A two-dimensional back-to-back broken-gap band arrangement phototransistor and a preparation method and application thereof

Abstract

The application belongs to the technical field of heterojunction phototransistors, and discloses a two-dimensional back-to-back broken-band phototransistor and a preparation method and application thereof. The structure of the phototransistor is SnSe2 / 2H-MoTe2 / SnSe2 heterojunction. The heterojunction is SnSe2 nanosheet on a p-SiO2 / Si substrate, then 2H-MoTe2 nanosheet is transferred to the SnSe2 nanosheet, and then SnSe2 nanosheet on PDMS is transferred to the 2H-MoTe2 nanosheet to form SnSe2 / 2H-MoTe2 / SnSe2 vertical heterojunction. The two layers of SnSe2 nanosheet are not in contact, and a metal bonding layer / Au electrode is evaporated on the SnSe2 nanosheet. The phototransistor has low dark current, tunability, high responsivity, fast response speed and the like, and is applied in the fields of optical communication, optical sensing, optical detection and optical imaging.

Description

Technical Field

[0001] This invention belongs to the field of back-to-back three-type heterojunction phototransistor technology, and more specifically, relates to a two-dimensional back-to-back broken bandgap phototransistor, its fabrication method, and its application. Background Technology

[0002] Since its discovery in 2004, graphene has become a research hotspot in the fields of electronics and optoelectronics due to its excellent thermal conductivity, ultra-high room-temperature mobility, and many other properties. However, the zero bandgap inherent in graphene makes it unsuitable for high-switching-ratio logic devices, limiting its development in electronics. Two-dimensional transition metal chalcogenides (TMCs) with layered structures, similar to graphene, have gradually come into focus. Their bandgap of 0.2-5 eV allows them not only to replace graphene in certain aspects but also to develop a wider range of applications. TMCs also possess excellent optical and electrical properties, exhibiting high carrier mobility; for example, MoS2 has a theoretical mobility of 2 × 10⁻⁶. 4 cm 2 ·V -1 ·s -1 With a mobility far exceeding that of silicon, two-dimensional transition metal chalcogenide thin films possess excellent flexibility, making them suitable for applications in flexible sensors, transistors, and photodetectors, with enormous potential.

[0003] Transition metal chalcogenides not only possess excellent optical, electrical, and physicochemical properties, but also boast abundant resources, low mining costs, and environmental friendliness, aligning with the values ​​of sustainable industrial development in the world today. This has greatly attracted researchers' interest, making transition metal chalcogenides a research hotspot in two-dimensional materials. Tin diselenide (SnSe2), as a typical group IV-VI narrow bandgap semiconductor, exhibits a wide spectral response, large specific surface area, and high electron mobility, demonstrating significant advantages and application value in the field of broad-spectrum optical detection. Due to its excellent electronic and optoelectronic properties, it has become a research hotspot.

[0004] SnSe2, with its low band gap, belongs to the two-dimensional layered post-transition metal chalcogenide family. It has an MX2 molecular structure, with each layer of molecules interconnected by covalent bonds. The SnSe2 crystal exhibits a hexagonal symmetry structure. The constituent elements Sn and Se have valences of +4 and -2, respectively, and a lattice constant of [missing value]. With an indirect bandgap of approximately 1 eV and an interlayer spacing of approximately 0.62 nm, the layers are interconnected by weak van der Waals forces, making them easily exfoliated into two-dimensional layered materials. However, due to strong interlayer forces, numerous deep-level defects, and strong electric field shielding effects between layers, these materials suffer from difficulties in mechanical exfoliation, slow optical response time, mediocre current switching characteristics, and limited optical absorption efficiency, severely hindering their development.

