N-polarity AlN / GaN epitaxial structure and preparation method thereof
By growing a metal polar AlN thick film on a low-cost sapphire substrate, and using a multi-layer strained SiN/AlN epitaxial structure to achieve polarity reversal, and finally growing an N-polar GaN epitaxial layer on the nitrogen polar AlN surface, the problem of difficulty in preparing high-quality nitrogen polar AlN/GaN epitaxial structure in the prior art is solved, and material preparation of excellent electrical properties and flat surfaces is achieved, reducing the preparation cost.
Patent Information
- Application Number
- CN202411748552.0
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2024-12-02
- Publication Date
- 2025-03-18
AI Technical Summary
The prior art is difficult to prepare high-quality nitrogen polar AlN/GaN epitaxial structure materials, and the preparation cost is high, which limits its application in industrial promotion.
By growing a metal polar AlN thick film on a relatively low-priced sapphire substrate, combined with a multi-layer polar inversion layer of strained SiN/AlN epitaxial structure, the metal polar AlN is transformed into nitrogen polar AlN, and finally, the N-polar GaN epitaxial layer is grown on the surface of nitrogen polar AlN to obtain the N-polar AlN/GaN epitaxial structure material on the flat surface.
It has achieved N-polar AlN/GaN epitaxial structure materials with excellent electrical properties of N-polarity and flat surfaces, reducing the preparation cost and promoting its application in industrial promotion.
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Figure CN119653801A_ABST
Abstract
Description
Technical Field
[0001] The present application relates to the field of semiconductor technology, and in particular to an N-polar AlN / GaN epitaxial structure and a preparation method thereof. Background Art
[0002] The third generation semiconductor GaAlInN compound has a wurtzite structure. Due to the lack of central inversion symmetry of the lattice, it exhibits a unique polarization effect in the C-axis direction. According to the polarization direction, nitrides can be divided into two categories: metal polarity (Al, Ga, In) and N polarity. GaN-HEMT (high electron mobility field effect transistor) is formed by the strain polarization effect between the heterojunction composed of AIN and GaN, which leads to the generation of two-dimensional electron gas (2DEG) on the GaN side of the interface. It has important applications in radio frequency and power electronic devices. At present, GaN-based materials and devices used in the industry, such as light-emitting diodes (LEDs) and high electron mobility transistors (HEMTs, High Electron Mobility Transistor), are basically metal polarity.
[0003] Compared with metal polarity, nitrogen-polarity GaN-HEMT has better electrical properties: (1) N-polarity GaN-based materials have many advantages over metal polarity GaN-based materials. In N-polarity HEMT epitaxial materials, two-dimensional electron gas (2DEG) is formed on the barrier layer with a wider bandgap width, so that the wide bandgap material forms a natural back barrier, increases the localization of 2DEG, improves the pinch-off characteristics of the device, and suppresses the short channel effect commonly found in Ga-polarity HEMT; (2) Ohmic contact is formed on the GaN channel layer with a smaller bandgap on the AlN layer, thereby achieving lower contact resistance between the source and the drain; (3) 2DEG is closer to the gate, allowing better gate control. At present, the maximum output power Pout of the reported N-polarity HEMT device at high frequency (W band) is twice that of the best Ga-polarity HEMT device of the same type; (4) In addition, for optoelectronic applications, the polarization field in the N-polarity LED structure is in the same direction as the pn junction depletion field, which can reduce the depletion region, reduce the turn-on voltage, and suppress leakage. In addition, the epitaxial growth temperature of N-polar InGaN is higher, and the epitaxial quality of InGaN with high In content is better.
[0004] The use of MOCVD technology to prepare epitaxial materials for metal-polar HEMTs is already relatively mature. However, due to the special properties of the nitrogen-polar GaN surface, the growth technology of nitrogen-polar HEMTs grown by MOCVD is very difficult, in addition to the use of tilted substrates, and it is difficult to obtain high-quality epitaxial materials. Plasma-assisted molecular beam epitaxy (PAMBE, plasma-assisted MBE) technology has shown its superiority. In addition to not having to use tilted substrates, the surface and interface of the materials grown by this technology are flat, and the device performance is excellent. However, due to the slow growth rate of this technology, homogeneous substrates must be used for epitaxy to show its superiority.
