Photoconductive solar-blind ultraviolet detector based on MgZnO / InGaO heterojunction photosensitive layer and preparation method of photoconductive solar-blind ultraviolet detector

Abstract

The invention discloses a high-performance photoconductive solar-blind ultraviolet detector based on an MgZnO / InGaO heterojunction photosensitive layer and a preparation method thereof, and belongs to the technical field of ultraviolet photoelectric detection. The device is composed of a SiO2 / FTO substrate, a MgZnO nano-film growing on the surface of FTO, an InGaO nano-film growing on the surface of the MgZnO nano-film, and Ag electrodes prepared on the surface of the InGaO nano-film and the surface of FTO from bottom to top, wherein the MgZnO nano-film and the InGaO nano-film form a heterojunction photosensitive layer. According to the photoconductive solar-blind ultraviolet detector prepared by the invention, when the photoconductive solar-blind ultraviolet detector is under an illumination condition, the composite film synergistically participates in photon absorption, and meanwhile, the photoconductive device has a high gain characteristic, so that the device has outstanding photocurrent and reaches a photoresponsivity peak value at a solar blind area (260nm); and a new thought is provided for the preparation of the high-performance solar blind ultraviolet detector.

Description

Technical Field

[0001] The invention belongs to the technical field of ultraviolet photoelectric detection, and specifically relates to a high-performance photoconductive solar-blind ultraviolet detector based on a MgZnO / InGaO heterojunction light-sensitive layer and a preparation method thereof. Background Art

[0002] Ultraviolet radiation is a type of invisible light with a wavelength of 10 to 400 nm, which can be divided into the following bands: UVA (320 to 400 nm), UVB (280 to 320 nm), UVC (200 to 280 nm), VUV (100 to 200 nm) and EUV (10 to 100 nm). Ultraviolet detection technology is an important means of converting ultraviolet light signals into electrical signals through external photoelectric effect or internal photoelectric effect. In the early days, ultraviolet detectors were mainly used in military fields, such as ultraviolet alarms, ultraviolet communications, and ultraviolet guidance. With the development of technology, their applications have gradually expanded to civilian fields, such as ultraviolet disinfection, fire detection, biomedicine, spectral analysis, and particle detection.

[0003] In the development process of UV detectors, solar-blind UV detectors have attracted much attention due to their unique advantages. Solar-blind UV detectors refer to photoelectric detectors that work in the solar-blind UV band of 200-280nm. Due to the strong absorption of 200-280nm ultraviolet light by the ozone layer in the atmosphere, UV detectors working in this solar-blind band exhibit low background noise characteristics and have the advantages of high sensitivity, strong anti-interference ability, and low false alarm rate. Wide bandgap semiconductor materials are the preferred materials for preparing solar-blind UV detectors, mainly including gallium oxide (Ga 2 O 3 ), aluminum gallium nitride (AlGaN), magnesium zinc oxide (MgZnO), etc. Single-material wide bandgap semiconductor ultraviolet detectors have advantages such as high breakdown electric field, high thermal conductivity and good chemical stability, but due to inherent traps and defects in the material, there are often problems such as excessive dark current and poor photoresponse. In order to break through this bottleneck, methods such as element doping of single-material devices, preparation of heterojunction photosensitive layers, and construction of electrode structures of different gold-semiconductor contact types can be used. The photoconductive detector prepared by the third method is an important type of ultraviolet detector. The structural feature is that a metal electrode that matches the semiconductor work function well and forms an ohmic contact is prepared on the surface of the semiconductor nanofilm of the ultraviolet light absorption layer. At this time, no barrier layer is formed at the gold-semiconductor contact interface, thereby significantly reducing the transport barrier to photogenerated carriers, so that this type of device exhibits higher photocurrent and responsivity, which is conducive to achieving significant photoelectric gain effects.

[0004] With the development of science and technology, the requirements for the performance parameters of ultraviolet photodetectors in various application fields are constantly increasing, including higher photocurrent, lower dark current and more outstanding responsivity. In order to solve the above problems, preparing photoconductive heterojunction light-sensitive layer devices is a simple and extremely effective method, that is, by synergizing the advantages of composite materials and the high gain characteristics of photoconductive devices to improve the shortcomings of single material devices and achieve an overall improvement in device performance. Summary of the invention

[0005] The purpose of the present invention is to provide a photoconductive solar-blind ultraviolet detector based on a MgZnO / InGaO heterojunction light-sensitive layer and a preparation method thereof.