[0005] Compared to traditional phototransistors, broken heterojunction phototransistors exhibit higher gain and sensitivity. This means they can more effectively convert optical signals into electrical signals and possess better amplification capabilities. They can still provide reliable detection and amplification functions under low-light conditions. Type III two-dimensional van der Waals heterostructures with broken bandgap arrangements readily exhibit tunneling currents due to their broken band offset and sharp band steps, provided they are applied to narrow band gaps. Narrower band gaps result in higher current densities, or carrier concentrations, facilitating tunneling. In Type III bandgap arrangements, the conduction band bottom on one side is lower than the valence band top on the other, creating a non-overlapping band gap between the two semiconductors. Charge transport occurs between the heterostructures via quantum tunneling from the band to the band. Therefore, it is widely used in the design of various high-speed, low-power devices. However, although its dark current is very low, the photocurrent is not high due to the low tunneling probability of the heterojunction. In addition, the responsivity of ordinary type III heterojunction phototransistors is not very high, and sometimes there are very few systems to choose from. The common ones are stacked in pairs without any structural innovation. Therefore, by constructing back-to-back type III heterojunctions to improve its performance, it can be found that the relative direction of the two electric fields of the back-to-back type III heterojunction can increase its photocurrent, effectively improving the problem that high responsivity and fast response time cannot coexist.

[0006] By employing different modulation methods, broken band arrangement van der Waals heterojunctions based on SnSe2 and other 2D materials have been realized. Currently, SnSe2 / 2H-MoTe2 / SnSe2 heterojunctions have not been reported. The combination of these three materials is expected to achieve a self-driven broadband detection function with both high responsivity and fast response speed, thus promoting the development of future two-dimensional high-performance optotransistors and their applications. Summary of the Invention

[0007] In order to overcome the shortcomings and disadvantages of the existing technology, the present invention aims to provide a two-dimensional back-to-back broken band arrangement phototransistor.

[0008] Another object of the present invention is to provide a method for fabricating the above-mentioned two-dimensional back-to-back broken bandgap phototransistor.

[0009] Another object of the present invention is to provide an application of the above-mentioned two-dimensional back-to-back broken bandgap phototransistor.

[0010] The objective of this invention is achieved through the following technical solution:

[0011] A two-dimensional back-to-back broken bandgap phototransistor is disclosed. The structure of the phototransistor is a SnSe2 / 2H-MoTe2 / SnSe2 heterojunction. The heterojunction is formed by N-type doped SnSe2 nanosheets on a p-SiO2 / Si substrate, then transferring 2H-MoTe2 nanosheets onto the SnSe2 nanosheets, and then transferring N-type doped SnSe2 nanosheets from PDMS onto the 2H-MoTe2 nanosheets to form a SnSe2 / 2H-MoTe2 / SnSe2 vertical heterojunction. The upper and lower SnSe2 nanosheets are not in contact. Finally, a metal bonding layer and Au electrodes are deposited on the SnSe2 nanosheets on both sides.

[0012] Preferably, the thickness of the SnSe2 nanosheets is 20–50 nm; the thickness of the 2H-MoTe2 nanosheets is 20–50 nm.

[0013] Preferably, the metal adhesion layer is Cr or Ti with a thickness of 3–15 nm, and the Au has a thickness of 50–100 nm.

[0014] The method for fabricating the two-dimensional back-to-back broken bandgap phototransistor includes the following specific steps:

[0015] S1. SnSe2 nanosheets were obtained by mechanical exfoliation on a cleaned SiO2 / Si substrate. The sample surface was then soaked in acetone to remove residual adhesive and other small organic molecules. SnSe2 nanosheets with a thickness of 20-50 nm were selected by optical metallographic microscopy.

[0016] S2. Using a dry transfer process, 2H-MoTe2 nanosheets with a thickness of 20-50 nm are stacked onto SnSe2 nanosheets through a three-dimensional micro-region transfer platform, and then SnSe2 nanosheets are stacked from PDMS film onto 2H-MoTe2 nanosheets to form SnSe2 / 2H-MoTe2 / SnSe2 vertical heterojunction;

[0017] S3. Photoresist is spin-coated onto the substrate surface and dried. Then, it is directly written onto the upper and lower SnSe2 nanosheets using ultraviolet lithography laser. The etched electrodes are developed with a developer. A metal adhesion layer and Au electrodes are deposited on the p-SiO2 / Si surface with the photolithographic electrode pattern by electron beam. After immersion in acetone to remove the electrode pattern and rinsing with deionized water, a SnSe2 / 2H-MoTe2 / SnSe2 heterojunction phototransistor with electrode pattern is obtained, which is a two-dimensional back-to-back broken bandgap phototransistor.

