Epitaxial structure and preparation method thereof

Abstract

The invention relates to the technical field of semiconductors, in particular to an epitaxial structure and a preparation method thereof.The epitaxial structure comprises a substrate, a combined buffer layer comprising at least one buffer unit and a drift layer which are sequentially stacked from bottom to top; wherein each buffer unit comprises a nitrogen-doped buffer unit and a phosphorus-doped buffer unit. According to the epitaxial structure provided by the invention, by performing nitrogen and phosphorus doping on the surface of the substrate to form epitaxy of the composite structure buffer layer, the induction probability of stacking fault in the initial epitaxy stage can be reduced, and meanwhile, a large amount of atomic phosphorus is doped in the epitaxy process, and the lattice constant can be expanded, so that the problem that the nitrogen-doped lattice constant is reduced in the mass production process is solved, and the yield of the product is improved. Therefore, the wafer warping is improved, and the yield is improved.

Description

Technical Field

[0001] The present invention relates to the field of semiconductor technology, and in particular to an epitaxial structure and a preparation method thereof. Background Art

[0002] Silicon carbide (SiC) compound semiconductors are considered to be the best performing power semiconductor materials that have been successfully mass-produced due to their wide bandgap, high breakdown electric field strength, and good thermal conductivity. The use of SiC-based power devices can significantly reduce the volume, weight, and cost of power electronics systems, improve power density, and meet the power electronics industry's demand for smaller, lighter, and more efficient high-power circuit systems. However, the road to SiC power devices being put on the car is not smooth, mainly because the SiC substrate material still has a high defect level, and the dislocation density is often more than 1,000 times that of silicon materials. Various types of crystal growth defects are difficult to effectively control during the physical vapor transport growth process, resulting in the power devices ultimately built on the SiC substrate being significantly behind silicon-based devices in terms of automotive reliability.

[0003] SiC epitaxy is a link in the SiC industry chain. It provides a core voltage-resistant layer for downstream SiC device manufacturing and also has the function of shielding certain substrate dislocations (base plane dislocations BPD). Different from the crystal growth process above 2000℃, the epitaxy process relies on the step flow growth method to reduce the growth temperature to below 1700℃, which is accompanied by an increase in the probability of epitaxy stacking fault (SF). At present, the industry cannot achieve "zero" stacking fault epitaxy. Improving the ability of epitaxy to suppress stacking faults is still the core topic of current SiC epitaxy process research.

[0004] Generally speaking, stacking fault defects are divided into Shockley type, Frank type and a mixture of the two. The stacking fault suppression methods commonly used in actual production are hydrogen etching treatment on the substrate surface and a slower growth rate in the initial growth stage. These methods have a good suppression effect on Shockley SF, but have a weaker suppression ability on Frank SF.

[0005] Traditional nitrogen-doped epitaxial growth has a high probability of inducing stacking faults. In addition, the lattice constant in the traditional nitrogen-doped epitaxial process is reduced to cause wafer warping, resulting in unsatisfactory epitaxial yield. Summary of the invention

[0006] In view of this, an object of the present invention is to provide an epitaxial structure and a preparation method thereof that can suppress stacking fault defects and improve wafer warpage.

[0007] In a first aspect, an embodiment of the present invention provides an epitaxial structure, which includes: a substrate stacked in sequence from bottom to top, a combined buffer layer including at least one buffer unit, and a drift layer; wherein each of the buffer units includes a nitrogen-doped buffer unit and a phosphorus-doped buffer unit.

[0008] In combination with the first aspect, the phosphorus-doped buffer unit and the nitrogen-doped buffer unit are stacked; the phosphorus-doped buffer unit includes at least one phosphorus-doped buffer layer; and the nitrogen-doped buffer unit includes at least one nitrogen-doped buffer layer.

[0009] In combination with the first aspect, the number of buffer units is less than or equal to ten.

[0010] In combination with the first aspect, the epitaxial structure further includes:

[0011] The modified layer is disposed between the substrate and the combined buffer layer; the modified layer is a co-doped diffusion layer containing phosphorus and nitrogen, the thickness of the modified layer is 2-20 nm, and the nitrogen doping concentration of the modified layer is 5×10 18 -1×10 19 / cm 3 ; Phosphorus doping concentration is 1×10 16 -1×10 19 / cm 3 .

