Photoconductive material and method of making same
By introducing a periodic superlattice structure into the photoconductive material, the problem of high carrier lifetime and low mobility was solved, enabling rapid carrier recombination and high mobility, thus improving the performance of terahertz devices.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- GUANGDONG UNIV OF TECH
- Filing Date
- 2026-01-14
- Publication Date
- 2026-05-29
AI Technical Summary
Conventional photoconductive materials have high carrier lifetimes but low mobility, which limits the operating frequency of terahertz devices, resulting in low radiation efficiency and insufficient detection sensitivity.
The photoconductive material employs a bottom-up structure, comprising a substrate, a buffer layer, a periodic superlattice layer, and a protective layer. The periodic superlattice layer consists of a drift transport layer and a composite trapping layer, and is grown using molecular beam epitaxy to optimize carrier mobility and lifetime.
It significantly shortens carrier lifetime, increases carrier mobility, enables wideband operation of high-efficiency terahertz devices, and improves system signal-to-noise ratio and radiation efficiency.
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Figure CN122121322A_ABST
Abstract
Description
Technical Field
[0001] This invention relates to the field of optoelectronic technology, and in particular to a photoconductive material and its preparation method. Background Technology
[0002] In the field of terahertz (THz) optoelectronics, photoconductive materials are the core carriers for realizing terahertz wave emission and detection. Due to its location in the special frequency range of the transition from electronics to photonics, the terahertz band exhibits unique physical properties, such as strong penetration of nonpolar materials, low ionization energy, and extremely high spectral bandwidth. To meet the stringent application requirements of high-performance terahertz devices in cutting-edge fields such as 6G ultra-high-speed wireless communication, semiconductor non-destructive testing, high-resolution biological imaging, and precise spectral analysis of complex molecules, ideal photoconductive materials need to possess both extremely short carrier lifetimes and high carrier mobility.
[0003] Specifically, as the source of photocurrent generation, the physical properties of photoconductive materials directly determine the overall performance limit of a terahertz system. An extremely short carrier lifetime is a prerequisite for achieving an ultra-wide spectrum response, ensuring that photogenerated carriers disappear rapidly at sub-picosecond speeds after excitation by an ultrashort pulse laser, thus avoiding distortion of the photocurrent waveform. Conversely, a high carrier mobility is crucial for improving the system's signal-to-noise ratio and radiation efficiency, determining the average drift velocity of carriers under ultra-high bias electric fields.
[0004] However, commonly used conventional photoconductive materials (such as low-temperature grown gallium arsenide LT-GaAs) have the following significant technical limitations in practical applications: First, conventional photoconductive materials have a relatively high carrier lifetime. Although conventional materials typically attempt to shorten the lifetime by introducing numerous As vacancies and other defects into the crystal lattice through low-temperature epitaxial growth processes, the carrier lifetime obtained in this way is often still too high due to the non-equilibrium dynamics of material growth during actual fabrication. A high carrier lifetime means that photogenerated carriers cannot recombine rapidly after stimulation, resulting in broadening and tailing of the terahertz pulse in the time domain, which directly limits the operating frequency and effective bandwidth of the device.
[0005] Second, conventional photoconductive materials generally have low mobility. Because these materials rely heavily on introducing a large number of deep-level defects into the crystal lattice to trap charge carriers, these defects inevitably become severe scattering centers during charge transport. This leads to intense scattering of photogenerated electrons during their drift under a bias electric field, resulting in a significant decrease in the mobility of conventional materials. Lower mobility can lead to a series of negative consequences: Low radiation efficiency: The electric field strength of terahertz radiation is proportional to the drift velocity of charge carriers. The low mobility limits the peak value of transient photocurrent, which in turn limits the output power of terahertz devices.
[0006] Thermal effect accumulation: Due to severe scattering, charge carriers generate a large amount of Joule heat during their movement, which not only reduces the breakdown electric field of the material, but also affects the long-term operating stability of the device.
[0007] Limited detection sensitivity: At the detection end, low mobility leads to a decrease in detector responsivity, making it difficult to achieve high signal-to-noise ratio signal extraction.