[0006] In order to achieve the above object, the technical solution of the present invention is:

[0007] A photoconductive solar-blind ultraviolet detector based on a MgZnO / InGaO heterojunction photosensitive layer, composed of SiO 2 / FTO substrate, MgZnO nanofilm grown on the FTO surface, InGaO nanofilm grown on the MgZnO nanofilm surface, Ag electrodes prepared on the InGaO nanofilm surface and FTO surface, MgZnO nanofilm and InGaO nanofilm constitute MgZnO / InGaO heterojunction photosensitive layer, thickness of 200 ~ 400nm; ultraviolet light to be measured from SiO 2 / Incident from one side of the FTO substrate.

[0008] The method for preparing a photoconductive solar-blind ultraviolet detector based on a MgZnO / InGaO heterojunction photosensitive layer of the present invention comprises the following steps:

[0009] (1) Substrate processing

[0010] SiO with a thickness of 1 to 4 mm 2 / FTO substrate (can be purchased directly) was immersed in acetone, ethanol, and deionized water for ultrasonic treatment for 5 to 15 minutes, and then dried with nitrogen and SiO 2 / The FTO substrate is placed in a UV ozone environment for 5 to 15 minutes to obtain a clean substrate;

[0011] (2) Preparation of MgZnO nanofilm

[0012] First, a MgZnO precursor solution is prepared: 0.5-3 g of zinc acetate, 0.1-1 g of magnesium acetate and 0.1-1 mL of ethanolamine are added to 30-50 mL of isopropanol in sequence and stirred continuously at 40-90° C. until a colorless transparent colloid is obtained, and the mixture is allowed to stand for 6-24 hours to obtain a MgZnO precursor solution;

[0013] After cleaning, SiO 2 Preparation of MgZnO nanofilm on SiO / FTO substrate surface: Spin coating was used to prepare MgZnO nanofilm on SiO 2 The MgZnO precursor solution is spin-coated on the FTO surface of the SiO / FTO substrate at a spin-coating speed of 2000-6000 rpm for 10-30 seconds, and then dried at 120-200°C for 5-15 minutes and cooled to room temperature; the above "spin coating-drying-cooling to room temperature" steps are repeated 5-10 times, and finally sintered at 300-600°C for 1-4 hours, thereby forming a SiO 2 / FTO substrates, the FTO surface is provided with a MgZnO nanofilm with a thickness of 100-200 nm;

[0014] (3) Preparation of InGaO precursor solution

[0015] 3-6 g of gallium nitrate nonahydrate, 0.5-2 g of indium nitrate tetrahydrate and 10-30 μL of polyoxyethylene lauryl ether are sequentially added to 10-30 mL of deionized water, and stirred in a water bath at 60-80° C. for 10-40 minutes. Finally, the solution is placed in a dark place and allowed to stand for 24-48 hours to obtain an InGaO precursor solution.

[0016] (4) Preparation of MgZnO / InGaO heterojunction photosensitive layer

[0017] The InGaO precursor solution is spin-coated on the surface of the MgZnO nanofilm prepared in step (2) at a spin-coating speed of 2000-3000 rpm for 10-30 seconds, and then the substrate is dried at 80-120°C for 5-15 minutes and then cooled to room temperature; the above "spin coating-drying-cooling to room temperature" steps are repeated 5-10 times, and finally sintered at 400-550°C for 1-4 hours, thereby forming a SiO 2 A MgZnO / InGaO heterojunction photosensitive layer with a thickness of 200 to 400 nm is obtained on the FTO surface of the FTO substrate;

[0018] (5) Preparation of Ag electrode

[0019] Using a mask with a hollow square pattern, conductive Ag slurry was drop-coated on the surface of the MgZnO / InGaO heterojunction photosensitive layer and the FTO surface, and wires were led out. The mixture was dried at 80-120°C for 5-20 minutes to obtain two square Ag electrodes with a thickness of 1-3 mm, thereby preparing an ultraviolet detector based on the MgZnO / InGaO heterojunction photosensitive layer.

