ZnMgO-based ultraviolet photodetector and preparation method thereof

Abstract

The application relates to the technical field of semiconductor photoelectric detection, and particularly relates to a ZnMgO-based ultraviolet photoelectric detector and a preparation method thereof. The detector comprises a single crystal substrate, an n-type ZnO film, an i-type ZnMgO film and a p-type NiO film which are tightly stacked from bottom to top. The i-type ZnMgO film is locally etched to form a step structure, and the surface of the exposed part of the n-type ZnO film at the step is exposed. A bottom electrode is arranged on the exposed n-type ZnO film. A top electrode is arranged on the surface of the p-type NiO film. The side of the i-type ZnMgO film, which is away from the single crystal substrate, is treated by hydrochloric acid wet etching to form a nano anti-reflection structure on the surface, and the interface oxygen vacancy defects are passivated. The advantages are that the preparation of the nano anti-reflection structure and the passivation of defects are simultaneously completed through hydrochloric acid wet etching and high-temperature annealing, the ultraviolet light interface reflection loss is reduced, the light absorption efficiency is improved, the oxygen vacancy recombination center is reduced, and the heterojunction electrical performance is improved.

Description

Technical Field

[0001] This invention relates to the field of semiconductor photoelectric detection technology, and in particular to a ZnMgO-based ultraviolet photodetector and its preparation method. Background Technology

[0002] Ultraviolet (UV) photodetectors have significant applications in space communication, environmental monitoring, biomedical imaging, and flame sensing. In space communication, their non-line-of-sight transmission characteristics enable the construction of secure communication links; in environmental monitoring, they can detect ozone layer depletion and atmospheric pollutants; in biomedical imaging, they aid in cell microscopy and lesion diagnosis; and in flame sensing, they are suitable for fire early warning and industrial combustion monitoring. Therefore, developing high-performance, highly stable, and low-power UV photodetectors has become a current research hotspot in photoelectric detection technology.

[0003] In recent years, ultraviolet photodetectors based on wide-bandgap semiconductor materials have attracted widespread attention. Wide-bandgap semiconductors, with their wide bandgap, high breakdown electric field, high carrier mobility, and excellent radiation resistance, overcome the limitations of traditional silicon-based detectors, such as weak response in the ultraviolet band, susceptibility to visible light interference, and insufficient stability. Representative materials include diamond, silicon carbide, gallium nitride, gallium oxide, and ZnMgO. Among them, diamond has extremely high thermal conductivity and breakdown electric field, making it suitable for ultraviolet detection in extreme environments; silicon carbide devices have mature manufacturing processes and good stability in solar-blind ultraviolet detection; gallium nitride-based materials have shown excellent integration potential in ultraviolet detector arrays and imaging systems; and gallium oxide, due to its ultra-wide bandgap and low growth cost, has become an important candidate material for solar-blind ultraviolet detection. These material systems provide a rich material foundation for improving the performance of ultraviolet photodetectors.

[0004] Among the aforementioned wide bandgap semiconductor materials, ZnMgO stands out as a highly promising candidate due to its excellent photoelectric properties, high radiation stability, and environmentally friendly characteristics. ZnMgO is a ternary alloy material developed based on ZnO, inheriting the advantages of ZnO such as high exciton binding energy, high electron mobility, and good visible light transparency. Furthermore, the bandgap width can be effectively controlled by introducing Mg. Compared to some wide bandgap materials, ZnMgO also features abundant raw materials, low toxicity, and the ability to be grown at low temperatures, aligning with the development trend of green electronic materials. By adjusting the Mg composition, Zn... 1- x Mg xThe bandgap of O can be continuously tuned from 3.37 eV to approximately 7.8 eV, enabling precise response to different bands of the ultraviolet spectrum. When the Mg content is low, the bandgap is close to that of ZnO, making it suitable for near-ultraviolet detection. When the Mg content is increased to a certain proportion, the bandgap can be extended to the deep ultraviolet and even the solar-blind band, meeting the requirements for high-sensitivity, low-background-noise solar-blind ultraviolet detection. This tunable bandgap characteristic allows ZnMgO to achieve direct selective response to specific ultraviolet bands without the need for complex filter structures, significantly improving the system's integration and reliability.

[0005] With the rapid development of the Internet of Things (IoT) and portable electronic devices, higher demands are being placed on the performance of ultraviolet (UV) photodetectors. They not only need high sensitivity, fast response speed, and excellent spectral selectivity, but also low power consumption or even self-powered (zero bias) operation to adapt to special application scenarios such as remote monitoring, mobile sensing, and implantable systems. Device structures based on built-in potential-driven photogenerated carrier separation (such as PN junctions, PIN junctions, and Schottky junctions) provide an effective physical mechanism for achieving self-powered detection. Among these structures, the PIN structure ZnMgO photodetector, with its strong built-in electric field, is considered an ideal architecture for achieving high-performance UV detection. However, this material system suffers from the difficulty of intrinsic doping, making it difficult to simultaneously obtain p-type and n-type conductive layers with matching mobility and stable performance. Therefore, related research has largely focused on ZnMgO-based heterojunction devices.

