CN113707749B - Epitaxial structure of avalanche focal plane detector and preparation method thereof - Google Patents
Epitaxial structure of avalanche focal plane detector and preparation method thereof Download PDFInfo
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- H01L31/00—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
- H01L31/08—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof in which radiation controls flow of current through the device, e.g. photoresistors
- H01L31/10—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof in which radiation controls flow of current through the device, e.g. photoresistors characterised by at least one potential-jump barrier or surface barrier, e.g. phototransistors
- H01L31/101—Devices sensitive to infrared, visible or ultraviolet radiation
- H01L31/102—Devices sensitive to infrared, visible or ultraviolet radiation characterised by only one potential barrier or surface barrier
- H01L31/107—Devices sensitive to infrared, visible or ultraviolet radiation characterised by only one potential barrier or surface barrier the potential barrier working in avalanche mode, e.g. avalanche photodiode
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- H01L31/035236—Superlattices; Multiple quantum well structures
- H01L31/035263—Doping superlattices, e.g. nipi superlattices
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- H01L31/18—Processes or apparatus specially adapted for the manufacture or treatment of these devices or of parts thereof
- H01L31/184—Processes or apparatus specially adapted for the manufacture or treatment of these devices or of parts thereof the active layers comprising only AIIIBV compounds, e.g. GaAs, InP
- H01L31/1844—Processes or apparatus specially adapted for the manufacture or treatment of these devices or of parts thereof the active layers comprising only AIIIBV compounds, e.g. GaAs, InP comprising ternary or quaternary compounds, e.g. Ga Al As, In Ga As P
- H01L31/1848—Processes or apparatus specially adapted for the manufacture or treatment of these devices or of parts thereof the active layers comprising only AIIIBV compounds, e.g. GaAs, InP comprising ternary or quaternary compounds, e.g. Ga Al As, In Ga As P comprising nitride compounds, e.g. InGaN, InGaAlN
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Abstract
The invention belongs to the field of wide bandgap semiconductors and optoelectronic devices, and particularly relates to an epitaxial structure of an avalanche focal plane detector and a preparation method thereof; the epitaxial structure comprises an unintentionally doped AlGaN absorption layer, an N-type AlGaN composition graded layer, an N-type AlGaN superlattice layer, an unintentionally doped AlN template layer and an AlN single crystal substrate from top to bottom. The epitaxial structure can solve the problems of high lattice mismatch and thermal mismatch of the sapphire substrate, high defect density of epitaxial materials, poor thermal conductivity and the like to a certain extent, and improves the sensitivity, response uniformity and reliability of the APD focal plane array.
Description
Technical Field
The invention belongs to the field of wide bandgap semiconductors and optoelectronic devices, and particularly relates to an epitaxial structure of an avalanche focal plane detector and a preparation method thereof.
Background
The ultraviolet avalanche focal plane detector is a novel all-solid-state image sensor based on nitride materials, has the characteristics of miniaturization, high sensitivity, strong environmental adaptability, low power consumption and the like, and is mainly used for detecting and imaging weak ultraviolet signals.
The photosensitive part of the ultraviolet avalanche focal plane detector is composed of a nitride avalanche diode (Avalanche Photo Diode, APD for short) array, namely each pixel comprises a unit APD and is responsible for converting ultraviolet light signals into electric signals. As shown in fig. 1, the ultraviolet detector is typically formed by epitaxially growing P-type and N-type multi-layer nitride materials on a sapphire substrate. However, when a high-voltage and high-current device such as an array-type ultraviolet APD is manufactured using a sapphire substrate, there are the following problems:
1) Lattice and thermal mismatch of nitrides, such as AlGaN epitaxial films, with substrate materials results in high threading dislocation density, which severely affects dark current and reliability of devices at high voltages;
2) The APD working voltage is high, the heat generation of the array chip is high, the thermal conductivity of the sapphire substrate is low, the APD working point is easy to drift, and the working voltage is unstable, so that the response uniformity and the detection efficiency of the focal plane detector are affected;
3) In the detector array, the lateral conduction distances from the pixels at different positions to the common N electrode are different, and as the array scale is enlarged, the lateral conduction resistance (total series resistance is counted) between the pixels and the N electrode has a larger influence on the transmission efficiency, and the response uniformity of the focal plane array is influenced.