[0020] The present invention constructs a heterojunction through a MgZnO / InGaO composite film. In the dark state, the presence of a built-in electric field depletes the majority carrier concentration, thereby effectively limiting the dark current of the device. In addition, the Ag electrode and the upper film material InGaO, as well as the FTO substrate and the lower film material MgZnO in the present invention, can form a suitable work function match to prepare an ohmic contact photoconductive device, and carriers can be transmitted unimpeded at the ohmic contact interface. When under illumination conditions, the composite film synergistically participates in photon absorption, and the photoconductive device has a high gain characteristic, so that the device has an outstanding photocurrent and reaches a peak light response at the solar blind region (260nm), providing a new idea for the preparation of a high-performance solar blind ultraviolet detector. At the same time, the preparation method is simple and easy, has few steps, low cost, and low equipment requirements. The prepared solar blind ultraviolet detector has a high response to ultraviolet light in the solar blind band, is suitable for mass production, and has important application value. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 : A schematic diagram of the structure of the ultraviolet photodetector based on the MgZnO / InGaO heterojunction photosensitive layer prepared by the present invention;

[0022] Figure 2 : UV-visible absorption graphs of the MgZnO nanofilm, InGaO nanofilm and MgZnO / InGaO heterojunction photosensitive layer prepared in Examples 1 to 3 of the present invention;

[0023] Figure 3 : Linear scale current-voltage characteristic curve of the ultraviolet photodetector based on the MgZnO / InGaO heterojunction photosensitive layer prepared in Example 3 of the present invention;

[0024] Figure 4 : Curves showing the variation of dark current with bias voltage of the ultraviolet photodetectors of the MgZnO nanofilm, InGaO nanofilm, and MgZnO / InGaO heterojunction photosensitive layer prepared in Examples 1 to 3 of the present invention;

[0025] Figure 5 : Curves of the photocurrent of the ultraviolet photodetector of the MgZnO nanofilm, InGaO nanofilm, and MgZnO / InGaO heterojunction photosensitive layer prepared in Examples 1 to 3 of the present invention as a function of bias voltage;

[0026] Figure 6 : Curves showing the spectral responsivity of the ultraviolet photodetectors of the MgZnO nanofilm, InGaO nanofilm, and MgZnO / InGaO heterojunction photosensitive layer prepared in Examples 1 to 3 of the present invention as a function of bias voltage.

[0027] like Figure 1 As shown, from bottom to top, SiO2 (2) / FTO (3) substrate, MgZnO nanofilm (4) grown on FTO, InGaO nanofilm (5) compositely grown on the surface of MgZnO nanofilm, Ag (6) electrodes prepared on the surface of InGaO nanofilm and FTO (3) substrate, the ultraviolet light (1) to be measured is emitted from SiO 2 / FTO substrate side incident, where SiO 2 / The thickness of the FTO substrate is 1 to 4 mm;

[0028] like Figure 2 As shown, it can be seen that the three nano-film materials all show good characteristics of high transmittance in the visible light region and high absorption in the ultraviolet light region. In the solar-blind band, the MgZnO / InGaO heterojunction photosensitive layer has higher absorbance than the two single-material nano-films of MgZnO and InGaO, indicating that the composite nano-film synergistically participates in photon absorption;

[0029] like Figure 3 As shown, the IV characteristic curve of the linear scale light-dark current of the ultraviolet photodetector based on the MgZnO / InGaO heterojunction photosensitive layer is approximately a straight line, and there is no rectification characteristic like the Schottky diode device, which proves that it is an ohmic contact photoconductive device. The flow of carriers at the ohmic contact interface will not be hindered by the potential barrier, ensuring that the device has a good photoelectric gain effect.

[0030] like Figure 4 As shown in the figure, under a bias voltage of 5V, the dark current of the MgZnO device is 8.26nA, the dark current of the InGaO device is 11.6nA, and the dark current of the MgZnO / InGaO heterojunction device is 0.27nA. This shows that the built-in electric field of the heterojunction device has a depletion effect on the majority carrier concentration in the dark state, effectively reducing the dark current of the device;

[0031] like Figure 5 As shown in the figure, the photocurrent of the MgZnO device under 5V bias is 2.02μA, the photocurrent of the InGaO device is 1.25μA, and the photocurrent of the MgZnO / InGaO heterojunction device is 2.62mA. This shows that the composite thin film photosensitive layer of the heterojunction device synergistically participates in photon absorption, further increasing the number of photogenerated carriers, thereby greatly improving the device conductivity, making the device show a higher photocurrent;