[0005] At present, there are mainly the following methods for preparing GaN-HEMT in the industry: (1) Using metal-organic chemical vapor deposition (MOCVD) to grow AlN thick films on sapphire substrates with a 0.2° (10-10) orientation has made rapid progress in recent years, and a dislocation density of 10 8 / cm 2 (2) Nitrogen-polar AlN thick films can be obtained by MOCVD on a sapphire substrate with a 2° off-angle under high temperature and low V / III ratio growth conditions. However, the PAMBE method cannot play its advantages on off-angle thick films because the sapphire substrate must be tilted to grow nitrogen-polar AlN thick films by MOCVD. However, the PAMBE method cannot play its advantages on off-angle thick films because the sapphire substrate must be tilted to grow nitrogen-polar AlN thick films by MOCVD. However, the PAMBE method cannot play its advantages on off-angle thick films by PAMBE. (1) The epitaxial growth method of AlN substrates is not suitable for the production of high-quality surface and interface epitaxial materials on AlN thick films with an offset angle. (2) The PAMBE method can be used to epitaxially grow nitrogen-polar GaN on sapphire substrates without an offset angle, but nitrogen-polar AlN cannot be directly obtained. (3) AlN substrates are prepared by sublimation method. The grown AlN can be used as metal-polar AlN or N-polar AlN. The homogeneous substrate can be nitrogen-polar GaN and AlN single crystal or thick film. Compared with GaN material substrates, HEMT materials grown on AlN substrates have good electrical properties. However, due to the high price of AlN single crystal substrates, the research and development speed on N-polar AlN substrates is affected, and the industrial application of HEMT on AlN substrates is also hindered.
[0006] In addition, one way is to use the MBE method to achieve the transformation of GaN from metal polarity to N polarity by heavily doping with Mg. However, since Mg is a p-type doped material in GaN, the holes introduced by the heavily doped Mg will have a serious negative impact on the subsequent material growth, making the defect density of the inversion layer ten times that of the original Ga-polar GaN, which ultimately seriously affects the performance of the prepared product. Another way is to achieve polarity reversal through micro-pit technology. The metal polarity AlN grown on a sapphire substrate tilted at 2° is converted into nitrogen polarity AlN through self-forming micro-pit technology. However, the nitrogen polarity AlN on the tilted substrate cannot be used for PAMBE growth because a flat surface and heterogeneous interface cannot be obtained, and therefore the advantages of PAMBE cannot be brought into play. Summary of the invention
[0007] In view of this, the embodiments of the present application provide an N-polar AlN / GaN epitaxial structural material having both excellent N-polar electrical properties and a flat surface to solve at least one problem existing in the background technology, and can control the preparation cost, greatly facilitating industrial promotion.
[0008] In a first aspect, an embodiment of the present application provides a method for preparing an N-polar AlN / GaN epitaxial structure, the method comprising:
[0009] Growing a metallic polar AlN film layer on a substrate;
[0010] epitaxially growing a metal polar AlN epitaxial layer on the metal polar AlN film layer;
[0011] A polarity inversion layer is epitaxially formed on the metal polarity AlN epitaxial layer, and the polarity inversion layer has a multi-layer strained SiN / AlN epitaxial structure;
[0012] epitaxially growing an N-polar AlN epitaxial layer on the polarity inversion layer;
[0013] An N-polar GaN epitaxial layer is grown on the N-polar AlN epitaxial layer.
[0014] In conjunction with the first aspect of the present application, in an optional implementation manner, growing a metal polar AlN film layer on a substrate includes:
[0015] A metal polar AlN thick film is deposited on a substrate by MOCVD, and a surface pretreatment is performed on the metal polar AlN thick film to obtain a flat metal polar AlN surface.
[0016] In combination with the first aspect of the present application, in an optional implementation, the substrate includes a Si substrate, a SiC substrate and / or a sapphire substrate.
[0017] In conjunction with the first aspect of the present application, in an optional implementation, the sapphire substrate used is sapphire with a 0.2° off-angle.
[0018] In conjunction with the first aspect of the present application, in an optional implementation manner, the thickness of the grown metal-polar AlN thick film is in the range of 2 um to 6 um.
[0019] In combination with the first aspect of the present application, in an optional implementation manner, the thickness of the grown metal-polar AlN thick film is 3 um.
[0020] In combination with the first aspect of the present application, in an optional embodiment, the surface pretreatment is a degassing and demoulding treatment.
[0021] In combination with the first aspect of the present application, in an optional embodiment, during surface pretreatment, the substrate temperature is above 800°C, and the Al beam of 6E-8Torr to 8E-8Torr is controlled to adsorb on the surface of the metal polar AlN thick film for 10 to 20 seconds, and then the Al beam is cut off, so that the Al atoms desorb on the surface for 20 to 30 seconds, and the deposited Al atoms are completely desorbed. The operation is repeated many times to obtain a clean and smooth metal polar AlN surface.
[0022] In conjunction with the first aspect of the present application, in an optional implementation manner, epitaxially growing a metal polarity AlN epitaxial layer on a metal polarity AlN film layer comprises:
[0023] PAMBE epitaxial AlN buffer layer was grown on the slightly Al-rich window of the metallic polar AlN film layer.
[0024] In combination with the first aspect of the present application, in an optional embodiment, the growth conditions of the PAMBE epitaxial AlN epitaxial layer include: the substrate temperature is above 850°C, the Al beam current is 2E-7Torr to 6E-7Torr, the nitrogen flow rate is 4SCCM to 10SCCM, and the RF source power is 300W to 600W.
