Preparation method of epitaxial material of back irradiation silicon-based barrier impurity band detector

Abstract

The invention discloses a preparation method of a back irradiation silicon-based barrier impurity band detector epitaxial material. Comprising the following steps: depositing a silicon oxide ion implantation protection layer on the surface of a high-purity silicon substrate through a plasma enhanced chemical vapor deposition method; selecting low implantation dose and energy parameters, performing ion implantation on a proper amount of N-type ions to form an ultrathin high-conductivity buried layer, performing lattice damage repair by adopting high-temperature rapid annealing, and then removing surface silicon oxide at normal temperature by using a buffer oxide etching solution BOE; epitaxially preparing a highly-doped active region absorption layer on the ultrathin high-conductivity buried layer through a chemical vapor deposition method; and then preparing an intrinsic silicon barrier layer through epitaxy by a chemical vapor deposition method. Compared with a barrier impurity band detector buried layer structure prepared by using a chemical vapor deposition method and a high-dose ion implantation method, the barrier impurity band detector buried layer structure with high doping, low diffusion and ultrathin barrier impurity band detector buried layer structure can be formed by adopting ion implantation parameters with lower dose and energy.

Description

Technical Field

[0001] The invention belongs to the technical field of medium-wave and long-wave infrared photoelectric detector material structure preparation, and in particular relates to a method for preparing a back-illuminated silicon-based impurity-barrier detector epitaxial material. Background Art

[0002] The frequency of the terahertz band is between 0.1-10THz, between far infrared and microwave. Due to its unique band characteristics, terahertz detection technology has broad application space in astronomical observation, high-frequency communication and non-destructive testing. The blocked impurity band (BIB) terahertz detector uses a heavily doped absorption layer to form an impurity band. The electrons in the impurity band absorb radiation and transition to the conduction band to form photogenerated carriers. Under the drive of an external bias, they are collected by the positive electrode through the curved conduction band. At the same time, the intrinsic blocking layer is used to effectively suppress the generation of dark current. Due to the special structural design of the BIB detector, it has the advantages of wide spectral response band, strong radiation resistance, high sensitivity, low dark current and easy preparation of large-area focal plane arrays. It has been applied to space satellites and space telescopes many times, such as the Cosmic Background Explorer (COBE satellite), the Infrared Astronomical Satellite (IRAS), and the James Webb Space Telescope (JWST) and Spitzer Space Telescope (SST) of NASA, all of which are equipped with large-scale focal plane array BIB detectors.

[0003] Silicon-based epitaxial materials with excellent performance and designed to meet the requirements are the basis and key to realizing highly sensitive silicon-based BIB detectors. The device structure of silicon-based barrier impurity band detectors is composed of a high-resistance silicon substrate, a high-conductivity buried layer, a highly doped active region absorption layer, and an intrinsic silicon barrier layer from bottom to top. The key parameters of the material include: material crystal orientation and parameters such as the resistivity and thickness of each layer. Chemical vapor deposition (CVD) is a generally recognized epitaxial technology in the current material growth process. Specifically, it refers to a process technology that controls the gas to form a solid film on the surface of the substrate by precisely controlling parameters such as gas flow, temperature, and pressure. However, due to the long epitaxial growth time and the high temperature of the preparation conditions, the impurities in the highly doped film are easily diffused and self-doped into the upper and lower adjacent structures, which is not conducive to the preparation of highly doped ultra-thin buried layers.

[0004] At present, the buried layer diffusion thickness of the silicon-based BIB detector prepared by chemical vapor deposition (CVD) is usually in the micron range, which results in the buried layer thickness being too thick when the back-illuminated silicon-based blocking impurity band detector is working. The 10~30μm far-infrared band is severely attenuated by the buried layer, and the incident light transmitted to the active area absorption layer is reduced, which seriously affects the detection performance of the silicon-based BIB detector. Summary of the invention