[0032] like Figure 6 As shown, at a wavelength of 250nm, the light intensity is 5.3μW / cm 2 Under ultraviolet light irradiation, the responsiveness of the MgZnO device at 5V bias is 6.92A / W. At a wavelength of 270nm and a light intensity of 9.7μW / cm 2Under ultraviolet light irradiation, the responsivity of the InGaO device at 5V bias is 3.2A / W. At a wavelength of 260nm, the light intensity is 6.8μW / cm 2 Under ultraviolet light irradiation, the MgZnO / InGaO heterojunction device exhibits excellent responsivity at 5V bias, with a value of 1.61×10 3 A / W, which is extremely important to its outstanding photocurrent. DETAILED DESCRIPTION

[0033] Example 1

[0034] (1) Substrate processing

[0035] The SiO 2 The SiO / FTO substrate was immersed in acetone, ethanol, and deionized water for ultrasonic treatment for 10 minutes, dried with nitrogen, and then 2 / The FTO substrate is placed in a UV-ozone environment for 10 minutes to obtain a clean substrate;

[0036] (2) Using the sol-gel method on SiO 2 Preparation of MgZnO nanofilm on FTO substrate

[0037] First, a MgZnO precursor solution was prepared: 1.6 g zinc acetate, 0.4 g magnesium acetate and 0.5 mL ethanolamine were added to 30 mL isopropanol in sequence and stirred continuously at 75°C until a colorless transparent colloid was obtained, and then aged for 24 hours to obtain a MgZnO precursor solution;

[0038] After cleaning, SiO 2 Preparation of MgZnO nanofilm on SiO / FTO substrate surface: Spin coating was used to prepare MgZnO nanofilm on SiO 2 The MgZnO precursor solution was spin-coated on the FTO surface of the SiO / FTO substrate at a spin-coating speed of 4000 rpm for 20 seconds; then dried at 160°C for 10 minutes, and the substrate was cooled to room temperature; the above "spin coating-drying-cooling to room temperature" steps were repeated 8 times, and finally sintered at 450°C for 2.5 hours, thereby forming a SiO / FTO substrate. 2 / FTO substrate FTO surface to obtain MgZnO nanofilm, thickness of 150nm;

[0039] (3) Preparation of Ag electrode

[0040] Using a mask with a hollow square pattern, a conductive Ag paste with a thickness of 2 mm was drop-coated on the surface of the MgZnO nanofilm and FTO, and wires were led out. The mixture was dried at 100°C for 15 minutes, and two square Ag electrodes were obtained on the surface of the MgZnO nanofilm, thereby preparing a MgZnO nanofilm ultraviolet detector.

[0041] After the UV detection device based on MgZnO nanofilm is prepared, the light and dark current and voltage characteristics of the device are tested. Figure 4 As shown in Figure 1, the dark current of the device is 8.26nA under 5V bias. Figure 5 As shown in the figure, the photocurrent of the device under 5V bias is 2.02μA, and the light-dark suppression ratio of the device is 244.55. Figure 6 As shown, at a wavelength of 250nm, the light intensity is 5.3μW / cm 2 Under ultraviolet light irradiation, the device's responsivity is 6.92A / W at 5V bias.

[0042] Example 2

[0043] (1) Substrate processing

[0044] The SiO 2 The SiO / FTO substrate was immersed in acetone, ethanol, and deionized water for ultrasonic treatment for 10 minutes, dried with nitrogen, and then 2 / The FTO substrate is placed in a UV-ozone environment for 10 minutes to obtain a clean substrate;

[0045] (2) Using the sol-gel method on SiO 2 Preparation of InGaO nanofilm on FTO substrate:

[0046] First, prepare an InGaO precursor solution: add 5 g of gallium nitrate nonahydrate, 1.2 g of indium nitrate tetrahydrate and 15 μL of polyoxyethylene lauryl ether to 30 mL of deionized water, stir in a 70°C water bath for 25 minutes, and finally place the solution in a dark place for 24 hours to obtain an InGaO precursor solution;

[0047] After cleaning, SiO 2 Preparation of InGaO nanofilm on SiO / FTO substrate surface: Spin coating method was used to prepare InGaO nanofilm on SiO 2 The InGaO precursor solution was spin-coated on the FTO surface of the SiO / FTO substrate at a spin-coating speed of 2500 rpm for 20 seconds; then dried at 100°C for 10 minutes, and the substrate was cooled to room temperature; the above "spin coating-drying-cooling to room temperature" steps were repeated 8 times, and finally sintered at 450°C for 2.5 hours, thereby forming a SiO / FTO substrate. 2 / InGaO nanofilm with a thickness of 150nm was obtained on the surface of FTO substrate;

[0048] (3) Preparation of Ag electrode: The specific implementation is the same as Example 1.