[0008] In summary, the inherent high carrier lifetime and low mobility of conventional photoconductive materials are mutually restrictive, forming a significant performance bottleneck. This material-level defect has become a major obstacle to the development of high-efficiency, wideband terahertz photoconductive devices. Summary of the Invention
[0009] To overcome the shortcomings of conventional photoconductive materials, which have high carrier lifetime but low mobility, this invention provides a photoconductive material and its preparation method.
[0010] To solve the above-mentioned technical problems, the technical solution of the present invention is as follows: This invention provides a photoconductive material comprising, from bottom to top, a substrate, a buffer layer, a periodic superlattice layer, and a protective layer; The periodic superlattice layer comprises n periods of drift transport layers and composite trapping layers arranged sequentially from bottom to top.
[0011] Preferably, the thickness of the periodic superlattice layer is in the range of 0.5μm≤d1≤4μm, and the period n is 10≤n≤200.
[0012] Preferably, the drift transport layer is made of undoped gallium arsenide or doped gallium arsenide, wherein the doping material of the gallium arsenide is any one or more of silicon, tellurium, selenium, sulfur, beryllium, carbon, and zinc, and the doping concentration is 1×10⁻⁶. 15 –1×10 19 cm - ³.
[0013] Preferably, the material of the composite trapping layer is Al. x Ga 1-x As and doped materials, wherein 0≤x≤1, and the doped materials are any one or more of silicon, tellurium, selenium, sulfur, beryllium, carbon, and zinc, with a doping concentration of 1×10⁻⁶. 16 –1×10 20 cm - ³.
[0014] Preferably, the doping method of the composite trapping layer is planar delta doping or bulk doping.
[0015] Preferably, the substrate is made of semi-insulating gallium arsenide.
[0016] Preferably, the material of the buffer layer is undoped gallium arsenide.
[0017] Preferably, the material of the protective layer is undoped gallium arsenide.
[0018] The present invention also provides a method for preparing a photoconductive material, comprising obtaining a substrate, cleaning and deoxidizing the substrate to obtain a treated substrate; At a first preset substrate temperature, a buffer layer is grown on the surface of the treated substrate using molecular beam epitaxy. At a second preset substrate temperature, a periodic superlattice layer is grown on the surface of the buffer layer, including: A drift transport layer is grown on the upper surface of the buffer layer, and a composite trapping layer is grown on the upper surface of the drift transport layer until n cycles are reached. A protective layer is grown on the surface of the nth composite trapping layer to obtain a photoconductive material.
[0019] Preferably, the first preset substrate temperature is 550℃≤T1≤680℃; The second preset substrate temperature range is 420℃≤T2≤680℃.
[0020] Compared with the prior art, the beneficial effects of the technical solution of the present invention are: The present invention comprises, from bottom to top, a substrate, a buffer layer, a periodic superlattice layer and a protective layer; The periodic superlattice layer comprises n periods of drift transport layers and composite trapping layers arranged sequentially from bottom to top; The drift transport layer has a complete lattice and low defect density, which can provide a high-speed drift channel with extremely low scattering for photogenerated electrons, thereby improving carrier mobility. The composite trapping layer rapidly traps and recombines photogenerated carriers, significantly shortening carrier lifetime.
[0021] The periodic superlattice layer reduces carrier lifetime while increasing carrier mobility, resulting in a photoconductive material with both low carrier lifetime and high mobility. Attached Figure Description
[0022] Figure 1 This is a schematic diagram of the structure of a photoconductive material in Example 1; Figure 2 The figures show the experimental results of carrier lifetime and mobility for conventional photoconductive materials and the photoconductive material of this application in Example 3. Detailed Implementation
[0023] The accompanying drawings are for illustrative purposes only and should not be construed as limiting the scope of this patent. To better illustrate this embodiment, some parts in the accompanying drawings may be omitted, enlarged, or reduced, and do not represent the actual product dimensions; It will be understood by those skilled in the art that certain well-known structures and their descriptions may be omitted in the accompanying drawings.
[0024] The technical solution of the present invention will be further described below with reference to the accompanying drawings and embodiments.
[0025] Example 1 This embodiment provides a photoconductive material, such as Figure 1 As shown, it includes: It includes, from bottom to top, a substrate 1, a buffer layer 2, a periodic superlattice layer 3, and a protective layer 4; The periodic superlattice layer 3 includes n periods of drift transport layer 301 and composite trapping layer 302 arranged sequentially from bottom to top.
