Superlattice structure for GaN HEMT, buffer layer, GaN HEMT epitaxial wafer and growth method

Abstract

The invention discloses a superlattice structure for a GaN HEMT (High Electron Mobility Transistor), a buffer layer, a GaN HEMT epitaxial wafer and a growth method. The superlattice structure comprises N AlN / GaN / AlGaN laminated structures, each AlN / GaN / AlGaN laminated structure comprises an AlN layer, a GaN layer and an AlGaN layer which are sequentially stacked and grown from bottom to top, and N is an integer larger than 1. According to the superlattice structure prepared by the invention, the GaN layer is inserted between the AlN layer and the AlGaN layer, the GaN layer not only can reduce the surface defects of the AlN layer, but also can counteract a part of pressure stress and reduce the stress defects, and the atom lateral migration capability of GaN during growth is stronger, so that the surface defects of the epitaxial layer can be obviously improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of semiconductor devices, and in particular relates to a superlattice structure, a buffer layer, a GaN HEMT epitaxial wafer and a growth method for GaN HEMT. Background Art

[0002] Gallium nitride high electron mobility transistor (GaN HEMT) is an electronic device based on the wide bandgap semiconductor material gallium nitride. It has the characteristics of high power density, high efficiency, high frequency operation and fast switching. It is mainly used in power adapters, car charging, data centers and other fields, and has gradually become the best solution for 5G base station power supply.

[0003] GaN HEMT contains AlGaN / GaN heterojunction. In the AlGaN / GaN heterojunction, due to the piezoelectric polarization and spontaneous polarization effects, a high-density electron layer, namely two-dimensional electron gas (2DEG), is formed at the interface between the AlGaN layer and the GaN layer. Applying a drain-source voltage can generate a lateral electric field. The two-dimensional electron gas is transported along the AlGaN / GaN heterojunction interface under the action of the lateral electric field to form a drain output current. The gate makes Schottky contact with the AlGaN barrier layer. By adjusting the gate voltage, the depth of the potential well in the AlGaN / GaN heterojunction can be controlled, thereby controlling the size of the drain output current.

[0004] In GaN HEMT technology, breakdown voltage refers to the rated maximum voltage that the source and drain of the device can withstand, and is a measure of the maximum voltage at which the device can safely operate. Increasing the breakdown voltage means that the device can withstand higher voltages, which is very important in many high-power applications, such as power management, electric vehicles, and renewable energy technologies.

[0005] In GaN HEMT devices, it is crucial to increase the breakdown voltage in the vertical direction of the epitaxial layer, as it determines the upper limit of the device's withstand voltage. Summary of the invention

[0006] The purpose of the present invention is to improve the vertical withstand voltage characteristics of GaN HEMT devices at low cost. By improving the structure and process of the buffer layer, not only the surface defects of the epitaxial layer are reduced, the leakage current is reduced, and the breakdown voltage in the vertical direction is increased, but also the difficulty of epitaxial growth is overcome and the production cost is reduced.

[0007] In order to achieve the above-mentioned object, the present invention provides a superlattice structure for GaN HEMT, comprising: N AlN / GaN / AlGaN stacked structures, wherein the AlN / GaN / AlGaN stacked structures comprise an AlN layer, a GaN layer and an AlGaN layer stacked and grown sequentially from bottom to top, where N is an integer greater than 1.

[0008] Optionally, the Al content in the AlGaN layer is 5% to 40% by molar ratio.

[0009] Optionally, the superlattice structure includes at least: a first superlattice structure and a second superlattice structure, the second superlattice structure is arranged above the first superlattice structure, and the Al component content in the AlGaN layer of the first superlattice structure is greater than the Al component content in the AlGaN layer of the second superlattice structure; the first superlattice structure includes m AlN / GaN / AlGaN stacked structures, and the second superlattice structure includes n AlN / GaN / AlGaN stacked structures, m and n are the same or different, and both are integers greater than 0.

[0010] Optionally, in the AlN / GaN / AlGaN stacked structure, the thickness of the AlN layer is 2nm-15nm, the thickness of the GaN layer is less than or equal to 5nm, and the thickness of the AlGaN layer is 10nm-40nm.

[0011] Optionally, the value range of N is 2 to 200.

[0012] Optionally, the superlattice structure is doped with acceptor doping elements to reduce the free electron concentration.

[0013] Optionally, the acceptor doping element includes at least one of iron (Fe) and carbon (C).