Disclosure of Invention
Aiming at the problems of high dark current, unstable working point, poor heat dissipation capability, high transverse conduction resistance and the like of a sapphire substrate ultraviolet avalanche focal plane detector under high voltage, the invention develops a novel monocrystal substrate APD focal plane detector epitaxial structure, solves the problems of high lattice mismatch and thermal mismatch of the sapphire substrate, high defect density of epitaxial materials, poor heat conductivity, high transverse conduction resistance of a large area array focal plane and the like, and finally improves the sensitivity, response uniformity and reliability of the APD focal plane.
The invention provides an epitaxial structure of an avalanche focal plane detector and a preparation method thereof.
In a first aspect thereof, the present invention provides an avalanche focal plane detector epitaxial structure comprising, from top to bottom, an unintentionally doped (Unintentional doping, UID) AlGaN (aluminum gallium nitride) absorber layer, an N-type AlGaN compositionally graded layer, an N-type AlGaN superlattice layer, an unintentionally doped AlN template layer, and an AlN (aluminum nitride) single crystal substrate.
Further, the unintentionally doped absorption layer adopts Al x1 Ga 1-x1 An N material; the thickness range is 0.1-0.5 mu m; wherein x1 is Al component, x1 is more than or equal to 0.2 and less than or equal to 0.6.
Further, the N-type component graded layer adopts Al x2 Ga 1-x2 An N material; the thickness range is 0.01-0.1 mu m; wherein x2 is an Al component, and x1 is more than or equal to x2 and less than or equal to x3.
Further, the N-type AlGaN superlattice layer comprises one or more periodic structures stacked along a direction perpendicular to the surface of the substrate; the periodic structure adopts two different N-type AlGaN materials which are alternated up and down; wherein the uppermost layer of the N-type AlGaN superlattice layer is Al x3 Ga 1-x3 N material, the lowest layer is Al x4 Ga 1-x4 N material with thickness of 0.005-0.01 μm; x3 and x4 respectively represent different Al components, x3 is more than or equal to 0.6 and less than or equal to 0.8, and x3+0.1 is more than or equal to x4 and less than or equal to 1.
Further, the thickness of the unintentionally doped AlN template layer serving as a buffer layer is 0.2-1 mu m.
Furthermore, the AlN single crystal substrate is made of AlN material, and the crystal orientation of the AlN single crystal substrate can be selected from (001).
In a second aspect of the present invention, the present invention further provides an avalanche focal plane detector, wherein the epitaxial wafer structure of the chip includes an epitaxial structure of the avalanche focal plane detector according to the first aspect of the present invention, and a diode array is formed on the epitaxial structure by processing, so as to obtain the avalanche focal plane detector chip by processing.
In a third aspect of the present invention, the present invention further provides a method for preparing an epitaxial structure of an avalanche focal plane detector, where the preparation method includes:
s1: cleaning and polishing the upper and lower surfaces of the AlN single crystal substrate;
s2: growing a layer of unintentionally doped AlN template layer on the surface of the AlN single crystal substrate by adopting a metal organic chemical vapor deposition technology;
s3: sequentially growing an N-type AlGaN superlattice layer on the unintended doped AlN template layer by adopting a metal organic chemical vapor deposition technology;
s4: growing a silicon doped N-type AlGaN composition graded layer on the N-type AlGaN superlattice layer by adopting a metal organic chemical vapor deposition technology;
s5: and (3) growing an unintentionally doped AlGaN absorption layer on the N-type AlGaN composition graded layer by adopting a metal organic chemical vapor deposition technology.
