SiC thin film surface inverted cone nano-pore anti-reflection structure, preparation method and application

Abstract

This invention relates to an antireflection structure, fabrication method, and application of inverted conical nanopores on the surface of SiC thin films, belonging to the field of surface engineering technology for photoelectric detectors. Addressing the problems of short optical path length and high interface reflection loss in existing SiC thin-film ultraviolet photodetectors based on heterogeneous substrates due to limited film thickness, a periodic array of inverted conical nanopores is constructed on the SiC thin film surface. The etching depth is controlled within the film thickness and does not penetrate the entire film, preserving a continuous electrical transmission channel. This structure creates a continuously gradient equivalent refractive index layer on the surface, reducing the surface reflectivity in the 200–400 nm wavelength range. The inverted conical morphology causes large-angle scattering of incident light, increasing the optical path length of ultraviolet light within the confined film and improving absorption. The array is fabricated using patterned masking technology combined with dry etching, suitable for SiC thin-film MSM or PIN detectors, with an average reflectivity of 7.58%, significantly improving the photoelectric conversion efficiency of the device.

Description

Technical Field

[0001] This invention belongs to the field of surface engineering technology for photoelectric detectors, specifically relating to an antireflection structure of inverted conical nanopores on the surface of a SiC thin film, its preparation method, and its application. Background Technology

[0002] Silicon carbide (SiC), as a wide-bandgap semiconductor material, is an ideal choice for fabricating high-performance ultraviolet photodetectors. In recent years, to further reduce dark current, decrease parasitic capacitance, and improve optoelectronic integration, building SiC thin-film detectors (such as thin-film MSM or PIN detectors) on heterogeneous substrates has gradually become an important development direction in this field. These thin-film devices typically integrate ultrathin SiC active layers onto heterogeneous substrates such as silicon, sapphire, or flexible polymers through thin-film transfer techniques (such as smart-cut lift-off) or heteroepitaxial growth processes.

[0003] However, due to the extremely limited film thickness, such SiC thin-film detectors based on heterostructure substrates face severe optical losses in practical applications, resulting in low responsivity. Specifically, this problem manifests as a dual limitation: on the one hand, the refractive index mismatch between the top SiC and air interface leads to extremely high surface Fresnel reflection loss; on the other hand, due to the limited thickness of the ultrathin active layer, the ultraviolet light absorption path into the film is too short, and unabsorbed photons can easily penetrate the film directly and reach the heterostructure interface at the bottom (such as the SiC / insulator interface or the SiC / substrate interface), resulting in low absorption.

[0004] To reduce reflection loss and improve device responsivity, existing technologies typically employ strategies such as transparent electrodes, specially designed electrode shapes, or the deposition of multilayer dielectric antireflection films. However, while achieving ideal antireflection effects over a wide wavelength range and wide incident angle, these methods often face challenges such as increased process complexity and limited compatibility with existing semiconductor device processes. Furthermore, they cannot excite sufficiently long light absorption paths within extremely thin SiC layers.

[0005] As semiconductor fabrication micro-nano patterning technology matures, costs have decreased to a manageable level. Therefore, there is an urgent need in this field for an anti-reflection scheme that is simple in structure, easy to fabricate on a large scale, and can solve the dual optical losses of SiC thin-film ultraviolet photodetectors without compromising the electrical properties of the active layer. Summary of the Invention

[0006] One of the objectives of this invention is to address the shortcomings of existing SiC thin-film ultraviolet photodetectors, such as short optical absorption path length, high reflection loss at the interface between SiC and air and the bottom heterogeneous material, and the inability of existing anti-reflection schemes to balance optical performance and the electrical integrity of the active layer. This invention provides an anti-reflection structure with inverted conical nanopores on the surface of a SiC thin film, which achieves effective capture and optical path enhancement of incident ultraviolet light without compromising the electrical integrity of the ultrathin active layer, thereby improving the light absorption rate and responsivity of the device.