[0014] Optionally, in the superlattice structure, at least one of the AlN / GaN / AlGaN stacked structures is doped with an acceptor doping element.

[0015] Optionally, in the AlN / GaN / AlGaN stacked structure, at least one of the AlN layer, the GaN layer or the AlGaN layer is doped with an acceptor doping element.

[0016] Optionally, the acceptor doping element includes carbon, and the carbon doping concentration is 1×10 17 / cm 3 ~1×10 20 / cm 3 .

[0017] The present invention also provides a buffer layer, comprising: the above-mentioned superlattice structure.

[0018] Optionally, the buffer layer further includes: an AlGaN transition layer, which is arranged below the superlattice structure.

[0019] Optionally, the Al component content in the AlGaN transition layer is not lower than the Al component content in the AlGaN layer of the superlattice structure.

[0020] Optionally, the Al content in the AlGaN transition layer is 50%-80%.

[0021] Optionally, the Al component content in the AlGaN transition layer remains constant; or, the Al component content in the AlGaN transition layer gradually decreases from bottom to top along the growth direction.

[0022] Optionally, the Al component content in the AlGaN transition layer gradually decreases from bottom to top along the growth direction.

[0023] Optionally, the buffer layer comprises at least two stacked AlGaN layers, and the Al component content in the AlGaN layers decreases gradually toward the superlattice direction.

[0024] The present invention also provides a GaN HEMT epitaxial wafer, comprising: substrate; The above-mentioned buffer layer; GaN channel; AlGaN barrier layer; Wherein, the GaN channel and the AlGaN barrier layer form a GaN / AlGaN heterojunction.

[0025] Optionally, the material of the substrate is at least one of SiC or Si.

[0026] Optionally, the epitaxial wafer further comprises an AlN nucleation layer, which is arranged between the substrate and the buffer layer.

[0027] Optionally, the epitaxial wafer further includes: a GaN semi-insulating layer, which is arranged between the buffer layer and the GaN channel.

[0028] The present invention also provides a method for growing a GaN HEMT epitaxial wafer, comprising: providing a substrate; forming the above-mentioned buffer layer on the substrate; A GaN channel and an AlGaN barrier layer are sequentially formed on the buffer layer.

[0029] Compared with the prior art, the technical solution of the present invention has at least the following beneficial effects: The improved buffer layer of the present invention comprises an improved superlattice structure, wherein the superlattice structure comprises: N AlN / GaN / AlGaN stacked structures, each of which comprises an AlN layer, a GaN layer and an AlGaN layer stacked and grown in sequence from bottom to top, that is, a step of forming a GaN layer is inserted after forming the AlN layer and before forming the AlGaN layer, so that, on the one hand, a part of the stress caused by the AlN / AlGaN lattice mismatch can be offset, and the difficulty of stress control can be reduced; on the other hand, the GaN growth process can improve the surface defects of the AlN layer, which is conducive to the formation of the AlGaN layer, and reduces the surface defects caused by the poor lateral migration ability of atoms during the growth of AlGaN. The improved superlattice structure of the present invention is used to form a buffer layer, which has fewer surface defects, can effectively reduce the leakage current of the epitaxial layer, improve the reliability of the device, and is easy to control in production, and has a low production cost.

[0030] Furthermore, the superlattice structure of the present invention comprises: a first superlattice structure and a second superlattice structure, wherein the second superlattice structure is arranged above the first superlattice structure, and the Al component content in the AlGaN layer of the first superlattice structure is greater than the Al component content in the AlGaN layer of the second superlattice structure. In the buffer layer, the Al component content in the AlGaN layer of the superlattice structure gradually decreases from bottom to top along the growth direction, which is beneficial to reducing the difficulty of stress control in epitaxial layer growth, improving surface defects of the epitaxial layer, and improving device quality. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 A schematic diagram of the structure of an AlGaN / GaN HEMT device.

[0032] Figure 2 It is a schematic structural diagram of a GaN HEMT epitaxial wafer of the present invention.

[0033] Figure 3 It is a schematic structural diagram of a superlattice structure of the present invention.

[0034] Figure 4 It is a schematic diagram of the structure of another superlattice structure of the present invention.

[0035] Figure 5 It is a schematic structural diagram of a buffer layer of the present invention.

[0036] Figure 6 It is a schematic diagram of the structure of another buffer layer of the present invention.

