Reverse airflow confined growth of β-Bi₂O₃ / Bi₂O₂Se heterojunction and its photodiode

Abstract

This invention provides a method for preparing a β-Bi₂O₃ / Bi₂O₂Se heterojunction and its photodiode via reverse airflow confinement growth, relating to the field of two-dimensional semiconductor materials and optoelectronic device fabrication technology. The method includes the following steps: S1. Providing a few-layer (or multi-layer) Bi₂O₂Se nanosheet; S2. Placing the Bi₂O₂Se nanosheet within a confinement space, which includes a cavity with an exhaust channel at the top and a heating platform with a reverse air supply port at the bottom; S3. Introducing a 10% (by volume) oxygen / argon mixture into the confinement space; S4. Oxidizing the Bi₂O₂Se nanosheet at an oxidation temperature of 260°C for 45 minutes to selectively generate a β-Bi₂O₃ layer with a thickness of 10–30 nm on its surface, thereby constructing a β-Bi₂O₃ / Bi₂O₂Se heterojunction structure at the interface; S5. Removing the heterojunction sample after cooling. Through the above steps, this method has the advantages of clear and stable heterojunction structure, significantly improved β-phase selective growth rate, greatly enhanced device performance, simple process, and strong compatibility.

Description

Technical Field

[0001] This invention relates to the field of two-dimensional semiconductor materials and their optoelectronic device fabrication technology, specifically to the reverse gas flow confined growth of β-Bi2O3 / Bi2O2Se heterojunctions and their photodiodes. Background Technology

[0002] Two-dimensional layered Bi₂O₂Se materials have attracted widespread attention in fields such as photodetectors, transistors, and neuromorphic devices due to their excellent electron mobility, photoelectric responsivity, and environmental stability. The significant band structure difference between Bi₂O₂Se and Bi₂O₃ gives Bi₂O₂Se / Bi₂O₃ heterojunctions potential advantages in band structure modulation and carrier separation, particularly in self-powered photodetectors.

[0003] However, because Bi2O2Se readily transforms completely into Bi2O3 under conventional oxidation conditions, phase boundary control is challenging, making it difficult to achieve high-quality, controllable interface construction of heterostructures. Existing technologies typically employ the following methods to attempt to oxidize the Bi2O2Se surface to form Bi2O3:

[0004] (1) Hot air annealing method: The Bi2O2Se film is exposed to air or oxygen and heat-treated at a high temperature (e.g., 350–450℃) to induce the oxidation process. However, the oxidation area is uncontrollable in this method, which can easily cause the entire Bi2O2Se film to be transformed into α- or β-Bi2O3, resulting in blurred phase boundaries and lack of pattern precision.

[0005] (2) Plasma treatment: Oxygen plasma is used to etch and oxidize the Bi2O2Se surface, which can achieve local modification. However, this method has the risk of physical damage, which can easily introduce defects and lattice damage, affecting the stability and repeatability of the electronic behavior of the subsequent heterojunction.

[0006] (3) Ultraviolet ozone oxidation method: The surface structure of Bi2O2Se is oxidized and transformed by UV-O3 source. Although this method has a certain degree of regional control capability, the oxidation path and depth are difficult to control precisely, which can easily lead to incomplete phase transformation or multiphase coexistence.

[0007] In summary, existing technologies for constructing heterostructures from Bi2O2Se to Bi2O3 mainly suffer from the following problems and shortcomings:

[0008] (1) There is a lack of controllable oxidation confinement methods, making it difficult to achieve patterned and region-selective growth;

[0009] (2) Oxidation process can easily lead to mixed phase structure and rough interface, which affects the quality of heterojunction;

[0010] (3) Traditional methods often produce the α-Bi2O3 phase instead of the β-Bi2O3 phase which has excellent photoelectric properties;

[0011] (4) The process window is narrow and it is sensitive to process conditions such as temperature and atmosphere, which is not conducive to industrial-scale preparation.

[0012] Therefore, there is an urgent need for a novel process to achieve controllable and confined oxidation of Bi2O2Se nanosheets under mild conditions, stably obtain the β-Bi2O3 phase, and form a clear, low-defect, high-quality heterojunction structure with the original Bi2O2Se, thereby further improving device performance and reliability. Summary of the Invention

[0013] (a) Technical problems to be solved

[0014] To address the shortcomings of existing technologies, this invention provides a reverse airflow confined growth of β-Bi2O3 / Bi2O2Se heterojunction and its photodiode, which solves the problems of lack of regional selectivity, unstable phase structure, unclear interface, and difficulty in stably obtaining the β-Bi2O3 phase in existing technologies.

