CN119403191A - Superlattice structure, buffer layer, GaN HEMT epitaxial wafer and growth method for GaN HEMT - Google Patents

Abstract

The invention discloses a superlattice structure for a GaN HEMT, a buffer layer, a GaN HEMT epitaxial wafer and a growth method. The superlattice structure comprises N AlN/GaN/AlGaN laminated structures, wherein the AlN/GaN/AlGaN laminated structures comprise AlN layers, gaN layers and AlGaN layers which are sequentially stacked and grown from bottom to top, and N is an integer larger than 1. The superlattice structure prepared by the invention is characterized in that the GaN layer is inserted between the AlN layer and the AlGaN layer, so that the GaN layer can not only reduce the surface defects of the AlN layer, but also offset a part of compressive stress and reduce stress defects, and the atomic lateral migration capability of GaN in growth is stronger, so that the surface defects of the epitaxial layer can be obviously improved.

Description

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

Technical Field

The invention belongs to the technical field of semiconductor devices, and particularly relates to a superlattice structure for a GaN HEMT, a buffer layer, a GaN HEMT epitaxial wafer and a growth method.

Background

Gallium nitride high electron mobility transistor (GaN HEMT) is an electronic device based on gallium nitride with wide forbidden band semiconductor material, has the characteristics of high power density, high efficiency, high frequency operation and fast switching, is mainly applied to the fields of power adapters, vehicle-mounted charging, data centers and the like, and is gradually becoming the best solution of a 5G base station power supply.

The GaN HEMT includes an AlGaN/GaN heterojunction in which a high-density electron layer, i.e., a two-dimensional electron gas (2 DEG), is formed at the interface between the AlGaN layer and the GaN layer due to piezoelectric polarization and spontaneous polarization effects. The application of the drain-source voltage can generate a transverse electric field, and the two-dimensional electron gas is transported along the AlGaN/GaN heterojunction interface under the action of the transverse electric field to form drain output current. The grid electrode and the AlGaN barrier layer are in Schottky contact, and the depth of a potential well in the AlGaN/GaN heterojunction can be controlled by adjusting the voltage of the grid electrode, so that the output current of the drain electrode is controlled.

The breakdown voltage refers to the rated maximum voltage which can be born by the two ends of the source and the drain of the device in the GaN HEMT technology, and is the maximum voltage value for measuring the safe operation of the device. Increasing the breakdown voltage means that the device is able to withstand higher voltages, which is important in many high power applications, such as power management, electric vehicles, and renewable energy technologies.

In a GaN HEMT device, it is important to increase the breakdown voltage of the epitaxial layer in the vertical direction, as it determines the upper voltage withstand of the device.

Disclosure of Invention

The invention aims to improve the vertical withstand voltage characteristic of a GaN HEMT device with low cost, and by improving the structure and the process of the buffer layer, the surface defect of the epitaxial layer is reduced, the leakage current is reduced, the breakdown voltage in the vertical direction is improved, the problem of difficult epitaxial growth is also solved, and the production cost is reduced.

In order to achieve the above purpose, the invention provides a superlattice structure for a GaN HEMT, which comprises N AlN/GaN/AlGaN laminated structures, wherein the AlN/GaN/AlGaN laminated structures comprise AlN layers, gaN layers and AlGaN layers which are sequentially stacked and grown from bottom to top, and N is an integer greater than 1.

Optionally, the component content of Al in the AlGaN layer is 5% -40%, and the molar ratio is calculated.

Optionally, 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, the Al component content in the AlGaN layer of the first superlattice structure is larger than the Al component content in the AlGaN layer of the second superlattice structure, the first superlattice structure comprises m AlN/GaN/AlGaN laminated structures, the second superlattice structure comprises n AlN/GaN/AlGaN laminated structures, and m is the same as n or different from n and is an integer larger than 0.

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

Optionally, the value range of N is 2-200.

Optionally, acceptor doping elements are doped in the superlattice structure to reduce the free electron concentration.