Further, in the step S1, the AlN single crystal substrate has a crystal orientation of (001), and the upper and lower surfaces thereof need to be cleaned and polished to ensure surface flatness.
Further, the step S3 comprises sequentially growing N-type Al with alternating high and low Al components doped with silicon on the unintentionally doped AlN template layer by adopting a metal organic chemical vapor deposition technology x Ga 1-x An N superlattice structure layer; wherein the growth temperature is 1000-1300 ℃, the growth pressure is 50-500torr, and the Al with high Al component x4 Ga 1-x4 The N component is x3+0.1.ltoreq.x4.ltoreq.1, and the Al component is Al with low Al content x3 Ga 1-x3 The N component is 0.6-0.3-0.8, the thickness is 0.005-0.01 μm, the symbiotic length is 5-10 periodic structures, and the electron concentration is 1×10 17 -1×10 19 /cm 3 。
Further, the step S4 includes growing a silicon doped N-type Al on the N-type AlGaN superlattice layer by a metal organic chemical vapor deposition technique x2 Ga 1-x2 An N-component gradual change layer; wherein x1 is less than or equal to x2 is less than or equal to x3, the growth temperature is 1000-1300 ℃, the growth pressure is 50-500torr, the thickness is 0.01-0.1 mu m, and the electron concentration is 1 multiplied by 10 17 -1×10 19 /cm 3 。
Further, the step S5 includes using metal organic chemical vapor deposition technique to deposit N-type Al x1 Ga 1- x1 Growth of unintentionally doped Al on N-component graded layer x1 Ga 1-x1 An N absorption layer; wherein x1 is more than or equal to 0.2 and less than or equal to 0.6, the growth temperature is 1000-1300 ℃, the growth pressure is 50-500torr, and the thickness is 0.1-0.5 mu m.
The invention has the beneficial effects that:
compared with the prior art, the invention uses the monocrystalline AlN substrate to replace the sapphire substrate, on one hand, the bit density in the epitaxial layer can be reduced, and the dark current can be reduced; on the other hand, the heat radiation capacity of the whole focal plane can be increased; the invention also replaces the conventional N-type AlGaN contact layer with the AlGaN two-dimensional superlattice structure layer with alternating N-type high-low Al components, on one hand, the high mobility two-dimensional electron gas generated by the nitride polarization effect can be utilized, the transverse conduction current density of the N-type layer is improved, and the problem of uneven series resistance of pixels in a large-scale focal plane array is solved; on the other hand, the superlattice structure is used as a material buffer layer, so that stress generated between an AlN template layer and an AlGaN two-dimensional superlattice structure layer due to lattice mismatch is reduced, and the quality of epitaxial materials and the sensitivity of an APD (avalanche photo diode) are improved; therefore, the epitaxial structure can solve the problems of high lattice mismatch and thermal mismatch of the sapphire substrate, high defect density of epitaxial materials, poor thermal conductivity and the like to a certain extent, and improves the sensitivity, response uniformity and reliability of the APD focal plane array.
Drawings
FIG. 1 is a cross-sectional view of a conventional avalanche focal plane detector epitaxial structure;
FIG. 2 is a cross-sectional view of an epitaxial structure of an avalanche focal plane detector in an embodiment of the present invention;
FIG. 3 is a schematic view of an avalanche focal plane detector in an embodiment of the present invention;
fig. 4 is a flow chart of the preparation of an epitaxial structure of an avalanche focal plane detector in an embodiment of the present invention.
Detailed Description
The following description of the embodiments of the present invention will be made clearly and completely with reference to the accompanying drawings, in which it is apparent that the embodiments described are only some embodiments of the present invention, but not all embodiments. 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.
For clarity, the various features of the drawings are not drawn to scale. Furthermore, some well-known portions may not be shown. For simplicity, the epitaxial structure obtained after several steps may be depicted in one figure.