[0015] (II) Technical Solution

[0016] To achieve the above objectives, the present invention provides the following technical solution: a method for preparing a β-Bi₂O₃ / Bi₂O₂Se heterojunction by reverse airflow confinement growth, comprising the following steps:

[0017] S1. A layer of Bi2O2Se nanosheets is provided, the nanosheets being grown on a mica substrate;

[0018] The substrate is selected with good thermal stability and optical transparency, such as mica sheets and sapphire. The two-dimensional Bi2O2Se single crystal is prepared by chemical vapor deposition (CVD) to obtain Bi2O2Se nanosheets with controllable thickness, which are then transferred to the target substrate and pre-cleaned.

[0019] S2. The Bi2O2Se nanosheets are placed in a confined space, which includes a cavity with an exhaust channel at the top and a heating platform with a reverse air supply port at the bottom;

[0020] A microcavity structure with spatial confinement, such as a mask layer or a reaction chamber with localized ventilation, is coated on the surface of Bi2O2Se nanosheets. The chamber can be made of quartz tube, metal frame or high-temperature stable polymer, and its shape matches the area to be oxidized. Gas channels are set in the reaction chamber to make the airflow flow in a directional and counter-directional manner from the edge to the inside of the chamber, forming a stable oxygen content gradient.

[0021] S3. In the confined space, a gas mixture of oxygen / argon with a volume fraction of 10% is introduced at a flow rate of 80 sccm to form a counter-diffusion gas flow from bottom to top;

[0022] The gas flow direction is opposite to the thermal diffusion direction, forming a stable region at the anti-diffusion interface;

[0023] S4. The Bi2O2Se nanosheets are oxidized at an oxidation temperature of 260 °C for 45 minutes to selectively generate a β-Bi2O3 layer with a thickness of 10~30 nm on its surface, thereby forming a β-Bi2O3 / Bi2O2Se heterojunction structure at the interface.

[0024] The sample was heated at a limited temperature (240 ℃–300 ℃) while oxygen-containing gas (such as O2 / Ar mixture, volume fraction 5%–30%) was slowly introduced, and the reaction time was maintained for 10–60 minutes. Through atmospheric confinement and temperature control, the local oxidation of Bi2O2Se was promoted to transform into β-Bi2O3, which has relatively stable crystals and excellent photoelectric properties.

[0025] S5. After cooling, the heterojunction sample is removed for use in the fabrication of optoelectronic devices;

[0026] After the reaction, the mixture was allowed to cool naturally in an inert gas atmosphere (such as argon) to prevent further oxidation. The presence of the β-Bi2O3 region and the clarity of the heterostructure interface were confirmed by Raman spectroscopy, optical microscopy (OM), and transmission electron microscopy (TEM).

[0027] Fabrication of photodiode devices: Source and drain electrodes were constructed in the Bi₂O₂Se and β-Bi₂O₃ regions using standard photolithography and metal evaporation techniques, respectively. The fabricated heterojunction structure was used as a pn-junction photodiode, and illumination was applied to achieve photodiode formation. I ds – V ds Tests were conducted to verify key performance parameters such as photoresponsivity, rectification ratio, open-circuit voltage, and response speed.

[0028] Preferably, the thickness of the Bi2O2Se nanosheets is 20~80 nm.

[0029] Preferably, the confined space is a cavity formed by a quartz cover and a controllable air intake module, and there is a gap of 0.5 to 2 mm between the cover and the heating platform.

[0030] Preferably, the oxidation treatment temperature is 280°C and the oxygen volume fraction is 20%.

[0031] A β-Bi₂O₃ / Bi₂O₂Se heterojunction structure has the following characteristics:

[0032] The heterostructure interface is clear, and the lattice continuity is good;

[0033] β-Bi2O3 is a layered crystal with a thickness of 20±5 nm and has typical β-phase Raman peaks;

[0034] The Bi2O2Se layer retains the original in-plane anisotropic electronic structure.

[0035] A photodiode with a β-Bi2O3 / Bi2O2Se heterojunction as the functional layer, wherein metal electrodes are deposited at both ends of the heterojunction to form Schottky contacts or ohmic contacts, and has self-driven photoresponse capability under no bias conditions, with a photoresponse rate of not less than 180 mA / W and a rectification ratio of not less than 450.