[0005] In view of the shortcomings of the buried layer structure of the silicon-based blocking impurity band detector prepared by the above-mentioned vapor deposition method (CVD), the purpose of the present invention is to provide a method for preparing a back-illuminated silicon-based blocking impurity band detector epitaxial material. The present invention finds that the ion implantation conditions are high in dose and high in implantation energy, and the buried layer junction depth formed after annealing is large and easy to form a plasma effect. The high-dose and high-energy implantation conditions are not only easy to damage the substrate crystal quality, but also the formed buried layer structure has serious attenuation of the incident light. The present invention adopts low-dose and low-energy phosphorus or arsenic ion implantation to prepare the buried layer structure of the blocking impurity band detector; by optimizing the ion implantation parameters, a high-doping, low-diffusion buried layer structure can be formed on the surface of the high-purity silicon substrate; without affecting the conductive performance, by adopting relatively low-dose and low-energy implantation conditions, an ultra-thin junction depth and a sharp distribution of doping elements can be formed on the surface of the high-resistance silicon substrate, while reducing the plasma effect, it also reduces the attenuation of the incident light by the buried layer prepared by ion implantation; in addition, by optimizing the thickness of the oxide layer grown before ion implantation (≤12nm), the lattice caused by ion implantation can be effectively reduced. damage problem, and at the same time regulate the position of the impurity concentration peak; at the same time, rapid high-temperature annealing (≤10s) is performed after ion implantation to effectively repair the lattice damage problem caused by ion implantation and reduce the impurity diffusion length; finally, a buffered oxide etchant (BOE) is used to remove the surface silicon oxide ion implantation protection layer. By replacing hydrofluoric acid with BOE, the damage to the surface of the high-resistance silicon material can be reduced. The above method can reduce the damage of ion implantation to the high-resistance silicon material while increasing the transmittance of the buried layer structure in the far-red band of 10~30μm, thereby improving the absorption of incident light by the back-illuminated silicon-based impurity-blocking band detector.

[0006] The objective of the present invention is achieved through the following technical solutions: The present invention provides a method for preparing a back-illuminated silicon-based impurity-barrier detector epitaxial material, comprising the following steps: S1. On the surface of the high-purity silicon substrate, a silicon oxide ion implantation protective layer (thickness ≤ 12 nm) is deposited on the surface of the high-purity silicon substrate by plasma enhanced chemical vapor deposition; S2. Select low implantation dose and energy parameters on the silicon oxide ion implantation protective layer, and implant an appropriate amount of N-type ions through an ion implanter to form an ultra-thin high-conductivity buried layer; use high-temperature rapid annealing to repair lattice damage, and then use buffered oxide etching solution BOE at room temperature to remove the surface silicon oxide ion implantation protective layer; S3, epitaxially preparing a highly doped active region absorption layer on the ultra-thin high conductivity buried layer by chemical vapor deposition; S4, epitaxially preparing an intrinsic silicon barrier layer on the highly doped active region absorption layer by chemical vapor deposition.

[0007] The high-purity silicon substrate crystal phase is <100> , thickness is 500~800um, and resistivity is greater than 20000 Ω·cm.

[0008] As an embodiment, in step S2, the implanted ions are phosphorus or arsenic ions.

[0009] As an embodiment, the ion implantation energy is 20-30 keV and the implantation dose is 1×10 14 ~3×10 14 cm -2 , the injection angle is 1 to 7 degrees.

[0010] As an embodiment, the high temperature annealing is performed at a temperature of 1000 to 1200° C. and for a time of 3 to 10 seconds.

[0011] As an embodiment, the thickness of the silicon oxide ion implantation protection layer is 5 to 12 nm.

[0012] As an embodiment, the thickness of the ultra-thin high conductivity buried layer is 0.1-0.5 um.

[0013] As an embodiment, the concentration of the buffered oxide etching solution BOE is 10% to 25%, and the surface silicon oxide is rinsed and removed, and the rinsing time is 30 to 60 seconds.

[0014] As an implementation scheme, in step S3, the doping element is Ga, the epitaxial thickness is 20-50 μm, and the doping concentration is 2×10 17 ~1×10 19 cm -3 .

[0015] As an implementation scheme, in step S4, the epitaxial thickness is 2-10 um, and the resistivity is required to be greater than 1000 Ω·cm.