[0006] Despite this, the performance of ZnMgO-based PIN heterojunction ultraviolet detectors remains limited by two key bottlenecks. First, high interface reflection loss caused by the refractive index mismatch between the semiconductor and air severely restricts the device's photon capture efficiency, resulting in the ineffective reflection of some incident ultraviolet light and directly reducing external quantum efficiency and responsivity. Second, the quality of the heterojunction interface is crucial to device performance. In traditional fabrication processes, high-density defect states (such as oxygen vacancies and dangling bonds) often exist at the interfaces of each layer. These defect states, acting as non-radiative recombination centers and carrier scattering centers, not only increase dark current and noise but also trap photogenerated carriers, delaying their transport process and thus degrading detectivity and response speed. Therefore, simultaneously improving the optical coupling efficiency and interface electrical quality of ZnMgO PIN devices is a key challenge for achieving performance breakthroughs.

[0007] Surface anti-reflective treatment has been proven to effectively improve the light absorption capacity of materials and is a common method for enhancing the light-harvesting capability of photosensitive devices. Compared with multilayer anti-reflective coatings that require precise control of refractive index and thickness, nanostructured surface anti-reflective textures prepared by top-down etching have advantages such as omnidirectionality, broadband anti-reflective properties, and simple fabrication process. In device fabrication, etchants such as HCl, HNO3, H3PO4, or H3PO4 / HAc / H2O have been used for wet chemical etching of ZnMgO-based materials. In addition, existing research shows that solution treatment, plasma treatment, and other methods can reduce defect states on the surface of ZnMgO materials and improve heterojunction contact interfaces, thereby further improving the performance of ZnMgO-based photodetectors. Summary of the Invention

[0008] To address the aforementioned problems, this invention provides a ZnMgO-based ultraviolet photodetector and its preparation method.

[0009] The primary objective of this invention is to provide a ZnMgO-based ultraviolet photodetector, comprising a single-crystal substrate, an n-type ZnO thin film, an i-type ZnMgO thin film, and a p-type NiO thin film stacked tightly from bottom to top; The i-type ZnMgO thin film is locally etched to form a stepped structure, and the surface of the n-type ZnO thin film is exposed at the step; a bottom electrode is provided on the exposed n-type ZnO thin film; a top electrode is provided on the surface of the p-type NiO thin film. The i-type ZnMgO thin film, on the side facing away from the single crystal substrate, is treated with hydrochloric acid to form a nano-anti-reflection structure on the surface, and the oxygen vacancy defects at the interface are passivated.

[0010] Preferably, the i-type ZnMgO thin film is a hexagonal wurtzite single crystal with a band gap that can be continuously controlled within the range of 3.37 eV to 5.15 eV, and a dislocation density of less than 102. 9 cm -2 .

[0011] Preferably, the thickness of the n-type ZnO film is 300~800 nm; the thickness of the i-type ZnMgO film is 300~600 nm; and the thickness of the p-type NiO film is 10~100 nm.

[0012] Preferably, the bottom electrode is a Ti electrode or a Ti / Au composite electrode with a thickness of 10~200nm; the top electrode is a Pt electrode with a mesh structure and a thickness of 50~200nm.

[0013] The second objective of this invention is to provide a method for preparing a ZnMgO-based ultraviolet photodetector, which specifically includes the following steps: S1. The single crystal substrate was ultrasonically cleaned sequentially with acetone, ethanol and deionized water, and dried with nitrogen after each cleaning. S2. An n-type ZnO thin film was prepared on a single-crystal substrate using low-temperature plasma-enhanced molecular beam epitaxy. S3. A type i-ZnMgO thin film was prepared on the surface of an n-type ZnO thin film by low-temperature plasma-enhanced molecular beam epitaxy. S4. A nano-antireflective structure is formed by wet etching of the i-type ZnMgO film with hydrochloric acid, and then the oxygen vacancy defects at the interface are passivated by annealing in an oxygen atmosphere; local i-type ZnMgO film is etched away to form steps and expose part of the n-type ZnO film. S5. A p-type NiO film was deposited on the surface of a modified i-type ZnMgO film using radio frequency magnetron sputtering. S6. The bottom electrode and the top electrode are fabricated by magnetron sputtering.

[0014] Preferably, step S2 specifically includes the following sub-steps: S21. Place the single-crystal substrate into the sample inlet chamber of the MBE equipment and evacuate the vacuum. S22. Transfer the sample to the growth chamber, then heat the tray containing the sample to 650~750℃ for isothermal pretreatment for 1.5~2.5h; after pretreatment, purge with liquid nitrogen, maintain the single crystal substrate temperature at 500~600℃ during growth, introduce high-purity oxygen at a flow rate of 2~3sccm; simultaneously turn on the radio frequency plasma power supply, control the zinc source temperature at 230~280℃, evaporate the Zn molecular beam, and continue epitaxial growth on the single crystal substrate surface for 2.5~3.5h to form an n-type ZnO thin film; S23. After growth is complete, turn off the radio frequency and oxygen flow, and reduce the temperature of the single crystal substrate and zinc source to 75~85℃.