[0012] In combination with the first aspect, the drift layer is a co-doped drift layer comprising phosphorus and nitrogen, the thickness of the drift layer is 5-100 nm, and the co-doping concentration of nitrogen and phosphorus is 1×10 14 -1×10 17 / cm 3 .

[0013] In a second aspect, the present application provides a method for preparing an epitaxial structure, which is used to prepare the above-mentioned epitaxial structure; the method comprises:

[0014] Providing a substrate, and performing etching pretreatment on the substrate;

[0015] In a hydrogen atmosphere, performing multiple epitaxial growths on the pretreated substrate to sequentially form multiple buffer units to obtain a combined buffer layer;

[0016] A drift layer is grown on top of the combined buffer layer to obtain the epitaxial structure as described above.

[0017] In combination with the second aspect, the substrate is a 4H-SiC substrate with the Si plane offset 2-6° in the [11-20] direction;

[0018] The step of performing etching pretreatment on the substrate comprises:

[0019] The 4H-SiC substrate is placed in a high-temperature furnace and the surface is etched in a H2 and / or HCl atmosphere to remove dirt and processing damage layers on the wafer surface.

[0020] In combination with the second aspect, in a hydrogen atmosphere, multiple epitaxial growths are performed on the pretreated substrate to sequentially form multiple buffer units before obtaining the combined buffer layer, further comprising:

[0021] In an environment of a temperature of 1700-2000° C., a phosphorus source and a silicon source are introduced into the pretreated substrate to diffuse and dope phosphorus elements on the substrate to form a modified layer on the surface of the substrate; wherein the partial pressure of the phosphorus source is 500-50000Pa, and the partial pressure of the silicon source is 0.5-50Pa.

[0022] In combination with the second aspect, in a hydrogen atmosphere, performing multiple epitaxial growths on the pretreated substrate to sequentially form multiple buffer units to obtain a combined buffer layer comprises:

[0023] Under preset conditions, a carbon source and a silicon source reactant are introduced into the reaction chamber and a nitrogen source or a phosphorus source is introduced alternately to alternately grow a phosphorus-doped buffer unit and a nitrogen-doped buffer unit to obtain a buffer unit;

[0024] Continue to alternately introduce a nitrogen source or a phosphorus source into the reaction chamber to periodically grow one or more buffer units on the buffer unit until a combined buffer layer including a first number of buffer units is obtained.

[0025] In combination with the second aspect, the step of growing a drift layer on top of the combined buffer layer includes:

[0026] A nitrogen dopant and a phosphorus dopant are simultaneously introduced into the reaction chamber to form a drift layer on the combined buffer layer.

[0027] The embodiments of the present invention bring about the following beneficial effects: the epitaxial structure provided in the present application can reduce the probability of inducing stacking faults in the early stage of epitaxy by performing nitrogen and phosphorus doping on the substrate surface to form an epitaxy of a combined structural buffer layer. At the same time, since a larger atomic phosphorus is doped during the epitaxy process, the lattice constant can be expanded, thereby compensating for the problem of reduced lattice constant of nitrogen doping in mass production processes, thereby improving wafer warping and improving yield rate.

[0028] Other features and advantages of the present invention will be described in the following description, and partly become apparent from the description, or understood by practicing the present invention. The purpose and other advantages of the present invention are realized and obtained by the structures particularly pointed out in the description, claims and drawings.

[0029] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, preferred embodiments are given below and described in detail with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] In order to more clearly illustrate the specific implementation methods of the present invention or the technical solutions in the prior art, the drawings required for use in the specific implementation methods or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are some implementation methods of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without paying any creative work.

[0031] Figure 1 A schematic diagram of an epitaxial structure provided by an embodiment of the present invention;

[0032] Figure 2 A schematic diagram of another epitaxial structure provided by an embodiment of the present invention;

[0033] Figure 3 A schematic diagram of another epitaxial structure provided by an embodiment of the present invention;

[0034] Figure 4 A flow chart of a method for preparing an epitaxial structure provided in an embodiment of the present invention.