[0037] Figure 7 It is a schematic structural diagram of another GaN HEMT epitaxial wafer of the present invention.

[0038] Figure 8 The present invention is a flow chart of a method for growing a GaN HEMT epitaxial wafer.

[0039] Fig. 9 The schematic diagram is a comparison of the optical microscope (OM) scanning results of the epitaxial wafers prepared in the embodiment and the comparative example of the present invention, wherein a is the epitaxial wafer prepared in the comparative example, and b is the epitaxial wafer prepared in the embodiment.

[0040] Fig.10 The schematic diagram is a comparison of the scanning results of the epitaxial wafers prepared in the embodiment of the present invention and the comparative example through a scanning electron microscope (SEM), wherein the SEM acceleration voltage is 10 kV, the magnification is 2.0k times, a is the epitaxial wafer prepared in the comparative example, and b is the epitaxial wafer prepared in the embodiment.

[0041] Figure ID: Substrate layer 1 Silicon substrate 10 AlN nucleation layer 10a GaN buffer layer 2 Buffer layer 20 GaN semi-insulating layer 20a Superlattice structure 21 First superlattice structure 21a Second superlattice structure 21b AlN / GaN / AlGaN stacked structure 210 AlGaN transition layer 22 GaN channel 3, 30 AlGaN barrier layers 4, 40 Two-dimensional electron gas 5, 50 Gate G Source S Drain D Micro defect 901. DETAILED DESCRIPTION

[0042] The technical solution of the present invention will be described clearly and completely below in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0043] In the description of the present invention, it should be noted that the terms "upper", "lower", "left", "right", "vertical", "horizontal", "inner", "outer", etc., indicating the orientation or positional relationship, are based on the orientation or positional relationship shown in the drawings, and are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as limiting the present invention. In addition, the terms "first", "second", and "third" are used for descriptive purposes only, and cannot be understood as indicating or implying relative importance.

[0044] In the description of the present invention, it should be noted that, unless otherwise clearly specified and limited, the terms "installed", "connected", and "connected" should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection; it can be a direct connection, or it can be indirectly connected through an intermediate medium, or it can be the internal communication of two components. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0045] The “epitaxial layer” described in this article includes a buffer layer, a GaN channel, an AlGaN barrier layer, an AlN nucleation layer, a GaN semi-insulating layer and other layer structures with different functions grown on a substrate by epitaxial technology.

[0046] The "superlattice" described in this article is a special microstructure material, which is composed of thin layers of at least two different semiconductor materials stacked alternately. The thickness of these thin layers can be at the nanometer level and have a very regular periodic arrangement. This structure can provide some physical properties that traditional single crystal materials do not have, so it has important applications in electronic and optoelectronic devices.

[0047] The "Al content" mentioned herein refers to the molar content or the atomic ratio.

[0048] like Figure 1 As shown, the basic structure of the AlGaN / GaN HEMT device, the bottom layer of which is the substrate layer 1, which serves as a carrier for the epitaxial growth of the device; a GaN buffer layer 2, a GaN channel 3, and an AlGaN barrier layer 4 are epitaxially grown from bottom to top on the surface of the substrate layer 1, wherein an AlGaN / GaN heterojunction is formed at the contact interface between GaN and AlGaN, and a two-dimensional electron gas 5 is formed on one side of the GaN channel 3; finally, a gate (G) of a Schottky contact is deposited on the AlGaN barrier layer 4; the source (S) and the drain (D) are highly doped and connected to the two-dimensional electron gas 5 in the channel to form an ohmic contact.

[0049] In order to increase the breakdown voltage of the epitaxial layer of GaN HEMT devices in the vertical direction, the following two methods can be adopted: one is to increase the thickness of the epitaxial layer, and the other is to reduce the leakage current of the GaN buffer layer. However, increasing the thickness of the epitaxial layer may bring about the difficulty of stress control and lead to increased costs. Therefore, reducing the leakage of the buffer layer becomes a more cost-effective solution.

[0050] The main method to reduce buffer layer leakage includes doping aluminum (Al) in the buffer layer, such as using an AlGaN buffer layer or an AlN / GaN superlattice buffer layer, especially using a superlattice as a buffer layer, which has a more obvious effect. However, in order to obtain a higher withstand voltage value under the same thickness of the epitaxial layer (that is, without increasing the thickness of the epitaxial layer), it is necessary to increase the doping amount of aluminum (Al), such as using an AlN / AlGaN superlattice instead of an AlN / GaN superlattice.