[0015] Preferably, in step S22, the temperature of the single crystal substrate is 550°C and the radio frequency power is 250~300W.

[0016] Preferably, step S3 specifically includes the following sub-steps: S31. Heat the tray after growing the n-type ZnO film to 600~700℃ and pre-treat at a constant temperature for 1.5~2.5h; S32. After pretreatment, the substrate growth temperature is controlled at 180~220℃; high-purity oxygen is introduced at a flow rate of 0.9~1.3 sccm; the radio frequency plasma power supply is turned on simultaneously at a power of 340~380W; the zinc source temperature is controlled at 230~280℃ and the magnesium source temperature at 260~300℃ respectively, and Zn and Mg molecular bundles are evaporated simultaneously. Epitaxial growth is co-deposited on the substrate surface for 2.5~3.5h to obtain type i ZnMgO thin film; S33. After the thin film epitaxy is completed, turn off the radio frequency plasma power supply and cut off the oxygen inlet valve in sequence; simultaneously cool the substrate, zinc source and magnesium source to 75~85℃.

[0017] Preferably, in step S4, a dilute hydrochloric acid solution with a mass fraction of 1-3 wt% is used to wet-etch the type i ZnMgO thin film for 3-10 s to prepare a nano-anti-reflection structure; then, the film is passivated by isothermal annealing at 800-900℃ for 0.8-1.5 h in an oxygen atmosphere to passivate oxygen vacancy defects on the film surface.

[0018] Preferably, in step S6, the local p-type NiO film and i-type ZnMgO film are first removed by photolithography and dilute hydrochloric acid etching to form a step and expose the n-type ZnO film; then, the bottom electrode and the mesh-like top electrode are prepared on the exposed n-type ZnO film and p-type NiO film respectively by magnetron sputtering.

[0019] Compared with the prior art, the present invention can achieve the following beneficial effects: (1) This invention simultaneously achieves optical anti-reflection and interface defect passivation, optimizing the basic performance of the thin film from both optical and electrical dimensions. The i-ZnMgO thin film is treated with dilute hydrochloric acid wet etching combined with oxygen annealing, which can generate nano-anti-reflection structures in situ on the surface of the thin film, alleviate the refractive index mismatch between the semiconductor and air, reduce ultraviolet light reflection loss, and improve light absorption utilization. At the same time, this treatment process can effectively eliminate oxygen vacancy defects on the surface of the thin film and the heterogeneous interface, reduce carrier recombination centers and scattering sites, and simultaneously improve the optical absorption capacity and interface electrical quality of the thin film.

[0020] (2) Significantly improve the photoelectric detection performance of the device and realize zero-bias self-powered solar-blind ultraviolet detection. A p-NiO / i-ZnMgO / n-ZnO heterojunction self-powered detector is constructed. Relying on the built-in electric field of the heterojunction, photogenerated carriers can be separated under zero bias voltage without the need for an external power supply. Compared with planar devices without nano-anti-reflection structures, the peak responsivity of the device is improved by about 100%, and the photoelectric conversion efficiency, ultraviolet detection sensitivity and response speed are significantly improved. Moreover, the device only responds to 295nm solar-blind ultraviolet light and has excellent visible light suppression effect.

[0021] (3) The thin film has excellent crystal quality, strong device stability, and simple preparation process for mass production. Pure hexagonal wurtzite ZnMgO single crystal thin film with a band gap of 3.37eV~5.15eV was prepared by molecular beam epitaxy. The dislocation density is low and the epitaxial interface is flat. The defect passivation process further ensures the repeatability of photocurrent under multiple light irradiation cycles and the environmental working stability is better. Attached Figure Description

[0022] Figure 1 This is a flowchart of a method for preparing a ZnMgO-based ultraviolet photodetector according to an embodiment of the present invention.

[0023] Figure 2 This is a schematic diagram of the structure of a ZnMgO-based ultraviolet photodetector provided in an embodiment of the present invention.

[0024] Figure 3 This is a schematic diagram of the fabrication process of a ZnMgO-based ultraviolet photodetector according to an embodiment of the present invention.

[0025] Figure 4 The results are characterization results of n-type ZnO thin films provided in the embodiments of the present invention.

[0026] Figure 5 The results are characterization results of the type i ZnMgO thin film provided according to the embodiments of the present invention.

[0027] Figure 6 This is a comparison chart of the photoelectric performance of a ZnMgO-based ultraviolet photodetector and a photodetector without a nano-anti-reflective structure provided according to an embodiment of the present invention.

[0028] Figure label: 1. Single crystal substrate; 2. n-type ZnO thin film; 3. Type i ZnMgO thin film; 4. p-type NiO thin film; 5. Bottom electrode; 6. Top electrode. Detailed Implementation

[0029] In the following description, embodiments of the invention will be described with reference to the accompanying drawings. In the description below, the same modules are denoted by the same reference numerals. Where the same reference numerals are used, their names and functions are also the same. Therefore, their detailed description will not be repeated.