It will be understood that when a layer, an area, or a structure of a device is described as being "on" or "over" another layer, another area, it can be referred to as being directly on the other layer, another area, or further layers or areas can be included between the other layer, another area, etc. And if the device is flipped, the one layer, one region, will be "under" or "beneath" the other layer, another region.
If, for the purposes of describing a situation directly overlying another layer, another region, the expression "directly overlying … …" or "overlying … … and adjoining" will be used herein.
Numerous specific details of the invention, such as device structures, materials, dimensions, processing techniques and technologies, are set forth in the following description in order to provide a thorough understanding of the invention. However, as will be understood by those skilled in the art, the present invention may be practiced without these specific details.
The invention may be embodied in various forms, some examples of which are described below.
FIG. 2 shows a cross-sectional view of an APD focal plane detector material epitaxial structure in an embodiment of the invention; as shown in fig. 2, the epitaxial structure in the embodiment of the present invention includes: an unintentionally doped AlGaN absorption layer, an N-type AlGaN composition graded layer, an N-type AlGaN superlattice layer, an unintentionally doped AlN template layer and an AlN single crystal substrate which are arranged from top to bottom.
In the embodiment, the aluminum nitride has the characteristics of higher carrier migration rate, ultrahigh breakdown field strength and the like, so that the AlN monocrystal substrate is adopted to replace the traditional sapphire substrate, and on one hand, the bit density in an epitaxial layer is reduced, so that dark current is reduced; in addition, the AlN single crystal substrate has high thermal conductivity and high thermal stability, so that the heat dissipation capacity of the whole focal plane can be increased; meanwhile, the crystal orientation of the AlN single crystal substrate used in this embodiment is (001), and of course, alN single crystal substrates having other crystal orientations may be used, which is not particularly limited in the present invention.
In the embodiment of the invention, the unintentionally doped AlN template layer is an undoped AlN buffer layer, and a buffer structure formed by the AlN buffer layer can release lattice mismatch stress; therefore, in order to achieve reasonable release requirements, the thickness of the material is set to be 0.2-1 μm, so that the release requirements of lattice adaptive stress in a device are met.
In an embodiment of the present invention, the N-type AlGaN superlattice layer includes one or more periodic structures stacked in a direction perpendicular to a substrate surface; the periodic structure adopts two different N-type AlGaN materials which are alternated up and down; therefore, the total layer number of the two N-type AlGaN materials is not less than 2 layers, wherein the uppermost layer of the N-type AlGaN superlattice layer is Al x3 Ga 1-x3 N material, the lowest layer is Al x4 Ga 1-x4 And the material N, x3 and x4 respectively represent different Al components, and x3 is more than or equal to 0.6 and less than or equal to 0.8, and x3 and x4 are more than or equal to 0.1 and less than or equal to 1.
The novel AlGaN two-dimensional superlattice structure layer with alternating N-type high-low Al components can improve the transverse conduction current density of the N-type layer and solve the problem of uneven series resistance of pixels in a large-scale focal plane array; meanwhile, the superlattice structure is used as a material buffer layer, so that stress generated between an AlN template layer and an AlGaN two-dimensional superlattice structure layer due to lattice mismatch is reduced, and the quality of epitaxial materials and the sensitivity of an APD are improved. The present application has the following advantages over the conventional technology, such as chinese patent No. CN105742387a, and the conventional technology is different from the present application in the manner and location of using the superlattice:
1) The patent uses a superlattice structure (layer number 104) as a multiplication layer, requires a low superlattice barrier and has higher carrier transport efficiency and response speed, and therefore a graded layer must be inserted into the structure, so that the N-type contact layer of the patent can only use a traditional single-layer material.