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

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

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

Optionally, the acceptor doping element comprises carbon, and the carbon doping concentration is 1×10 ¹⁷/cm³~1×10²⁰/cm³.

The invention also provides a buffer layer which comprises the superlattice structure.

Optionally, the buffer layer further comprises an AlGaN transition layer disposed below the superlattice structure.

Optionally, the composition content of Al in the AlGaN transition layer is not lower than the composition content of Al in the AlGaN layer of the superlattice structure.

Optionally, the component content of Al in the AlGaN transition layer is 50% -80%.

Optionally, the component content of Al in the AlGaN transition layer is constant, or the component content of Al in the AlGaN transition layer is gradually reduced from bottom to top along the growth direction.

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

Optionally, the buffer layer includes at least two AlGaN layers stacked, and the content of Al in the AlGaN layers decreases in order toward the superlattice.

The invention also provides a GaN HEMT epitaxial wafer, which comprises:

A substrate;

The buffer layer;

A GaN channel;

an AlGaN barrier layer;

wherein, the GaN channel and the AlGaN barrier layer form a GaN/AlGaN heterojunction.

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

Optionally, the epitaxial wafer further comprises an AlN nucleation layer disposed between the substrate and the buffer layer.

Optionally, the epitaxial wafer further comprises a GaN semi-insulating layer arranged between the buffer layer and the GaN channel.

The invention also provides a growth method of the GaN HEMT epitaxial wafer, which comprises the following steps:

providing a substrate;

Forming the buffer layer on the substrate;

and forming a GaN channel and an AlGaN barrier layer on the buffer layer in sequence.

Compared with the prior art, the technical scheme of the invention has at least the following beneficial effects:

The improved buffer layer comprises an improved superlattice structure, wherein 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 from bottom to top, namely, the step of forming the GaN layer is inserted after the AlN layer is formed and before the AlGaN layer is formed, so that on one hand, stress caused by lattice mismatch of the AlN/AlGaN can be counteracted, the difficulty of stress control is reduced, and on the other hand, the surface defect of the AlN layer can be improved in the GaN growing process, the AlGaN layer is formed, and the surface defect caused by poor transverse migration capability of atoms during AlGaN growth is reduced. The buffer layer is formed by adopting the improved superlattice structure, has fewer surface defects, can effectively reduce the leakage current of the epitaxial layer, improves the reliability of the device, and has easy control of production and lower production cost.

Further, the superlattice structure 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 an AlGaN layer of the first superlattice structure is larger than the Al component content in an AlGaN layer of the second superlattice structure. In the buffer layer, the content of Al components in the AlGaN layer from bottom to top in the growth direction of the superlattice structure is gradually reduced, so that the stress control difficulty of epitaxial layer growth is reduced, the surface defect of the epitaxial layer is improved, and the quality of a device is improved.

Drawings

Fig. 1 is a schematic structural diagram of an AlGaN/GaN HEMT device.

Fig. 2 is a schematic structural diagram of a GaN HEMT epitaxial wafer according to the present invention.

Fig. 3 is a schematic structural diagram of a superlattice structure in accordance with the invention.

Fig. 4 is a schematic structural diagram of another superlattice structure in accordance with the invention.

Fig. 5 is a schematic structural diagram of a buffer layer according to the present invention.

FIG. 6 is a schematic diagram of another buffer layer according to the present invention.

Fig. 7 is a schematic structural diagram of a GaN HEMT epitaxial wafer according to still another embodiment of the present invention.

Fig. 8 is a flowchart of a method for growing a GaN HEMT epitaxial wafer according to the present invention.

Fig. 9 is a schematic diagram showing comparison of scanning results of an Optical Microscope (OM) of epitaxial wafers prepared in examples and comparative examples according to the present invention, wherein a is an epitaxial wafer prepared in comparative example, and b is an epitaxial wafer prepared in example.