[0030] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and do not constitute a limitation thereof.

[0031] This invention provides a ZnMgO-based ultraviolet photodetector, comprising a single crystal substrate, an n-type ZnO thin film, an i-type ZnMgO thin film, and a p-type NiO thin film stacked tightly from bottom to top; The i-type ZnMgO thin film is removed by photolithography etching to form a stepped structure, exposing part of the n-type ZnO thin film surface; The exposed n-type ZnO film surface has a bottom electrode; the p-type NiO film surface has a mesh-like top electrode. The i-type ZnMgO thin film has a nanoscale anti-reflection structure on the surface away from the substrate, and the internal and interface oxygen vacancy defects are passivated. The i-type ZnMgO thin film has a hexagonal wurtzite single crystal structure, the band gap can be tuned to 5.15 eV, and the dislocation density is less than 102. 9 cm -2 .

[0032] Specifically, n-type ZnO thin films and i-type ZnMgO thin films were prepared by low-temperature plasma-enhanced molecular beam epitaxy; p-type NiO thin films were prepared by radio frequency magnetron sputtering. The single-crystal substrate is a c-plane (0001) α-alumina substrate; The n-type ZnO thin film is the bottom conductive layer with a thickness of 300~800nm, preferably 400~500nm; The i-type ZnMgO thin film is the intermediate layer with a thickness of 300~600nm, preferably 300~400nm; the nano-anti-reflection structure on the surface of the i-type ZnMgO thin film is prepared by hydrochloric acid wet etching and oxygen annealing passivation; the band gap of the i-type ZnMgO thin film is 4.8 eV. The thickness of the p-type NiO thin film is 10~100nm, and the preferred thickness is 30~40nm; The bottom electrode is a Ti electrode or a Ti / Au composite electrode with a thickness of 10~200nm; The top electrode is a Pt electrode with a mesh structure (preferably a spider web structure). The annular region is partially transparent and partially opaque, which can effectively collect charge carriers; the thickness is 50~200nm.

[0033] The preparation method of the above-mentioned ZnMgO-based ultraviolet photodetector specifically includes the following steps: S1. Clean the single crystal substrate with acetone, ethanol and deionized water for 10 minutes each, and dry it with nitrogen after each cleaning. In some embodiments, the single-crystal substrate is a c-plane (0001) α-alumina (c-plane sapphire) substrate.

[0034] S2. An n-type ZnO thin film is prepared on a single-crystal substrate using low-temperature plasma-enhanced molecular beam epitaxy; specifically including the following sub-steps: S21. Place the single crystal substrate into the sample inlet chamber of the MBE equipment. First, use the mechanical pump as the backing pump to pump for 10 minutes, then turn on the molecular pump to pump for 2 hours. After that, open the valve and place the tray on the sample transfer cart. Use the sample transfer cart to put the tray into the growth chamber and close the valve. No oxygen, plasma or any other process gases are introduced in this step; only vacuum pumping is used. S22. Heat the sample tray in the growth chamber to 650~750℃ and maintain the temperature for 1.5~2.5 hours to remove adsorbed water vapor, organic impurities and surface defects from the substrate surface; after the pretreatment, purge with liquid nitrogen. During growth, maintain the temperature of the single crystal substrate at 500~600℃ and introduce high-purity oxygen at a flow rate of 2~3 sccm; simultaneously turn on the radio frequency plasma power supply at 250~300W to ionize oxygen and generate active oxygen plasma; control the temperature of the zinc source at 230~280℃ to evaporate the Zn molecular bundle and continue epitaxial growth on the single crystal substrate surface for 2.5~3.5 hours to form an n-type ZnO thin film; Specifically, the sample tray was heated to 700℃ and pretreated for 2 hours; the single crystal substrate temperature was 550℃ during growth; the oxygen flow rate was set to 2.25 sccm; the RF power was 260W; the zinc source temperature was controlled at 250℃, and epitaxial growth was carried out for 3 hours. S23. After growth is complete, turn off the radio frequency and oxygen flow, and reduce the temperature of the single crystal substrate and zinc source (referring to the pure substance of chemical element Zn placed in a crucible) to 75~85℃ (preferably 80℃).