[0025] In conjunction with the first aspect of the present application, in an optional implementation manner, epitaxially growing a polarity reversal layer on a metal polarity AlN epitaxial layer comprises:
[0026] A polarity reversal layer having a multi-layer strained SiN / AlN epitaxial structure is grown on the metal polar AlN epitaxial layer by PAMBE, so that the metal polar AlN is transformed into nitrogen polar AlN.
[0027] In conjunction with the first aspect of the present application, in an optional implementation, when generating a multilayer strained SiN / AlN epitaxial structure, SiN growth and AlN growth operations are performed and the SiN growth and AlN growth are repeated at least three times.
[0028] In combination with the first aspect of the present application, in an optional embodiment, the growth conditions of SiN in the PAMBE epitaxial polarity reversal layer include: substrate temperature 600°C~800°C, Si beam current 3E-8Torr~8E-8Torr, nitrogen flow rate 2SCCM~5SCCM, and RF source power of 50W-300W.
[0029] In combination with the first aspect of the present application, in an optional embodiment, the growth conditions of AlN in the polarity reversal layer in the PAMBE slightly metal-rich growth mode include: substrate temperature 850°C ~ 900°C, Al beam 2E-7Torr ~ 6E-7Torr, nitrogen flow 4SCCM ~ 10SCCM, and RF source power of 300W-600W.
[0030] In conjunction with the first aspect of the present application, in an optional implementation manner, epitaxially growing an N-polarity AlN epitaxial layer on the polarity inversion layer includes:
[0031] An N-polar AlN buffer layer was grown on the polarity inversion layer by PAMBE.
[0032] In combination with the first aspect of the present application, in an optional embodiment, the conditions for growing an N-polar AlN buffer layer in a PAMBE slightly metal-rich growth mode include: substrate temperature 870°C to 900°C, nitrogen flow rate 4SCCM to 10SCCM, Al beam current 8E-8Torr to 8E-7Torr, and RF source power 300W to 600W.
[0033] In combination with the first aspect of the present application, in an optional embodiment, the growth conditions for growing an N-polar GaN epitaxial layer on an N-polar AlN epitaxial layer include: substrate temperature 680°C to 740°C, nitrogen flow rate 4SCCM to 10SCCM, Ga beam current 3E-7Torr to 1E-6Torr, and RF source power 300W to 600W.
[0034] In conjunction with the first aspect of the present application, in an optional implementation, dual verification of metal polarity and N polarity is performed in combination with RHEED monitoring and corrosion testing.
[0035] In a second aspect, an embodiment of the present application provides an N-polar AlN / GaN epitaxial structure, which is prepared by the N-polar AlN / GaN epitaxial structure preparation method according to any of the above embodiments.
[0036] In a third aspect, an embodiment of the present application provides an N-polarity AlN / GaN epitaxial structure, comprising a substrate, a metal-polarity AlN film layer, a metal-polarity AlN epitaxial layer, a polarity reversal layer, an N-polarity AlN epitaxial layer and an N-polarity GaN epitaxial layer stacked in sequence, wherein the polarity reversal layer has a multilayer strained SiN / AlN epitaxial structure.
[0037] The N-polarity AlN / GaN epitaxial structure and preparation method thereof provided in the embodiment of the present application utilize a relatively low-cost substrate (sapphire, Si substrate, SiC substrate, etc.) to replace an expensive AlN substrate, first grow a metal polarity AlN thick film, and combine a metal polarity AlN epitaxial layer to reduce dislocations and release stress; then, on this basis, a polarity reversal layer having a multi-layer strained SiN / AlN epitaxial structure is transitionally epitaxially grown to transform the metal polarity AlN into nitrogen polarity AlN; and, a N-polarity AlN epitaxial layer is further grown on the polarity reversal layer to reduce dislocations and release stress, and finally GaN is grown on the surface of the N-polarity AlN, i.e., an N-polarity GaN epitaxial layer, to finally obtain an N-polarity AlN / GaN epitaxial structure material having both excellent N-polarity electrical properties and a flat surface, and the preparation cost can be controlled, thereby being more convenient for industrial promotion.
[0038] Additional aspects and advantages of the present application will be given in part in the description below, and in part will become apparent from the description below, or will be learned through the practice of the present application. BRIEF DESCRIPTION OF THE DRAWINGS
[0039] The drawings described herein are used to provide a further understanding of the present application and constitute a part of the present application. The illustrative embodiments of the present application and their descriptions are used to explain the present application and do not constitute an improper limitation on the present application. In the drawings:
[0040] Figure 1 A schematic flow chart of a method for preparing an N-polar AlN / GaN epitaxial structure provided in some embodiments of the present application;
[0041] Figure 2 for Figure 1 Schematic diagram of RHEED image monitoring results corresponding to the core process steps in the preparation process;
[0042] Figure 3 for Figure 1 Schematic diagram of corrosion test results corresponding to the core process steps in the preparation process;
[0043] Figure 4 for Figure 1 Schematic diagram of another RHEED image monitoring result after the S5 preparation process;
[0044] Figure 5 Schematic diagram of N-polar AlN / GaN epitaxial structure provided for some embodiments of the present application. DETAILED DESCRIPTION
[0045] The exemplary embodiments disclosed in the present application will be described in more detail below with reference to the accompanying drawings. Although the exemplary embodiments of the present application are shown in the accompanying drawings, it should be understood that the present application can be implemented in various forms and should not be limited by the specific embodiments described herein. On the contrary, these embodiments are provided to enable a more thorough understanding of the present application and to fully convey the scope disclosed in the present application to those skilled in the art.