Fig. 10 is a schematic diagram showing comparison of scanning results of the epitaxial wafers prepared in the examples and the comparative examples according to the present invention by a Scanning Electron Microscope (SEM), wherein the SEM acceleration voltage is 10kV, the magnification is 2.0k times, a is the epitaxial wafer prepared in the comparative examples, and b is the epitaxial wafer prepared in the examples.

The attached drawings are identified:

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 laminated structure 210

AlGaN transition layer 22

GaN channels 3, 30

AlGaN barrier layers 4, 40

Two-dimensional electron gas 5, 50

Grid G

Source S

Drain electrode D

Micro defect 901.

Detailed Description

The following description of the embodiments of the present invention will be made apparent and fully in view of the accompanying drawings, in which some, but not all embodiments of the invention are shown. All other embodiments, which can be made by those skilled in the art based on the embodiments of the invention without making any inventive effort, are intended to be within the scope of the invention.

In the description of the present invention, it should be noted that the directions or positional relationships indicated by the terms "upper", "lower", "left", "right", "vertical", "horizontal", "inner", "outer", etc. are directions or positional relationships based on the drawings, are merely for convenience of describing the present invention and simplifying the description, and do not indicate or imply that the apparatus or elements referred to must have a specific orientation, be constructed and operated in a specific orientation, and thus should not be construed as limiting the present invention. Furthermore, the terms "first," "second," and "third" are used for descriptive purposes only and are not to be construed as indicating or implying relative importance.

In the description of the present invention, unless explicitly specified and limited otherwise, the terms "mounted," "connected," and "connected" are to be construed broadly, and may be, for example, fixedly connected, detachably connected, or integrally connected, mechanically connected, directly connected, indirectly connected via an intermediate medium, or in communication between two elements. The specific meaning of the above terms in the present invention will be understood in specific cases by those of ordinary skill in the art.

The term "epitaxial layer" as used herein includes various functional layer structures such as buffer layers, gaN channels, alGaN barrier layers, alN nucleation layers, gaN semi-insulating layers, etc., grown on a substrate by epitaxial techniques.

The "superlattice" as described herein is a particular microstructured material formed by alternating thin layers of at least two different semiconductor materials, which may be on the order of nanometers thick, and have a very regular periodic arrangement. Such a structure can provide physical properties that are not possessed by some conventional single crystal materials, and thus has important applications in electronic and optoelectronic devices.

As used herein, "component content of Al" refers to the molar content, or atomic ratio.

As shown in fig. 1, the basic structure of an AlGaN/GaN HEMT device is characterized in that the bottommost layer is a substrate layer 1 which is used as a carrier for epitaxial growth of the device, a GaN buffer layer 2, a GaN channel 3 and an AlGaN barrier layer 4 are sequentially grown on the surface of the substrate layer 1 from bottom to top in an epitaxial manner, wherein an AlGaN/GaN heterojunction is formed at a contact interface of GaN and AlGaN, two-dimensional electron gas 5 is formed at one side of the GaN channel 3, a grid electrode (G) which is in Schottky contact is deposited on the AlGaN barrier layer 4, and a source electrode (S) and a drain electrode (D) are subjected to high-concentration doping and are connected with the two-dimensional electron gas 5 in the channel to form ohmic contact.

In order to improve the breakdown voltage of the epitaxial layer of the GaN HEMT device in the vertical direction, two methods can be adopted, namely, the thickness of the epitaxial layer is increased, and the leakage current of the GaN buffer layer is reduced. However, increasing the epitaxial layer thickness may present a difficult problem of stress control and result in increased costs. Thus, reducing buffer layer leakage becomes a more cost effective solution.

The main method for reducing the leakage of the buffer layer comprises doping aluminum (Al) in the buffer layer, such as an AlGaN buffer layer or an AlN/GaN superlattice buffer layer, and particularly, the buffer layer is formed by adopting a superlattice, so that the effect is obvious. However, in order to obtain a higher withstand voltage value under the same thickness of the epitaxial layer (i.e., without additionally increasing the thickness of the epitaxial layer), it is necessary to increase the doping amount of aluminum (Al), for example, to use an AlN/AlGaN superlattice instead of an AlN/GaN superlattice.