[0016] Compared with the prior art, the present invention has the following beneficial effects: Compared with the buried layer structure of the blocking impurity band detector prepared by chemical vapor deposition and high-dose ion implantation, the present invention can form a high-doped, low-diffusion, ultra-thin buried layer structure of the blocking impurity band detector by adopting ion implantation parameters with lower dose and energy. The present invention optimizes the ion implantation parameters, reduces the implantation dose and energy, and forms a high charge collection efficiency buried layer structure on the surface of the high-resistance silicon substrate, while also forming a non-damaged buried layer structure with extremely high doping concentration within an extremely thin range; the high-doped buried layer of the blocking impurity band detector prepared by epitaxy of chemical vapor deposition is usually diffused at high temperature, and the diffusion thickness can reach the micron level; while the high-doped buried layer of the blocking impurity band detector prepared by high-dose and high-energy ion implantation has a large buried layer junction depth after annealing and is easy to form a plasma effect. The high-dose and high-energy implantation conditions are not only easy to damage the substrate crystal quality, but also the prepared buried layer structure has serious attenuation of the incident light. The present invention adopts a low-dose and low-energy ion implantation method to form a high-doped, low-diffusion, ultra-thin buried layer structure of the blocking impurity band detector, with a thickness of hundreds of nanometers, and the diffusion thickness is effectively suppressed. When the back-illuminated silicon-based impurity-blocking band detector is working, the thicker the buried layer diffusion thickness, the more serious the absorption and attenuation phenomenon of the buried layer in the 10~30μm far-infrared band, resulting in less incident light transmitted to the active region absorption layer, which seriously affects the detection performance of the silicon-based BIB detector. Therefore, the preparation of the epitaxial material buried layer of the back-illuminated silicon-based impurity-blocking band detector by low-dose and low-energy ion implantation can effectively increase the transmittance of the buried layer structure in the 10~30μm far-red band and improve the absorption of the incident light by the back-illuminated silicon-based impurity-blocking band detector. At the same time, by reducing the thickness of the oxide layer grown before ion implantation and reducing the high-temperature annealing time after ion implantation, the lattice damage caused by ion implantation can be effectively repaired, reducing the impact on the quality of the subsequent absorption layer and barrier layer crystals prepared by chemical vapor deposition epitaxy. The test results show that the buried layer structure of the blocking impurity band detector prepared on a high-purity silicon substrate by ion implantation can reduce the reflection and absorption of the incident light by the buried layer structure. At the same time, by adjusting and optimizing the parameters such as the implantation angle, implantation energy and implantation dose of the ion implantation, the transmittance in the far-red band of 10~30μm can be optimized, and the transmittance can reach more than 40%, thereby improving the absorption of incident light by the back-illuminated silicon-based blocking impurity band detector. Other features, objects and advantages of the present invention will become more apparent from the detailed description of non-limiting embodiments made with reference to the following drawings: Figure 1 This is a structural diagram of the back-illuminated silicon-based impurity-barrier detector epitaxial material preparation proposed by the present invention, wherein 1 is a high-purity silicon substrate, 2 is an ion-implanted buried layer (ultra-thin high-conductivity buried layer), 3 is an active region absorption layer, and 4 is an intrinsic barrier layer; Figure 2The dopant atomic concentration distribution data of the buried layer material of the back-illuminated silicon-based barrier impurity band detector prepared by chemical vapor deposition (CVD) on a high-resistance silicon substrate; Figure 3 The dopant atomic concentration distribution data of the buried layer material of the back-illuminated silicon-based barrier impurity band detector prepared by low-dose and low-energy ion implantation on a high-resistance silicon substrate; Figure 4 The transmittance test results of the back-illuminated silicon-based impurity-blocking detector buried layer prepared by the chemical vapor deposition (CVD) method and the back-illuminated silicon-based impurity-blocking detector buried layer prepared by the low-dose and low-energy ion implantation method in the range of 2 to 50 μm are shown in FIG. 1 , wherein the black solid line is the test result of the high-purity silicon material; the red solid line is the test result of the buried layer material prepared by the chemical vapor deposition method 1 (CVD), and the buried layer thickness is 1 μm; the green solid line is the test result of the buried layer material prepared by the chemical vapor deposition method 2 (CVD), and the buried layer thickness is 1.5 μm; the dark blue solid line is the test result of the buried layer material prepared by the chemical vapor deposition method 3 (CVD). Test results, the buried layer thickness is 2um; the light blue solid line is the test result of the buried layer material prepared by chemical vapor deposition method 4 (CVD), and the buried layer thickness is 3um; the red dotted line is the test result of the buried layer material prepared by ion implantation scheme 1; the green dotted line is the test result of the buried layer material prepared by ion implantation scheme 2; the dark blue dotted line is the test result of the buried layer material prepared by ion implantation scheme 3; the light blue dotted line is the test result of the buried layer material prepared by ion implantation scheme 4; among them, ion implantation scheme 1 and ion implantation scheme 2 are high-dose and high-energy implantation parameters, and ion implantation scheme 3 and ion implantation scheme 4 are low-dose and low-energy implantation parameters; Figure 5 The reflectivity test results of the back-illuminated silicon-based impurity-blocking detector buried layer prepared by chemical vapor deposition (CVD) and the back-illuminated silicon-based impurity-blocking detector buried layer prepared by ion implantation within 2~50um are shown in Figure 1. Figure 4 ; Figure 6 The test results of the absorption rate of the back-illuminated silicon-based impurity-blocking detector buried layer prepared by chemical vapor deposition (CVD) and the back-illuminated silicon-based impurity-blocking detector buried layer prepared by ion implantation within 2~50um are shown in Figure 1, where each line represents the same Figure 4 . DETAILED DESCRIPTION