[0036] Preferably, the electrode material is Au, Ti / Au, or Pd / Au, and the electrode spacing is 5~20 μm.

[0037] Preferably, its own optical response time is less than 30 ms, which is suitable for short-wavelength or visible light fast response scenarios.

[0038] The flexible or integrated optoelectronic array device constructed according to this technical solution has array scalability, heterogeneous structure repeatability, and stable operating performance.

[0039] Working principle: This invention is based on the principle of anti-diffusion confined atmosphere. By adjusting the local oxygen partial pressure in the reaction zone through reverse airflow, the oxidation depth and range of Bi2O2Se are effectively controlled. The low-temperature oxidation-induced preferential growth mechanism of β phase is used to avoid the formation of α-Bi2O3, which is more thermodynamically stable but has poor photoelectric performance. The heterojunction formation process does not destroy the original crystal structure, has fewer interface defects, and the Schottky barrier is tunable, which is more conducive to the construction of high-performance optoelectronic devices.

[0040] (III) Beneficial Effects

[0041] This invention provides a reverse-flow confined growth β-Bi₂O₃ / Bi₂O₂Se heterojunction and its photodiode. It has the following beneficial effects:

[0042] 1. The heterojunction structure is clear and stable. The β-Bi2O3 / Bi2O2Se heterostructure prepared by the method of this invention forms a highly ordered van der Waals heterojunction interface at the interface with high lattice matching degree and no obvious voids and diffusion layers at the interface, which effectively reduces the interface state density and scattering centers, providing a basis for the high-performance operation of the device.

[0043] 2. The selective growth rate of the β phase is significantly improved. Compared with the traditional thermal oxidation method (treatment in air at about 300 °C for 60 minutes, which mainly forms α-Bi2O3), the process of this invention achieves a β-Bi2O3 phase content of more than 85% under the conditions of oxygen volume fraction of 5–30% and temperature control at 240–300 °C.

[0044] 3. Significantly improved device performance: The β-Bi₂O₃ / Bi₂O₂Se heterojunction photodiode constructed based on this invention exhibits excellent rectification characteristics and photoelectric response performance. A typical device achieves a current ratio (rectification ratio) as high as 10 under reverse bias (-2 V). 5 The device exhibits a photoresponsivity of up to 180 mA / W under 532 nm laser irradiation, with a response time of less than 30 ms. Compared to the unoptimized device, the photocurrent is increased by about 3.5 times. The introduction of the β phase significantly improves the carrier separation and interface barrier modulation capabilities.

[0045] 4. The process is simple and highly compatible, with a low overall fabrication temperature (<300 ℃), making it suitable for various flexible or CMOS-compatible substrates. It possesses good scalability and practical application potential. No external metal catalysts or high vacuum environment are required, resulting in a simple process flow and low cost. Attached Figure Description

[0046] Figure 1 This is a schematic diagram of the reverse airflow confined CVD device structure for the reverse airflow confined growth of β-Bi2O3 / Bi2O2Se heterojunction and its photodiode proposed in this invention.

[0047] Figure 2 This is a schematic diagram illustrating the formation principle of the β-Bi2O3 / Bi2O2Se heterojunction and its photodiode in the reverse airflow confined growth method proposed in this invention.

[0048] Figure 3 Optical microscope images of the β-Bi2O3 / Bi2O2Se heterojunction and its photodiode grown under reverse airflow confinement as proposed in this invention.

[0049] Figure 4 Raman spectrum analysis of the β-Bi2O3 / Bi2O2Se heterojunction and its photodiode grown under reverse airflow confinement proposed in this invention;

[0050] Figure 5 STEM image analysis of the β-Bi2O3 / Bi2O2Se heterojunction and its photodiode grown under reverse airflow confinement as proposed in this invention;

[0051] Figure 6 This is a schematic diagram of the device structure and band alignment of the β-Bi2O3 / Bi2O2Se heterojunction and its photodiode grown under reverse airflow confinement proposed in this invention.