[0007] The second objective of this invention is to address the shortcomings of existing anti-reflection structure fabrication processes, such as complexity, poor compatibility with existing semiconductor processes, low yield, and high cost, which fail to meet the needs of industrial mass production. This invention provides a fabrication method adapted to the aforementioned anti-reflection structure. This method is simple, feasible, and compatible with existing semiconductor fabrication processes, ensuring accurate fabrication of the anti-reflection structure while also considering the fabrication cost, thus meeting the needs of industrial mass production.

[0008] The third objective of this invention is to address the shortcomings of existing anti-reflection structures, such as unclear application methods, ineffective implementation, and easy damage to the electrical performance of devices after application, making it difficult to fully realize the advantages of anti-reflection. This invention provides the application of the aforementioned anti-reflection structure in SiC thin-film ultraviolet photodetectors, achieving a synergistic improvement in the optical and electrical performance of the detector and meeting actual detection requirements.

[0009] To achieve the above objectives, the technical solution provided by the present invention is as follows:

[0010] An antireflection structure with inverted conical nanopores on the surface of a SiC thin film includes a SiC thin film and a periodic array of inverted conical nanopores on the surface of the SiC thin film. The array of inverted conical nanopores is arranged perpendicular to the surface of the SiC thin film and penetrates only the surface layer of the SiC thin film. The etching depth is less than the total thickness of the SiC thin film and does not penetrate the entire thickness of the SiC thin film, thus preserving the continuous electrical transport channels inside the SiC thin film. This is used to avoid damaging the electrical transport channels of the SiC active layer and ensure the stability of the device's electrical performance. The array of inverted conical nanopores has a geometric morphology that is wider at the top and narrower at the bottom, with the duty cycle gradually increasing along the depth direction. Its structural parameters include spatial period, top diameter, and etching depth. The configuration of these parameters allows the antireflection structure to form an equivalent refractive index layer that is continuously and gradually changes along the normal direction on the surface of the SiC thin film, thereby suppressing interface reflection in the target ultraviolet band (200-400 nm).

[0011] Furthermore, the inverted conical nanopore array has a geometric morphology that is wider at the top and narrower at the bottom, and its specific structural parameters range from 100 to 500 nm, with a spatial period of 1 to 100 nm, a top diameter of 1 to 100 nm, and an etching depth of 100 to 500 nm.

[0012] Furthermore, with a spatial period of 100–200 nm, a top diameter of 70–90 nm, and an etching depth of 100–200 nm, this preferred parameter range can achieve optimal anti-reflection and light-trapping effects in the target ultraviolet band, while taking into account both structural stability and optical performance.

[0013] Furthermore, the inverted conical nanopore array is an array structure with a wide upper and narrow lower geometry formed by anisotropic dry etching of the SiC thin film under mask protection.

[0014] Furthermore, the sidewall tilt angle of the inverted conical nanopore array is 70° to 85°. This tilt angle is controlled by the dry etching process parameters and matches the crystal orientation of the SiC thin film, ensuring the structural stability and optical performance of the inverted conical nanopores, while providing feasible parameter basis for subsequent etching processes.

[0015] Furthermore, the SiC thin film is a single-crystal SiC thin film with a thickness of 0.5–2 μm. The etching depth of the inverted conical nanopore array is less than 1 / 5 of the thickness of the SiC thin film, ensuring that the active layer structure and electrical transmission channels of the SiC thin film are not damaged, thereby achieving synergistic optimization of optical and electrical performance.

