NITRIDE EPITAXIAL STRUCTURE AND SEMICONDUCTOR DEVICE

Abstract

A nitride epitaxial structure is provided, including: a substrate; a nucleation layer, formed on the substrate, where the nucleation layer is an aluminum nitride layer or a gallium nitride layer; a buffer layer, formed on the nucleation layer, including K stacked group-III nitride double-layer structures, K ≥ 3, each double-layer structure includes an upper layer and a lower layer that are stacked, a band gap difference of each double-layer structure is a difference between a band gap of a material of the upper layer and a band gap of a material of the lower layer, and band gap differences of the K double-layer structures generally present a gradient trend along a thickness direction of the buffer layer; and an epitaxial layer, formed on the buffer layer, where a material of the epitaxial layer includes group-III nitride. A semiconductor device is further provided, including the nitride epitaxial structure.

Claims

1. A nitride epitaxial structure, comprising: a substrate; a nucleation layer, formed on the substrate, wherein the nucleation layer is an aluminum nitride layer or a gallium nitride layer; a buffer layer, formed on the nucleation layer, wherein the buffer layer comprises K stacked group-III nitride double-layer structures, K ≥ 3, each double-layer structure comprises an upper layer and a lower layer that are stacked, a band gap of a material of the upper layer is greater than a band gap of a material of the lower layer, a band gap difference of each double-layer structure is a difference between the band gap of the material of the upper layer and the band gap of the material of the lower layer, and band gap differences of the K double-layer structures generally present a gradient trend along a thickness direction of the buffer layer; and an epitaxial layer, formed on the buffer layer, wherein a material of the epitaxial layer comprises group-III nitride.

2. The nitride epitaxial structure according to claim 1, wherein the material of the upper layer and the material of the lower layer each comprise at least one of GaN, AlN, InN,.

3. The nitride epitaxial structure according to claim 1, wherein a thickness of the lower layer is greater than twice a thickness of the upper layer.

4. The nitride epitaxial structure according to claim 1, wherein the band gap differences of the K double-layer structures gradually decrease from one side of the nucleation layer to one side of the epitaxial layer.

5. The nitride epitaxial structure according to claim 1, wherein a difference between a largest band gap difference and a smallest band gap difference in the K double-layer structures is greater than 20% of a band gap difference between a group-III nitride with a largest band gap and a group-III nitride with a smallest band gap that constitute the double-layer structure.

6. The nitride epitaxial structure according to claim 1, wherein the K double-layer structures comprise GaN and AlN, and an average Al component content in each double-layer structure is 5% to 50%.

7. The nitride epitaxial structure according to claim 6, wherein the average Al component content in each double-layer structure is the same.

8. The nitride epitaxial structure according to claim 6, wherein average Al component contents of the K double-layer structures present a gradient trend along the thickness direction of the buffer layer.

9. The nitride epitaxial structure according to claim 1, wherein a thickness of each double-layer structure is less than 100 nm.

10. The nitride epitaxial structure according to claim 1, wherein a material of the epitaxial layer comprises one or more of GaN, AlN, InN, AlGaN, InGaN, InAlN and InAlGaN.

11. The nitride epitaxial structure according to claim 1, wherein a thickness of the epitaxial layer is greater than or equal to 300 nm.

12. The nitride epitaxial structure according to claim 1, wherein the substrate comprises a silicon substrate, a sapphire substrate, a silicon-on-insulator substrate, a gallium nitride substrate, a gallium arsenide substrate, an indium phosphide substrate, an aluminum nitride substrate, a silicon carbide substrate, a quartz substrate, or a diamond substrate.

13. The nitride epitaxial structure according to claim 1, wherein a thickness of the nucleation layer is 10 nm to 300 nm.

14. A semiconductor device, comprising: at least one input and one output; and a nitride epitaxial structure that further includes: a substrate; a nucleation layer, formed on the substrate, wherein the nucleation layer is an aluminum nitride layer or a gallium nitride layer; a buffer layer, formed on the nucleation layer, wherein the buffer layer comprises K stacked group-III nitride double-layer structures, K ≥ 3, each double-layer structure comprises an upper layer and a lower layer that are stacked, a band gap of a material of the upper layer is greater than a band gap of a material of the lower layer, a band gap difference of each double-layer structure is a difference between the band gap of the material of the upper layer and the band gap of the material of the lower layer, and band gap differences of the K double-layer structures generally present a gradient trend along a thickness direction of the buffer layer; and an epitaxial layer, formed on the buffer layer, wherein a material of the epitaxial layer comprises group-III nitride.

15. The semiconductor device according to claim 14, wherein the semiconductor device comprises a power device, a radio frequency device, or a photoelectric device.