[0035] S3. An intrinsic type i ZnMgO thin film is prepared on the surface of an n-type ZnO thin film using low-temperature plasma-enhanced molecular beam epitaxy; specifically including the following sub-steps: S31. Heat the tray after growing the n-type ZnO film to 600~700℃ and pre-treat at a constant temperature for 1.5~2.5h to remove adsorbed impurities and interface contaminants on the surface of the n-type ZnO film. Specifically, the sample tray was heated to 650℃ and pretreated at this temperature for 2 hours. Epitaxial growth of S32.i type ZnMgO thin film: After pretreatment, the substrate growth temperature is controlled at 180~220℃; high-purity oxygen is introduced at a flow rate of 0.9~1.3 sccm; radio frequency plasma power supply is turned on simultaneously at a power of 340~380W to ionize oxygen and generate active oxygen plasma; the zinc source temperature is controlled at 230~280℃ and the magnesium source temperature is controlled at 260~300℃ respectively, and Zn and Mg molecular bundles are evaporated simultaneously. Epitaxial growth is carried out on the substrate surface for 2.5~3.5h to obtain a hexagonal wurtzite intrinsic i type ZnMgO thin film with continuously adjustable bandgap. Specifically, the substrate growth temperature was controlled at 200℃, the oxygen flow rate was 1.1 sccm, and the RF power was 360W; the zinc source was kept at a constant temperature of 250℃, the magnesium source at a constant temperature of 280℃, and the epitaxial growth was carried out continuously for 3 hours. S33. Cooling after growth: After the thin film epitaxy is completed, turn off the radio frequency plasma power supply and cut off the oxygen inlet valve in sequence; simultaneously cool the substrate, zinc source and magnesium source to 75~85℃ (preferably 80℃) and maintain the ultra-high vacuum of the growth chamber throughout the process; the zinc source and magnesium source are respectively high-purity Zn and high-purity Mg elements placed in the temperature-controlled crucible.

[0036] S4. The i-type ZnMgO film is etched by hydrochloric acid wet etching followed by high-temperature annealing in oxygen; specifically including the following sub-steps: S41. Hydrochloric acid wet etching: Prepare a 1-3 wt% dilute hydrochloric acid etching solution, immerse the i-type ZnMgO film for 3-10 seconds to form a nano-anti-reflection structure in situ on the ZnMgO film surface, and simultaneously remove surface oxygen vacancy defects; immediately rinse with deionized water after etching and dry with nitrogen for later use; perform photolithography and wet etching to etch away local i-type ZnMgO film to form steps and expose part of the n-type ZnO film; Specifically, a 2.08 wt% dilute hydrochloric acid corrosion solution was selected, and the immersion time was 6 seconds. S42. High-temperature annealing and passivation in oxygen atmosphere: The etched sample is placed in a tube annealing furnace, and high-purity oxygen is introduced to form a continuous oxygen atmosphere. The temperature is raised to 800~900℃ and held at a constant temperature for annealing for 0.8~1.5h. The high-temperature oxygen atmosphere fully passivates the oxygen vacancy defects in the bulk phase of the ZnMgO film and the heterostructure interface, and optimizes the subsequent p-NiO / i-ZnMgO interface contact quality. Specifically, the annealing temperature is 850℃, and the annealing is carried out at a constant temperature for 1 hour in an oxygen atmosphere.

[0037] S5. A p-type NiO film is deposited on the surface of the modified i-ZnMgO film using radio frequency magnetron sputtering; specifically including the following sub-steps: S51. p-type NiO thin film deposition: The treated film is placed in a magnetron sputtering vacuum chamber, the NiO target is fixed, the growth temperature is set to room temperature, and the chamber is evacuated; an argon-oxygen mixed protective gas is introduced, with a total flow rate of 50~70 sccm, and the radio frequency power is set to 90~110W. Deposition is carried out continuously at room temperature for 8~12 min to generate a p-type NiO thin film on the surface of the i-type ZnMgO thin film. Specifically, the total flow rate of the mixed gas was 60 sccm, the radio frequency power was 100 W, and the deposition time was 10 min at room temperature. S52. Post-processing of the chamber: After the thin film deposition is completed, turn off the radio frequency power supply and cut off the argon and oxygen inlet valves in sequence; when the molecular pump speed drops to below 100 r / min, turn off the molecular pump and mechanical pump, fill the chamber with high-purity nitrogen to break the vacuum, and take out the deposited sample; This step includes the following sub-steps before depositing the p-type NiO thin film: S50. Pre-sputtering cleaning of target material: Place the treated film into the magnetron sputtering vacuum chamber and fix the NiO target material; after evacuating the chamber to a high background vacuum, introduce an argon-oxygen mixed protective gas, turn on the RF power supply to pre-sputter the NiO target material for 15~25 minutes, peel off the oxide and impurity layers on the target material surface, and eliminate the impurity source of the deposited film. Specifically, the pre-sputtering time is 20 minutes.

[0038] S6. Fabrication of the bottom and top electrodes by magnetron sputtering; specifically including the following sub-steps: S61. Photoresist spin coating, pre-baking and exposure development: Place the sample with deposited p-type NiO thin film in a spin coater, pre-rotate at 900~1100 r / min for 8~12s, then spin coat the photoresist at 5000~7000 r / min for 15~25s; after spin coating, place it on a heating plate at 85~95℃ for 5~7min; expose it with a UV lithography machine for 30~40s, immerse it in the developer for 10~15s, rinse with deionized water for 1~3min, and blow dry with nitrogen to form a pattern mask for the electrode area; Specifically, the pre-rotation speed is 1000 r / min for 10 s, the actual rotation speed is 6000 r / min for 20 s; the baking temperature is 90℃ for 6 min; the exposure time is 35 s, the development time is 12 s, and the deionized water rinse time is 2 min. S62. A bottom electrode was fabricated on a bare n-type ZnO thin film using magnetron sputtering, and a mesh-like top electrode was fabricated on a p-type NiO thin film, thus completing the detector fabrication. See the fabrication flowchart below. Figure 1 .