[0035] Reference numerals:

[0036] 1-substrate, 2-modified layer, 3-combined buffer layer, 31-nitrogen-doped buffer unit, 32-phosphorus-doped buffer unit, 4-drift layer. DETAILED DESCRIPTION

[0037] In order to make the purpose, technical solution and advantages of the embodiments of the present invention clearer, the technical solution of the present invention will be clearly and completely described below in conjunction with the accompanying drawings. Obviously, the described embodiments are 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 those skilled in the art without creative work are within the scope of protection of the present invention.

[0038] In order to facilitate the understanding of this embodiment, the technical terms designed in this application are briefly introduced below.

[0039] Epitaxy, depositing a single crystal layer on a single crystal substrate, the newly deposited layer becomes the epitaxial layer.

[0040] Stacking fault (SF) refers to the change in the stacking sequence of atoms. The formation of Frank-type SF is closely related to the step interaction in crystal growth: the large step formed by step aggregation covers the surface step of the screw dislocation or mixed dislocation during the advancement process, causing the full dislocation to decompose in the base vector plane to form a stacking fault. Therefore, improving the stability of step advancement during epitaxial growth can significantly reduce the probability of Frank SF formation.

[0041] Step Flow Mode refers to the process in which crystal growth relies on the deposition of reactants at the edge of surface steps.

[0042] After introducing the technical terms involved in this application, the application scenarios and design concepts of the embodiments of this application are briefly introduced.

[0043] Traditional nitrogen-doped epitaxial growth has a high probability of inducing stacking faults. In addition, the lattice constant in the traditional nitrogen-doped epitaxial process is reduced to cause wafer warping, resulting in unsatisfactory epitaxial yield.

[0044] Based on this, an embodiment of the present application provides an epitaxial structure and a method for preparing the same.

[0045] Example 1

[0046] The present application provides an epitaxial structure, which includes: a substrate 1, a combined buffer layer 3 including at least one buffer unit, and a drift layer 4 stacked in sequence from bottom to top; wherein each of the buffer units includes a nitrogen-doped buffer unit 31 and a phosphorus-doped buffer unit 32.

[0047] In this embodiment, phosphorus diffusion doping modification treatment is performed on the surface of the substrate 1 to form the epitaxy of the nitrogen-phosphorus co-doped combined buffer layer 3 to stabilize the step flow, thereby reducing the probability of inducing stacking faults in the early stage of epitaxy. At the same time, by doping with larger atomic phosphorus during the epitaxy process, the lattice constant can be expanded, thereby compensating for the problem of reduced lattice constant of nitrogen doping in the mass production process, thereby improving wafer warpage.

[0048] It can be understood that when the number of buffer units in the combined buffer layer 3 is greater than or equal to 2, the buffer units are stacked in sequence.

[0049] In combination with the first aspect, the phosphorus-doped buffer unit 32 and the nitrogen-doped buffer unit 31 are stacked.

[0050] Nitrogen (N) and phosphorus (P) are both N-type doping elements of SiC, with high ionization energy, where N mainly replaces C atoms, and its covalent radius is 0.74 angstroms, while P mainly replaces Si atoms, and its covalent radius is 1.1 angstroms. The nitrogen-doped and phosphorus-doped atmospheres in epitaxial growth can improve the step flow stability of 4H-SiC epitaxial growth. Therefore, the present application grows nitrogen-doped and phosphorus-doped epitaxial layers, and while optimizing the step flow, it can also modulate the epitaxial lattice constant to control the mismatch stress between the epitaxial layer and the substrate, thereby improving wafer warping and inhibiting the formation of stacking faults, thereby improving the yield of the epitaxial structure, and laying a solid foundation for the subsequent improvement of the yield and reliability of silicon carbide semiconductor devices.

[0051] In this embodiment, the epitaxial structure includes at least one buffer unit. When the number of buffer units is greater than 1, the nitrogen-doped buffer units 31 and the phosphorus-doped buffer units 32 are alternately and stacked. After multiple implementations, it was found that the density of stacking faults can be significantly reduced to 50% by alternating phosphorus-doping and nitrogen-doping. Therefore, the prepared epitaxial structure with alternating phosphorus-doped buffer units 32 and nitrogen-doped buffer units 31 has a high yield rate and stacking fault defects are suppressed.