[0018] Preferably, the thickness of SiO2 in the SiO2 / Si substrate described in step S1 is 285–315 nm.

[0019] Preferably, the drying temperature in step S3 is 100°C, and the drying time is 4–30 min; the volume ratio of the developing solution to water in the developing solution is 1:(6–8); and the developing solution is disodium tetraborate sodium hydroxide.

[0020] The aforementioned two-dimensional back-to-back broken bandgap phototransistor is used in the fields of optical communication, optical sensing, optical detection, optical imaging, or two-dimensional high-performance broadband optoelectronic devices.

[0021] This invention utilizes a SnSe2 / 2H-MoTe2 / SnSe2 heterostructure to construct a phototransistor. Molybdenum ditelluride (MoTe2) is a two-dimensional material belonging to the transition metal dichalcogenides. MoTe2 is a layered structure formed by molybdenum (Mo) and tellurium (Te) atoms, with each molybdenum atom surrounded by six tellurium atoms. This two-dimensional structure gives MoTe2 high tunability and surface sensitivity. The interlayer interactions of MoTe2 are relatively weak, allowing the coupling between layers to be tunable through stacking or external conditions. This characteristic provides the possibility for constructing two-dimensional heterostructures and devices. MoTe2 possesses rich optical properties, including absorption and emission of visible and near-infrared light. The three-layer structure, with its additional layer, provides stronger current control capabilities. This means that during current injection and collection, carrier transport and recombination can be controlled more effectively, thereby improving device performance and efficiency. Due to the presence of the multilayer structure, back-to-back three-type heterojunction phototransistors typically exhibit higher gain and sensitivity. This enables more efficient conversion of optical signals into electrical signals and provides more reliable detection and amplification functions under low-light conditions. The two built-in electric fields created by the three-layer structure help reduce dark current generation. Dark current is the current generated in the absence of light illumination, which can reduce the signal-to-noise ratio and sensitivity of a device. By optimizing the material composition and interface properties, back-to-back type III bandgap van der Waals heterojunctions can reduce the level of dark current, increase its photocurrent, and effectively improve high responsivity and fast response time.

[0022] Compared with the prior art, the present invention has the following beneficial effects:

[0023] 1. The two-dimensional back-to-back broken bandgap phototransistor of the present invention, namely the SnSe2 / 2H-MoTe2 / SnSe2 heterojunction-based phototransistor, significantly improves the photoelectric performance of the phototransistor by utilizing the unique back-to-back built-in electric field direction. This solves the problem in this field where the responsivity and response time of phototransistors cannot be simultaneously satisfied.

[0024] 2. The phototransistor based on SnSe2 / 2H-MoTe2 / SnSe2 heterojunction of the present invention also effectively improves the broadband absorption range, and has obvious photoresponse in the 405-1310nm band.

[0025] 3. The phototransistor based on the SnSe2 / 2H-MoTe2 / SnSe2 heterojunction, fabricated through structural innovation in this invention, exhibits excellent near-infrared self-driven photoelectric performance (its maximum self-driven photoresponsivity reaches 6.17 A·W under 808 nm laser irradiation). -1 The maximum specific detectivity reached 8.8×10 12 Jones (with a response speed of up to 25 μs) compares to photodetectors based on single-walled carbon nanotubes / graphene / Al₂O₃ / InP (which achieve a responsivity of 154.3 mA·W at 808 nm wavelength and under zero bias). -1 The detection rate reached 1.27×10 12 Jones and rising time τ r and descent time τ f Compared to 40μs and 180μs respectively, the responsivity of this invention is increased by 60 times, the detectivity is increased by 7 times, and the response speed is also accelerated. It is expected to realize a self-driven broadband detection function with both high responsivity and fast response speed, and promote the development of future two-dimensional high-performance phototransistors and their applications. Attached Figure Description

[0026] Figure 1 This is an optical microscope image of the SnSe2 / 2H-MoTe2 / SnSe2 heterojunction based on Example 1.