[0026] The thickness of the periodic superlattice layer 3 is in the range of 0.5μm≤d1≤4μm, and the period n is 10≤n≤200.
[0027] The drift transport layer 301 is made of undoped gallium arsenide or gallium arsenide, wherein the gallium arsenide doping material is any one or more of silicon, tellurium, selenium, sulfur, beryllium, carbon, and zinc, and the doping concentration is 1×10⁻⁶. 15 –1×10 19 cm - ³.
[0028] The composite trapping layer 302 is made of Al. x Ga 1-x As and doped materials, wherein 0≤x≤1, and the doped materials are any one or more of silicon, tellurium, selenium, sulfur, beryllium, carbon, and zinc, with a doping concentration of 1×10⁻⁶. 16 –1×10 20 cm - ³.
[0029] The doping method for the composite trapping layer is planar delta doping or bulk doping.
[0030] The substrate 1 is made of semi-insulating gallium arsenide.
[0031] The material of the buffer layer 2 is undoped gallium arsenide.
[0032] The material of the protective layer 4 is undoped gallium arsenide.
[0033] The drift transport layer in the periodic superlattice layer of this invention has a complete lattice and low defect density, which can provide a high-speed drift channel with extremely low scattering for photogenerated electrons, thereby improving carrier mobility. The composite trapping layer rapidly traps and recombines photogenerated carriers, significantly shortening carrier lifetime. While reducing carrier lifetime, the periodic superlattice layer also improves carrier mobility, resulting in a photoconductive material of this invention with both low carrier lifetime and high mobility.
[0034] Example 2 This embodiment provides a method for preparing a photoconductive material, including: Obtain substrate 1, and perform cleaning and deoxidation treatment on substrate 1 to obtain treated substrate 1; At a first preset substrate temperature, a buffer layer 2 is grown on the surface of the treated substrate 1 using molecular beam epitaxy. At a second preset substrate temperature, a periodic superlattice layer 3 is grown on the upper surface of the buffer layer 2, including: A drift transport layer 301 is grown on the upper surface of the buffer layer 2, and a composite trapping layer 302 is grown on the upper surface of the drift transport layer 301 until n cycles are reached. A protective layer 4 is grown on the surface of the nth composite trapping layer 302 to obtain a photoconductive material.
[0035] Preferably, the first preset substrate temperature is 550℃≤T1≤680℃; The second preset substrate temperature range is 420℃≤T2≤680℃.
[0036] Example 3 This embodiment provides a method for preparing a photoconductive material, including: A semi-insulating gallium arsenide crystal-oriented substrate 1 was selected and subjected to conventional cleaning and deoxidation treatments to obtain the treated substrate 1.
[0037] It should be noted that in this embodiment, the first preset substrate temperature is 600°C. At 600°C, a 500nm thick undoped gallium arsenide buffer layer 2 is grown by molecular beam epitaxy, so that the substrate surface reaches atomic level flatness, providing a foundation for the subsequent growth of high-quality periodic superlattice layers.
[0038] It should be noted that in this embodiment, the second preset substrate temperature is 580°C. At 580°C, a periodic superlattice layer 3 consisting of 50 cycles is grown, and each cycle includes a drift transport layer 301 and a composite trapping layer 302. It should be noted that, in this embodiment, the drift transport layer 301 is a 15nm thick undoped gallium arsenide layer with high crystal quality and low defect density, providing a high-mobility drift channel for photogenerated carriers.
[0039] It should be noted that, in this embodiment, the composite trapping layer 302 is a 5nm thick Al0.3Ga0.7As layer, with beryllium as the doping material and a doping concentration of 1×10⁻⁶. 18 cm - ³. This layer utilizes the relatively wide band gap of AlGaAs to provide a high potential barrier, while high-concentration doping introduces a large number of recombination centers to achieve rapid capture and recombination of photogenerated carriers.
[0040] A 10 nm thick undoped gallium arsenide protective layer is grown on the surface of the topmost periodic superlattice layer 3 to prevent AlGaAs from oxidizing when exposed to air.