2) The superlattice structure is utilized as the N-type contact layer, and the graded layer cannot be inserted like the conventional technology, wherein the graded layer is not suitable for the following reasons: the AlGaN superlattice structure can only form obvious two-dimensional electron gas by using abrupt components (high-low phase), and utilizing spontaneous (or piezoelectric) polarization effect generated by component difference between layers, thereby improving the transverse conductivity of an N-type contact layer, achieving the effect of the invention, and the superlattice layer with gradual components can not form obvious two-dimensional electron gas, and periodic potential wells and potential barriers; the meaning of the graded layer is that: the stress between two layers with large component difference is reduced, and the carrier transmission efficiency and response speed are improved. The superlattice layer does not absorb ultraviolet light and is only used as a conductive layer, so that a gradual change layer is not needed to improve transmission efficiency and response speed; in addition, the composition difference between the superlattices is small, and the composition difference between the superlattices and the absorption layer is large, so that the lattice mismatch can be reduced by using a gradual change layer between the top layer of the superlattice and the absorption layer.
3) The superlattice layer is doped with N+ type, so that the lower electrode (N electrode of the diode) of the avalanche focal plane detector is prepared on the layer; the upper electrode of the avalanche focal plane detector is directly manufactured on the i-type (UID) absorption layer without a P-type layer, compared with the prior art, the mode can reduce the proportion of electrons participating in avalanche multiplication, and simultaneously reduces the tunneling current of the narrow band gap P-type layer under high voltage, so that the APD has lower avalanche noise and dark current.
Wherein the number of the periodic structures is 5-10, and the thickness is 0.005-0.01 mu m; the number and thickness of the periodic structures determined in the application are obtained through trial and error, taking into consideration
If the thickness is too thick, the barrier of the superlattice (multi-quantum well) becomes large, and the transport of carriers is affected; if the thickness is too thin, the growth process is not easy to control, the material has poor crystallization quality and defects, and the performance of the device is affected; meanwhile, the number of periodic structures is considered, and if the number is too small, the difficulty in etching the electrode is high; if the number is too large, the total thickness is too thick, which affects the internal quantum efficiency of the device. The number of cycles and the thickness range of each layer were thus obtained on the basis of the previous tests.
In the embodiment of the invention, the unintentionally doped absorption layer adopts Al x1 Ga 1-x1 An N material; the thickness range is 0.1-0.5 mu m; wherein x1 is Al component, x1 is more than or equal to 0.2 and less than or equal to 0.6.
In the embodiment of the invention, the N-type component graded layer adopts Al x2 Ga 1-x2 An N material;the thickness range is 0.01-0.1 mu m; wherein x2 is an Al component, and x1 is more than or equal to x2 and less than or equal to x3.
Wherein the Al component x2 in the N-type component gradual change layer can gradually change from x1 to x3; such gradual fashion includes, but is not limited to, linear variation, step variation, exponential variation, trigonometric variation, etc.; the change pattern is a monotonically increasing change pattern.
In the traditional technology, the gradual change layer is inserted between different component layers and is used for reducing lattice mismatch stress, so that the growth quality of materials is improved, the superlattice is generally not gradual change in the traditional technology, otherwise, the effect of reducing the lattice mismatch stress cannot be achieved, and in the application, the superlattice structure is only required to be used as a material buffer layer, so that the stress generated by lattice mismatch between an AlN template layer and an AlGaN two-dimensional superlattice structure layer is reduced, and the quality of epitaxial materials and the sensitivity of APD are improved.
Fig. 3 is an avalanche focal plane detector in an embodiment of the present invention, including, as shown in fig. 3.
Fig. 4 is a flowchart of a preparation process of an epitaxial structure of an avalanche focal plane detector in an embodiment of the present invention, as shown in fig. 4, where the preparation process includes:
s1: cleaning and polishing the upper and lower surfaces of the AlN single crystal substrate;
in the step S1, the crystal orientation of the AlN single crystal substrate is (001), but it is needless to say that substrates with other crystal orientations may be selected, and the upper and lower surfaces thereof need to be cleaned and polished to ensure a flat surface.
S2: growing a layer of unintentionally doped AlN template layer on the surface of the AlN single crystal substrate by adopting a metal organic chemical vapor deposition technology;
in the step S2, a metal organic chemical vapor deposition technology is adopted to grow an AlN buffer layer with the thickness of 0.2-1 mu m on the surface of the AlN monocrystalline substrate silicon substrate.