16. The semiconductor device according to claim 14, wherein the semiconductor device comprises a field-effect transistor, a light emitting diode, or a laser diode.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0028] FIG. 1 is a schematic diagram of a structure of a nitride epitaxial structure according to an implementation of an embodiment;

[0029] FIG. 2 is a schematic diagram of a structure of a nitride epitaxial structure according to another implementation of an embodiment;

[0030] FIG. 3 is a schematic diagram of a structure of a nitride epitaxial structure according to still another implementation of an embodiment;

[0031] FIG. 4 is a schematic diagram of a structure of a buffer layer according to an implementation of an embodiment;

[0032] FIG. 5 is a schematic diagram of a structure of a nitride epitaxial structure according to an implementation of an embodiment;

[0033] FIG. 6 is a flowchart of a preparation process for a nitride epitaxial structure according to an implementation of an embodiment;

[0034] FIG. 7A is a transmission electron microscope (TEM) graph of a double-layer structure, of a buffer layer, that is close to a side of a substrate in a nitride epitaxial structure according to Embodiment 2;

[0035] FIG. 7B is an energy dispersive spectroscopy (EDS) graph of a double-layer structure, of a buffer layer, that is close to a side of a substrate in a nitride epitaxial structure according to Embodiment 2;

[0036] FIG. 8A is a TEM graph of a double-layer structure, of a buffer layer, that is close to a side of an epitaxial layer in a nitride epitaxial structure according to Embodiment 2; and

[0037] FIG. 8B is an EDS graph of a double-layer structure, of a buffer layer, that is close to a side of an epitaxial layer in a nitride epitaxial structure according to Embodiment 2.

DESCRIPTION OF EMBODIMENTS

[0038] The following describes an embodiment of this disclosure with reference to accompanying drawings in this disclosure.

[0039] As shown in FIG. 1 and FIG. 2, an an embodiment of this disclosure provides a nitride epitaxial structure 100, including a substrate 10, a nucleation layer 20, a buffer layer 30, and an epitaxial layer 40. The nucleation layer 20 is an AlN layer or a GaN layer, and is formed on the substrate 10. The buffer layer 30 is formed on the nucleation layer 20. The epitaxial layer 40 is formed on the buffer layer 30. A material of the epitaxial layer 40 includes a group-III nitride. The buffer layer 30 includes K stacked group-III nitride double-layer structures 200, where K ≥ 3. Each double-layer structure 300 includes a lower layer 302 and an upper layer 301. A band gap of a material of the upper layer 301 is greater than a band gap of a material of the lower layer 302, to generate a band gap difference. The band gap difference of each double-layer structure 300 is a difference between the band gap of the material of the upper layer 301 and the band gap of the material of the lower layer 302. Band gap differences of the K double-layer structures generally present a gradient trend along a thickness direction of the buffer layer 30. The nitride epitaxial structure provided in an embodiment of this disclosure has an epitaxial layer with good uniformity and high crystal quality, and may be used in a semiconductor device to improve device performance. The nitride epitaxial structure may have a nitride epitaxial layer with a large size of at least 6 inches and a thickness of at least 5 micrometers, to meet a requirement of a large-size epitaxial structure.

[0040] It should be noted that, for the upper layer and the lower layer of the double-structure in an embodiment, the “upper” and the “lower” do not represent specific orientations. In the art, for a superlattice double-layer structure, usually, a layer with a larger band gap is written as an upper layer, and a layer with a smaller band gap is written as a lower layer. Even if the two layers are exchanged in order during actual growth, the writing is not changed.

[0041] In an implementation of an embodiment, the substrate 10 may be a silicon substrate, a sapphire substrate, a silicon-on-insulator substrate (SOI substrate), a gallium nitride substrate, a gallium arsenide substrate, an indium phosphide substrate, an aluminum nitride substrate, a silicon carbide substrate, a quartz substrate, or a diamond substrate, or may be any known conventional substrate that can be used to prepare a group-III nitride film. A crystal orientation of the silicon substrate is not limited, for example, may be a silicon substrate with a crystal facet index of (111), or may be a silicon substrate with a crystal facet index of (100), or may be a silicon substrate with another crystal facet index.

[0042] In an implementation of an embodiment, the nucleation layer 20 is a layer of aluminum nitride or gallium nitride film, and the nucleation layer 20 fully covers the substrate 10. The nucleation layer 20 may provide a nucleation center for subsequent growth of the nitride epitaxial layer, may alleviate stress generated between the substrate 10 and the epitaxial layer 40 due to a lattice mismatch, and may further effectively prevent impurities brought by the substrate 10 from affecting subsequent growth of the nitride epitaxial layer, to reduce lattice defects, reduce dislocation density, and improve crystal quality of the nitride epitaxial layer. In addition, the nucleation layer 20 is thin, and is monocrystalline or quasi-monocrystalline. Therefore, stress generated between the substrate 10 and the epitaxial layer 40 due to a lattice mismatch can be alleviated, without affecting subsequent crystal quality of the nitride epitaxial layer, and costs can also be effectively controlled. In some implementations of this disclosure, a thickness of the nucleation layer 20 may be 10 nm to 300 nm. In some other implementations of this disclosure, a thickness of the nucleation layer 20 may be 20 nm to 200 nm. In some other implementations of this disclosure, a thickness of the nucleation layer 20 may be alternatively 50 nm to 150 nm.