[0052] In combination with the first aspect, the number of buffer units is less than or equal to ten.

[0053] Taking into account the process complexity and production efficiency of periodic nitrogen doping and phosphorus doping, the number of buffer units is preferably 1-10, and the prepared epitaxial structure has a high yield and can suppress stacking fault defects.

[0054] As an practicable method, the phosphorus-doped buffer unit 32 is disposed below the nitrogen-doped buffer unit 31. Figure 2 shown.

[0055] As another practicable method, the phosphorus-doped buffer unit 32 is disposed above the nitrogen-doped buffer unit 31. Figure 1 shown.

[0056] Both of the above-mentioned methods can be implemented. In this embodiment, the phosphorus-doped buffer unit 32 is disposed above the nitrogen-doped buffer unit 31 .

[0057] In combination with the first aspect, the phosphorus-doped buffer unit 32 includes at least one phosphorus-doped buffer layer; and the nitrogen-doped buffer unit 31 includes at least one nitrogen-doped buffer layer.

[0058] In this embodiment, the number of buffer units in the combined buffer layer 3 is one or more, and each buffer unit includes a nitrogen-doped buffer unit 31 and a phosphorus-doped buffer unit 32. The nitrogen-doped buffer unit 31 includes one or more phosphorus-doped buffer layers; the nitrogen-doped buffer unit 31 includes one or more nitrogen-doped buffer layers.

[0059] It can be understood that the number of the above-mentioned phosphorus-doped buffer layer, nitrogen-doped buffer layer, and buffer unit can be set according to actual needs and is not limited here.

[0060] As an practicable manner, the number of buffer units is n, where n≥1. The n buffer units are stacked in sequence from bottom to top. Since each buffer unit includes nitrogen-doped buffer units 31 and phosphorus-doped buffer units 32 that are alternately arranged, multiple buffer units are stacked in the height direction, and the nitrogen-doped buffer units 31 and the phosphorus-doped buffer units 32 are in a periodic alternating form. It can be understood that as the number of phosphorus-doped buffer units 32 and nitrogen-doped buffer units 31 increases, the complexity of the epitaxial structure increases significantly, the number of preparation steps increases, and the process is relatively complicated.

[0061] The nitrogen-doped buffer unit 31 is disposed above or below the phosphorus-doped buffer unit 32 .

[0062] As an example, there are two buffer units, which are the first buffer unit and the second buffer unit from bottom to top in the height direction, and each buffer unit includes a nitrogen-doped buffer unit 31 composed of three nitrogen-doped buffer layers and a phosphorus-doped buffer unit 32 arranged below the nitrogen-doped buffer unit 31, and the phosphorus-doped buffer unit includes two phosphorus-doped buffer layers. Combined with the above example, the combined buffer layer 3 in this example includes a first phosphorus-doped buffer layer, a second phosphorus-doped buffer layer, a first nitrogen-doped buffer layer, a second nitrogen-doped buffer layer, a third nitrogen-doped buffer layer, a third phosphorus-doped buffer layer, a fourth phosphorus-doped buffer layer, a fourth nitrogen-doped buffer layer, a fifth nitrogen-doped buffer layer, and a sixth nitrogen-doped buffer layer arranged in sequence from bottom to top.

[0063] As another example, there is one buffer unit in the combined buffer layer, and one or more phosphorus-doped buffer layers constitute a phosphorus-doped buffer unit 32, and one or more nitrogen-doped buffer layers constitute a nitrogen-doped buffer unit 31; then the phosphorus-doped buffer unit 32 is arranged above or below the nitrogen-doped buffer unit 31.

[0064] Combination Figure 3 As shown, in the epitaxial structure provided in the present application, the combined buffer layer 3 includes a plurality of buffer units, each of which includes a phosphorus-doped buffer unit 32 and a nitrogen-doped buffer unit 31. Compared with the single nitrogen-doped silicon carbide epitaxial structure in the prior art, the stacking fault density can be reduced, thereby improving the yield rate. As the number of buffer units increases, a periodic structure of phosphorus-doped buffer unit 32, nitrogen-doped buffer unit 31 ... phosphorus-doped buffer unit 32, nitrogen-doped buffer unit 31 is formed.