[0049] After the preparation of the InGaO nanofilm-based ultraviolet detection device was completed, the light and dark current-voltage characteristics of the device were tested.

[0050] Example 3

[0051] (1) Substrate processing

[0052] The SiO 2 The SiO / FTO substrate was immersed in acetone, ethanol, and deionized water for ultrasonic treatment for 10 minutes, dried with nitrogen, and then 2 / The FTO substrate is placed in a UV-ozone environment for 10 minutes to obtain a clean substrate;

[0053] (2) Using the sol-gel method on SiO 2 Preparation of MgZnO nanofilm on FTO substrate: The specific implementation is the same as Example 1.

[0054] (3) Preparation of InGaO precursor solution: The specific implementation is the same as Example 2.

[0055] (4) Preparation of MgZnO / InGaO heterojunction photosensitive layer:

[0056] The surface of the MgZnO nanofilm prepared in step (2) was spin-coated with an InGaO precursor solution at a spin-coating speed of 2500 rpm for 20 seconds; the substrate was then dried at 100°C for 10 minutes and then cooled to room temperature; the above "spin coating-drying-cooling to room temperature" steps were repeated 8 times, and finally sintered at 450°C for 2.5 hours, thereby forming a SiO 2 A MgZnO / InGaO heterojunction photosensitive layer with a thickness of 300 nm was obtained on the surface of the FTO substrate;

[0057] (5) Preparation of Ag electrode: The specific implementation is the same as Example 1.

[0058] After the UV detection device based on the MgZnO / InGaO heterojunction photosensitive layer is prepared, the light and dark current voltage characteristics of the device are tested. Figure 4 As shown in Figure 1, the dark current of the device is 0.27nA under 5V bias. Figure 5 As shown in the figure, the photocurrent of the device under 5V bias is 2.62mA, and the device achieves an extremely high light-dark suppression ratio of 9.7×10 6 It can be seen that the dark current of the device is lower than that of MgZnO UV detector and InGaO UV detector, and the photocurrent is greatly improved. Figure 6 As shown, at a wavelength of 260nm in the solar blind zone, the light intensity is 6.8μW / cm 2 Under ultraviolet light irradiation, the device responsivity at 5V bias is 1.61×10 3A / W, the UV / visible responsivity suppression ratio of the device is 5.37×10 4 , showing excellent spectral selectivity.

[0059] The ultraviolet photodetector based on the MgZnO / InGaO heterojunction photosensitive layer prepared by the present invention combines the high gain characteristics of the ohmic contact photoconductive device and the limitation of the majority carrier concentration in the dark state by the built-in electric field of the heterojunction photosensitive layer device, so that the device has a relatively ideal light-dark current characteristic. At the same time, the device also exhibits excellent spectral responsivity in the solar-blind band. Therefore, the present invention produces a high-performance photoconductive solar-blind ultraviolet photodetector based on the MgZnO / InGaO heterojunction photosensitive layer.

[0060] At the same time, the process of the invention is simple, the ultraviolet photoelectric detector prepared is small in size, suitable for mass production, and thus has important application value. The preparation method of the invention has the characteristics of simple preparation method, low cost, and promising large-scale production, and has good detection performance for ultraviolet light.

[0061] The above-described contents are only specific implementation modes of the present invention and cannot be used to limit the scope of implementation of the present invention. All equivalent changes and improvements made within the scope of application of the present invention should still fall within the scope covered by the present invention.

Claims

1. A photoconductive solar-blind ultraviolet detector based on a MgZnO / InGaO heterojunction photosensitive layer, characterized in that: From bottom to top, it consists of a SiO2 / FTO substrate, a MgZnO nanofilm grown on the FTO surface, an InGaO nanofilm grown on the MgZnO nanofilm surface, and Ag electrodes prepared on the InGaO nanofilm surface and the FTO surface. The MgZnO nanofilm and the InGaO nanofilm constitute a MgZnO / InGaO heterojunction photosensitive layer with a thickness of 200 to 400 nm. The ultraviolet light to be measured is incident from one side of the SiO2 / FTO substrate.