[0017] The specific embodiments of the present invention are described in detail below in conjunction with the accompanying drawings. The following examples will help those skilled in the art to further understand the present invention, but are not intended to limit the present invention in any form. It should be noted that, for those of ordinary skill in the art, several variations and improvements can be made without departing from the concept of the present invention. These all belong to the protection scope of the present invention.

[0018] Example 1 This embodiment relates to a method for preparing a back-illuminated silicon-based impurity-barrier detector epitaxial material, the preparation structure diagram of which is shown in FIG. Figure 1 As shown, the specific steps include: Step 1: In the crystal phase <100> A silicon oxide ion implantation protective layer is deposited on the surface of a high-purity silicon substrate (1) having a thickness of 750 μm and a resistivity greater than 20,000 Ω·cm by plasma enhanced chemical vapor deposition (PECVD), wherein the deposition temperature is 350° C., the reaction gases are silane and nitrous oxide, the growth time is 5 seconds, and the obtained silicon oxide ion implantation protective layer has a thickness of 10 nm; Step 2: On the silicon oxide ion implantation protective layer, an ion implanter is used to implant N-type ions (arsenic ions or phosphorus ions) with a lower dose and lower energy to form an ultra-thin buried layer (2). The implantation energy is 20 keV and the implantation dose is 2×10 14 cm -2 The implantation angle is 6 degrees, and a high-temperature annealing is performed in a nitrogen atmosphere in a rapid annealing furnace at a temperature of 1000°C for 5 seconds. Then, a 20% buffered oxide etchant (BOE) is used to rinse and remove the surface silicon oxide ion implantation protective layer. The rinsing time is 45 seconds; Step 3: epitaxially prepare a highly doped active region absorption layer (3) on the buried layer (2) by chemical vapor deposition; the doping element is Ga, the epitaxial thickness is 30 μm, and the doping concentration is 1×10 18 cm -3 .

[0019] Step 4: epitaxially preparing an intrinsic silicon barrier layer (4) on the highly doped active region absorption layer (3) by chemical vapor deposition; the epitaxial thickness is 4 um, and the resistivity is required to be greater than 1000 Ω·cm.

[0020] Figure 2 and Figure 3 The measured data of the concentration distribution of doped atoms in the buried layer prepared by chemical vapor deposition (CVD) and low-dose, low-energy ion implantation on the high-resistance silicon substrate are shown in Figure 2. The structure is the high-resistance silicon substrate + buried layer structure. The peak doping concentration of the buried layer of the silicon-based barrier impurity band detector prepared by chemical vapor deposition (CVD) is designed to be 2×10 19 cm -3 , the thickness is as thin as possible, the growth gas is SiH4, the growth temperature is 1050℃, and the growth time is 19s; the parameters of low-dose and low-energy ion implantation are as described in step 2; the test results show that the doping concentration is greater than 1×10 19 cm -3The thickness of the buried layer prepared by chemical vapor deposition (CVD) was measured to be 1um, and the thickness of the buried layer prepared by low-dose, low-energy ion implantation was measured to be 0.33um.