[0046] In the following description, a large number of specific details are given to provide a more thorough understanding of the present application. However, it is obvious to those skilled in the art that the present application can be implemented without one or more of these details. In other examples, in order to avoid confusion with the present application, some technical features known in the art are not described; that is, all features of the actual embodiments are not described here, and well-known functions and structures are not described in detail.
[0047] In the drawings, the sizes of layers, regions, elements and their relative sizes may be exaggerated for clarity. Like reference numerals denote like elements throughout.
[0048] It should be understood that when an element or layer is referred to as "on ...", "adjacent to ...", "connected to" or "coupled to" other elements or layers, it can be directly on, adjacent to, connected to or coupled to other elements or layers, or there can be intervening elements or layers. On the contrary, when an element is referred to as "directly on ...", "directly adjacent to ...", "directly connected to" or "directly coupled to" other elements or layers, there is no intervening element or layer. It should be understood that although the terms first, second, third, etc. can be used to describe various elements, components, regions, layers and / or parts, these elements, components, regions, layers and / or parts should not be limited by these terms. These terms are only used to distinguish one element, component, region, layer or part from another element, component, region, layer or part. Therefore, without departing from the teachings of the present application, the first element, component, region, layer or part discussed below can be represented as the second element, component, region, layer or part. And when the second element, component, region, layer or part is discussed, it does not indicate that the present application necessarily has the first element, component, region, layer or part.
[0049] Spatially relative terms such as "under", "beneath", "below", "under", "above", "above", etc., may be used here for convenience of description to describe the relationship between an element or feature shown in the figure and other elements or features. It should be understood that in addition to the orientation shown in the figure, the spatial relationship terms are intended to also include different orientations of the device in use and operation. For example, if the device in the accompanying drawings is turned over, then the elements or features described as "under other elements" or "under it" or "under it" will be oriented as "on" other elements or features. Therefore, the exemplary terms "under" and "under" may include both upper and lower orientations. The device can be oriented otherwise (rotated 90 degrees or other orientations) and the spatial descriptors used herein are interpreted accordingly.
[0050] The purpose of the terms used herein is only to describe specific embodiments and is not intended to be limiting of the present application. When used herein, the singular forms "one", "an" and "said / the" are also intended to include plural forms, unless the context clearly indicates otherwise. It should also be understood that the terms "consisting of" and / or "comprising", when used in this specification, determine the presence of the features, integers, steps, operations, elements and / or parts, but do not exclude the presence or addition of one or more other features, integers, steps, operations, elements, parts and / or groups. When used herein, the term "and / or" includes any and all combinations of the relevant listed items.
[0051] In order to thoroughly understand the present application, detailed steps and detailed structures will be presented in the following description to illustrate the technical solution of the present application. The preferred embodiments of the present application are described in detail below, but in addition to these detailed descriptions, the present application may also have other implementation methods.
[0052] In view of the current state of the art described in the background technology, the inventors of the present application have conducted in-depth research on how to combine the advantages of the preparation process to obtain an N-polarity AlN / GaN epitaxial structure solution that has both excellent N-polarity electrical properties and appropriate cost control, and is convenient for AlN / GaN HEMT material preparation and device development and promotion to industrial application.
[0053] Since the inventors noticed that metal-polarity GaN and AlIn thick films with low dislocation density can be grown on a sapphire substrate with an extremely small off-angle of 0.2 degrees, if a method is explored to convert metal-polarity GaN and AlIn into nitrogen-polarity materials, then a new N-polarity AlN / GaN epitaxial structure with smooth surface and interface and excellent performance can be prepared by combining appropriate process advantages. In addition, the AlIn / GaN heterojunction has a stronger strain polarization effect. Obviously, the HEMT grown on the N-polarity AlIn single crystal or thick film can obtain a higher 2DEG concentration and a stronger back barrier effect.
[0054] Furthermore, through in-depth exploration and repeated technical verification, the inventors found that relatively inexpensive substrates (sapphire, Si substrate, SiC substrate, etc.) can be used instead of expensive AlN substrates to first grow a metal polarity AlN thick film, and combine it with a metal polarity AlN epitaxial layer to reduce dislocations and release stress; then, on this basis, a polarity reversal layer with a multi-layer strained SiN / AlN epitaxial structure is transitionally epitaxially grown to transform the metal polarity AlN into nitrogen polarity AlN; and, a N polarity AlN epitaxial layer is further grown on the polarity reversal layer to reduce dislocations and release stress, and finally GaN is grown on the surface of the N polarity AlN, that is, an N polarity GaN epitaxial layer, to finally obtain an N polarity AlN / GaN epitaxial structure material that has both excellent N polarity electrical properties and a flat surface, and can control the preparation cost, thereby being more convenient for industrial promotion.