[0027] Figure 2 This is a side view of the phototransistor based on the SnSe2 / 2H-MoTe2 / SnSe2 heterojunction in Example 1.

[0028] Figure 3 The current-voltage curve is shown for the phototransistor based on the SnSe2 / 2H-MoTe2 / SnSe2 heterojunction in Example 1.

[0029] Figure 4 The graph shows the transfer curve of the phototransistor based on the SnSe2 / 2H-MoTe2 / SnSe2 heterojunction in Example 1 under a bias voltage of -1V.

[0030] Figure 5 The graph shows the relationship between photoresponsivity, photocurrent, and optical power density of the phototransistor based on the SnSe2 / 2H-MoTe2 / SnSe2 heterojunction in Example 1 under 808nm incident light and a bias voltage of 0V.

[0031] Figure 6The graph shows the self-driven response time of the phototransistor based on the SnSe2 / 2H-MoTe2 / SnSe2 heterojunction in Example 1 at different optical power densities when the gate voltage is 0V in an 808nm laser.

[0032] Figure 7 The Vo of the SnSe2 / 2H-MoTe2 / SnSe2 heterojunction-based phototransistor of Example 1 under different optical power densities with 808nm incident light and a bias voltage of 0V is shown. oc / I sc Line graph.

[0033] Figure 8 The graph shows the relationship between external quantum efficiency, specific detectivity, and optical power density of the phototransistor based on the SnSe2 / 2H-MoTe2 / SnSe2 heterojunction in Example 1 under 808nm incident light and a bias voltage of 0V.

[0034] Figure 9 The graph shows the response speed of the phototransistor based on the SnSe2 / 2H-MoTe2 / SnSe2 heterojunction in Example 1 under 808nm incident light and a bias voltage of 0V. Detailed Implementation

[0035] The technical solutions in the embodiments of the present invention will be clearly and completely described with reference to the accompanying drawings, but should not be construed as limiting the present invention. Unless otherwise specified, the experimental methods described in the following embodiments are conventional methods; the reagents and materials described, unless otherwise specified, can be obtained from publicly available commercial sources. The present invention will now be described in further detail.

[0036] Example 1

[0037] 1. Using SiO2 / Si (a 2-inch Si-doped p-type SiO2 substrate is cut into 1cm×1cm pieces with a SiO2 thickness of 285–315nm) as the substrate, semiconductor SnSe2 nanosheets are prepared by tape peeling. SnSe2 nanosheets with a thickness of 20–40nm are selected by optical microscopy. 2H-MoTe2 nanosheets prepared by PDMS are aligned with the selected SnSe2 nanosheets in pairs by dry transfer method. SnSe2 nanosheets are then prepared on PDMS by tape peeling. Light yellow or yellow SnSe2 nanosheets are stacked on 2H-MoTe2 nanosheets, with no direct contact between the upper and lower SnSe2 nanosheets. The overlapping part of the three forms a SnSe2 / 2H-MoTe2 / SnSe2 heterojunction.

[0038] 2. The transferred substrate is suspended in a photoresist suspender and heated at 100°C for 4 minutes. Then, the electrode pattern is directly written onto the surface of the p-SiO2 / Si substrate using a photolithography laser. The substrate is then developed using a developing solution (the volume ratio of sodium hydroxide, disodium tetraborate, and water is 1:7). A maskless ultraviolet lithography system and vapor deposition process are used to fabricate 10 nm Cr and 50 nm Au electrodes on the SnSe2 / 2H-MoTe2 / SnSe2 van der Waals heterojunction. After immersion in acetone to remove the electrode pattern and rinsing with deionized water, a device with electrode pattern is obtained. This results in a phototransistor based on the SnSe2 / 2H-MoTe2 / SnSe2 heterojunction, which is a two-dimensional back-to-back broken bandgap phototransistor.

