the AlN layer, two crystals with different lattice constants, and thus causes crystal deformation, the present invention directly deposits AlN on the GaN surface by magnetron sputtering, which is equivalent to directly covering the GaN with a layer of AlN, thereby having less stress.

[0035] On the other hand, thermal annealing technology can promote the fusion of grains, gradually transforming polycrystalline AlN into single crystal AlN, and forming a stable and unified polarization electric field inside the AlN barrier layer, which is conducive to the formation of heterojunction two-dimensional electron gas, and can therefore be used to manufacture high-current and high-power electronic power devices. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 It is a schematic diagram of growing an AlN / GaN heterojunction according to the present invention.

[0037] Figure 2 The present invention is made Figure 1 Schematic diagram of the process flow of the meso-epitaxial structure. DETAILED DESCRIPTION

[0038] The present invention will be described in further detail below with reference to the accompanying drawings.

[0039] The purpose of the present invention is to provide a method for preparing a high-conductivity AlN / GaN heterojunction, which utilizes the characteristics of thermal annealing technology to improve the crystal quality of the magnetron sputtered AlN layer, combines the low stress of magnetron sputtered AlN and the thermal annealing technology to transform polycrystalline AlN into a single crystal state to obtain a high-quality AlN / GaN heterojunction.

[0040] A high-conductivity AlN / GaN heterojunction, comprising, from bottom to top, a sapphire substrate, an AlN buffer layer, a GaN channel layer, and an AlN barrier layer;

[0041] The total polarization intensity within the AlN material of the GaN channel layer is much greater than the total polarization intensity within the GaN material of the AlN barrier layer. A net positive charge appears at the interface, which attracts free electrons and forms a triangular potential well on the GaN side near the interface. The electrons are quantum confined in this potential well, forming a high-density two-dimensional electron gas.

[0042] The sapphire substrate has a thickness of approximately 200-1000 μm, providing mechanical support and a lattice matching foundation. The AlN buffer layer has a thickness of 20-50 nm, which is used to alleviate the lattice mismatch between the substrate and the functional layer. The GaN channel layer has a thickness of 800-3000 nm, which is the main body of the heterojunction. The AlN barrier layer has a thickness of approximately 20-50 nm, which acts as a barrier layer to enhance carrier confinement.

[0043] The length of the heterojunction is about 100-2000nm.

[0044] The difference in polarization effect strength between the GaN channel layer and the AlN barrier layer is that the total polarization strength in the AlN material is much greater than that in the GaN material, and is determined by σ 表面 =P AlN -P GaN It can be seen that a net positive charge appears at the interface. The positive charge attracts free electrons, forming a triangular potential well on the GaN side near the interface. The electrons are quantum confined in the potential well, forming a high-density two-dimensional electron gas.

[0045] like Figure 2As shown, a high conductivity AlN / GaN heterojunction is prepared based on thermal annealing of magnetron sputtered aluminum nitride, and the technology used in the present invention is described.

[0046] First, the AlN buffer layer is prepared. A 25nm AlN buffer layer is grown by magnetron sputtering on a substrate that has been cleaned and high-temperature nitrided. Secondly, a 1200nm GaN channel layer is epitaxially grown on the AlN buffer layer using MOCVD technology. This is followed by the growth of the AlN barrier layer. A 25nm AlN barrier layer is also grown using magnetron sputtering, and then the polycrystalline AlN obtained by magnetron sputtering is converted into single crystal AlN through thermal annealing to form an AlN / GaN heterojunction. The AlN / GaN heterojunction formed by the single crystal AlN barrier layer obtained in this way is conducive to increasing the two-dimensional electron gas concentration and realizing high-current and high-power electronic power devices.

[0047] The specific implementation steps are as follows:

[0048] 1) Substrate cleaning: First, ultrasonically clean the sapphire (0001) substrate in HF acid or HCl acid for 5-10 minutes, then ultrasonically clean it in acetone solution for 5-10 minutes, then ultrasonically clean it in anhydrous ethanol solution for 5-10 minutes, then ultrasonically clean it in deionized water for 5-10 minutes, and finally blow dry it with nitrogen gas;

[0049] 2) Magnetron sputtering AlN buffer layer: The substrate was placed in a magnetron sputtering reaction chamber, and the chamber was evacuated to a vacuum degree of 5e-6 Torr. The substrate was heated to 570°C and nitrogen and argon were introduced to make the Ar:N ratio 1:2. A 20-50 nm AlN film was sputtered at a sputtering power of 130 W to obtain a magnetron sputtered AlN buffer layer.