However, the Al component content of AlGaN in AlN/AlGaN superlattice is generally less than 50%. When an AlGaN layer is directly grown on the surface of an AlN layer, the AlGaN layer is subjected to larger compressive stress due to larger lattice mismatch, and the AlGaN layer is poor in atom lateral migration capability during growth, so that the grown epitaxial layer buffer layer is more in surface defects. Therefore, the difficulty of growing AlN/AlGaN superlattice is high, and the surface defects are high, which is a technical challenge to be overcome.

The research shows that at the working temperature of epitaxial growth (generally 900-1200 ℃), alN mainly grows in a three-dimensional mode, concave-convex fluctuation is easy to form on the surface, alGaN mainly grows in a two-dimensional mode and coexists with the three-dimensional growth, but the atomic lateral migration capability is poor, so that the AlN/AlGaN superlattice structure is difficult to generate, and the surface defects are more. And GaN grows in a basically two-dimensional manner, and atoms have higher lateral migration capability. When a GaN layer is firstly generated on the AlN surface, the concave defects of the AlN surface can be effectively filled by the two-dimensional grown GaN, the formed epitaxial layer is smoother in surface, the subsequent AlGaN growth can be cooperatively promoted, and the generated AlGaN surface defects are fewer.

Therefore, the method for forming the superlattice structure is improved, the GaN layer is inserted between the AlN layer and the AlGaN layer, the roughness of the surface of the AlN layer can be effectively improved, part of compressive stress between the AlN layer and the AlGaN layer can be counteracted, the stress defect of the superlattice structure can be reduced, in addition, the atomic lateral migration capability of GaN in the growth process of the surface of the AlN layer is stronger, a flatter surface can be easily obtained, and the method is beneficial to promoting the growth of AlGaN and improving the surface defect of the AlGaN layer. The following description is made with reference to the accompanying drawings.

As shown in fig. 2, 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 grown on the substrate in this order. The GaN channel 30 contacts the AlGaN barrier layer 40 to form a heterojunction, forming a potential well at the heterojunction interface by piezoelectric polarization and spontaneous polarization effects, causing electrons to accumulate at the interface to form a two-dimensional electron gas (2 DEG) 50,2DEG that can form a channel between the drain and source of the power device.

The buffer layer 20 includes at least one superlattice 21 for reducing background carrier concentration and leakage current.

As shown in fig. 3, the superlattice structure 21 of the present invention includes N AlN/GaN/AlGaN stacked structures 210, where the AlN/GaN/AlGaN stacked structures 210 include an AlN layer, a GaN layer, and an AlGaN layer that are sequentially stacked from bottom to top, and N is an integer greater than 1, and can be adjusted according to the quality requirement or the thickness requirement of the superlattice structure. In some embodiments, the superlattice structure comprises 2-200 of the AlN/GaN/AlGaN stacked structures 210.

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

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 be deteriorated. As an example, the content of the Al component is 5% -40% in terms of molar ratio.

In some embodiments, as shown in fig. 4, the superlattice structure 21 at least comprises 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 composition content in the AlGaN layer of the first superlattice structure 21a is greater than the Al composition content in the AlGaN layer of the second superlattice structure 21 b. 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, each being an integer greater than 0, and n=m+n.

To further reduce leakage current, the superlattice structure may be doped with certain elements, such as acceptor doping elements, which add acceptors to trap more free electrons, thereby reducing leakage current. The doping element includes any one of iron (Fe) and carbon (C). In this example, the superlattice structure may be pretreated by doping carbon (C), and the doped C may serve as an acceptor to capture free electrons, thereby reducing free electrons and reducing leakage current. However, too high doping concentrations may result in increased crystal structure defects of the material, affecting the performance of the device. Appropriate doping concentrations may optimize the electrical properties of the semiconductor to better suit the specific application requirements. As an example, the C doping concentration is 1×10 ¹⁷/cm³~1×10²⁰/cm³.