[0016] A method for preparing an inverted conical nanopore antireflection structure on the surface of a SiC thin film includes the following steps:

[0017] S1: Perform surface cleaning and pretreatment on the substrate containing SiC thin film to remove oil, oxide layer and impurities from the substrate surface, ensuring that the SiC thin film surface is clean and flat, and avoiding impurities from affecting the structural morphology and optical performance;

[0018] S2: Forming a patterned mask on the surface of a SiC thin film;

[0019] S3: Using a mask as a barrier layer, anisotropic etching of SiC thin films is performed using dry etching technology to form an inverted conical nanopore array that is wider at the top and narrower at the bottom;

[0020] S4: Remove the mask, clean and dry the sample to obtain the anti-reflection structure of the inverted conical nanopore array on the SiC surface.

[0021] Furthermore, in step S2, the pattern of the mask is consistent with the pattern of the target inverted conical nanopore array. The mask material is an AAO template, polymer photoresist, or metal thin film, ensuring that the mask has good etching blocking performance and providing multiple mask fabrication routes to adapt to different fabrication scenarios and precision requirements.

[0022] Furthermore, in step S2, the fabrication method of the patterned mask can be any of the following, to suit different precision and mass production requirements:

[0023] Process Route A (AAO Aluminum Oxide Template Method): A PMMA-supported double-through ultra-thin AAO template is selected. The template is attached to the SiC substrate surface with the porous side facing down. The PMMA support layer is dissolved and removed to achieve a tight transfer between the AAO template and the SiC surface, which is suitable for large-area mass production.

[0024] Process route B (nanoimprint method): Spin-coating an imprint resist onto the SiC surface, using a corresponding template for hot imprinting or UV imprinting, curing and demolding, followed by oxygen plasma to remove residual adhesive, forming a regular polymer mask hole array, balancing precision and mass production efficiency.

[0025] Process route C (electron beam exposure method): Electron beam photoresist is spin-coated on the SiC surface, followed by pre-baking, electron beam direct writing patterning exposure, post-baking and development to obtain a high-precision mask pattern, which is suitable for high-precision structure fabrication.

[0026] Furthermore, in step S3, by adjusting the etching process parameters, the etching time and etching rate of the dry etching technology are matched, so that the etching rate of the SiC film during the etching process exhibits anisotropic etching characteristics of fast at the top and slow at the bottom, thereby accurately forming an inverted conical nanopore array that is wider at the top and narrower at the bottom. By matching the etching time and etching rate, the etching depth of the final inverted conical nanopore array is less than the total thickness of the SiC film, ensuring that the active layer structure and electrical transport channels of the SiC film are not damaged, ensuring that the electrical performance of the device is not affected, and strengthening the core constraints of the fabrication process.

[0027] Further, in step S3, the dry etching technique is ICP etching (inductively coupled plasma etching), and the etching gas is SF6; the ICP power is 200-400W, the RF power is 20-50W, and the etching gas pressure is 10-50mTorr; by matching the etching time and etching rate of the dry etching technique, the etching depth of the final inverted conical nanopore array is less than the total thickness of the SiC film. Specifically, the etching time is 30-60s, and the etching rate is 2-10nm / s. By clearly defining the etching parameters, the inverted conical morphology can be precisely controlled. Under the above parameters, an inverted conical morphology with a sidewall tilt angle of 70°-85° can be stably formed, and the etching depth is guaranteed to be less than the total thickness of the SiC film, thereby achieving precise control of the inverted conical nanopore structure and protecting the SiC active layer.

[0028] Further, in step S4, the mask removal method is selected according to the different mask materials: if it is an AAO template, the sample is immersed in a 5 wt.% phosphoric acid solution at room temperature for 10-20 min to dissolve and remove the AAO template; if it is a polymer mask, it is dissolved and removed by immersion in acetone solution for 10-20 min, or by oxygen plasma ashing (plasma power of 100-200W, processing time of 30-60s); if it is a metal thin film mask, it is dissolved and removed by the corresponding metal etching solution; after removing the mask, the sample is washed with deionized water 3-5 times, 5-10 min each time, and then dried with nitrogen to obtain the antireflection structure of the inverted conical nanopore array on the SiC surface, which is adapted to different mask materials to ensure that the mask is removed thoroughly without damaging the structure.