[0039] Example 1: This embodiment provides a ZnMgO-based ultraviolet photodetector. (See [link to documentation]) Figure 2 It includes a single crystal substrate 1, an n-type ZnO thin film 2, an i-type ZnMgO thin film 3, and a p-type NiO thin film 4, which are stacked tightly from bottom to top; Single crystal substrate 1 is a -Al2O3 substrate; The i-type ZnMgO film 3 and the p-type NiO film 4 are partially removed by photolithography etching to form a stepped structure, exposing part of the surface of the n-type ZnO film 2; the exposed n-type ZnO film 2 surface is provided with a bottom electrode 5; the p-type NiO film 4 surface is provided with a mesh-like top electrode 6. The i-type ZnMgO thin film 3 was treated with hydrochloric acid on the side away from the substrate to form a nano-anti-reflection structure on the surface, and the oxygen vacancy defects at the interface were passivated. The bottom electrode 5 is a Ti / Au composite electrode, and the top electrode 6 is a Pt electrode.

[0040] Preparation methods include: S1. The single crystal substrate is ultrasonically cleaned sequentially with acetone, ethanol and deionized water, and dried with nitrogen after each cleaning. S2. An n-type ZnO thin film was prepared by epitaxial growth on a pretreated single-crystal substrate using a low-temperature plasma-enhanced molecular beam epitaxy process. S3. An intrinsic i-type ZnMgO thin film is epitaxially grown on the surface of an n-type ZnO thin film using a low-temperature plasma-enhanced molecular beam epitaxy process. S4. The i-type ZnMgO thin film is subjected to short-time wet etching with dilute hydrochloric acid to generate a nano-anti-reflection structure on the film surface. Then, it is placed in an oxygen atmosphere for high-temperature annealing to passivate the oxygen vacancy defects inside the film and at the interface. Photolithography and wet etching are performed to remove local i-type ZnMgO thin film to form steps and expose part of n-type ZnO thin film. S5. First, the NiO target is pre-sputtered to remove impurities, and then a p-type NiO film is deposited on the surface of the modified i-type ZnMgO film by radio frequency magnetron sputtering. S6. Spin-coating photoresist onto the sample, pre-baking, UV exposure, and development to form a patterned mask for the electrode region; using magnetron sputtering, fabricate the Ti / Au bottom electrode and the mesh Pt top electrode respectively, to obtain a complete ZnMgO-based UV photodetector. The fabrication process is as follows: Figure 3 As shown.

[0041] Systematic performance characterization was performed on the n-type ZnO thin film serving as the bottom conductive layer. The characterization results are as follows: Figure 4 As shown, four testing methods were used to analyze the film morphology, thickness, surface roughness, optical properties and crystal quality: scanning electron microscopy (SEM), atomic force microscopy (AFM), ultraviolet-visible spectrophotometry and X-ray diffraction (XRD).

[0042] Figure 4 (a) is a cross-sectional SEM image of the ZnO thin film. The cross-sectional morphology clearly shows that the n-type ZnO thin film prepared by molecular beam epitaxy (MBE) has a uniform thickness of about 500 nm. The interface between the n-type ZnO thin film and the single crystal substrate is clear and flat, without obvious bulk defects such as pores and cracks, and the overall continuity of the film is good. Figure 4 (b) shows the AFM surface morphology test results of the n-type ZnO thin film. The test results show that the root mean square (RMS) roughness of the film is 0.44 nm, which proves that the n-type ZnO thin film prepared by this process has a smooth and flat surface and dense and uniform grain arrangement, which can provide a high-quality flat interface for the subsequent epitaxial growth of i-ZnMgO thin film. Figure 4 In the middle (c), the ultraviolet-visible transmission spectrum of the n-type ZnO thin film is obtained. The band gap of the ZnO thin film is 3.37 eV, which is calculated by fitting the embedded Tauc plot method. The thin film has high light transmittance in the visible light range and is suitable for the use of the bottom conductive layer of ultraviolet photodetectors. Figure 4The middle (d) shows the X-ray rocking curve of the ZnO(002) crystal plane. The XRD test results show that, in addition to the diffraction peak of the sapphire substrate, the sample has a characteristic ZnO(002) diffraction peak at the 34.7° position. The full width at half maximum (FWHM) of the rocking curve corresponding to this crystal plane is about 0.07°. The extremely small FWHM value indicates that the ZnO film has excellent preferred orientation along the c-axis, low internal dislocation density, and excellent single crystal crystal quality.

[0043] Systematic characterization of the intermediate i-type ZnMgO thin film was carried out, and the test results are as follows: Figure 5 As shown, type i ZnMgO thin films with different Mg composition contents were prepared, and ultraviolet-visible transmission spectroscopy and X-ray diffraction tests were carried out to verify the band gap control range, crystal phase and crystal quality of the thin films.