[0065] In combination with the first aspect, the epitaxial structure further includes: a modified layer 2 .

[0066] The modified layer 2 is disposed between the substrate 1 and the combined buffer layer 3; the modified layer 2 is a co-doped diffusion layer containing phosphorus and nitrogen, the thickness of the modified layer 2 is 2-20 nm, and the nitrogen doping concentration of the modified layer 2 is 5×10 18 -1×1019 / cm 3 ; Phosphorus doping concentration is 1×10 16 -1×10 19 / cm 3 The above doping concentration can ensure that the conductive properties and defect level of this layer do not change significantly.

[0067] In this way, phosphorus and nitrogen co-diffusion is performed on the substrate 1 to form a modified layer 2, which provides nitrogen doping and phosphorus doping buffering effects for epitaxial growth of a buffer unit or periodic epitaxial growth of multiple buffer units, which is beneficial to the epitaxial growth of the combined buffer layer 3.

[0068] In combination with the first aspect, the drift layer 4 is a nitrogen and phosphorus co-doped silicon carbide drift layer, the thickness of the drift layer is 5-100 nm, and the nitrogen and phosphorus co-doping concentration is 1×10 14 -1×10 17 / cm 3 , thickness and doping concentration settings vary with device voltage levels.

[0069] In this way, based on the characteristic that phosphorus doping can expand the lattice constant, the problem of nitrogen doping lattice constant reduction in mass production process can be compensated. In this way, without changing the doping type, the degree of freedom of lattice constant regulation is increased, thereby improving wafer warpage. In a second aspect, the present application provides a method for preparing an epitaxial structure, which is used to prepare the above-mentioned epitaxial structure. Figure 4 As shown, the method includes:

[0070] S110, providing a substrate, and performing etching pretreatment on the substrate.

[0071] S120, performing multiple epitaxial growths on the pretreated substrate in a hydrogen atmosphere to sequentially form multiple buffer units to obtain a combined buffer layer.

[0072] S130, growing a drift layer on top of the combined buffer layer to obtain an epitaxial structure.

[0073] In the present application, after pre-treatment of the substrate, multiple epitaxial growths are performed, and then a drift layer is grown on top of the combined buffer layer obtained by the epitaxial growth to obtain the above-mentioned epitaxial structure. Nitrogen and phosphorus elements are alternately doped on the surface of the substrate to reduce the probability of inducing stacking faults in the early stage of epitaxy, and the lattice constant can be expanded based on phosphorus doping to compensate for the problem of shrinking lattice constants of nitrogen-doped mass production processes. Without changing the doping type, the freedom of lattice constant control is increased, thereby improving wafer warping.

[0074] In combination with the second aspect, the substrate is a 4H-SiC substrate with the Si surface offset 2-6° toward the [11-20] direction.

[0075] In step S110, etching pretreatment of the substrate specifically includes:

[0076] The 4H-SiC substrate is placed in a high-temperature furnace and the surface is etched in a H2 and / or HCl atmosphere to remove dirt and processing damage layers on the wafer surface.

[0077] In conjunction with the second aspect, before step S120, the method further includes:

[0078] S111, in an environment of a temperature of 1700-2000°C, introducing a phosphorus source and a silicon source into the pretreated substrate to diffuse and dope phosphorus elements on the substrate to form a modified layer on the surface of the substrate; wherein the partial pressure of the phosphorus source is 500-50000Pa, and the partial pressure of the silicon source is 0.5-50Pa.