2. A photoconductive solar-blind ultraviolet detector based on a MgZnO / InGaO heterojunction photosensitive layer as claimed in claim 1, characterized in that: The thickness of the SiO2 / FTO substrate is 1-4 mm, the thickness of the MgZnO / InGaO heterojunction photosensitive layer is 200-400 nm, and the thickness of the Ag electrode is 1-3 mm.

3. A method for preparing a photoconductive solar-blind ultraviolet detector based on a MgZnO / InGaO heterojunction photosensitive layer, the steps of which are as follows: (1) Substrate processing The SiO2 / FTO substrate with a thickness of 1 to 4 mm is immersed in acetone, ethanol, and deionized water for ultrasonic treatment for 5 to 15 minutes, dried with nitrogen, and then placed in an ultraviolet ozone environment for treatment for 5 to 15 minutes to obtain a clean substrate; (2) Preparation of MgZnO nanofilm First, a MgZnO precursor solution is prepared: 0.5-3 g of zinc acetate, 0.1-1 g of magnesium acetate and 0.1-1 mL of ethanolamine are added to 30-50 mL of isopropanol in sequence and stirred continuously at 40-90° C. until a colorless transparent colloid is obtained, and the mixture is allowed to stand for 6-24 hours to obtain a MgZnO precursor solution; A MgZnO nanofilm is prepared on the surface of the cleaned SiO2 / FTO substrate: a MgZnO precursor solution is spin-coated on the FTO surface of the SiO2 / FTO substrate by a spin coating method, and the substrate is cooled to room temperature after drying; the above "spin coating-drying-cooling to room temperature" steps are repeated 5 to 10 times, and finally sintered at 300 to 600°C for 1 to 4 hours, thereby obtaining a MgZnO nanofilm on the FTO surface of the SiO2 / FTO substrate with a thickness of 100 to 200 nm; (3) Preparation of InGaO precursor solution 3-6 g of gallium nitrate nonahydrate, 0.5-2 g of indium nitrate tetrahydrate and 10-30 μL of polyoxyethylene lauryl ether are sequentially added to 10-30 mL of deionized water, and stirred in a water bath at 60-80° C. for 10-40 minutes. Finally, the solution is placed in a dark place and allowed to stand for 24-48 hours to obtain an InGaO precursor solution. (4) Preparation of MgZnO / InGaO heterojunction photosensitive layer Spin-coat the InGaO precursor solution on the surface of the MgZnO nanofilm prepared in step (2), dry the substrate and cool it to room temperature; repeat the above "spin-coat-dry-cool to room temperature" steps 5 to 10 times, and finally sinter at 400 to 550° C. for 1 to 4 hours, thereby obtaining a MgZnO / InGaO heterojunction photosensitive layer on the FTO surface of the SiO2 / FTO substrate with a thickness of 200 to 400 nm; (5) Preparation of Ag electrode Using a mask with a hollow square pattern, conductive Ag slurry was drop-coated on the surface of the MgZnO / InGaO heterojunction photosensitive layer and the FTO surface, and wires were led out. The mixture was dried at 80-120°C for 5-20 minutes to obtain two square Ag electrodes with a thickness of 1-3 mm, thereby preparing an ultraviolet detector based on the MgZnO / InGaO heterojunction photosensitive layer.

4. The method for preparing a photoconductive solar-blind ultraviolet detector based on a MgZnO / InGaO heterojunction photosensitive layer according to claim 3, characterized in that: The spin coating speed in step (2) is 2000-6000 rpm, the spin coating time is 10-30 seconds, and then the substrate is dried at 120-200° C. for 5-15 minutes and then cooled to room temperature.

5. The method for preparing a photoconductive solar-blind ultraviolet detector based on a MgZnO / InGaO heterojunction photosensitive layer according to claim 3, characterized in that: The spin coating speed in step (4) is 2000-3000 rpm, and the spin coating time is 10-30 seconds; then the substrate is dried at 80-120° C. for 5-15 minutes and then cooled to room temperature.

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