[0041] It should be noted that, in this embodiment, an experiment was conducted to compare the mobility and carrier lifetime of conventional photoconductive materials and photoconductive materials with periodic superlattice layers. The experimental results are as follows: Figure 2 As shown in the figure, the carrier lifetime of conventional photoconductive materials is 1.5 ps, while the mobility is only 53.9 cm⁻¹. 2 The carrier lifetime of the photoconductive material of this invention is only 0.93 ps, while its mobility is 1101 cm⁻¹. 2 / V·s. The photoconductive material of the present invention has a lower carrier lifetime and a higher mobility compared to conventional photoconductive materials. The photoconductive material with a periodic superlattice layer provided by the present invention reduces the carrier lifetime while increasing the mobility, resulting in a photoconductive material with both low carrier lifetime and high mobility.
[0042] Example 4 This embodiment provides a method for preparing a photoconductive material, including: A semi-insulating gallium arsenide crystal-oriented substrate 1 was selected and subjected to conventional cleaning and deoxidation treatments to obtain the treated substrate 1.
[0043] It should be noted that in this embodiment, the first preset substrate temperature is 600°C. At 600°C, a 500nm thick undoped gallium arsenide buffer layer 2 is grown by molecular beam epitaxy, so that the substrate surface reaches atomic level flatness, providing a foundation for the subsequent growth of high-quality periodic superlattice layers.
[0044] It should be noted that in this embodiment, the second preset substrate temperature is 580°C. At 580°C, a periodic superlattice layer 3 is grown, and each period includes a drift transport layer 301 and a composite trapping layer 302. It should be noted that in this embodiment, neither the composite trapping layer nor the drift transport layer introduces Al; that is, both the composite trapping layer 302 and the drift transport layer 301 are made of GaAs, thus forming a homogeneous superlattice structure. This avoids interface scattering caused by the interface barrier and improves carrier mobility. In this embodiment, the total thickness of a single period of the periodic superlattice layer 3 is 10 nm, and the number of periods is 150. The composite trapping layer 302 is a heavily doped p-GaAs with a thickness of 2 nm, using Be as the dopant, and the doping concentration is approximately 1 × 10¹. 9 cm - ³ is used to introduce high-density recombination centers to shorten carrier lifetime.
[0045] The drift transport layer 301 is GaAs with a thickness of 8 nm.
[0046] A 10 nm thick undoped gallium arsenide protective layer (4) is grown on the surface of the topmost periodic superlattice layer 3 to prevent GaAs from being oxidized when exposed to air.
[0047] Example 5 This embodiment provides a method for preparing a photoconductive material, including: A semi-insulating gallium arsenide crystal-oriented substrate 1 was selected and subjected to conventional cleaning and deoxidation treatments to obtain the treated substrate 1.
[0048] It should be noted that in this embodiment, the first preset substrate temperature is 600°C. At 600°C, a 500nm thick undoped gallium arsenide buffer layer 2 is grown by molecular beam epitaxy, so that the substrate surface reaches atomic level flatness, providing a foundation for the subsequent growth of high-quality periodic superlattice layers.
[0049] It should be noted that in this embodiment, the second preset substrate temperature is 580°C. At 580°C, a periodic superlattice layer 3 is grown, and each period includes a drift transport layer 301 and a composite trapping layer 302. It should be noted that in this embodiment, Ga is not introduced into the composite trapping layer material, that is, the composite trapping layer 302 is made of AlAs. A 10 nm thick undoped gallium arsenide protective layer (4) is grown on the surface of the topmost periodic superlattice layer 3 to prevent AlAs from being oxidized when exposed to air.
[0050] Example 6 This embodiment provides a method for preparing a photoconductive material, including: A semi-insulating gallium arsenide crystal-oriented substrate 1 was selected and subjected to conventional cleaning and deoxidation treatments to obtain the treated substrate 1.
[0051] It should be noted that in this embodiment, the first preset substrate temperature is 550°C. At 550°C, a 500nm thick undoped gallium arsenide buffer layer 2 is grown by molecular beam epitaxy, so that the substrate surface reaches atomic level flatness, providing a foundation for the subsequent growth of high-quality periodic superlattice layers.
[0052] It should be noted that in this embodiment, the second preset substrate temperature is 420°C. At 420°C, a periodic superlattice layer 3 consisting of 50 cycles is grown, and each cycle includes a drift transport layer 301 and a composite trapping layer 302. It should be noted that, in this embodiment, the drift transport layer 301 is a 15nm thick undoped gallium arsenide layer with high crystal quality and low defect density, providing a high-mobility drift channel for photogenerated carriers.