[0043] In an implementation of an embodiment, the nucleation layer 20 may be prepared in a manner of metal-organic chemical vapor deposition or molecular beam epitaxy. The metal-organic chemical vapor deposition (MOCVD) is a chemical vapor deposition technology for growing a film through vapor phase epitaxy by using a thermal decomposition reaction of an organic metal compound. Generally, an organic compound of group-III or group-II elements, a hydride of group-V or group-VI elements, and the like may be used as source materials for crystal growth of a metal organic compound, to grow a group-III-V or group II-VI compound film on a substrate by using a thermal decomposition reaction. The manner of metal-organic chemical vapor deposition can improve subsequent crystal quality of the nitride epitaxial layer.

[0044] In an implementation of an embodiment, the buffer layer 30 includes K stacked group-III nitride double-layer structures 300. In some implementations, a value of K may be 3 to 100. In some other implementations, a value of K may be 10 to 60. In some other implementations, a value of K may be alternatively 20 to 50. In each double-layer structure, the material of the upper layer 301 and the material of the lower layer 302 each may be selected from one of GaN, AlN, InN, or a combination thereof: AlGaN, InGaN, InAlN, or InAlGaN. AlGaN is a combination of two types of group-III nitrides: GaN and AlN. InGaN is a combination of two types of group-III nitrides: GaN and InN. InAlN is a combination of two types of group-III nitrides: AlN and InN. InAlGaN is a combination of three types of group-III nitrides: GaN, AlN, and InN. A band gap of GaN is 3.4 eV, a band gap of AlN is 6.2 eV, and a band gap of InN is 0.7 eV. Generally, a combination of the materials of the upper layer and the lower layer of the double-layer structure 300 may be AlN/GaN, AlGaN/GaN, AlN/AlGaN, or AlGaN/AlGaN.

[0045] In an implementation of an embodiment, a thickness of the buffer layer 30 may be set based on a withstand voltage level. For example, if the withstand voltage level is 100 V, the buffer layer usually needs to be disposed to be 2 .Math.m to 3 .Math.m; or if the withstand voltage level is 600 V, the buffer layer needs to be disposed to be greater than 5 .Math.m. In some implementations of this disclosure, a thickness of the buffer layer 30 is greater than 300 nm. A thickness of each double-layer structure is less than 100 nm. Generally, the thickness of each double-layer structure may be 10 nm to 80 nm, or 20 nm to 60 nm. An appropriate thickness of the double-layer structure helps avoid relaxation. In each double-layer structure 300, a thickness of the lower layer 302 is greater than twice a thickness of the upper layer 301. A thick lower layer and a thick upper layer are disposed, so that lattice relaxation can be effectively avoided. For heteroepitaxy, when the thickness of the upper layer is smaller, the material is in a strained state, to be specific, a lattice constant of the material of the upper layer is the same as that of the material of the lower layer after the material of the upper layer is stretched or compressed by the material of the lower layer, so that a superlattice can effectively function; or when the thickness of the upper layer is greater, the material is restored to a lattice constant of the material, causing lattice relaxation. In the K double-layer structures 300, thicknesses of all upper layers may be the same, and thicknesses of all lower layers may be the same.

[0046] In some implementations of this disclosure, band gap differences of the K double-layer structures 300 gradually decrease from one side of the nucleation layer 200 to one side of the epitaxial layer 400. When the band gap difference is large, a difference between lattice constants of the materials of the double-layer structure is large, thereby helping filter dislocations. However, because a polarization effect is also strong, electric leakage is likely to occur. On the contrary, when the band gap difference is small, this helps reduce a leakage current, but is not conducive to dislocation filtering. A double-layer structure with a large band gap difference is disposed on a side close to the nucleation layer, thereby helping filter dislocations. In addition, because the double-layer structure is far away from an AlGaN barrier layer and a channel layer, adverse impact caused by electric leakage is small. A double-layer structure with a small band gap difference is disposed on a side close to the epitaxial layer, thereby helping reduce a polarization effect and improve voltage withstand performance. In some other implementations, alternatively, the K double-layer structures 300 may be arranged, according to a requirement, in a manner that band gap differences gradually increase from one side of the nucleation layer 200 to one side of the epitaxial layer 400.