[0055] The following specific examples describe in detail the N-polar AlN / GaN epitaxial structure and preparation method provided in the embodiments of the present application. In the following embodiments, the steps can be performed in the same growth chamber by way of example, and without departing from the inventive concept of the present application, by providing the corresponding necessary growth conditions, the growth method is not limited to PAMBE, MBE or MOCVD, etc., that is, any feasible growth method can be used to generate it. In addition, the metal polar AlN thick film substrate can be appropriately temperature controlled, exemplarily controlled in the temperature range of 20°C-1250°C; a source furnace of Al, Ga, In, and Si can be provided, and in the example, the Group III metal is heated to a certain temperature by a thermal evaporation source to form an Al beam, and the beam can be calibrated using a beam meter; a source furnace of N can be provided, and N can be exemplarily excited by a radio frequency signal of a certain power (for example, 0-1500W) to excite N2, and the gas flow rate of N2 can be adjusted (0-25SCCM) to form a nitrogen plasma, which is sprayed onto the substrate surface to participate in the reaction process.
[0056] In addition, it should be noted that the growth chamber is equipped with a reflection high-energy electron diffraction (RHEED), through which the epitaxial growth of the nitride surface can be observed during the growth of the organizational structure, so as to timely judge and determine the completion of each growth process. Exemplarily, the RHEED device consists of an electron gun and a fluorescent screen for collecting diffraction spots. The electron gun can generate a 10keV to 50keV single-energy electron beam focused on the surface. The electron beam reaches the material surface at an angle of less than 2 degrees. The interaction between the high-energy electrons and the surface is limited to a few atoms on the surface, so that the information on the material surface can be fully reflected. The electrons reflected and diffracted by the atoms on the material surface are collected on the fluorescent screen to form diffraction spots, and then the pattern can be digitized by using a CCD camera.
[0057] like Figure 2 and Figure 4 As shown, the polarity is judged by in-situ observation of GaN surface reconstruction using RHEED. When the surface is 2×2 reconstructed, it is a Ga-polar GaN characteristic reconstruction, which can be determined as a metal polar material by this reconstruction. At the same time, when the surface is 3×3 reconstructed, it is an N-polar GaN characteristic reconstruction. The ×3 reconstruction on [1-100] is a necessary and sufficient condition for the GaN surface to be nitrogen polar. In addition, 2×2 and 3×3 reconstructions at low temperatures are necessary and sufficient conditions for judging the polarity of the GaN surface, and the temperature conditions can be appropriately controlled during the judgment. In addition, corrosion tests (such as alkaline solution corrosion) are also important criteria for distinguishing metal polar AlN and GaN from N polar AlN and GaN based on their resistance to alkaline corrosion, and are also a common practice recognized internationally. The surface polarity of GaN and AlN is judged through alkaline corrosion tests. N polar GaN and AlN can be corroded, while metal polar GaN and AlN are difficult to be corroded by alkali. The reason is that the metal polar surface has three dangling bonds that can resist OH - , while the nitrogen polar material has only one dangling bond on its surface to resist OH - In a preferred exemplary embodiment, a dual verification method of RHEED image monitoring and alkaline corrosion test is used to determine polarity conversion, so as to fully and accurately ensure that the N-polar AlN / GaN epitaxial structure prepared by the N-polar AlN / GaN epitaxial structure preparation scheme provided in the embodiment of the present application has achieved the expected polarity conversion effect.
[0058] Specifically, Figure 1 As shown, the preparation process of the N-polar AlN / GaN epitaxial structure provided in the embodiment of the present application includes the following steps.
[0059] S1, growing a metal polar AlN film layer on a substrate;
[0060] A metal polarity AlN thick film is grown on a Si substrate, a SiC substrate and / or a sapphire substrate by MOCVD, and the metal polarity AlN thick film is subjected to appropriate surface pretreatment (such as degassing and demolding treatment, etc.) to obtain a flat and clean metal polarity AlN surface, so that a 1x1 surface reconstruction of RHEED can be observed on the surface. That is, a metal polarity AlN thick film is generated as a metal polarity AlN film layer, and the thickness ranges from 2um to 6um, preferably 3um. The thickness of this AlN thick film can meet the preparation process requirements, and the appropriate thickness value is more convenient for controlling the growth process cost. It should be noted that any feasible surface pretreatment method can be used to obtain a metal polarity AlN thick film layer with a flat surface, and the embodiments of the present application are not particularly limited. For example, the temperature of the sapphire substrate can be raised to above 800°C (e.g., 800°C, 830°C, etc.) for surface pretreatment, and the Al beam (i.e., Al source furnace beam) with a flow rate of 6E-8Torr to 8E-8Torr is controlled to adsorb on the surface of the metal polar AlN thick film for 10 to 20 seconds, and then the shutter is closed to cut off the Al beam, so that the Al atoms are desorbed on the surface for 20 to 30 seconds, and the deposited Al atoms are completely desorbed, and this operation is repeated many times to obtain a clean and flat metal polar AlN surface. In this process, the adsorption and desorption time can be strictly controlled by RHEED intensity monitoring combined with corrosion testing to obtain a better metal polar AlN film preparation effect.