[0021] Figure 4 The black solid line in the middle is the spectral transmission test data of the high-purity silicon substrate (resistivity greater than 10000Ω·cm), and the transmittance is between 50% and 55% within 10 to 30um; the red solid line is the spectral transmission test data of the N-type impurity heavily doped buried layer epitaxial material prepared by chemical vapor deposition (CVD) on the high-resistance silicon substrate as described above, that is, Figure 2 Corresponding structural materials, the doping concentration is 1×10 19 cm -3 When the thickness is 1um, the transmittance is between 15% and 20% within 10 to 30um. The green solid line is the spectral transmission test data of the N-type impurity heavily doped buried epitaxial material prepared by chemical vapor deposition (CVD) on the high-purity silicon substrate as described above, with a doping concentration of 1×10 19 cm -3 , its thickness is 1.5um, and the transmittance is between 10% and 15% within 10 to 30um; the dark blue solid line is the spectral transmission test data of the N-type impurity heavily doped buried epitaxial material prepared by chemical vapor deposition (CVD) on the high-purity silicon substrate as mentioned above, with a doping concentration of 1×10 19 cm -3 , its thickness is 2um, and the transmittance is between 5% and 10% within 10-30um; the light blue solid line is the spectral transmission test data of the N-type impurity heavily doped buried epitaxial material prepared by chemical vapor deposition (CVD) on the high-purity silicon substrate as mentioned above, with a doping concentration of 1×10 19 cm -3 , its thickness is 3um, and the transmittance is between 1% and 5% within 10-30um; in addition, the red dotted line is the spectral transmission test data of the N-type impurity heavily doped buried epitaxial material prepared by ion implantation parameter scheme 1 on the high-purity silicon substrate as described above, that is, Figure 3 For the corresponding structural material, the implantation energy is 20keV and the implantation dose is 2×10 14 cm -2 The implantation angle is 6 degrees, and a high temperature annealing is performed in a nitrogen atmosphere through a rapid annealing furnace, the annealing temperature is 1000℃, and the time is 5s. Then, a 20% buffered oxide etchant (BOE) is used to rinse and remove the surface silicon oxide ion implantation protective layer. The rinsing time is 45s; the green dotted line is the spectral transmission test data of the N-type impurity heavily doped buried layer epitaxial material prepared on the high-purity silicon substrate as described above using the ion implantation parameter scheme 2, with an implantation energy of 20keV and an implantation dose of 2×10 14 cm -2The implantation angle is 6 degrees, and the high temperature annealing is carried out in a nitrogen atmosphere by a rapid annealing furnace, the annealing temperature is 1000℃, and the time is 10s. The dark blue dotted line is the spectral transmission test data of the N-type impurity heavily doped buried layer epitaxial material prepared on the high-purity silicon substrate as described above using the ion implantation parameter scheme 3, the implantation energy is 40keV, the implantation dose is 5×10 14 cm -2 The implantation angle is 6 degrees, and the high temperature annealing is carried out in a nitrogen atmosphere by a rapid annealing furnace, the annealing temperature is 1000℃, and the time is 10s. The light blue dotted line is the spectral transmission test data of the N-type impurity heavily doped buried epitaxial material prepared on the high-purity silicon substrate as described above using the ion implantation parameter scheme 4, the implantation energy is 40keV, the implantation dose is 5×10 14 cm -2 The implantation angle is 6 degrees, and high-temperature annealing is performed in a nitrogen atmosphere in a rapid annealing furnace with an annealing temperature of 1000°C and a time of 5s.

[0022] Figure 5 and Figure 6 The implementation conditions of relevant parameters are the same as Figure 4 .Depend on Figure 2 , 3 , 4, 5, and 6 show that, compared with the buried structure of the blocking impurity band detector prepared by chemical vapor deposition and high-energy, high-dose ion implantation, the present invention can form a highly doped, low-diffusion, ultra-thin buried structure of the blocking impurity band detector by adopting lower-dose and energy ion implantation parameters, while effectively reducing the absorption rate and reflectivity of the buried structure to the incident light, and increasing the transmittance of the buried structure in the far-red band of 10~30μm, which can reach a maximum transmittance of more than 40%.