[0047] In an embodiment, a difference between a largest band gap difference and a smallest band gap difference in the K double-layer structures is greater than 20% of a band gap difference between a group-III nitride with a largest band gap and a group-III nitride with a smallest band gap that constitute the double-layer structure. For example, in K AlGaN/AlGaN double-layer structures, group-III nitrides that constitute the double-layer structure include GaN and AlN, where a band gap of GaN is 3.4 eV, and a band gap of AlN is 6.2 eV. In this case, 20% of a band gap difference between a group-III nitride with a largest band gap and a group-III nitride with a smallest band gap that constitute the double-layer structure is 20% × (6.2 - 3.4) eV. For another example, in K InAlGaN/InAlGaN double-layer structures, group-III nitrides that constitute the double-layer structure include InN, GaN, and AlN, where a band gap of InN is 0.7 eV, a band gap of GaN is 3.4 eV, and a band gap of AlN is 6.2 eV. In this case, 20% of a band gap difference between a group-III nitride with a largest band gap and a group-III nitride with a smallest band gap that constitute the double-layer structure is 20% × (6.2 - 0.7) eV.

[0048] In some implementations of this disclosure, the K double-layer structures 200 include two types of group-III nitrides: GaN and AlN, and a combination of the upper layer and the lower layer of the double-layer structure may be expressed as Al.sub.xGa.sub.1-xN/Al.sub.yGa.sub.1-yN (0 < x ≤ 1, y > 0). In the double-layer structures, a gradient trend of band gap differences may be equivalent to a gradient trend of Al component content differences. A change trend of band gaps and a change trend of an Al component content in AlGaN are approximately in a linear relationship. To be specific, a higher Al component content in AlGaN indicates a larger band gap. Therefore, a gradient trend of band gap differences of the double-layer structures may be equivalent to a gradient trend of Al component content differences of the double-layer structures. It should be noted that the Al component content in an embodiment is a molar percentage of an Al element in group-III metal elements. A band gap of Al.sub.xGa.sub.1-xN may be approximately equal to [6.2x + (1 - x) 3.4] eV. Therefore, Al component contents in an upper layer and a lower layer of each double-layer structure are controlled, so that the Al component content differences of the double-layer structures are in a gradient trend, and therefore the band gap differences of the K double-layer structures can be gradient. For example, as shown in FIG. 2 and FIG. 3, the K double-layer structures 200 include a first double-layer structure Al.sub.x1Ga.sub.1-.sub.x1N/Al.sub.y1Ga.sub.1-y1N, a second double-layer structure Al.sub.x2Ga.sub.1-x2N/Al.sub.y2Ga.sub.1-y2N, a third double-layer structure Al.sub.x3Ga.sub.1-x3N/Al.sub.y3Ga.sub.1-y3N, ..., and a Kth double-layer structure Al.sub.xkGa.sub.1-xkN/Al.sub.ykGa.sub.1-ykN. If band gap differences gradually decrease from the first double-layer structure to the Kth double-layer structure, x1 - y1 > x2 - y2 > x3 - y3 > ... > xk - yk, that is, Al component content differences gradually decrease from the first double-layer structure to the Kth double-layer structure. In this implementation, any two adjacent double-layer structures have different band gap differences. Generally, any two adjacent double-layer structures have different components. In some other implementations of this disclosure, in the K double-layer structures, some adjacent double-layer structures may have a same band gap difference. A repetition periodicity of double-layer structures that are adjacently stacked and that have a same band gap difference may be 1 to 10. Repetition periodicities of double-layer structures with different band gap differences may be the same or different. For example, as shown in FIG. 4, in a buffer layer constituted by Al.sub.xGa.sub.1-xN/Al.sub.yGa.sub.1-yN, repetition periodicities of double-layer structures with different band gap differences are the same, that is, are all 2. This may be as follows: a band gap difference of the first double-layer structure = a band gap difference of the second double-layer structure > a band gap difference of the third double-layer structure = a band gap difference of the fourth double-layer structure ... > a band gap difference of the Kth double-layer structure. Repetition periodicities of double-layer structures with different band gap differences are different, for example, a band gap difference of the first double-layer structure = a band gap difference of the second double-layer structure = a band gap difference of the third double-layer structure > a band gap difference of the fourth double-layer structure = a band gap difference of the fifth double-layer structure > a band gap difference of the sixth double-layer structure ... > a band gap difference of the Kth double-layer structure.

[0049] In an implementation of this application, the K double-layer structures include two types of group-III nitrides: GaN and AlN, and an average Al component content in each double-layer structure may be 5% to 50%. In some implementations, an average Al component content in each double-layer structure is 8% to 38%. In some other implementations, an average Al component content in each double-layer structure is 15% to 30%. In some other implementations, an average Al component content in each group-III nitride double-layer structure is 20% to 25%. The average Al component content is an average molar percentage of an Al element in group-III metal elements in each double-layer structure. The average Al component content of each double-layer structure is controlled within an appropriate range, to ensure that the buffer layer has good crystal quality. For the Al.sub.xGa.sub.1-xN/Al.sub.yGa.sub.1-yN double-layer structure, an average Al component content in each double-layer structure may be expressed as [(T.sub.upper × x + T.sub.lower × y)/(T.sub.upper + T.sub.lower)] × 100%, where T.sub.upper and T.sub.lower indicate thicknesses of an upper layer and a lower layer of the double-layer structure respectively, and x and y indicate Al component contents of the upper layer and the lower layer respectively. In some implementations of this disclosure, the average Al component content in each double-layer structure is the same. An Al component remains unchanged. Therefore, when pre-compensation is designed for high-temperature warpage, only impact of a thickness of a film layer needs to be considered, and impact of a change of the Al component does not need to be considered, thereby simplifying a parameter design. In some other implementations of this disclosure, average Al component contents of the K double-layer structures present a gradient trend along the thickness direction of the buffer layer. Average Al component contents of a plurality of double-layer structures are designed to be gradient, thereby facilitating stress adjustment.