[0079] Wherein, the phosphorus source is a phosphorus-containing gas. Under the above conditions, the 4H-SiC substrate is diffusely doped with phosphorus to obtain a nitrogen and phosphorus co-doped diffusion layer with a thickness of 2-200nm, which serves as a subsequent epitaxial growth surface. In this process, since phosphorus atoms are relatively large, phosphorus doping can expand the lattice constant and make up for the problem of lattice parameter reduction in the nitrogen doping process of the mass production process. In this way, the lattice constant is increased by doping with phosphorus in step S120, so that the lattice constant can be freely adjusted, and both the doped phosphorus element and the doped nitrogen element are N-type doping. Under the premise of not changing the doping type, the lattice constant loss can be compensated, which is conducive to improving the problem of epitaxial warping exceeding the specification.

[0080] In conjunction with the second aspect, step S120 specifically includes:

[0081] S121, under preset conditions, introducing a carbon source and a silicon source reactant into the reaction chamber and alternately introducing a nitrogen source or a phosphorus source to alternately grow a phosphorus-doped buffer unit and a nitrogen-doped buffer unit to obtain a buffer unit.

[0082] S132, continue to alternately introduce a nitrogen source or a phosphorus source into the reaction chamber to periodically grow one or more buffer units on the buffer unit until a combined buffer layer including a first number of buffer units is obtained.

[0083] Specifically, the 4H-SiC substrate with the modified layer is placed in a reaction chamber used in the epitaxial process, and the reaction chamber is adjusted to the preset conditions, namely: the temperature of the reaction chamber is raised to 1500-1700°C, the pressure in the reaction chamber is 50-200mbar, and hydrogen is introduced into the reaction chamber with a flow rate of 80-130slm.

[0084] Afterwards, a carbon source and a silicon source reactant are introduced into the reaction chamber, and then a nitrogen source or a phosphorus source is introduced alternately. When the nitrogen source is introduced, the nitrogen-doped buffer unit 31 is grown; when the phosphorus source is introduced, the phosphorus-doped buffer unit 32 is grown.

[0085] It is understandable that if the nitrogen source is introduced first, the nitrogen-doped buffer unit 31 is located below the phosphorus-doped buffer unit 32. Conversely, if the phosphorus source is introduced first, the phosphorus-doped buffer unit 32 is located below the nitrogen-doped buffer unit 31.

[0086] In the process of introducing nitrogen source for epitaxy, the nitrogen doping concentration is 1×10 16 -1×10 19 / cm 3 ; During the process of introducing phosphorus source for epitaxy, the phosphorus doping concentration is also 1×10 16 -1×10 19 / cm 3 The thickness of the nitrogen-doped buffer unit 31 and the phosphorus-doped buffer unit 32 obtained by epitaxy is 5-500 nm.

[0087] Afterwards, in step S122, a nitrogen source or a phosphorus source is alternately introduced into the reaction chamber again to repeat the epitaxial growth operation of step S121, thereby preparing another buffer unit on an existing buffer unit. By repeating step S121 multiple times, a combined buffer layer 3 containing a first number of buffer units is prepared. Specifically, the first number can be set according to actual needs and is not limited here.

[0088] In combination with the second aspect, step S130 includes: simultaneously introducing a nitrogen dopant and a phosphorus dopant into the reaction chamber to form a drift layer on the combined buffer layer.

[0089] In the present application, nitrogen dopant and phosphorus dopant are introduced into the reaction chamber simultaneously to deposit a 5-100 nm thick layer on top of the combined buffer layer 3, with a nitrogen and phosphorus co-doping concentration of 1×10 14 -1×10 17 / cm 3 The drift layer 4 is used as the drift layer 4 or the voltage-resistant layer of the chip device to improve the voltage-resistant performance of the chip device.

[0090] Those skilled in the art can clearly understand that, for the convenience and brevity of description, the specific working process of the system and device described above can refer to the corresponding process in the aforementioned method embodiment, and will not be repeated here.

[0091] In addition, in the description of the embodiments of the present invention, 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 or an electrical connection; it can be a direct connection or an indirect connection through an intermediate medium, and it can be the internal communication of two components. For those skilled in the art, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0092] In the description of the present invention, it should be noted that the terms "center", "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.