[0052] Figure 7 The reverse airflow confined growth of β-Bi₂O₃ / Bi₂O₂Se heterojunction and its photodiode proposed in this invention is based on the I... ds -V ds Curves and light response performance graphs. Detailed Implementation

[0053] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0054] Example 1:

[0055] This invention provides a reverse-flow confined growth method for β-Bi₂O₃ / Bi₂O₂Se heterojunction and its photodiode. First, high-quality Bi₂O₂Se single crystals are prepared using conventional chemical vapor deposition (CVD). Specifically, Bi₂O₃ powder is placed as the evaporation source in the high-temperature zone (820 °C) at the center of the CVD furnace tube, while Se particles are placed in the low-temperature zone (250 °C) near the furnace opening. The carrier gas is high-purity argon gas at 30 sccm. After purging the reaction system for 30 minutes to replace the air, the temperature is increased for a growth time of 20 minutes. A Si / SiO₂ (300 nm) substrate is placed 12–14 cm downstream of the Bi source. After the reaction is complete and the substrate is allowed to cool naturally, a uniform Bi₂O₂Se film with a thickness of approximately 10–15 nm is obtained. Raman spectroscopy shows that the sample has clear argon deposition. 1g The (160 cm⁻¹) mode peak position confirms that it is a high-quality Bi₂O₂Se single crystal.

[0056] Based on the obtained Bi₂O₂Se single crystals, a β-Bi₂O₃ / Bi₂O₂Se heterostructure was constructed by introducing a reverse airflow confined oxidation technique. During the oxidation process, a semi-enclosed confined structure was set up in the sample region, with flow-limiting baffles installed on both sides. With the assistance of a low-speed reverse airflow, a local micro-convection region was formed, maintaining a low concentration of oxygen on the Bi₂O₂Se surface and ensuring a continuous supply. The oxidation conditions were: oxygen volume fraction of 20% (O₂:Ar = 1:4), total flow rate of 50 sccm, temperature of 280℃, and treatment time of 30 minutes. Under these conditions, the Bi₂O₂Se surface was selectively oxidized to the β-Bi₂O₃ phase. Raman spectroscopy showed E... g A 1g 315 cm -1 and 456 cm -1 Three characteristic vibrational modes were observed. Transmission electron microscopy (TEM) further confirmed that the heterojunction interface was clear, with a thickness of approximately 15–25 nm, and no obvious amorphous regions or polycrystalline inclusions were observed, indicating that the oxidation process of this invention has good phase selectivity and structure control.

[0057] Subsequently, the prepared β-Bi₂O₃ / Bi₂O₂Se heterostructure was fabricated into a lateral photodiode device using micro / nano fabrication methods. First, Ti / Au (10 nm / 60 nm) electrodes were deposited on the sample surface using photolithography and electron beam evaporation, with an electrode spacing of 5 μm. The device was annealed at 150 °C for 10 minutes in a nitrogen atmosphere to optimize electrode contact. The electrical performance of the device was measured using a Keithley 4200 parameter analyzer, and the rectification ratio (-2 V) was measured to be 10 at room temperature within the negative voltage range. 5 The above demonstrates excellent unidirectional conductivity. Under 532 nm laser irradiation, the photoresponsivity reaches 180 mA / W, with response times of 30 ms (rise) and 25 ms (fall), which are superior to conventionally thermally oxidized α-Bi₂O₃ / Bi₂O₂Se devices (with a responsivity of 74 mA / W and a rectification ratio of 23). This indicates that the heterojunction prepared in this invention has significant advantages in photoelectric conversion efficiency and rectification performance.

[0058] To further verify the effectiveness and stability of this method, comparative experiments were designed under different oxygen concentrations and temperatures.

[0059] Example 2:

[0060] The preparation method is the same as the overall process and Example 1, except that the oxygen concentration in the oxidation step is adjusted to 10% (O2:Ar = 1:9), the total flow rate is 80 sccm, the oxidation temperature is set to 260 ℃, and the treatment time is extended to 45 minutes. Under these conditions, only a β-Bi2O3 layer with a thickness of about 10 nm is formed. TEM images show that this layer has small grains but still has a striped structure characteristic of the β phase, and the interface continuity is acceptable. Device test results show that its rectification ratio is approximately ~10. 4 The photoresponsivity was 101 mA / W and the response time was 200 ms, indicating that β-phase selective oxidation can be achieved at lower temperatures and oxygen concentrations, but the generation rate is slower and the device performance is slightly reduced.