[0050] In addition, in this application, that the band gap differences of the K double-layer structures generally present a gradual tend along the thickness direction of the buffer layer may be that the band gap differences strictly present a change of gradually increasing or gradually decreasing along the thickness direction of the buffer layer, or may be that the band gap differences generally present a change of gradually increasing or gradually decreasing, but there are a few special cases in which a change is opposite to the overall gradient trend, for example, in a buffer layer in which band gap differences generally gradually increase, there are a few double-layer structures whose band gap differences gradually decrease.

[0051] In an implementation of this application, a material of the epitaxial layer includes one or more of GaN, AlN, InN, AlGaN, InGaN, InAlN, and InAlGaN. In an implementation of this application, a material of the epitaxial layer 40 includes a group-III nitride, and may be generally, for example, one or more of GaN, AlN, InN, AlGaN, InGaN, InAlN, and InAlGaN. A thickness of the epitaxial layer 40 is greater than or equal to 300 nm. A thickness of a conventional gallium nitride epitaxial layer is usually small due to a limitation of stress. However, the nitride epitaxial structure in an embodiment of this disclosure can well eliminate stress, and therefore is applicable to preparation of a thick-film epitaxial layer, and theoretically may have an infinite thickness. In some implementations of this disclosure, a thickness of the epitaxial layer may be greater than or equal to 5 .Math.m, or may be greater than or equal to 10 .Math.m, for example, 15 .Math.m to 100 .Math.m. The epitaxial layer 40 may fully cover the nucleation layer 20, or may partially cover the nucleation layer 20.

[0052] In an implementation of this application, different nitride epitaxial layers are applicable to different semiconductor devices. For example, GaN, AlGaN, and AlN are applicable to power devices, and a nitride epitaxial layer including In is applicable to photoelectric devices.

[0053] In this implementation of this application, to meet a usability requirement, another element may be alternatively added to the epitaxial layer 40. For example, to improve insulation, carbon may be added to achieve a high resistance and improve voltage withstand performance.

[0054] As shown in FIG. 5, the nitride epitaxial structure 100 further includes a transition layer 50 disposed between the nucleation layer 30 and the epitaxial layer 40, and the transition layer 50 may be an AlGaN layer. A thickness of the transition layer may be 10 nm to 300 nm. The transition layer 50 is disposed, thereby helping adjust stress applied to the epitaxial structure.

[0055] In an implementation of this application, as shown in FIG. 5, the nitride epitaxial structure 100 further includes other functional layers 60 disposed on the epitaxial layer 40. A specific structural composition of the other functional layers 60 may be arranged according to an actual application requirement. In an implementation of this application, the other functional layers 60 may generally include an AlN insertion layer 61, an AlGaN barrier layer 62, and a P-type GaN layer 63 that are sequentially disposed on the epitaxial layer 40. In some other implementations, the other functional layers 60 may alternatively have another structure.

[0056] As shown in FIG. 6, an embodiment of this disclosure further provides a method for preparing a nitride epitaxial structure, including the following steps.

[0057] S01: Form a nucleation layer on a substrate, where the nucleation layer is an AlN layer or a GaN layer.

[0058] Generally, the nucleation layer 20 may be prepared on the substrate 10 in a manner of metal-organic chemical vapor deposition or molecular beam epitaxy. Before preparation of the nucleation layer 20, conventional cleaning may be performed on the substrate 10.

[0059] In a specific implementation of this application, the nucleation layer 20 is prepared on the substrate 10 in a manner of metal-organic chemical vapor deposition. Details may be as follows: The substrate 10 is placed in a metal-organic chemical vapor deposition reaction chamber, and hydrogen and ammonia are injected for 3 min to 5 min at a temperature of 900° C. to 1100° C. and under a pressure of 30 to 60 Torr, to obtain a processed substrate 10. Then hydrogen, ammonia, and an aluminum source or a gallium source are injected, and an aluminum nitride or a gallium nitride is deposited on the processed substrate 10, to obtain the nucleation layer 20. In this implementation of this application, the parameters in the deposition process are not limited to the foregoing ranges. The gallium source includes but is not limited to trimethyl gallium and triethyl gallium. The aluminum source includes but is not limited to trimethyl aluminum and triethyl aluminum.

[0060] S02: Form a buffer layer on the nucleation layer.