In some embodiments, at least one of the AlN/GaN/AlGaN stack structures 210 is C doped.

In some embodiments, at least one of the AlN layer, the GaN layer, and the AlGaN layer in the AlN/GaN/AlGaN stacked structure 210 is subjected to C (carbon) doping treatment. As an example, in one AlN/GaN/AlGaN stack structure, only the AlN layer is C-doped, only the GaN layer is C-doped, only the AlGaN layer is C-doped, or both the AlN layer and the GaN layer are C-doped, and the AlN layer, the GaN layer, and the AlGaN layer are C-doped.

As shown in fig. 5, a buffer layer 20 of the present invention is composed of the superlattice structure 21 and an AlGaN transition layer 22, and the AlGaN transition layer 22 is disposed below the superlattice structure 21 and closer to the silicon substrate 10. The AlGaN transition layer 22 can effectively control the warpage (bowing) problem that may occur in the epitaxial growth of the superlattice structure 21, and reduce cracks. The superlattice structure 21 may include N AlN/GaN/AlGaN stacked structures 210, or may include at least a first superlattice structure 21a and a second superlattice structure 21b.

The Al composition content in the AlGaN transition layer 22 is not lower than the Al composition content in the AlGaN layer of the superlattice structure 21. As an example, the Al component in the AlGaN transition layer 22 may be 50% -80%.

In some embodiments, the Al composition in the AlGaN transition layer 22 may be constant.

In some embodiments, the Al content in the AlGaN transition layer 22 gradually decreases from bottom to top from the surface of the silicon substrate 10. For example, a graded concentration AlGaN layer may be formed by controlling the molar ratio of the aluminum source to be gradually reduced during the epitaxial growth of AlGaN transition layer 22.

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

As shown in fig. 7, in order to avoid the corrosion back melting of gallium (Ga) to silicon (Si) in the silicon substrate 10 during epitaxial growth, the GaN HEMT epitaxial wafer of the present invention further comprises an AlN nucleation layer 10a disposed between the silicon substrate 10 and the buffer layer 20. The AlN nucleation layer 10a also serves to reduce stress defects caused by lattice mismatch between the silicon substrate 10 and the buffer layer 20.

In some embodiments, the GaN HEMT epitaxial wafer of the invention further comprises a GaN semi-insulating layer 20a 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, the GaN semi-insulating layer 20a may be doped with carbon (C) or iron (Fe) element.

As shown in fig. 8, the present invention provides a method for growing a GaN HEMT epitaxial wafer, comprising:

step S1, a substrate is provided.

The material of the substrate may be silicon carbide (SiC) or silicon (Si) material, in this case silicon.

And S2, forming the buffer layer on the silicon substrate, wherein the buffer layer at least comprises a superlattice structure, and the superlattice structure comprises a plurality of AlN/GaN/AlGaN laminated structures.

The aluminum source material forming the AlN/GaN/AlGaN laminated structure can be Trimethylaluminum (TMAL), the gallium source material can be Trimethylgallium (TMG), the nitrogen source material can be ammonia gas (NH ₃), and the process temperature is less than 1300 ℃ and can be 900-1200 ℃ by way of example.

In some embodiments, the superlattice structure is prepared using metal-organic chemical vapor deposition (MOCVD) techniques by alternately growing AlN layers, gaN layers, and AlGaN layers on a substrate to form the superlattice structure. In the MOCVD process, high-quality superlattice materials can be grown by precisely controlling parameters such as temperature, pressure, flow and the like. Alternatively, to better control the growth of the superlattice and reduce stress, pulsed MOCVD techniques may be employed to optimize the growth process by adjusting the supply pattern of the source material.