[0029] The application of an inverted conical nanopore antireflection structure on the surface of a SiC thin film in a SiC thin film ultraviolet photodetector involves placing the aforementioned inverted conical nanopore array antireflection structure on the surface of the light-receiving area in the light incident path of the detector without covering the electrode area of ​​the detector. This ensures both antireflection and light trapping effects without affecting the electrical transmission of the detector, achieving a synergistic improvement in optical and electrical performance.

[0030] The SiC thin-film ultraviolet photodetector comprises, from bottom to top, at least a heterogeneous substrate, an insulating layer (optional), and a SiC thin film located on the heterogeneous substrate or insulating layer. The thickness of the SiC thin film is greater than the etching depth of the inverted conical nanopore array, ensuring that the inverted conical nanopores do not penetrate the SiC thin film, thus avoiding damage to the electrical performance of the device. This maintains consistency with the design constraints of the anti-reflection structure and is compatible with different types of SiC thin-film ultraviolet photodetectors, such as MSM structure detectors and lateral PIN structure detectors, providing broad application compatibility.

[0031] The heterogeneous substrate may be a silicon substrate, a sapphire substrate, or a flexible polymer substrate; the insulating layer may be a SiO2 insulating layer with a thickness of 100–500 nm.

[0032] Compared with the prior art, the present invention has the following outstanding technical effects and advantages:

[0033] 1. This invention employs an inverted conical nanopore array structure. On one hand, the inverted conical morphology allows the duty cycle of the SiC material on the horizontal cross-section to gradually increase from the top to the bottom of the interface. Based on the equivalent medium theory, a continuous gradient layer with an equivalent refractive index that smoothly transitions from air (1.0) to bulk SiC (2.6-2.8) is constructed on the SiC surface, greatly suppressing surface Fresnel reflection loss. On the other hand, the tilt angle of the inverted conical sidewalls allows vertically incident ultraviolet light to undergo large-angle scattering, significantly increasing the lateral optical path of photons within the confined SiC film. This promotes multiple internal reflections of photons between the SiC film and the heterostructure interface below, effectively suppressing transmission escape from the bottom interface and fundamentally solving the problem of insufficient light absorption in ultrathin SiC films. Experimental results show that the antireflection structure of the present invention can reduce the average reflectance of SiC thin films in the 200-400nm ultraviolet band from 15.84% to 7.58%, and the reflectance at the 280nm representative wavelength from 14.79% to 6.89%, while increasing the absorption rate to 78.97%, demonstrating significant antireflection and light trapping effects.

[0034] 2. This invention ensures that the etching depth of the inverted conical nanopores only penetrates the surface of the SiC thin film and does not penetrate the entire thickness of the SiC thin film, thus avoiding damage to the electrical transport channels of the SiC active layer. At the same time, the fabrication process adopts mature micro-nano fabrication technologies such as AAO template method and nanoimprinting method, which have good compatibility with existing semiconductor device fabrication processes. No additional complex equipment is required, and it can be directly integrated into the existing SiC thin film detector fabrication process without affecting the carrier transport efficiency and fabrication yield of the device.

[0035] 3. The antireflection structure of the present invention is a periodic inverted conical nanopore array, which is simple in structure and does not require complex multilayer structure design. The preparation process can use low-cost processes such as AAO template method, which does not require high-precision expensive equipment, making it easy to achieve large-area preparation, with controllable preparation cost, and suitable for industrial mass production.

[0036] 4. The inverted conical nanopore array of the present invention achieves good anti-reflection effect in the entire ultraviolet band of 200-400nm through the design of an equivalent refractive index gradient layer, and has strong adaptability to incident angle. It can still maintain excellent anti-reflection performance in a wide incident angle range (0°-45°), thus expanding the application scenarios of the device. Attached Figure Description

[0037] Figure 1 This is a schematic diagram of the morphology of the AAO template after it has been transferred to the SiC substrate surface according to the present invention;

[0038] Figure 2 The image shows the morphology of the antireflection structure of the inverted conical nanopore array on the SiC surface prepared in this invention.