[0061] In an implementation of this application, the buffer layer 30 may be prepared in a manner of metal-organic chemical vapor deposition. Generally, the substrate with the nucleation layer that is obtained in step SOI is placed in a metal-organic chemical vapor deposition reaction chamber. Then hydrogen, ammonia, and a group-III metal source are injected at a temperature of 900° C. to 1100° C. and under a pressure of 30 to 60 Torr, and a group-III nitride is obtained through epitaxial growth on the buffer layer 30, to obtain the buffer layer 30. The group-III metal source is an organic compound including a group-III metal element, for example, trimethyl gallium, triethyl gallium, trimethyl aluminum, or triethyl aluminum. A flux of the group-III metal source may be changed to change a content of each group-III nitride in the buffer layer, and deposition time may be controlled to obtain nitride layers of different thicknesses.

[0062] S03: Epitaxially grow a group-III nitride on the buffer layer to form an epitaxial layer.

[0063] In an implementation of this application, the epitaxial layer 40 may be prepared in a manner of metal-organic chemical vapor deposition. Generally, the substrate 10 obtained in step S02 is placed in a metal-organic chemical vapor deposition reaction chamber, and hydrogen and ammonia are injected for 3 min to 5 min at a temperature of 900° C. to 1100° C. and under a pressure of 30 to 60 Torr, to obtain a processed substrate 10. Then hydrogen, ammonia, and a group-III metal source are injected, and a group-III nitride is obtained through epitaxial growth on the buffer layer 30, to form the epitaxial layer 40. The group-III nitride may be generally, for example, one or more of GaN, AlN, InN, AlGaN, InGaN, InAlN, and InAlGaN. The group-III metal source is an organic compound including a group-III metal element, for example, trimethyl gallium, triethyl gallium, trimethyl aluminum, or triethyl aluminum.

[0064] In an implementation of this application, the preparation method may further include: forming a transition layer 50 between the buffer layer 30 and the epitaxial layer 40. To be specific, before step S03, the transition layer 50 is first prepared on the buffer layer 30, and then the epitaxial layer 40 is grown on the transition layer 50. A material of the transition layer 50 may be an AlGaN layer.

[0065] One embodiment of this disclosure further provides a semiconductor device, including the foregoing nitride epitaxial structure described here in this disclosure. The nitride epitaxial structure in may be directly used as a part of the semiconductor device, or it may be separately used in the semiconductor device. The semiconductor device includes but is not limited to a power device (namely, an electronic power device), a radio frequency device, or a photoelectric device. The power device or the radio frequency device may be a transistor, and may be generally a field-effect transistor, for example, a high electron mobility transistor (HEMT). The photoelectric device is, for example, a light emitting diode (LED) or a laser diode (LD), and may be generally a nitride-based light emitting diode or a nitride-based quantum well laser diode.

[0066] The following further describes an embodiment of this disclosure by illustrating a plurality of embodiments.

Embodiment 1

[0067] A nitride epitaxial structure includes a substrate, and a nucleation layer, a buffer layer, an epitaxial layer, and other functional layers that are sequentially disposed on the substrate. A material of the substrate is Si, sapphire, GaN, SiC, diamond, SOI, or the like. The nucleation layer is an AlN nucleation layer with a thickness of 50 nm to 400 nm. The buffer layer is a structural layer with gradient band gap differences, and includes 11 double-layer structures with an upper layer and a lower layer of Al.sub.xGa.sub.1-xN/Al.sub.yGa.sub.1-yN. An average Al component content of each double-layer structure is 20%. Table 1 shows values of x and y and thicknesses T.sub.upper and T.sub.lower in each double-layer structure. The epitaxial layer is a GaN layer, may include a carbon-doped structure such as GaN or AlGaN, and has a thickness of 100 nm to 3 .Math.m. The other functional layers may include the following layers that are sequentially disposed on the epitaxial layer: an AlN insertion layer with a thickness of 1 nm; an AlGaN barrier layer with an Al component range of 10% to 30% and a thickness of 10 nm to 30 nm; and a p-GaN layer, where a P-type impurity is implemented through Mg doping, and a thickness range is 30 nm to 120 nm.

TABLE-US-00001 Buffer layer parameters in Embodiment 1 Double-layer structure sequence number x y Al component content difference T.sub.upper (nm) T.sub.lower (nm) Average Al component content Periodicity 1 0.5 0.17 0.33 2 20 20% 5 2 0.55 0.165 0.385 2 20 20% 5 3 0.6 0.16 0.44 2 20 20% 5 4 0.65 0.155 0.495 2 20 20% 5 5 0.7 0.15 0.55 2 20 20% 5 6 0.75 0.145 0.605 2 20 20% 5 7 0.8 0.14 0.66 2 20 20% 5 8 0.85 0.135 0.715 2 20 20% 5 9 0.9 0.13 0.77 2 20 20% 5 10 0.95 0.125 0.825 2 20 20% 5 11 1.0 0.12 0.88 2 20 20% 5