[0061] Combination Figure 2 and Figure 3 The RHEED image monitoring result data and the corrosion test result data are displayed respectively. According to the observation of the 1x1 surface reconstruction of RHEED, the AlN film layer at this time is of metal polarity, and the surface morphology of the AlN film layer does not change and the roughness remains unchanged after being immersed in a KOH solution with a concentration of 1 mol / L for 1 to 20 minutes, that is, it is difficult to be corroded by alkali. Combined with the aforementioned alkaline corrosion principle, it can be further confirmed that it is of metal polarity.
[0062] S2, epitaxially growing a metal polar AlN epitaxial layer on the metal polar AlN film layer;
[0063] PAMBE epitaxially grows an AlN buffer layer on the slightly Al-rich window of the metal polar AlN film layer, that is, the AlN buffer layer is grown by PAMBE high-temperature window epitaxial AlN method, that is, the AlN buffer layer is epitaxially grown on the AlN film layer. The presence of the AlN buffer layer can reduce dislocations, release stress, and optimize interlayer structure performance. Exemplarily, the growth conditions of the AlN epitaxial layer epitaxially grown by PAMBE are: the substrate temperature is maintained above 850°C, the Al beam flow rate is 2E-7Torr~6E-7Torr, the nitrogen flow rate is 4SCCM~10SCCM, and the RF source power is 300W-600W. It should be noted that without departing from the inventive concept of the present application, any feasible epitaxial growth method can be used to generate the metal polar AlN epitaxial layer, and it is not limited to the above-mentioned PAMBE method.
[0064] S3, epitaxially growing a polarity inversion layer on the metal polarity AlN epitaxial layer, wherein the polarity inversion layer has a multi-layer strained SiN / AlN epitaxial structure;
[0065] After the growth of the metal polar AlN epitaxial layer is completed, the polarity reversal layer (i.e., the polarity reversal layer) having a multilayer strained SiN / AlN epitaxial structure is grown by PAMBE. Figure 2 , Figure 3 The polarity reversal transition layer shown in ) is used to transform the metal polarity AlN into nitrogen polarity AlN. Wherein, when generating a multilayer strained SiN / AlN epitaxial structure, it is preferred to repeat the SiN / AlN growth (including the growth of SiN and the growth of AlN) for more than three times, and finally a triple strained structure of SiN single atomic layer / 5 to 10 nm as a period of AlN can be formed. The technical principle of using the polarity reversal layer as a multilayer strained SiN / AlN polarity reversal transition layer or SiN / AlN inversion insertion layer to achieve polarity transformation by changing the lattice structure is that the single-layer SiN structure grown on the AlN layer changes the surface energy of the adsorbed Al atoms on the original AlN layer, and the binding sites of the adsorbed Al atoms are changed, causing lattice distortion, so that the AlN subsequently deposited on the SiN interface produces an inversion domain, and the metal polarity AlN is transformed into N polarity AlN at the position of the inversion domain, so that after the growth of the multilayer strained SiN, the metal polarity AlN is completely transformed into N polarity AlN. Combined with Figure 2 , Figure 3 The changes in the lattice structure, surface morphology and surface roughness show that the polarity has changed from metal to N polarity.
[0066] For example, the growth conditions of SiN in the epitaxial polarity reversal layer using PAMBE are: substrate temperature 600℃~800℃, Si beam flow rate 3E-8Torr~8E-8Torr, nitrogen flow rate 2SCCM~5SCCM, and RF source power 50W-300W. During this growth process, the growth time can be strictly controlled in combination with RHEED monitoring, such as Figure 2 As shown, SiN is grown so that the thickness of the SiN layer is within a single atomic layer. When the RHEED image shows a micro-point shape, the AlN growth window in the PAMBE micro-rich metal growth mode is used to perform a growth transition from SiN to AlN until the thickness of AlN reaches 5 to 10 nm, so that the RHEED line becomes smooth. Exemplarily, the growth conditions of AlN in the polarity reversal layer in the PAMBE micro-rich metal growth mode are: substrate temperature 850°C to 900°C, Al beam current 2E-7Torr to 6E-7Torr, nitrogen flow rate 4SCCM to 10SCCM, and RF source power 300W-600W.
[0067] S4, epitaxially growing an N-polar AlN epitaxial layer on the polarity inversion layer;
[0068] After the polarity reversal layer is epitaxially grown, the AlN growth window in the PAMBE micro-metal-rich growth mode is used at an appropriate substrate temperature, so that the epitaxial N-polar AlN epitaxial layer is further used as an AlN buffer layer to reduce dislocations and release stress. Exemplarily, the growth conditions of this process are as follows: the substrate temperature is 870°C to 900°C, the nitrogen gas flow rate is 4SCCM to 10SCCM, the RF source power is 300W-600W, and the Al beam flow rate is 8E-8Torr to 8E-7Torr.