[0051] However, the Al content of AlGaN in the AlN / AlGaN superlattice is generally less than 50%. When the AlGaN layer is grown directly on the surface of the AlN layer, the AlGaN layer is subjected to greater compressive stress due to the large lattice mismatch, and the AlGaN layer has poor lateral atomic migration ability during growth, resulting in more surface defects in the grown epitaxial buffer layer. Therefore, it is difficult to grow the AlN / AlGaN superlattice and there are many surface defects, which is a technical challenge that needs to be overcome.

[0052] The study found that at the working temperature of epitaxial growth (generally 900℃~1200℃), AlN mainly grows in three-dimensional growth, and the surface is prone to unevenness, while AlGaN grows in a coexistence of three-dimensional growth and two-dimensional growth. Although two-dimensional growth is dominant, its atomic lateral migration ability is poor, so it is difficult to generate AlN / AlGaN superlattice structure, and there are many surface defects. GaN grows in two-dimensional growth, and its atomic lateral migration ability is high. When a GaN layer is first generated on the AlN surface, the two-dimensionally grown GaN can effectively fill the concave defects on the AlN surface, and the surface of the epitaxial layer formed is relatively smooth, which can not only synergistically promote the subsequent growth of AlGaN, but also generate fewer surface defects in the AlGaN.

[0053] To this end, the present invention improves the formation method of the superlattice structure, inserts a GaN layer between the AlN layer and the AlGaN layer, and the GaN layer can not only effectively improve the roughness of the AlN layer surface, but also the GaN layer located between the AlN layer and the AlGaN layer can offset part of the compressive stress, which can reduce the stress defects of the superlattice structure. In addition, when GaN grows on the surface of the AlN layer, the atomic lateral migration ability is stronger, and it is easy to obtain a smoother surface, which is conducive to promoting the growth of AlGaN and improving the surface defects of the AlGaN layer. The following is an explanation in conjunction with the accompanying drawings.

[0054] like Figure 2 As shown, a GaN HEMT epitaxial wafer of the present invention comprises: a silicon substrate 10; and a buffer layer 20, a GaN channel 30 and an AlGaN barrier layer 40 sequentially grown on the substrate. The GaN channel 30 contacts the AlGaN barrier layer 40 to form a heterojunction, and a potential well is formed at the heterojunction interface through piezoelectric polarization and spontaneous polarization effects, causing electrons to accumulate at the interface to form a two-dimensional electron gas (2DEG) 50, and the 2DEG can form a channel between the drain and source of the power device.

[0055] The buffer layer 20 at least includes a superlattice structure 21 for reducing background carrier concentration and leakage current.

[0056] like Figure 3 As shown, the superlattice structure 21 of the present invention includes: N AlN / GaN / AlGaN stacked structures 210, wherein the AlN / GaN / AlGaN stacked structures 210 include: AlN layers, GaN layers, and AlGaN layers stacked and grown sequentially from bottom to top, wherein N is an integer greater than 1 and can be adjusted according to the quality requirements or thickness requirements of the superlattice structure. In some embodiments, the superlattice structure includes 2-200 AlN / GaN / AlGaN stacked structures 210.

[0057] In the AlN / GaN / AlGaN stacked structure 210 , the thickness of the AlN layer is 2 nm to 15 nm, the thickness of the GaN layer is less than or equal to 5 nm, and the thickness of the AlGaN layer is 10 nm to 40 nm.

[0058] The higher the concentration of Al in the AlGaN layer, the higher the breakdown voltage of the formed epitaxial layer, but the crystal quality may deteriorate. As an example, the Al component content is 5% to 40% by molar ratio.

[0059] like Figure 4As shown, in some embodiments, the superlattice structure 21 at least includes: a first superlattice structure 21a and a second superlattice structure 21b, wherein the second superlattice structure 21b is disposed above the first superlattice structure 21a, and the Al component content in the AlGaN layer of the first superlattice structure 21a is greater than the Al component content in the AlGaN layer of the second superlattice structure 21b. The first superlattice structure 21a includes m AlN / GaN / AlGaN stacked structures 210, and the second superlattice structure 21b includes n AlN / GaN / AlGaN stacked structures 210, where m and n are the same or different, and are both integers greater than 0, and N=m+n.