[0039] Figure 1 This is an optical microscope image of the phototransistor based on the SnSe2 / 2H-MoTe2 / SnSe2 heterojunction from Example 1. Figure 1 It can be seen that the structure of the phototransistor is a SnSe2 / 2H-MoTe2 / SnSe2 heterojunction. This heterojunction is formed by N-type doped SnSe2 nanosheets on a p-SiO2 / Si substrate, then transferring 2H-MoTe2 nanosheets onto SnSe2 nanosheets, and finally transferring N-type doped SnSe2 nanosheets from PDMS onto 2H-MoTe2 nanosheets to form a SnSe2 / 2H-MoTe2 / SnSe2 vertical heterojunction. The upper and lower SnSe2 nanosheets are not in contact, and the Au electrodes are located on the SnSe2 on both sides. Figure 2 This is a side view of the phototransistor based on the SnSe2 / 2H-MoTe2 / SnSe2 heterojunction in Example 1. Near the substrate are SnSe2 nanosheets, then a layer of 2H-MoTe2 nanosheets is stacked on top of the SnSe2 nanosheets, and finally, a final layer of SnSe2 nanosheets is stacked on top, forming a back-to-back broken heterojunction structure. Figure 3 This is the current-voltage curve of the phototransistor based on the SnSe2 / 2H-MoTe2 / SnSe2 heterojunction in Example 1. Since it uses symmetrical electrodes, the source and drain are connected to SnSe2 nanosheets at both ends. The results show that the current ratio of this heterojunction from -1V to 1V is approximately 10. 5 This indicates that the phototransistor based on the SnSe2 / 2H-MoTe2 / SnSe2 heterojunction exhibits significant rectification behavior. Figure 4 This is a transfer curve of the phototransistor based on the SnSe2 / 2H-MoTe2 / SnSe2 heterojunction in Example 1 at a bias voltage of -1V. The on / off ratio of this heterojunction is 10 from a gate voltage of -60V to 60V. Figure 5This is a graph showing the photoresponsivity-photocurrent versus optical power density of the SnSe2 / 2H-MoTe2 / SnSe2 heterojunction phototransistor of Example 1 under 808nm incident light and a bias voltage of 0V. Figure 5 It can be known that at 2.19 mW·cm -2 Under these conditions, the photoresponsivity reaches its maximum value of 6.17 A·W. -1 As optical power density increases, photoresponsivity first increases and then decreases. Figure 6 This is a current-time curve of the SnSe2 / 2H-MoTe2 / SnSe2 heterojunction phototransistor of Example 1 under different optical powers with an 808nm laser gate voltage of 0V. Figure 6 It can be seen that as the optical power increases, the current also increases. Figure 7 This is a VOC / ISC curve of the phototransistor based on the SnSe2 / 2H-MoTe2 / SnSe2 heterojunction in Example 1 under different optical power densities with 808nm incident light and a bias voltage of 0V. Figure 7 It can be seen that the open-circuit voltage and short-circuit current increase continuously with the increase of optical power density. Figure 8 This is a graph showing the external quantum efficiency-specific detectivity versus optical power density of the SnSe2 / 2H-MoTe2 / SnSe2 heterojunction-based phototransistor of Example 1 under 808nm incident light and a bias voltage of 0V. Figure 8 It can be seen that the specific detectivity and external quantum efficiency of the phototransistor based on the SnSe2 / 2H-MoTe2 / SnSe2 heterojunction both increase to a maximum value with increasing optical power density, which is 2.19 mW / cm². 2 At that time, the maximum specific detectivity and external quantum efficiency were 8.8 × 10⁻⁶. 12 Jones, 949%, then gradually decreased. Figure 9 This is a graph showing the response speed of the phototransistor based on the SnSe2 / 2H-MoTe2 / SnSe2 heterojunction in Example 1 under 808nm incident light and a bias voltage of 0V. Figure 9 It can be seen that at a frequency of 5kHz, the rise and fall speeds of this phototransistor are 74μs and 25μs, respectively, indicating a very fast response time.