[0044] Figure 5 In the middle (a), the ultraviolet-visible transmission spectra of type i ZnMgO thin films with different Mg compositions are shown. As the proportion of Mg composition in the film increases, the ultraviolet absorption edge of the film continues to shift towards the short wavelength direction, which intuitively shows that the band gap of the film continues to widen with the increase of Mg composition. The film has high overall transmittance in the visible light range and good optical consistency. Figure 5 In Figure (b), the corresponding sample Tauc fitting curve is shown. According to linear fitting calculation, the band gap of the series of thin films can be continuously adjusted in the range of 3.37eV to 5.15eV. Among them, the target ratio ZnMgO thin film with a band gap of about 4.8eV can be obtained, realizing the precise control of the composition of wide band gap solar blind ultraviolet materials. Figure 5 (c) shows the full X-ray diffraction (XRD) spectra of type i ZnMgO thin films with different Mg compositions. All samples except... α In addition to the diffraction peaks of the Al2O3 substrate and the bottom ZnO(002) diffraction peaks, only the characteristic diffraction peaks of hexagonal wurtzite ZnMgO(002) appeared, and no cubic phase impurity peaks were generated, proving that ZnMgO films with different Mg compositions can stably maintain the pure hexagonal wurtzite crystal structure and have high phase purity. Figure 5 (d) shows the X-ray rocking curve of the (002) crystal plane of the i-type ZnMgO thin film with the optimal ratio. The diffraction peak is symmetrical and sharp, and the measured full width at half maximum (FWHM) is 0.10°. The small FWHM value indicates that the i-type ZnMgO thin film has strong preferred orientation along the c-axis, low internal defect and dislocation density, and excellent single crystal crystal quality. This series of high-quality i-type ZnMgO epitaxial thin films with continuously adjustable band gaps provides a reliable core functional layer foundation for the subsequent preparation of high-performance ZnMgO-based heterojunction ultraviolet photodetectors.

[0045] The photoelectric performance of the nano-antireflective ZnMgO-based ultraviolet photodetector and the planar ZnMgO-based ultraviolet photodetector prepared by this invention were tested, and the test results are as follows: Figure 6As shown, the effects of nanoscale anti-reflective structures on improving the photoelectric detection performance of devices are compared and analyzed.

[0046] Figure 6 In Figure (a), the spectral responsivity curves of the planar ZnMgO device and the nano-anti-reflective ZnMgO device of this invention are shown under zero bias (V=0V) conditions. The response cutoff edges of the two devices match the ZnMgO bandgap, and the peak response wavelengths are both located in the solar-blind ultraviolet band of approximately 295 nm. The peak responsivity of the planar device is 22.4 mA / W. The peak responsivity of the device formed by etching the nano-anti-reflective structure with hydrochloric acid in this invention can reach 50.5 mA / W, with a responsivity improvement of approximately 100%. This directly verifies that the nano-anti-reflective structure on the thin film surface can effectively enhance the absorption and utilization rate of ultraviolet light and significantly improve the photoelectric conversion capability of the device. The embedded logarithmic coordinate spectral curves further show that both devices have excellent solar-blind ultraviolet spectral selectivity, and the response in the visible light region is almost completely suppressed. Figure 6 (b) Irradiation with zero bias and 295nm ultraviolet light (optical power density P=521μW / cm²) 2 Under these conditions, the current-time (FOT) of the two types of devices I - t Cyclic switching characteristic curves; results of multiple periodic switching illumination tests show that both the planar structure device and the nano-anti-reflective structure device have good photocurrent repeatability and operational stability, with high consistency in the baseline of each switching response; under the same test conditions, the photocurrent amplitude of the nano-anti-reflective structure device is always significantly higher than that of the planar structure device, further confirming that the ZnMgO surface nano-anti-reflective modification process introduced in this invention can effectively optimize the overall photoelectric detection performance of the heterojunction detector.

[0047] It should be understood that the various forms of processes shown above can be used to reorder, add, or delete steps. For example, the steps described in this invention disclosure can be executed in parallel, sequentially, or in different orders, as long as the desired result of the technical solution disclosed in this invention can be achieved, and this is not limited herein.

[0048] The specific embodiments described above do not constitute a limitation on the scope of protection of this invention. Those skilled in the art should understand that various modifications, combinations, sub-combinations, and substitutions can be made according to design requirements and other factors. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of this invention should be included within the scope of protection of this invention.

Claims

1. A ZnMgO-based ultraviolet photodetector, characterized in that: It includes a single crystal substrate, an n-type ZnO thin film, an i-type ZnMgO thin film, and a p-type NiO thin film that are tightly stacked from bottom to top; The i-type ZnMgO thin film is locally etched to form a stepped structure, and the surface of the n-type ZnO thin film is exposed at the step; a bottom electrode is provided on the exposed n-type ZnO thin film; a top electrode is provided on the surface of the p-type NiO thin film. The i-type ZnMgO thin film, on the side facing away from the single crystal substrate, is etched with hydrochloric acid to form a nano-anti-reflection structure on the surface, and the oxygen vacancy defects at the interface are passivated.