[0060] In order to further reduce the leakage current, the superlattice structure can be doped with some specific elements, such as acceptor doping elements, to increase the acceptors to capture more free electrons, thereby reducing the leakage current. The doping elements include any one of iron (Fe) and carbon (C). In this example, the superlattice structure can be pre-doped with carbon (C), and the doped C can act as an acceptor to capture free electrons, thereby reducing free electrons and reducing leakage current. However, too high a doping concentration may cause an increase in the crystal structure defects of the material, affecting the performance of the device. The appropriate doping concentration can optimize the electrical properties of the semiconductor, making it more suitable for specific application requirements. As an example, the C doping concentration is 1×10 17 / cm 3 ~1×10 20 / cm 3 .

[0061] In some embodiments, at least one of the AlN / GaN / AlGaN stacked structures 210 is C-doped.

[0062] In some embodiments, in the AlN / GaN / AlGaN stacked structure 210, at least one of the AlN layer, the GaN layer, and the AlGaN layer is doped with C (carbon). As an example, in an AlN / GaN / AlGaN stacked structure, only the AlN layer may be doped with C, only the GaN layer may be doped with C, only the AlGaN layer may be doped with C, or both the AlN layer and the GaN layer may be doped with C, and the AlN layer, the GaN layer, and the AlGaN layer may be doped with C.

[0063] like Figure 5As shown, a buffer layer 20 of the present invention is composed of the superlattice structure 21 and the AlGaN transition layer 22, and the AlGaN transition layer 22 is arranged below the superlattice structure 21 and closer to the silicon substrate 10. The AlGaN transition layer 22 can effectively control the bowing problem that may be generated by the epitaxial growth of the superlattice structure 21 and reduce cracks. The superlattice structure 21 can include N AlN / GaN / AlGaN stacked structures 210; or can include at least a first superlattice structure 21a and a second superlattice structure 21b.

[0064] The Al content in the AlGaN transition layer 22 is not less than the Al content in the AlGaN layer of the superlattice structure 21. As an example, the Al content in the AlGaN transition layer 22 may be 50% to 80%.

[0065] In some embodiments, the content of the Al component in the AlGaN transition layer 22 may be constant.

[0066] In some embodiments, the content of the Al component in the AlGaN transition layer 22 gradually decreases from the surface of the silicon substrate 10 to the top. For example, in the process of epitaxially growing the AlGaN transition layer 22, a concentration gradient AlGaN layer can be formed by gradually reducing the molar ratio of the aluminum source.

[0067] like Figure 6 As shown, the AlGaN transition layer 22 includes M AlGaN layers with different concentrations, and the content of the Al component decreases in the direction away from the silicon substrate 10 (i.e., the direction close to the superlattice structure 21). The AlGaN layers with different concentrations can be grown using the same process or different processes according to product requirements.

[0068] like Figure 7 As shown, in order to prevent gallium (Ga) from corroding and melting back silicon (Si) in the silicon substrate 10 during epitaxial growth, the GaN HEMT epitaxial wafer of the present invention further includes an AlN nucleation layer 10a, which is disposed between the silicon substrate 10 and the buffer layer 20. The AlN nucleation layer 10a also plays a role in reducing stress defects caused by lattice mismatch between the silicon substrate 10 and the buffer layer 20.

[0069] In some embodiments, the GaN HEMT epitaxial wafer of the present invention further comprises: a GaN semi-insulating layer 20a, which is disposed between the buffer layer 20 and the GaN channel 30. In order to increase the resistivity of the GaN semi-insulating layer 20a and reduce leakage current, carbon (C) or iron (Fe) elements may be doped into the GaN semi-insulating layer 20a.

[0070] like Figure 8 As shown, the present invention provides a method for growing a GaN HEMT epitaxial wafer, comprising: Step S1, providing a substrate.

[0071] The material of the substrate may be silicon carbide (SiC) or silicon (Si) material, and in this example, silicon.

[0072] Step S2, forming the buffer layer on the silicon substrate; the buffer layer at least comprises a superlattice structure, and the superlattice structure comprises a plurality of AlN / GaN / AlGaN stacked structures.

[0073] The aluminum source material for forming the AlN / GaN / AlGaN stacked structure may be trimethylaluminum (TMAl), the gallium source material may be trimethylgallium (TMG), and the nitrogen source material may be ammonia (NH3); the process temperature is less than 1300°C, and as an example may be 900°C~1200°C.