[0061] Example 3:

[0062] The preparation method was the same as the overall process and Example 1, except that the oxidation temperature was increased to 300 °C, the oxygen volume fraction was 30% (O2:Ar = 3:7), and the processing time was 20 minutes. Due to the high oxygen concentration and temperature, although the reverse gas flow confinement slowed the oxidation rate to some extent, the β-Bi2O3 layer on the Bi2O2Se surface rapidly reached a thickness of approximately 30 nm, and signs of phase mixing (α / β mixing) were observed in some areas. TEM showed slightly disordered grain boundaries. The rectification ratio of the optoelectronic device was 6.0 × 10⁻⁶. 2 The photoresponsivity is approximately 108 mA / W, and the response time is extended to 300 ms. Although it still has rectification and photoresponse capabilities, its overall performance is worse than that under intermediate conditions due to decreased phase purity and increased interface defects.

[0063] The results show that without reverse airflow confinement (conventional thermal oxidation), the obtained Bi2O3 is mainly α phase, and the device rectification ratio and photoresponse performance are significantly reduced. However, when the confinement strategy proposed in this invention is adopted, a stable β phase can be preferentially grown, and the heterostructure interface is more regular, resulting in higher device performance stability and repeatability.

[0064] In summary, the reverse airflow confined growth method proposed in this invention features strong controllability of the oxidation process, good phase selectivity, and high interface quality. It can effectively construct stable β-Bi₂O₃ / Bi₂O₂Se heterostructures and significantly improve the optoelectronic performance of devices. It is suitable for the large-scale, low-cost fabrication of novel two-dimensional material heterojunction devices, and has promising industrial application prospects and promotional value. This technical route can also be universally adapted for further expansion of heterojunction material systems or application in multi-band detectors, demonstrating strong versatility and scalability.

[0065] Although embodiments of the 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 to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for preparing a reverse-gas-flow confined growth β-Bi2O3 / Bi2O2Se heterojunction, characterized in that: The preparation steps include the following: ​ S1. A layer of Bi2O2Se nanosheets is provided, the nanosheets being grown on a mica substrate; S2. The Bi2O2Se nanosheets are placed in a confined space, which includes a cavity with an exhaust channel at the top and a heating platform with a reverse air supply port at the bottom; S3. In the confined space, a mixture of oxygen and argon with a volume fraction of 5% to 30% is introduced at a gas flow rate of 40 sccm to 100 sccm to form a counter-diffusion gas flow from bottom to top; S4. The Bi2O2Se nanosheets are oxidized at an oxidation temperature of 240~300 ℃ for 45 minutes to selectively generate a β-Bi2O3 layer with a thickness of 10~30 nm on its surface, thereby constructing a β-Bi2O3 / Bi2O2Se heterojunction structure at the interface. S5. After cooling, the heterojunction sample is removed for use in the fabrication of optoelectronic devices.

2. The method for preparing β-Bi₂O₃ / Bi₂O₂Se heterojunctions by reverse airflow confinement growth according to claim 1, characterized in that: The thickness of the Bi2O2Se nanosheets is 20~80 nm.

3. The method for preparing β-Bi₂O₃ / Bi₂O₂Se heterojunctions by reverse airflow confinement growth according to claim 1, characterized in that: The confined space is a cavity formed by a quartz cover and a controllable air intake module, with a gap of 0.5 to 2 mm between the cover and the heating platform.

4. The method for preparing β-Bi₂O₃ / Bi₂O₂Se heterojunctions by reverse airflow confinement growth according to claim 1, characterized in that: The oxidation treatment temperature is 280°C, and the oxygen volume fraction is 20%.

5. A β-Bi2O3 / Bi2O2Se heterojunction structure prepared by the method of claim 1-4, wherein the β-Bi2O3 / Bi2O2Se heterojunction structure is prepared by the method of claim 1-4. It has the following characteristics: heterogeneous The junction interface is clear and the lattice continuity is good; β-Bi2O3 is a layered crystal with a thickness of 20±5 nm and has typical β-phase Raman peaks; The Bi2O2Se layer retains the original in-plane anisotropic electronic structure.

6. A photodiode constructed based on the heterojunction structure of claim 5, characterized in that: Its β-Bi2O3 / Bi2O2Se heterojunction serves as a functional layer, and metal electrodes are deposited at both ends of the heterojunction to form Schottky contacts or ohmic contacts. It has self-driven photoresponse capability under no bias conditions, with a photoresponse rate of not less than 180 mA / W and a rectification ratio of not less than 450.

7. The photodiode of claim 6, wherein: The electrode material is Au, Ti / Au, or Pd / Au, and the electrode spacing is 5~20 μm.

8. The photodiode according to claim 7, characterized in that: Its own response time is less than 30 ms.

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