[0023] In summary, the present invention adopts low-dose and low-energy phosphorus or arsenic ion implantation to prepare the buried layer structure of the impurity blocking band detector; by optimizing the ion implantation parameters, a high-doping, low-diffusion buried layer structure can be formed on the surface of a high-purity silicon substrate; while not affecting the conductive performance, by adopting relatively low-dose and low-energy implantation conditions, an ultra-thin junction depth and a sharp distribution of doping elements can be formed on the surface of the high-resistance silicon substrate, while reducing the plasma effect, it also reduces the attenuation of the incident light by the buried layer prepared by ion implantation; in addition, by optimizing the thickness of the oxide layer grown before ion implantation (≤12nm), the lattice damage problem caused by ion implantation can be effectively reduced. At the same time, the position of the impurity concentration peak is regulated; at the same time, rapid high-temperature annealing (≤10s) is performed after ion implantation to effectively repair the lattice damage caused by ion implantation and reduce the impurity diffusion length; finally, a buffered oxide etchant (BOE) is used to remove the surface silicon oxide ion implantation protection layer. By replacing hydrofluoric acid with BOE, the damage to the surface of the high-resistance silicon material can be reduced. The above method can reduce the damage to the high-resistance silicon material caused by ion implantation while increasing the transmittance of the buried layer structure in the far-red band of 10~30μm. The transmittance can reach up to more than 40%, thereby improving the absorption of incident light by the back-illuminated silicon-based impurity-blocking band detector.

[0024] The above describes the specific embodiments of the present invention. It should be understood that the present invention is not limited to the above specific embodiments, and those skilled in the art may make various modifications or variations within the scope of the claims, which do not affect the essence of the present invention.

Claims

1. A method for preparing a back-illuminated silicon-based impurity-barrier detector epitaxial material, characterized in that: The method comprises the following steps: S1. Depositing a silicon oxide ion implantation protective layer on the surface of a high-purity silicon substrate by plasma enhanced chemical vapor deposition; S2. Select low implantation dose and energy parameters on the silicon oxide ion implantation protective layer, and implant an appropriate amount of N-type ions by an ion implanter to form an ultra-thin high-conductivity buried layer; use high-temperature rapid annealing to repair lattice damage, and then use buffered oxide etching solution BOE at room temperature to remove the surface silicon oxide ion implantation protective layer; S3, epitaxially preparing a highly doped active region absorption layer on the ultra-thin high conductivity buried layer by chemical vapor deposition; S4, epitaxially preparing an intrinsic silicon barrier layer on the highly doped active region absorption layer by chemical vapor deposition.

2. The method for preparing back-illuminated silicon-based impurity-barrier detector epitaxial material according to claim 1, characterized in that: The high-purity silicon substrate crystal phase is <100> , thickness is 500~800um, and resistivity is greater than 20000 Ω·cm.

3. The method for preparing back-illuminated silicon-based impurity-barrier detector epitaxial material according to claim 1, characterized in that: In step S2, the implanted ions are phosphorus or arsenic ions.

4. The method for preparing back-illuminated silicon-based impurity-barrier detector epitaxial material according to claim 3, characterized in that: The injection energy of ion implantation is 20~30keV, and the injection dose is 1×10 14 ~3×10 14 cm -2 , the injection angle is 1 to 7 degrees.

5. The method for preparing back-illuminated silicon-based impurity-barrier detector epitaxial material according to claim 1, characterized in that: The high temperature annealing is performed at a temperature of 1000-1200° C. and for a time of 3-10 seconds.

6. The method for preparing back-illuminated silicon-based impurity-barrier detector epitaxial material according to claim 1, characterized in that: The thickness of the silicon oxide ion implantation protection layer is 5-12 nm.

7. The method for preparing back-illuminated silicon-based impurity-barrier detector epitaxial material according to claim 1, characterized in that: The thickness of the ultra-thin high-conductivity buried layer is 0.1-0.5 um.

8. The method for preparing back-illuminated silicon-based impurity-barrier detector epitaxial material according to claim 1, characterized in that: The concentration of the buffered oxide etching solution BOE is 10% to 25%, and the surface silicon oxide is removed by rinsing, and the rinsing time is 30 to 60 seconds.

9. The method for preparing back-illuminated silicon-based impurity-barrier detector epitaxial material according to claim 1, characterized in that: In step S3, the doping element is Ga, the epitaxial thickness is 20-50 μm, and the doping concentration is 2×10 17 ~1×10 19 cm -3 .

10. The method for preparing back-illuminated silicon-based impurity-barrier detector epitaxial material according to claim 1, characterized in that: In step S4, the epitaxial thickness is 2-10 um, and the resistivity is required to be greater than 1000 Ω·cm.

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