S3: sequentially growing an N-type AlGaN superlattice layer on the unintended doped AlN template layer by adopting a metal organic chemical vapor deposition technology;
the step S3 comprises adopting a metal organic chemical vapor deposition technologySequentially growing silicon-doped N-type Al with alternating high and low Al components on the unintentionally doped AlN template layer x Ga 1-x An N superlattice structure layer 3; wherein the growth temperature is 1000-1300 ℃, the growth pressure is 50-500torr, and the Al with high Al component x4 Ga 1-x4 The N component is x3+0.1.ltoreq.x4.ltoreq.1, and the Al component is Al with low Al content x3 Ga 1-x3 The N component is 0.6-0.3-0.8, the thickness is 0.005-0.01 μm, the symbiotic length is 5-10 periodic structures, and the electron concentration is 1×10 17 -1×10 19 /cm 3 。
S4: growing a silicon doped N-type AlGaN composition graded layer on the N-type AlGaN superlattice layer by adopting a metal organic chemical vapor deposition technology;
the step S4 comprises the step of growing silicon doped N-type Al on the N-type AlGaN superlattice layer by adopting a metal organic chemical vapor deposition technology x2 Ga 1-x2 An N-component gradual change layer; wherein x1 is less than or equal to x2 is less than or equal to x3, the growth temperature is 1000-1300 ℃, the growth pressure is 50-500torr, the thickness is 0.01-0.1 mu m, and the electron concentration is 1 multiplied by 10 17 -1×10 19 /cm 3 。
S5: and (3) growing an unintentionally doped AlGaN absorption layer on the N-type AlGaN composition graded layer by adopting a metal organic chemical vapor deposition technology.
The step S5 comprises adopting metal organic chemical vapor deposition technology to perform N-type Al x1 Ga 1-x1 Growth of unintentionally doped Al on N-component graded layer x1 Ga 1-x1 An N absorption layer; wherein x1 is more than or equal to 0.2 and less than or equal to 0.6, the growth temperature is 1000-1300 ℃, the growth pressure is 50-500torr, and the thickness is 0.1-0.5 mu m.
It is to be understood that the avalanche focal plane detector epitaxial structure, the avalanche focal plane detector and the avalanche focal plane detector method of the present invention all belong to the same concept of the present invention, and the corresponding features thereof can be cited with each other.
In the description of the present invention, it should be understood that the terms "coaxial," "bottom," "one end," "top," "middle," "another end," "upper," "one side," "top," "inner," "outer," "front," "center," "two ends," etc. indicate or are based on the orientation or positional relationship shown in the drawings, merely to facilitate description of the invention and simplify the description, and do not indicate or imply that the devices or elements referred to must have a specific orientation, be configured and operated in a specific orientation, and therefore should not be construed as limiting the invention.
In the present invention, unless explicitly specified and limited otherwise, the terms "mounted," "configured," "connected," "secured," "rotated," and the like are to be construed broadly, and may be, for example, fixedly connected, detachably connected, or integrally formed; can be mechanically or electrically connected; either directly or indirectly through intermediaries, or in communication with each other or in interaction with each other, unless explicitly defined otherwise, the meaning of the terms described above in this application will be understood by those of ordinary skill in the art in view of the specific circumstances.
Although embodiments of the present invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made therein without departing from the principles and spirit of the invention, the scope of which is defined in the appended claims and their equivalents.
Claims (7)
1. An epitaxial structure of an avalanche focal plane detector is characterized by comprising an unintentionally doped AlGaN absorption layer, an N-type AlGaN composition gradual change layer, an N-type AlGaN superlattice layer, an unintentionally doped AlN template layer and an AlN single crystal substrate from top to bottom; the N-type AlGaN superlattice layer comprises one or more periodic structures stacked along the direction vertical to the surface of the substrate; the periodic structure adopts two different N-type AlGaN materials which are alternated up and down; wherein the uppermost layer of the N-type AlGaN superlattice layer is Al x3 Ga 1-x3 N material, the lowest layer is Al x4 Ga 1-x4 N material with thickness of 0.005-0.01 μm; x3 and x4 respectively represent different Al components, x3 is more than or equal to 0.6 and less than or equal to 0.8, and x3+0.1 is more than or equal to x4 and less than or equal to 1.