[0074] In some embodiments, the superlattice structure is prepared by metal organic chemical vapor deposition (MOCVD) technology, and the superlattice structure is formed by alternately growing AlN layers, GaN layers and AlGaN layers on a substrate. In the MOCVD process, high-quality superlattice materials can be grown by precisely controlling parameters such as temperature, pressure, and flow rate. Optionally, in order to better control the growth of the superlattice and reduce stress, pulsed MOCVD technology may be used to optimize the growth process by adjusting the supply mode of the source material.

[0075] In some embodiments, a superlattice structure is grown using molecular beam epitaxy (MBE) technology, and by precisely controlling the evaporation rates of various source materials, a superlattice including multiple AlN / GaN / AlGaN stacked structures can be deposited layer by layer on a substrate.

[0076] In some embodiments, during the process of growing the superlattice structure, C may be doped. The C doping source is selected from carbon tetrachloride (CCl4), carbon bromide (CBr4), methane (CH4), ethane (C2H6), propane (C3H8) or butane (C4H 10 ) and other carbon-containing compounds.

[0077] Step S3, forming a GaN channel and an AlGaN barrier layer in sequence on the buffer layer.

[0078] The GaN channel 30 and the AlGaN barrier layer 40 may be grown in sequence by metal organic compound chemical vapor deposition or molecular beam epitaxy growth technology to form an AlGaN / GaN heterojunction.

[0079] Comparative Example A silicon substrate is provided, trimethylaluminum (TMAl) is used as an Al source; trimethylgallium (TMG) is used as a Ga source; ammonia (NH3) is used as a nitrogen source; the process temperature is 900°C to 1200°C; a 5nm AlN layer and a 30nm AlGaN layer are sequentially grown on the silicon substrate by MOCVD, and the process is repeated 50 times to form a superlattice structure. The superlattice structure is used as a buffer layer. GaN is grown as a GaN channel and AlGaN is grown as an AlGaN barrier layer on the buffer layer by MOCVD technology to obtain an epitaxial wafer.

[0080] The epitaxial wafer prepared above was scanned by optical microscope (OM), and the results were as follows Fig. 9 a, 901 represents a micro pit. It can be seen that the surface of the epitaxial wafer is densely covered with micro pits 901. The epitaxial wafer is observed by electron microscope. The results are as follows Fig.10 As shown in a, there are many hexagonal defects on the surface of the epitaxial wafer.

[0081] Example A silicon substrate is provided, trimethylaluminum (TMAl) is used as an Al source; trimethylgallium (TMG) is used as a Ga source; ammonia (NH3) is used as a nitrogen source, and C3H8 is used as a C source; the process temperature is 900°C to 1200°C; a 5nm AlN layer, a 2nm GaN layer, and a 28nm AlGaN layer are sequentially grown on the silicon substrate by MOCVD, and the process is repeated 50 times to form a superlattice structure. The superlattice structure is used as a buffer layer. GaN is grown as a GaN channel and AlGaN is grown as an AlGaN barrier layer on the buffer layer by MOCVD technology to obtain an epitaxial wafer.

[0082] The epitaxial wafer prepared above was scanned by optical microscope (OM), and the results were as follows Fig. 9 b, 901 represents a micro pit. It can be seen that the micro pit 901 on the surface of the epitaxial wafer prepared in this embodiment is significantly reduced. The epitaxial wafer is observed by electron microscope. The results are as follows Fig.10 As shown in b, no hexagonal defects are observed.

[0083] It can be seen that compared with the comparative example, the epitaxial wafer prepared in the embodiment of the present invention has significantly fewer surface defects, which can significantly reduce the leakage current of the epitaxial wafer and improve the device quality.

[0084] In summary, the superlattice structure prepared by the present invention inserts a GaN layer between the AlN layer and the AlGaN layer. The GaN layer can not only reduce the surface defects of the AlN layer, but also offset part of the compressive stress and reduce stress defects. In addition, the atomic lateral migration ability of GaN during growth is stronger, so that the surface defects of the epitaxial layer can be significantly improved. The superlattice structure prepared by the present invention can be used for lateral structure GaN HEMT.

[0085] Although the content of the present invention has been described in detail through the above preferred embodiments, it should be appreciated that the above description should not be considered as a limitation of the present invention. After reading the above content, it will be apparent to those skilled in the art that various modifications and substitutions of the present invention will occur. Therefore, the protection scope of the present invention should be limited by the appended claims.