[0050] 3) MOCVD epitaxial GaN layer: A sapphire substrate with an AlN layer sputtered deposited thereon is placed in an MOCVD reaction chamber. The chamber temperature is set to 1070°C, and trimethylgallium (TMGa) and ammonia (NH3) are introduced as Ga and N sources, respectively. H2 is used as a carrier gas, with an ammonia flow rate of 2500-3000sccm and a gallium source flow rate of 150-180sccm. A GaN film with a thickness of 800-3000nm is grown to obtain an MOCVD epitaxial GaN layer.

[0051] 4) Cleaning the epitaxial wafer: First, ultrasonically clean the epitaxial wafer with acetone for 5-10 minutes to remove organic matter. Then, heat the wafer in a constant temperature water bath at 60°C for 5-10 minutes. Next, ultrasonically clean the wafer in isopropyl alcohol. Finally, rinse the wafer with deionized water to remove any residual isopropyl alcohol and dust. Then, blow dry the wafer with nitrogen.

[0052] 5) Magnetron sputtering of AlN on the epitaxial GaN channel layer: The GaN epitaxial wafer was placed in a magnetron sputtering reaction chamber. After the chamber was evacuated to a vacuum of 5e-6 Torr, the GaN epitaxial wafer was heated to 570°C and nitrogen and argon were introduced to make the Ar:N ratio 1:2. A 25nm thick AlN film was sputtered at a sputtering power of 130W to obtain an AlN barrier layer.

[0053] 6) Thermal annealing of the AlN barrier layer: The sample was placed in a tube furnace in a normal-pressure N2 environment. The annealing temperature was set to 800-1500°C, the heating rate was set to 13°C / min, and the holding time was set to 5 minutes. This was done to thermally anneal the AlN barrier layer on the GaN epitaxial layer, completing the preparation of a high-performance AlN / GaN heterojunction obtained by thermal annealing technology based on magnetron sputtering AlN.

[0054] The AlN buffer layer deposited on a sapphire substrate by magnetron sputtering can effectively reduce the dislocation density of the GaN layer. The AlN barrier layer is deposited on a GaN channel layer grown by MOCVD by magnetron sputtering. Since the deposition temperature is low and the lattice constant of the GaN layer is not distorted during the process, the stress in the material can be reduced. By thermally annealing the AlN barrier layer, the recrystallization of AlN can be promoted and the internal stress of samples with poor crystallization quality can be released, providing energy to the material and thus rearranging the crystals. After thermal annealing, the surfaces of the GaN layer and the AlN barrier layer form a high-density two-dimensional electron gas at the interface due to the unified polarization electric field of the two, and have a high electron mobility due to the low dislocation density at the surface.

[0055] In addition, AlN obtained by magnetron sputtering does not require the high-temperature film formation conditions of the MOCVD process, and will not cause lattice constant distortion on the GaN surface; and because the result of magnetron sputtering is polycrystalline AlN, it has less stress than single-crystalline AlN, and maintains the low stress characteristics after thermal annealing, so it has higher quality.

[0056] The present invention uses magnetron sputtering to deposit the AlN barrier layer, and transforms the polycrystalline AlN obtained by magnetron sputtering into single-crystalline AlN through thermal annealing, thereby forming a two-dimensional electron gas on the surface of the heterojunction. The single-crystalline AlN obtained by conventional MOCVD deposition has a huge lattice mismatch with GaN, resulting in excessive stress inside the crystal; while the polycrystalline AlN obtained by magnetron sputtering has very low stress, but it is difficult to form a two-dimensional electron gas at the heterojunction. The invention transforms polycrystalline AlN into single-crystalline AlN through the thermal annealing technology of magnetron sputtering, retaining its low stress characteristic while also being able to form a two-dimensional electron gas on the surface, which is also the protective point of the invention.

Claims

1. A high-conductivity AlN / GaN heterojunction, characterized in that: From bottom to top, there are sapphire substrate, AlN buffer layer, GaN channel layer and AlN barrier layer; The total polarization intensity within the AlN material of the GaN channel layer is much greater than the total polarization intensity within the GaN material of the AlN barrier layer. A net positive charge appears at the interface, which attracts free electrons and forms a triangular potential well on the GaN side near the interface. The electrons are quantum confined in this potential well, forming a high-density two-dimensional electron gas.