2. The epitaxial structure of the avalanche focal plane detector of claim 1, wherein said unintentionally doped absorption layer is Al x1 Ga 1-x1 An N material; the thickness range is 0.1-0.5 μm; wherein x1 is Al component, x1 is more than or equal to 0.2 and less than or equal to 0.6.
3. The epitaxial structure of the avalanche focal plane detector of claim 1, wherein the N-type graded layer is Al x2 Ga 1-x2 An N material; the thickness range is 0.01-0.1 mu m; wherein x2 is Al component, x1 is less than or equal to x2 is less than or equal to x3, x1 is unintentionally doped with Al component in the absorption layer, and x3 is the Al component of the uppermost layer of the N-type AlGaN superlattice layer.
4. An avalanche focal plane detector, characterized in that the epitaxial wafer structure of the chip comprises an epitaxial structure of the avalanche focal plane detector according to any one of claims 1-3, and a diode array is formed on the epitaxial structure in a processing way, so that the avalanche focal plane detector chip is obtained.
5. The preparation method of the epitaxial structure of the avalanche focal plane detector is characterized by comprising the following steps of:
s1: cleaning and polishing the upper and lower surfaces of the AlN single crystal substrate;
s2: growing a layer of unintentionally doped AlN template layer on the surface of the AlN single crystal substrate by adopting a metal organic chemical vapor deposition technology;
s3: sequentially growing an N-type AlGaN superlattice layer on the unintended doped AlN template layer by adopting a metal organic chemical vapor deposition technology;
sequentially growing silicon-doped N-type Al with alternating high and low Al components on the unintentionally doped AlN template layer by adopting a metal organic chemical vapor deposition technology x Ga 1-x An N superlattice structure layer; wherein the growth temperature is 1000-1300 ℃ and the growth pressure is 50-500torr, wherein x represents the Al component in the superlattice structure layer;al of high Al component x4 Ga 1-x4 The N component is x3+0.1.ltoreq.x4.ltoreq.1, and the Al component is Al with low Al content x3 Ga 1-x3 The N component is 0.6-x 3-0.8, the thickness is 0.005-0.01 μm, the symbiotic length is 5-10 periodic structures, and the electron concentration is 1×10 17 -1×10 19 /cm 3 ；
S4: growing a silicon doped N-type AlGaN composition graded layer on the N-type AlGaN superlattice layer by adopting a metal organic chemical vapor deposition technology;
s5: and (3) growing an unintentionally doped AlGaN absorption layer on the N-type AlGaN composition graded layer by adopting a metal organic chemical vapor deposition technology.
6. The method of claim 5, wherein the step S4 includes growing silicon doped N-type Al on the N-type AlGaN superlattice layer by metal organic chemical vapor deposition (mocvd) technique x2 Ga 1-x2 An N-component gradual change layer; wherein x1 is less than or equal to x2 is less than or equal to x3, x1 is an unintentionally doped Al component in the absorption layer, the growth temperature is 1000-1300 ℃, the growth pressure is 50-500torr, the thickness is 0.01-0.1 mu m, and the electron concentration is 1X 10 17 -1×10 19 /cm 3 。
7. The method of claim 5, wherein the step S5 includes growing unintentionally doped Al on the N-type AlGaN graded layer by metal organic chemical vapor deposition x1 Ga 1-x1 An N absorption layer; wherein x1 is more than or equal to 0.2 and less than or equal to 0.6, the growth temperature is 1000-1300 ℃, the growth pressure is 50-500torr, and the thickness is 0.1-0.5 mu m.
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