In some embodiments, a superlattice structure is grown using Molecular Beam Epitaxy (MBE) techniques, and a superlattice comprising a plurality of AlN/GaN/AlGaN stacked structures may be deposited layer-by-layer on a substrate by precisely controlling the evaporation rates of the various source materials.

In some embodiments, C may be doped during the process of growing the superlattice structure. The C doping source is selected from carbon-containing compounds such as carbon tetrachloride (CCl ₄), carbon bromide (CBr ₄), methane (CH ₄), ethane (C ₂H₆), propane (C ₃H₈) or butane (C ₄H₁₀).

And S3, sequentially forming a GaN channel and an AlGaN barrier layer on the buffer layer.

The AlGaN/GaN heterojunction can be formed by growing the GaN channel 30, the AlGaN barrier layer 40 in sequence by metal organic chemical vapor deposition or molecular beam epitaxial growth techniques.

Comparative example

Providing a silicon substrate, using Trimethylaluminum (TMAL) as an Al source, trimethylgallium (TMG) as a Ga source, ammonia (NH ₃) as a nitrogen source, and sequentially growing a 5nm AlN layer and a 30nm AlGaN layer on the silicon substrate by MOCVD at the process temperature of 900-1200 ℃ to form a superlattice structure. The superlattice structure acts as a buffer layer. And growing GaN on the buffer layer by adopting an MOCVD technology to serve as a GaN channel, and growing AlGaN on the buffer layer to serve as an AlGaN barrier layer, so as to obtain the epitaxial wafer.

As a result of scanning the epitaxial wafer prepared above by an Optical Microscope (OM), as shown in a of fig. 9, 901 represents micro defects (micro pit), it can be seen that micro defects 901 are densely distributed on the surface of the epitaxial wafer, and as a result of observing the epitaxial wafer by an electron microscope, as shown in a of fig. 10, the surface of the epitaxial wafer is distributed with more hexagonal defects.

Examples

Providing a silicon substrate, using Trimethylaluminum (TMAL) as an Al source, trimethylgallium (TMG) as a Ga source, ammonia (NH ₃) as a nitrogen source, C ₃H₈ as a C source, and sequentially growing 5nmAlN layers, 2nmGaN layers and 28nmAlGaN layers on the silicon substrate by MOCVD for 50 times to form a superlattice structure. The superlattice structure acts as a buffer layer. And growing GaN on the buffer layer by adopting an MOCVD technology to serve as a GaN channel, and growing AlGaN on the buffer layer to serve as an AlGaN barrier layer, so as to obtain the epitaxial wafer.

As a result of scanning the epitaxial wafer prepared above with an Optical Microscope (OM), b and 901 in fig. 9 represent micro defects (micro pit), it can be seen that the micro defects 901 on the surface of the epitaxial wafer prepared in this embodiment are significantly reduced, and the result of observing the epitaxial wafer with an electron microscope is shown in b in fig. 10, and no hexagonal defects are seen.

Compared with the comparative example, the epitaxial wafer prepared by the embodiment of the invention has obviously fewer surface defects, can obviously reduce the leakage current of the epitaxial wafer and improve the quality of devices.

In summary, the superlattice structure prepared by the invention inserts the GaN layer between the AlN layer and the AlGaN layer, so that the GaN layer can not only reduce the surface defects of the AlN layer, but also offset a part of compressive stress, reduce the stress defects, and the atomic lateral migration capability of GaN in growth is stronger, so that the surface defects of the epitaxial layer can be obviously improved. The superlattice structure prepared by the invention can be used for a GaN HEMT with a transverse structure.

While the present invention has been described in detail through the foregoing description of the preferred embodiment, it should be understood that the foregoing description is not to be considered as limiting the invention. Many modifications and substitutions of the present invention will become apparent to those of ordinary skill in the art upon reading the foregoing. Accordingly, the scope of the invention should be limited only by the attached claims.