2. The high-conductivity AlN / GaN heterojunction according to claim 1, characterized in that: The thickness of the sapphire substrate is 200-1000 μm; the thickness of the AlN buffer layer is 20-50 nm, which is used to alleviate the lattice mismatch between the substrate and the functional layer; the thickness of the GaN channel layer is 800-3000 nm, which is the main body of the heterojunction; the thickness of the AlN barrier layer is about 20-50 nm, which acts as a barrier layer to enhance carrier confinement; In the two-dimensional planar structure of the heterojunction, the horizontal length of the heterojunction is 100-2000nm; The spontaneous polarization intensity of the AlN material in the GaN channel layer is three times that of the GaN material in the AlN barrier layer; the high density is 10 13 cm -2 .

3. A method for preparing a high-conductivity AlN / GaN heterojunction, characterized in that: The following steps are included: Step 1): The sapphire substrate is first ultrasonically cleaned in HF acid or HCl acid, then ultrasonically cleaned in acetone solution, then ultrasonically cleaned in anhydrous ethanol, then ultrasonically cleaned in deionized water, and finally blown dry with nitrogen; Step 2): placing the cleaned substrate in a magnetron sputtering reaction chamber, evacuating the chamber, heating the substrate and introducing N2 and Ar2, and sputtering the AlN film under sputtering power conditions; Step 3): Place the sapphire substrate with the AlN film sputtered and deposited in the MOCVD reaction chamber, introduce trimethylgallium (TMGa) and ammonia (NH3) as Ga and N sources, respectively, and H2 as the carrier gas; Step 4): First, ultrasonically clean the GaN channel layer of the epitaxial wafer with acetone to remove organic matter on the epitaxial wafer, then use a constant temperature water bath to heat and clean it, then place the sample in isopropyl alcohol and ultrasonically clean it, and finally rinse the residual isopropyl alcohol and dust on the surface of the sample with deionized water, and blow dry with nitrogen; Step 5): placing the cleaned GaN epitaxial wafer obtained in step 4) in a magnetron sputtering reaction chamber, evacuating the chamber, heating the GaN epitaxial wafer, introducing N2 and Ar2, and sputtering an AlN film under sputtering power conditions; Step 6): Place the sample obtained in step 5) in a tube furnace in a normal pressure N2 environment to achieve thermal annealing of the AlN barrier layer on the GaN epitaxial layer, completing the preparation of a high-performance AlN / GaN heterojunction with low stress and low dislocation based on the thermal annealing technology of magnetron sputtering AlN.

4. The method for preparing a high-conductivity AlN / GaN heterojunction according to claim 3, characterized in that: In the step 1), ultrasonic cleaning is performed in HF acid or HCl acid for 5-10 minutes, ultrasonic cleaning is performed in acetone solution for 5-10 minutes, ultrasonic cleaning is performed in anhydrous ethanol for 5-10 minutes, and ultrasonic cleaning is performed in deionized water for 5-10 minutes.

5. The method for preparing a high-conductivity AlN / GaN heterojunction according to claim 3, characterized in that: In step 2), the chamber is evacuated to a vacuum degree of 5×10 -6 After 300-4000 ℃, the substrate is heated to 400-700℃ and N2 and Ar2 are introduced to make the Ar:N ratio 1:1-1:

2. A 20-50nm AlN film is sputtered at a sputtering power of 100-250W.

6. The method for preparing a high-conductivity AlN / GaN heterojunction according to claim 3, characterized in that: In the step 3), the temperature of the reaction chamber is 800-1200° C., the flow rate of ammonia is 2500-3000 sccm, the flow rate of gallium source is 150-180 sccm, and the epitaxial GaN layer has a thickness of 800-3000 nm.

7. The method for preparing a high-conductivity AlN / GaN heterojunction according to claim 3, characterized in that: In the step 4), ultrasonic cleaning is performed for 5-10 minutes, the temperature is kept at 50-70° C., and the heating time is 5-10 minutes.

8. The method for preparing a high-conductivity AlN / GaN heterojunction according to claim 3, characterized in that: In step 5), the vacuum degree reaches 5×10 -6 Torr, heat the GaN epitaxial wafer to 400-700℃ and introduce N2 and Ar2 to make the Ar:N ratio 1:2, and sputter 20-50nm AlN film under the condition of sputtering power 100-250W.

9. The method for preparing a high-conductivity AlN / GaN heterojunction according to claim 3, characterized in that: In step 6), the annealing temperature is set to 800-1500° C., the heating rate is set to 10-25° C. / min, and the holding time is set to 5-10 min.

10. The high-conductivity AlN / GaN heterojunction according to any one of claims 1 to 9, characterized in that: The high-quality AlN / GaN heterojunction is used to prepare high-power, high-current power electronic devices.

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