[0053] It should be noted that, in this embodiment, the composite trapping layer 302 is a 5nm thick Al0.3Ga0.7As layer, with beryllium as the doping material and a doping concentration of 1×10⁻⁶. 18 cm - ³. This layer utilizes the relatively wide band gap of AlGaAs to provide a high potential barrier, while high-concentration doping introduces a large number of recombination centers to achieve rapid capture and recombination of photogenerated carriers.
[0054] A 10 nm thick undoped gallium arsenide protective layer is grown on the surface of the topmost periodic superlattice layer 3 to prevent AlGaAs from oxidizing when exposed to air.
[0055] Example 7 This embodiment provides a method for preparing a photoconductive material, including: A semi-insulating gallium arsenide crystal-oriented substrate 1 was selected and subjected to conventional cleaning and deoxidation treatments to obtain the treated substrate 1.
[0056] It should be noted that in this embodiment, the first preset substrate temperature is 680°C. At 680°C, a 500nm thick undoped gallium arsenide buffer layer 2 is grown by molecular beam epitaxy, so that the substrate surface reaches atomic level flatness, providing a foundation for the subsequent growth of high-quality periodic superlattice layers.
[0057] It should be noted that in this embodiment, the second preset substrate temperature is 680°C. At 680°C, a periodic superlattice layer 3 consisting of 50 cycles is grown, and each cycle includes a drift transport layer 301 and a composite trapping layer 302. It should be noted that, in this embodiment, the drift transport layer 301 is a 15nm thick undoped gallium arsenide layer with high crystal quality and low defect density, providing a high-mobility drift channel for photogenerated carriers.
[0058] It should be noted that, in this embodiment, the composite trapping layer 302 is a 5nm thick Al0.3Ga0.7As layer, with beryllium as the doping material and a doping concentration of 1×10⁻⁶. 18 cm - ³. This layer utilizes the relatively wide band gap of AlGaAs to provide a high potential barrier, while high-concentration doping introduces a large number of recombination centers to achieve rapid capture and recombination of photogenerated carriers.
[0059] A 10 nm thick undoped gallium arsenide protective layer is grown on the surface of the topmost periodic superlattice layer 3 to prevent AlGaAs from oxidizing when exposed to air.
[0060] Example 8 This embodiment provides a method for preparing a photoconductive material, including: A semi-insulating gallium arsenide crystal-oriented substrate 1 was selected and subjected to conventional cleaning and deoxidation treatments to obtain the treated substrate 1.
[0061] It should be noted that in this embodiment, the first preset substrate temperature is 550°C. At 550°C, a 500nm thick undoped gallium arsenide buffer layer 2 is grown by molecular beam epitaxy, so that the substrate surface reaches atomic level flatness, providing a foundation for the subsequent growth of high-quality periodic superlattice layers.
[0062] It should be noted that in this embodiment, the second preset substrate temperature is 680°C. At 680°C, a periodic superlattice layer 3 consisting of 50 cycles is grown, and each cycle includes a drift transport layer 301 and a composite trapping layer 302. It should be noted that, in this embodiment, the drift transport layer 301 is a 15nm thick undoped gallium arsenide layer with high crystal quality and low defect density, providing a high-mobility drift channel for photogenerated carriers.
[0063] It should be noted that, in this embodiment, the composite trapping layer 302 is a 5nm thick Al0.3Ga0.7As layer, with beryllium as the doping material and a doping concentration of 1×10⁻⁶. 18 cm - ³. This layer utilizes the relatively wide band gap of AlGaAs to provide a high potential barrier, while high-concentration doping introduces a large number of recombination centers to achieve rapid capture and recombination of photogenerated carriers.
[0064] A 10 nm thick undoped gallium arsenide protective layer is grown on the surface of the topmost periodic superlattice layer 3 to prevent AlGaAs from oxidizing when exposed to air.
[0065] Example 9 This embodiment provides a method for preparing a photoconductive material, including: A semi-insulating gallium arsenide crystal-oriented substrate 1 was selected and subjected to conventional cleaning and deoxidation treatments to obtain the treated substrate 1.
[0066] It should be noted that in this embodiment, the first preset substrate temperature is 680°C. At 680°C, a 500nm thick undoped gallium arsenide buffer layer 2 is grown by molecular beam epitaxy, so that the substrate surface reaches atomic level flatness, providing a foundation for the subsequent growth of high-quality periodic superlattice layers.