[0068] Double-layer structures whose sequence numbers are 1 to 11 are sequentially disposed in a stacked manner. A double-layer structure with a sequence number of 1 is disposed close to the epitaxial layer, and a double-layer structure with a sequence number of 11 is disposed close to the nucleation layer. The periodicity 5 in Table 1 means that a double-layer structure with each sequence number is repeated five times. To be specific, double-layer structures with different band gap differences have a same repetition periodicity, to form 11 groups of double-layer structures, and each group includes five same double-layer structures that are stacked. To be specific, the buffer layer includes five stacked Al.sub.0.5Ga.sub.0.5N/Al.sub.0.17Ga.sub.0.83N double-layer structures, five stacked Al.sub.0.55Ga.sub.0.45N/Al.sub.0.165Ga.sub.0.835N double-layer structures, and so on. It can be learned from Table 1 that, in the nitride epitaxial structure in Embodiment 1, Al component content differences of the 11 groups of double-layer structures of the buffer layer gradually decrease from one side of the nucleation layer to one side of the epitaxial layer, that is, band gap differences of the 11 groups of double-layer structures gradually decrease from one side of the nucleation layer to one side of the epitaxial layer. In addition, an average Al component content of each group of double-layer structures in Embodiment 1 is the same, and is 20%.

Embodiment 2

[0069] A difference from Embodiment 1 lies only in that the buffer layer includes 51 double-layer structures with an upper-layer and a lower layer of Al.sub.xGa.sub.1-xN/Al.sub.yGa.sub.1-yN. Table 2 shows values of x and y, an upper-layer thickness T.sub.upper, and a lower-layer thickness T.sub.lower in each double-layer structure.

TABLE-US-00002 Buffer layer parameters in Embodiment 2 Double-layer structure sequence number x y Al component content difference T.sub.upper (nm) T.sub.lower (nm) Average Al component content Periodicity 1 0.5 0.170 0.330 2 20 20% 1 2 0.51 0.169 0.341 2 20 20% 1 3 0.52 0.168 0.352 2 20 20% 1 4 0.53 0.167 0.363 2 20 20% 1 5 0.54 0.166 0.374 2 20 20% 1 6 0.55 0.165 0.385 2 20 20% 1 7 0.56 0.164 0.396 2 20 20% 1 8 0.57 0.163 0.407 2 20 20% 1 9 0.58 0.162 0.418 2 20 20% 1 10 0.59 0.161 0.429 2 20 20% 1 11 0.6 0.160 0.440 2 20 20% 1 12 0.61 0.159 0.451 2 20 20% 1 13 0.62 0.158 0.462 2 20 20% 1 14 0.63 0.157 0.473 2 20 20% 1 15 0.64 0.156 0.484 2 20 20% 1 16 0.65 0.155 0.495 2 20 20% 1 17 0.66 0.154 0.506 2 20 20% 1 18 0.67 0.153 0.517 2 20 20% 1 19 0.68 0.152 0.528 2 20 20% 1 20 0.69 0.151 0.539 2 20 20% 1 21 0.7 0.150 0.550 2 20 20% 1 22 0.71 0.149 0.561 2 20 20% 1 23 0.72 0.148 0.572 2 20 20% 1 24 0.73 0.147 0.583 2 20 20% 1 25 0.74 0.146 0.594 2 20 20% 1 26 0.75 0.145 0.605 2 20 20% 1 27 0.76 0.144 0.616 2 20 20% 1 28 0.77 0.143 0.627 2 20 20% 1 29 0.78 0.142 0.638 2 20 20% 1 30 0.79 0.141 0.649 2 20 20% 1 31 0.80 0.140 0.660 2 20 20% 1 32 0.81 0.139 0.671 2 20 20% 1 33 0.82 0.138 0.682 2 20 20% 1 34 0.83 0.137 0.693 2 20 20% 1 35 0.84 0.136 0.704 2 20 20% 1 36 0.85 0.135 0.715 2 20 20% 1 37 0.86 0.134 0.726 2 20 20% 1 38 0.87 0.133 0.737 2 20 20% 1 39 0.88 0.132 0.748 2 20 20% 1 40 0.89 0.131 0.759 2 20 20% 1 41 0.90 0.130 0.770 2 20 20% 1 42 0.91 0.129 0.781 2 20 20% 1 43 0.92 0.128 0.792 2 20 20% 1 44 0.93 0.127 0.803 2 20 20% 1 45 0.94 0.126 0.814 2 20 20% 1 46 0.95 0.125 0.825 2 20 20% 1 47 0.96 0.124 0.836 2 20 20% 1 48 0.97 0.123 0.847 2 20 20% 1 49 0.98 0.122 0.858 2 20 20% 1 50 0.99 0.121 0.869 2 20 20% 1 51 1.00 0.120 0.880 2 20 20% 1