Claims

1. A superlattice structure for a GaN HEMT, characterized in that: The invention comprises: N AlN / GaN / AlGaN stacked structures, wherein the AlN / GaN / AlGaN stacked structures comprise an AlN layer, a GaN layer and an AlGaN layer stacked and grown in sequence from bottom to top, and N is an integer greater than 1.

2. The superlattice structure for GaN HEMT according to claim 1, characterized in that: The Al content in the AlGaN layer is 5% to 40% by mole ratio.

3. The superlattice structure for GaN HEMT according to claim 1, characterized in that: The superlattice structure at least comprises: a first superlattice structure and a second superlattice structure, wherein the second superlattice structure is arranged above the first superlattice structure, and the Al component content in the AlGaN layer of the first superlattice structure is greater than the Al component content in the AlGaN layer of the second superlattice structure; The first superlattice structure includes m AlN / GaN / AlGaN stacked structures, and the second superlattice structure includes n AlN / GaN / AlGaN stacked structures, where m and n are the same or different and are both integers greater than 0.

4. The superlattice structure for GaN HEMT according to claim 1, characterized in that: In the AlN / GaN / AlGaN stacked structure, the thickness of the AlN layer is 2nm-15nm, the thickness of the GaN layer is less than or equal to 5nm, and the thickness of the AlGaN layer is 10nm-40nm.

5. The superlattice structure for GaN HEMT according to claim 1, characterized in that: The value range of N is 2~200.

6. The superlattice structure for GaN HEMT according to any one of claims 1 to 5, characterized in that: The superlattice structure is doped with acceptor doping elements to reduce the free electron concentration.

7. The superlattice structure for GaN HEMT according to claim 6, characterized in that: The acceptor doping element includes at least one of iron and carbon.

8. The superlattice structure for GaN HEMT according to claim 6, characterized in that: In the superlattice structure, at least one of the AlN / GaN / AlGaN stacked structures is doped with an acceptor doping element.

9. The superlattice structure for GaN HEMT according to claim 8, characterized in that: In the AlN / GaN / AlGaN stacked structure, at least one of the AlN layer, the GaN layer or the AlGaN layer is doped with an acceptor doping element.

10. The superlattice structure for GaN HEMT according to claim 8, characterized in that: The acceptor doping element includes carbon, and the carbon doping concentration is 1×10 17 / cm 3 ~1×10 20 / cm 3 .

11. A buffer layer, characterized in that: Comprising: a superlattice structure for GaN HEMT as claimed in any one of claims 1 to 10.

12. The buffer layer according to claim 11, characterized in that It also includes: at least one AlGaN transition layer, which is arranged below the superlattice structure.

13. The buffer layer according to claim 12, characterized in that The Al content in the AlGaN transition layer is not lower than the Al content in the AlGaN layer of the superlattice structure.

14. The buffer layer according to claim 13, characterized in that The Al content in the AlGaN transition layer is 50%-80%.

15. The buffer layer according to claim 13, characterized in that The Al component content in the AlGaN transition layer remains constant; or, the Al component content in the AlGaN transition layer gradually decreases from bottom to top along the growth direction.

16. The buffer layer according to any one of claims 12 to 15, characterized in that: The buffer layer comprises at least two stacked AlGaN layers, and the Al component content in the AlGaN layers decreases in sequence toward the superlattice direction.

17. A GaN HEMT epitaxial wafer, characterized in that: Include: substrate; The buffer layer according to any one of claims 11 to 16; GaN channel; AlGaN barrier layer; Wherein, the GaN channel and the AlGaN barrier layer form a GaN / AlGaN heterojunction.

18. The GaN HEMT epitaxial wafer according to claim 17, wherein: The material of the substrate is at least one of SiC and Si.

19. The GaN HEMT epitaxial wafer according to claim 17, wherein: The invention also comprises an AlN nucleation layer disposed between the substrate and the buffer layer.

20. The GaN HEMT epitaxial wafer according to claim 17, wherein: The invention also comprises: a GaN semi-insulating layer, which is arranged between the buffer layer and the GaN channel.

21. A method for growing a GaN HEMT epitaxial wafer, characterized in that: Include: providing a substrate; forming a buffer layer as claimed in any one of claims 11 to 16 on the substrate; A GaN channel and an AlGaN barrier layer are sequentially formed on the buffer layer.

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