[0069] Combined with the corrosion test, the surface morphology of the AlN epitaxial layer that has undergone polarity reversal transition changed after being immersed in a 1 mol / L KOH solution for 1 minute. Hexagonal hillocks appeared and the surface roughness changed greatly. The large island morphology after corrosion can reach more than 50nm, which proves that the epitaxial layer is N-polar AlN, which once again verifies that the transition from metal polarity to N polarity has been achieved after polarity reversal layer epitaxy.
[0070] S5, growing an N-polar GaN epitaxial layer on the N-polar AlN epitaxial layer;
[0071] After the N-polar AlN epitaxial layer is grown, RHEED can be used to generate flat N-polar GaN. For example, the specific growth conditions are as follows: the substrate temperature is reduced to 680°C to 740°C, the nitrogen flow rate is 4SCCM to 10SCCM, the Ga beam flow rate is 3E-7Torr to 1E-6Torr, and the RF source power is 300W to 600W.
[0072] Combination Figures 2 to 4 As shown, when the surface temperature drops to below 350°C, a 3×3 surface reconstruction can be clearly observed, thereby determining that the surface is of N polarity; then the surface roughness is 2.3nm before alkaline corrosion, and the surface roughness is 6.5nm after immersion in a 1mol / L KOH solution for 1 minute. Therefore, the GaN epitaxially grown on the AlN after polarity reversal is once again confirmed to be of N polarity in combination with corrosion theory.
[0073] In addition, if Figure 5 As shown, some embodiments of the present application also provide an N-polarity AlN / GaN epitaxial structure, including a substrate, a metal polarity AlN film layer, a metal polarity AlN epitaxial layer, a polarity inversion layer, an N-polarity AlN epitaxial layer and an N-polarity GaN epitaxial layer stacked in sequence, wherein the polarity inversion layer has a multilayer strained SiN / AlN epitaxial structure. Preferably, the N-polarity AlN / GaN epitaxial structure is prepared by the N-polarity AlN / GaN epitaxial structure preparation method described in any of the above embodiments.
[0074] Since AlN / GaN HEMT materials are grown on N-polar AlN substrates, they have the advantages of high two-dimensional electron gas concentration and more significant back barrier effect compared to GaN substrates. However, AlN substrates are expensive, and there is no available N-polar AlN thick film using other methods, which restricts the development of HEMT on N-polar AlN substrates. This type of material is called the next generation device platform. The AlN thick film on sapphire used in this application scheme has a lower price after batch production of 2 inches, the polarity reversal process only takes 30 to 45 minutes of machine time, and the conversion rate is 100%, thus providing a valuable opportunity for the development of excellent performance GaN / AlN heterojunction HEMT. More importantly, this technology can be used on metal polar AlN thick films on sapphire substrates up to six inches. The six-inch N-polar AlN thick film will open up the situation for this type of next-generation AlN / GaN HEMT devices to move from the laboratory to industrialization, greatly facilitating the formation of industrial scale and development and promotion.
[0075] It should be understood that the above embodiments are exemplary and are not intended to include all possible implementations included in the claims. Various modifications and changes may be made on the basis of the above embodiments without departing from the scope of the present disclosure. Similarly, the various technical features of the above embodiments may be arbitrarily combined to form other embodiments of the present application that may not be explicitly described. Therefore, the above embodiments only express several implementations of the present application and do not limit the scope of protection of the patent of this application.
Claims
1. A method for preparing an N-polar AlN / GaN epitaxial structure, characterized in that: The method comprises: Growing a metallic polar AlN film layer on a substrate; epitaxially growing a metal polar AlN epitaxial layer on the metal polar AlN film layer; A polarity inversion layer is epitaxially formed on the metal polarity AlN epitaxial layer, and the polarity inversion layer has a multi-layer strained SiN / AlN epitaxial structure; epitaxially growing an N-polar AlN epitaxial layer on the polarity inversion layer; An N-polar GaN epitaxial layer is grown on the N-polar AlN epitaxial layer.
2. The method for preparing an N-polar AlN / GaN epitaxial structure according to claim 1, characterized in that: Growing a metal polar AlN film layer on a substrate, comprising: A metal polar AlN thick film is deposited on a substrate by MOCVD, and a surface pretreatment is performed on the metal polar AlN thick film to obtain a flat metal polar AlN surface.
3. The method for preparing an N-polar AlN / GaN epitaxial structure according to claim 2, characterized in that: The substrate includes a Si substrate, a SiC substrate and / or a sapphire substrate.
4. The method for preparing an N-polar AlN / GaN epitaxial structure according to claim 3, characterized in that: The sapphire substrate used is sapphire with a 0.2° off-angle.
5. The method for preparing an N-polar AlN / GaN epitaxial structure according to claim 2, characterized in that: The thickness of the grown metallic polar AlN thick film ranges from 2um to 6um.