[0070] Double-layer structures whose sequence numbers are 1 to 51 are sequentially disposed in a stacked manner. A double-layer structure with a sequence number of 1 is disposed close to the epitaxial layer, and a double-layer structure with a sequence number of 51 is disposed close to the nucleation layer. The periodicity 1 in Table 2 means that there is only one double-layer structure with each sequence number. It can be learned from Table 2 that, in the nitride epitaxial structure in Embodiment 2, Al component content differences of the 51 double-layer structures of the buffer layer gradually decrease from one side of the nucleation layer to one side of the epitaxial layer, that is, band gap differences of the 51 double-layer structures gradually decrease from one side of the nucleation layer to one side of the epitaxial layer. In addition, an average Al component content of each double-layer structure in Embodiment 2 is the same, and is 20%. Compared with Embodiment 1, a gradient spacing between band gap differences of the buffer layer in Embodiment 2 is smaller, so that stress between the substrate and the epitaxial layer can be better adjusted.

[0071] FIG. 7A is a TEM graph of a double-layer structure, of the buffer layer, that is close to a side of the substrate in the nitride epitaxial structure according to Embodiment 2. FIG. 7B is an EDS graph of a double-layer structure, of the buffer layer, that is close to a side of the substrate in the nitride epitaxial structure according to Embodiment 2. The double-layer structure in FIG. 7B corresponds to three double-layer structures whose sequence numbers are 49 to 51 in Table 2. It can be learned from FIG. 7A and FIG. 7B that, in a double-layer structure close to a side of the substrate, an Al component in an upper layer with a large band gap is close to 100%, and Al components present a gradual decrease trend toward a side away from the substrate. FIG. 8A is a TEM graph of a double-layer structure, of the buffer layer, that is close to a side of the epitaxial layer in the nitride epitaxial structure according to Embodiment 2. FIG. 8B is an EDS graph of a double-layer structure, of the buffer layer, that is close to a side of the epitaxial layer in the nitride epitaxial structure according to Embodiment 2. The double-layer structure in FIG. 8B corresponds to three double-layer structures whose sequence numbers are 1 to 3 in Table 2. It can be learned from FIG. 7B and FIG. 8B that, an Al content difference of a double-layer structure close to a side of the substrate is greater than an Al content difference of a double-layer structure close to a side of the epitaxial layer.

[0072] Note: The EDS graph includes curves for three elements: Ga, Al, and N. The N element accounts for approximately 50%. Therefore, a vertical-coordinate value of each point on a curve for the Al element multiplied by 2 is an Al component content. Due to factors such as measurement errors, a measured value may be different from a designed value.

Embodiment 3

[0073] A difference from Embodiment 1 lies in that the buffer layer includes 11 groups of double-layer structures with an upper-layer and a lower layer of Al.sub.xGa.sub.1-xN/Al.sub.yGa.sub.1-yN. Table 3 shows values of x and y, an upper-layer thickness T.sub.upper, and a lower-layer thickness T.sub.lower in each group of double-layer structures.

TABLE-US-00003 Buffer layer parameters in Embodiment 3 Double-layer structure sequence number x y Al component content difference T.sub.upper (nm) T.sub.lower (nm) Average Al component content Periodicity 1 0.50 0.2 0.30 2 20 22.73% 5 2 0.55 0.2 0.35 2 20 23.18% 5 3 0.60 0.2 0.40 2 20 23.64% 5 4 0.65 0.2 0.45 2 20 24.09% 5 5 0.70 0.2 0.50 2 20 24.55% 5 6 0.75 0.2 0.55 2 20 25.00% 5 7 0.80 0.2 0.60 2 20 25.45% 5 8 0.85 0.2 0.65 2 20 25.91% 5 9 0.90 0.2 0.70 2 20 26.36% 5 10 0.95 0.2 0.75 2 20 26.82% 5 11 1.00 0.2 0.80 2 20 27.27% 5

[0074] Double-layer structures whose sequence numbers are 1 to 11 are sequentially disposed in a stacked manner. A double-layer structure with a sequence number of 1 is disposed close to the epitaxial layer, and a double-layer structure with a sequence number of 11 is disposed close to the nucleation layer. The periodicity 5 in Table 1 means that a double-layer structure with each sequence number is repeated five times. To be specific, double-layer structures with different band gap differences have a same repetition periodicity, to form 11 groups of double-layer structures, and each group includes five same double-layer structures that are stacked. It can be learned from Table 3 that, in the nitride epitaxial structure in Embodiment 3, Al component content differences of the 11 groups of double-layer structures of the buffer layer gradually decrease from one side of the nucleation layer to one side of the epitaxial layer, that is, band gap differences of the 11 groups of double-layer structures gradually decrease from one side of the nucleation layer to one side of the epitaxial layer. In addition, average Al component contents of the 11 groups of double-layer structures in Embodiment 3 also gradually decrease from one side of the nucleation layer to one side of the epitaxial layer.