[0093] Finally, it should be noted that the above embodiments are only specific implementations of the present invention, which are used to illustrate the technical solutions of the present invention, rather than to limit them. The protection scope of the present invention is not limited thereto. Although the present invention is described in detail with reference to the above embodiments, those skilled in the art should understand that any person skilled in the art can still modify the technical solutions recorded in the above embodiments within the technical scope disclosed by the present invention, or can easily think of changes, or make equivalent replacements for some of the technical features therein; and these modifications, changes or replacements do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of the embodiments of the present invention, and should be included in the protection scope of the present invention. Therefore, the protection scope of the present invention shall be based on the protection scope of the claims.

Claims

1. An epitaxial structure, characterized in that: The epitaxial structure comprises: a substrate, a combined buffer layer including at least one buffer unit, and a drift layer which are stacked in sequence from bottom to top; wherein each of the buffer units comprises a nitrogen-doped buffer unit and a phosphorus-doped buffer unit.

2. The epitaxial structure according to claim 1, characterized in that: The phosphorus-doped buffer unit and the nitrogen-doped buffer unit are stacked; the phosphorus-doped buffer unit includes at least one phosphorus-doped buffer layer; and the nitrogen-doped buffer unit includes at least one nitrogen-doped buffer layer.

3. The epitaxial structure according to claim 1, characterized in that: The number of the buffer units is less than or equal to ten.

4. The epitaxial structure according to claim 1, characterized in that: The epitaxial structure further comprises: A modified layer is provided between the substrate and the combined buffer layer; the modified layer is a co-doped diffusion layer containing phosphorus and nitrogen, the thickness of the modified layer is 2-20 nm, and the nitrogen doping concentration of the modified layer is 5×10 18 -1×10 19 / cm 3 ; Phosphorus doping concentration is 1×10 16 -1×10 19 / cm 3 .

5. The epitaxial structure according to claim 1, characterized in that: The drift layer is a co-doped drift layer containing phosphorus and nitrogen. The thickness of the drift layer is 5-100 nm, and the co-doping concentration of nitrogen and phosphorus is 1×10 14 -1×10 17 / cm 3 .

6. A method for preparing an epitaxial structure, characterized in that: Used to prepare the epitaxial structure according to any one of claims 1 to 5; the method comprises: Providing a substrate, and performing etching pretreatment on the substrate; In a hydrogen atmosphere, performing multiple epitaxial growths on the pretreated substrate to sequentially form multiple buffer units to obtain a combined buffer layer; A drift layer is grown on top of the combined buffer layer to obtain the epitaxial structure as claimed in any one of claims 1 to 5.

7. The method according to claim 6, characterized in that The substrate is a 4H-SiC substrate with the Si surface offset 2-6° in the [11-20] direction; The step of performing etching pretreatment on the substrate comprises: The 4H-SiC substrate is placed in a high-temperature furnace, and the surface is etched in a H2 and / or HCl atmosphere to remove dirt and processing damage layers on the wafer surface.

8. The method according to claim 6, characterized in that In a hydrogen atmosphere, multiple epitaxial growths are performed on the pretreated substrate to sequentially form multiple buffer units to obtain a combined buffer layer, and the method further includes: In an environment of a temperature of 1700-2000° C., a phosphorus source and a silicon source are introduced into the pretreated substrate to diffuse and dope phosphorus elements on the substrate to form a modified layer on the surface of the substrate; wherein the partial pressure of the phosphorus source is 500-50000Pa, and the partial pressure of the silicon source is 0.5-50Pa.

9. The method according to claim 6, characterized in that The step of performing multiple epitaxial growths on the pretreated substrate in a hydrogen atmosphere to sequentially form multiple buffer units to obtain a combined buffer layer comprises: Under preset conditions, a carbon source and a silicon source reactant are introduced into the reaction chamber and a nitrogen source or a phosphorus source is introduced alternately to alternately grow a phosphorus-doped buffer unit and a nitrogen-doped buffer unit to obtain a buffer unit; Continue to alternately introduce a nitrogen source or a phosphorus source into the reaction chamber to periodically grow one or more buffer units on the buffer unit until a combined buffer layer including a first number of buffer units is obtained.

10. The method according to claim 9, characterized in that The step of growing a drift layer on top of the combined buffer layer comprises: A nitrogen dopant and a phosphorus dopant are introduced into the reaction chamber simultaneously to form a drift layer on the combined buffer layer.

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