[0067] It should be noted that in this embodiment, the second preset substrate temperature is 420°C. At 420°C, a periodic superlattice layer 3 consisting of 50 cycles is grown, and each cycle includes a drift transport layer 301 and a composite trapping layer 302. It should be noted that, in this embodiment, the drift transport layer 301 is a 15nm thick undoped gallium arsenide layer with high crystal quality and low defect density, providing a high-mobility drift channel for photogenerated carriers.
[0068] It should be noted that, in this embodiment, the composite trapping layer 302 is a 5nm thick Al0.3Ga0.7As layer, with beryllium as the doping material and a doping concentration of 1×10⁻⁶. 18 cm - ³. This layer utilizes the relatively wide band gap of AlGaAs to provide a high potential barrier, while high-concentration doping introduces a large number of recombination centers to achieve rapid capture and recombination of photogenerated carriers.
[0069] A 10 nm thick undoped gallium arsenide protective layer is grown on the surface of the topmost periodic superlattice layer 3 to prevent AlGaAs from oxidizing when exposed to air.
[0070] The same or similar labels correspond to the same or similar parts; The terms used to describe positional relationships in the accompanying drawings are for illustrative purposes only and should not be construed as limiting this patent. Obviously, the above embodiments of the present invention are merely examples for clearly illustrating the present invention, and are not intended to limit the implementation of the present invention. Those skilled in the art can make other variations or modifications based on the above description. It is neither necessary nor possible to exhaustively describe all embodiments here. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the claims of the present invention.
Claims
1. A photoconductive material, characterized in that, It includes a substrate (1), a buffer layer (2), a periodic superlattice layer (3), and a protective layer (4) arranged sequentially from bottom to top; The periodic superlattice layer (3) includes n periods of drift transport layer (301) and composite trapping layer (302) arranged sequentially from bottom to top.
2. The photoconductive material according to claim 1, characterized in that, The thickness of the periodic superlattice layer (3) is in the range of 0.5μm≤d1≤4μm, and the period n is 10≤n≤200.
3. The photoconductive material according to claim 1, characterized in that, The drift transport layer (301) is made of undoped gallium arsenide or gallium arsenide, wherein the gallium arsenide doping material is any one or more of silicon, tellurium, selenium, sulfur, beryllium, carbon, and zinc, and the doping concentration is 1×10⁻⁶. 15 –1×10 19 cm - ³.
4. The photoconductive material according to claim 1, characterized in that, The composite trapping layer (302) is made of Al. x Ga 1-x As and doped materials, wherein 0≤x≤1, and the doped materials are any one or more of silicon, tellurium, selenium, sulfur, beryllium, carbon, and zinc, with a doping concentration of 1×10⁻⁶. 16 –1×10 20 cm - ³.
5. The photoconductive material according to claim 4, characterized in that, The doping method for the composite trapping layer is planar delta doping or bulk doping.
6. A photoconductive material according to any one of claims 1-5, characterized in that, The substrate (1) is made of semi-insulating gallium arsenide.
7. A photoconductive material according to any one of claims 1-5, characterized in that, The material of the buffer layer (2) is undoped gallium arsenide.
8. A photoconductive material according to any one of claims 1-5, characterized in that, The material of the protective layer (4) is undoped gallium arsenide.
9. A method for preparing a photoconductive material, used to prepare the photoconductive material described in claims 1-8, characterized in that, include: Obtain a substrate (1), and perform cleaning and deoxidation treatment on the substrate (1) to obtain a treated substrate (1). At a first preset substrate temperature, a buffer layer (2) is grown on the surface of the treated substrate (1) using molecular beam epitaxy. At a second preset substrate temperature, a periodic superlattice layer (3) is grown on the upper surface of the buffer layer (2), including: A drift transport layer (301) is grown on the upper surface of the buffer layer (2), and a composite trapping layer (302) is grown on the upper surface of the drift transport layer until n cycles are reached; A protective layer (4) is grown on the surface of the nth composite trapping layer (302) to obtain a photoconductive material.
10. A method for preparing a photoconductive material according to claim 9, characterized in that, The first preset substrate temperature is 550℃≤T1≤680℃; The second preset substrate temperature range is 420℃≤T2≤680℃.