2. The ZnMgO-based ultraviolet photodetector according to claim 1, characterized in that: The i-type ZnMgO thin film is a hexagonal wurtzite single crystal with a band gap that can be continuously controlled within the range of 3.37 eV to 5.15 eV, and a dislocation density of less than 102. 9 cm -2 .

3. The ZnMgO-based ultraviolet photodetector according to claim 1, characterized in that: The thickness of the n-type ZnO film is 300~800nm; the thickness of the i-type ZnMgO film is 300~600nm; and the thickness of the p-type NiO film is 10~100nm.

4. The ZnMgO-based ultraviolet photodetector according to claim 1, characterized in that: The bottom electrode is a Ti electrode or a Ti / Au composite electrode with a thickness of 10~200nm; the top electrode is a Pt electrode with a mesh structure and a thickness of 50~200nm.

5. A method for preparing a ZnMgO-based ultraviolet photodetector, used to prepare the ZnMgO-based ultraviolet photodetector according to claim 1, characterized in that: Specifically, the steps include the following: S1. The single crystal substrate was ultrasonically cleaned sequentially with acetone, ethanol and deionized water, and dried with nitrogen after each cleaning. S2. n-type ZnO thin films were prepared on single-crystal substrates using low-temperature plasma-enhanced molecular beam epitaxy. S3. A type i-ZnMgO thin film was prepared on the surface of an n-type ZnO thin film by low-temperature plasma-enhanced molecular beam epitaxy. S4. A nano-antireflective structure is formed by wet etching of type i ZnMgO film with hydrochloric acid, and then the oxygen vacancy defects at the interface are passivated by annealing in an oxygen atmosphere. Etching removes localized i-type ZnMgO films to form steps and exposes some n-type ZnO films; S5. A p-type NiO film was deposited on the surface of a modified i-type ZnMgO film using radio frequency magnetron sputtering. S6. The bottom electrode and the top electrode are prepared by magnetron sputtering.

6. The method for preparing a ZnMgO-based ultraviolet photodetector according to claim 5, characterized in that: Step S2 specifically includes the following sub-steps: S21. Place the single-crystal substrate into the sample inlet chamber of the MBE equipment and evacuate the vacuum. S22. Transfer the sample to the growth chamber, and then heat the tray containing the sample to 650~750℃ for constant temperature pretreatment for 1.5~2.5h; After pretreatment, liquid nitrogen is introduced. During growth, the temperature of the single crystal substrate is 500~600℃, and high-purity oxygen is introduced with an oxygen flow rate of 2~3 sccm. Simultaneously, the radio frequency plasma power supply is turned on, and the temperature of the zinc source is controlled at 230~280℃ to evaporate the Zn molecular beam. Epitaxial growth is carried out on the surface of the single crystal substrate for 2.5~3.5 hours to form an n-type ZnO thin film. S23. After growth is complete, turn off the radio frequency and oxygen flow, and reduce the temperature of the single crystal substrate and zinc source to 75~85℃.

7. The method for preparing a ZnMgO-based ultraviolet photodetector according to claim 6, characterized in that: In step S22, the temperature of the single crystal substrate is 550℃ and the radio frequency power is 250~300W.

8. The method for preparing a ZnMgO-based ultraviolet photodetector according to claim 5, characterized in that: Step S3 specifically includes the following sub-steps: S31. Heat the tray after growing the n-type ZnO film to 600~700℃ and pre-treat at a constant temperature for 1.5~2.5h; S32. After pretreatment, the substrate growth temperature is controlled at 180~220℃; high-purity oxygen is introduced at a flow rate of 0.9~1.3 sccm; the radio frequency plasma power supply is turned on simultaneously at a power of 340~380W; the zinc source temperature is controlled at 230~280℃ and the magnesium source temperature at 260~300℃ respectively, and Zn and Mg molecular bundles are evaporated simultaneously. Epitaxial growth is co-deposited on the substrate surface for 2.5~3.5h to obtain type i ZnMgO thin film; S33. After the thin film epitaxy is completed, turn off the radio frequency plasma power supply and cut off the oxygen inlet valve in sequence; simultaneously cool the substrate, zinc source and magnesium source to 75~85℃.

9. The method for preparing a ZnMgO-based ultraviolet photodetector according to claim 5, characterized in that: In step S4, a dilute hydrochloric acid solution with a mass fraction of 1-3 wt% is used to wet-etch the type i ZnMgO thin film for 3-10 s to prepare a nano-anti-reflection structure; then, the film is passivated by isothermal annealing at 800-900℃ for 0.8-1.5 h in an oxygen atmosphere to passivate oxygen vacancy defects on the film surface.

10. The method for preparing a ZnMgO-based ultraviolet photodetector according to claim 5, characterized in that: In step S6, the local p-type NiO film and i-type ZnMgO film are first removed by photolithography and dilute hydrochloric acid etching to form steps and expose the n-type ZnO film; then, the bottom electrode and the mesh-like top electrode are prepared on the exposed n-type ZnO film and p-type NiO film respectively by magnetron sputtering.

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