[0040] The above embodiments are preferred embodiments of the present invention, but the embodiments of the present invention are not limited to the above embodiments. Any changes, modifications, substitutions, combinations and simplifications made without departing from the spirit and principle of the present invention shall be considered equivalent substitutions and shall be included within the protection scope of the present invention.

Claims

1. A two-dimensional back-to-back broken bandgap phototransistor, characterized in that, The structure of the phototransistor is a SnSe2 / 2H-MoTe2 / SnSe2 heterojunction. This heterojunction is formed by N-type doped SnSe2 nanosheets on a p-SiO2 / Si substrate, then transferring 2H-MoTe2 nanosheets onto SnSe2 nanosheets, and then transferring N-type doped SnSe2 nanosheets from PDMS onto 2H-MoTe2 nanosheets to form a SnSe2 / 2H-MoTe2 / SnSe2 vertical heterojunction. The upper and lower SnSe2 nanosheets are not in contact. Finally, a metal bonding layer and Au electrodes are deposited on the SnSe2 nanosheets on both sides.

2. The two-dimensional back-to-back broken bandgap phototransistor according to claim 1, characterized in that, The thickness of the SnSe2 nanosheets is 20~50 nm; the thickness of the 2H-MoTe2 nanosheets is 20~50 nm.

3. The two-dimensional back-to-back broken bandgap phototransistor according to claim 1, characterized in that, The metal bonding layer is Cr or Ti with a thickness of 3-15 nm, and the Au electrode has a thickness of 50-100 nm.

4. The method for fabricating a two-dimensional back-to-back broken bandgap phototransistor according to any one of claims 1-3, characterized in that, The specific steps include the following: S1. SnSe2 nanosheets were obtained by mechanical exfoliation on a cleaned SiO2 / Si substrate. The sample surface was then soaked in acetone to remove residual adhesive and other small organic molecules. SnSe2 nanosheets with a thickness of 20-50 nm were selected by optical metallographic microscopy. S2. Using a dry transfer process, 2H-MoTe2 nanosheets with a thickness of 20~50nm are stacked onto SnSe2 nanosheets through a three-dimensional micro-region transfer platform, and then SnSe2 nanosheets are stacked from PDMS film onto 2H-MoTe2 nanosheets to form SnSe2 / 2H-MoTe2 / SnSe2 vertical heterojunction; S3. Spin-coat photoresist onto the substrate surface and dry it. Then, directly write the electrode pattern onto the upper and lower SnSe2 nanosheets using ultraviolet lithography laser. Develop the etched electrode pattern and deposit a metal bonding layer and Au electrode onto the p-SiO2 / Si surface with the etched electrode pattern using an electron beam. After immersion in acetone to remove the etched electrode and rinsing with deionized water, a SnSe2 / 2H-MoTe2 / SnSe2 heterojunction phototransistor is obtained, which is a two-dimensional back-to-back broken bandgap phototransistor.

5. The method for fabricating a two-dimensional back-to-back broken bandgap phototransistor according to claim 4, characterized in that, The thickness of SiO2 in the SiO2 / Si substrate mentioned in step S1 is 285~315 nm.

6. The method for fabricating a two-dimensional back-to-back broken bandgap phototransistor according to claim 4, characterized in that, The drying temperature in step S3 is 100℃, and the drying time is 4~30min.

7. The application of the two-dimensional back-to-back broken bandgap phototransistor according to any one of claims 1-3 in the fields of optical communication, optical sensing, optical detection or optical imaging.

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