6. The method for preparing an N-polar AlN / GaN epitaxial structure according to claim 5, characterized in that: The thickness of the grown metallic polar AlN thick film is 3 um.
7. The method for preparing an N-polar AlN / GaN epitaxial structure according to claim 2, characterized in that: The surface pretreatment is degassing and demoulding treatment.
8. The method for preparing an N-polar AlN / GaN epitaxial structure according to claim 7, characterized in that: During surface pretreatment, the substrate temperature is above 800°C, and the Al beam of 6E-8Torr to 8E-8Torr is controlled to adsorb on the surface of the metal polar AlN thick film for 10 to 20 seconds, and then the Al beam is cut off to allow Al atoms to desorb on the surface for 20 to 30 seconds. The deposited Al atoms are completely desorbed, and the operation is repeated many times to obtain a clean and smooth metal polar AlN surface.
9. The method for preparing an N-polar AlN / GaN epitaxial structure according to claim 1, characterized in that: A metal polar AlN epitaxial layer is epitaxially grown on the metal polar AlN film layer, comprising: PAMBE epitaxial AlN buffer layer was grown on the slightly Al-rich window of the metallic polar AlN film layer.
10. The method for preparing an N-polar AlN / GaN epitaxial structure according to claim 9, characterized in that: The growth conditions of the PAMBE epitaxial AlN epitaxial layer include: substrate temperature above 850°C, Al beam current of 2E-7Torr to 6E-7Torr, nitrogen flow rate of 4SCCM to 10SCCM, and RF source power of 300W to 600W.
11. The method for preparing an N-polar AlN / GaN epitaxial structure according to claim 1, characterized in that: The polarity reversal layer is epitaxially grown on the metal polarity AlN epitaxial layer, comprising: A polarity reversal layer having a multi-layer strained SiN / AlN epitaxial structure is grown on the metal polar AlN epitaxial layer by PAMBE, so that the metal polar AlN is transformed into nitrogen polar AlN.
12. The method for preparing an N-polar AlN / GaN epitaxial structure according to claim 11, characterized in that: When a multilayer strained SiN / AlN epitaxial structure is formed, the SiN growth and AlN growth operations are performed and repeated at least three times.
13. The method for preparing an N-polar AlN / GaN epitaxial structure according to claim 11, characterized in that: The growth conditions of SiN in the PAMBE epitaxial polarity reversal layer include: substrate temperature 600°C to 800°C, Si beam current 3E-8Torr to 8E-8Torr, nitrogen flow rate 2SCCM to 5SCCM, and RF source power 50W to 300W.
14. The method for preparing an N-polar AlN / GaN epitaxial structure according to claim 11, characterized in that: The growth conditions of AlN in the polarity reversal layer in the PAMBE slightly metal-rich growth mode include: substrate temperature 850°C to 900°C, Al beam current 2E-7Torr to 6E-7Torr, nitrogen flow rate 4SCCM to 10SCCM, and RF source power 300W to 600W.
15. The method for preparing an N-polar AlN / GaN epitaxial structure according to claim 1, characterized in that: Epitaxially growing an N-polar AlN epitaxial layer on the polarity inversion layer, comprising: An N-polar AlN buffer layer was grown on the polarity inversion layer by PAMBE.
16. The method for preparing an N-polar AlN / GaN epitaxial structure according to claim 15, characterized in that: The growth conditions of the N-polar AlN buffer layer in the PAMBE slightly metal-rich growth mode include: substrate temperature 870°C to 900°C, nitrogen flow rate 4SCCM to 10SCCM, Al beam current 8E-8Torr to 8E-7Torr, and RF source power 300W to 600W.
17. The method for preparing an N-polar AlN / GaN epitaxial structure according to any one of claims 1 to 16, characterized in that: The growth conditions for growing an N-polar GaN epitaxial layer on an N-polar AlN epitaxial layer include: a substrate temperature of 680° C. to 740° C., a nitrogen flow rate of 4 SCCM to 10 SCCM, a Ga beam current of 3E-7 Torr to 1E-6 Torr, and a radio frequency source power of 300W to 600W.
18. The method for preparing an N-polar AlN / GaN epitaxial structure according to any one of claims 1 to 17, characterized in that: Combine RHEED monitoring and corrosion testing to perform dual verification of metal polarity and N polarity.
19. An N-polar AlN / GaN epitaxial structure, characterized in that: The N-polar AlN / GaN epitaxial structure is prepared by the method for preparing the N-polar AlN / GaN epitaxial structure according to any one of claims 1 to 18.
20. An N-polar AlN / GaN epitaxial structure, characterized in that: It includes a substrate, a metal polarity AlN film layer, a metal polarity AlN epitaxial layer, a polarity inversion layer, an N polarity AlN epitaxial layer and an N polarity GaN epitaxial layer which are stacked in sequence, wherein the polarity inversion layer has a multi-layer strained SiN / AlN epitaxial structure.