Claims (21)

1. A superlattice structure for a GaN HEMT is characterized by comprising N AlN/GaN/AlGaN laminated structures, wherein the AlN/GaN/AlGaN laminated structures comprise AlN layers, gaN layers and AlGaN layers which are sequentially stacked and grown from bottom to top, and N is an integer larger than 1.

2. The superlattice structure for a GaN HEMT of claim 1, wherein said AlGaN layer has a composition of Al of 5% -40% by mole.

3. The superlattice structure for a GaN HEMT as recited in claim 1, wherein the superlattice structure comprises at least a first superlattice structure and a second superlattice structure, the second superlattice structure is disposed above the first superlattice structure, 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 comprises m AlN/GaN/AlGaN stacked structures, the second superlattice structure comprises n AlN/GaN/AlGaN stacked structures, and m is the same as n or different from n and is an integer greater than 0.

4. The superlattice structure for a GaN HEMT of claim 1, wherein in said AlN/GaN/AlGaN stacked structure, the thickness of the AlN layer is 2nm to 15nm, the thickness of the GaN layer is 5nm or less, and the thickness of the AlGaN layer is 10nm to 40nm.

5. The superlattice structure for a GaN HEMT of claim 1 wherein N has a value in the range of 2 to 200.

6. The superlattice structure for a GaN HEMT according to any one of claims 1-5 wherein said superlattice structure is doped with an acceptor doping element to reduce free electron concentration.

7. The superlattice structure for a GaN HEMT of claim 6, wherein said acceptor doping element comprises at least one of iron and carbon.

8. The superlattice structure for a GaN HEMT of claim 6 wherein at least one of said AlN/GaN/AlGaN stacked structures is doped with an acceptor doping element.

9. The superlattice structure for a GaN HEMT of claim 8, wherein at least one of an AlN layer, a GaN layer, or an AlGaN layer in said AlN/GaN/AlGaN stacked structure is doped with an acceptor doping element.

10. The superlattice structure for a GaN HEMT of claim 8, wherein said acceptor doping element comprises carbon at a carbon doping concentration of 1 x10 ¹⁷/cm³~1×10²⁰/cm³.

11. A buffer layer comprising the superlattice structure for a GaN HEMT as recited in any one of claims 1-10.

12. The buffer layer of claim 11, further comprising at least one AlGaN transition layer disposed below the superlattice structure.

13. The buffer layer of claim 12, wherein the composition content of Al in the AlGaN transition layer is not lower than the composition content of Al in the AlGaN layer of the superlattice structure.

14. The buffer layer of claim 13, wherein the AlGaN transition layer has a composition of Al of 50% to 80%.

15. The buffer layer of claim 13, wherein the composition content of Al in the AlGaN transition layer is constant or gradually decreases from bottom to top along the growth direction.

16. The buffer layer according to any one of claims 12 to 15, wherein the buffer layer comprises at least two AlGaN layers arranged in a stack, the composition content of Al in the AlGaN layers decreasing in order toward the superlattice.

17. A GaN HEMT epitaxial wafer is characterized by comprising:

A substrate;

A buffer layer as claimed in any one of claims 11 to 16;

A GaN channel;

an AlGaN barrier layer;

wherein, the GaN channel and the AlGaN barrier layer form a GaN/AlGaN heterojunction.

18. The GaN HEMT epitaxial wafer of claim 17, wherein the substrate is at least one of SiC or Si.

19. The GaN HEMT epitaxial wafer of claim 17, further comprising an AlN nucleation layer disposed between the substrate and the buffer layer.

20. The GaN HEMT epitaxial wafer of claim 17, further comprising a GaN semi-insulating layer disposed between said buffer layer and said GaN channel.

21. A method for growing GaN HEMT epitaxial wafer is characterized by comprising the following steps:

providing a substrate;

Forming a buffer layer as claimed in any one of claims 11 to 16 on the substrate;

and forming a GaN channel and an AlGaN barrier layer on the buffer layer in sequence.