[0039] Figure 3This is a cross-sectional schematic diagram of the fabrication process of the SiC surface inverted conical nanopore array antireflection structure of the present invention;

[0040] Figure 4 The diagram shows the integration of the antireflection structure of the present invention in different types of SiC thin-film ultraviolet photodetectors, where (a) shows its application in a thin-film MSM structure detector and (b) shows its application in a thin-film lateral PIN structure detector.

[0041] Figure 5 This is a schematic diagram of the three-dimensional FDTD optical simulation model of the SiC surface inverted conical nanopore antireflection structure of the present invention;

[0042] Figure 6 The simulated absorption and reflectance spectra of the optimal parameters of the inverted conical nanopore array antireflection structure of the present invention and planar SiC are compared.

[0043] Figure 7 The images show a comparison of the cross-sectional steady-state optical field distribution of planar SiC and the antireflection structure of the inverted conical nanopore array of the present invention at a wavelength of 280 nm; where (a) is the cross-sectional optical field distribution of the planar SiC film without surface structure, and (b) is the cross-sectional optical field distribution of the SiC film after introducing the inverted conical nanopore array.

[0044] Figure 8 This is a comparison of the measured reflectance spectra of the inverted conical nanopore array antireflection structure of the present invention and a planar SiC sample. Detailed Implementation

[0045] To make the technical problems, technical solutions, and beneficial effects of this invention clearer and more understandable, the specific embodiments of this invention are described below, and the invention is further explained in detail. It should be noted that the illustrations provided in this embodiment are only schematic representations of the basic concept of this invention. Therefore, the drawings only show components related to this invention and are not drawn according to the actual number, shape, and size of the components in implementation. In actual implementation, the type, quantity, and proportion of each component can be arbitrarily changed, and the component layout may also be more complex. All inventions utilizing the concept of this invention are protected.

[0046] Example 1: Preparation of antireflection structure with inverted conical nanopores on SiC surface

[0047] This embodiment provides several feasible specific fabrication process routes for the antireflection structure of the inverted conical nanopore array on the SiC surface.

[0048] Step 1: Prepare a heterostructure containing a SiC thin film

[0049] The photodetector in this invention is a thin-film device; the SiC thin-film substrate is prepared by methods including but not limited to: smart-cut technology, heteroepitaxial growth, or wafer bonding and grinding thinning method; as a preferred embodiment, a silicon carbide-on-insulator (SiCOI) wafer prepared by smart-cut technology is used as the substrate (which, from bottom to top, includes a Si support substrate, a SiO2 insulating layer, and a top single-crystal SiC thin film with a thickness of submicron to micron, such as 1 μm).

[0050] The substrate was ultrasonically cleaned for 10 min each with acetone, anhydrous ethanol, and deionized water in sequence. Then, it was cleaned for 10 min each with RCA-III solution (volume ratio H2SO4:H2O2=4:1), RCA-I solution (volume ratio H2O2:NH3·H2O:H2O=1:1:4), and RCA-II solution (volume ratio H2O2:HCl:H2O=1:1:4) heated to 85℃. After each cleaning solution, the substrate was immersed in HF solution (volume ratio HF:H2O=1:20) for 4 min, and then dried with nitrogen gas for later use.

[0051] Step 2: Fabricating a patterned mask on the surface of the SiC thin film

[0052] To obtain a periodic nanopore array on the SiC surface, this embodiment can employ any of the following masking techniques:

[0053] Process Route A (AAO Aluminum Oxide Template Method): A PMMA-supported double-channel ultrathin AAO template with a period of 100nm, a pore size of 80nm, and a thickness of 200nm is selected; the template is attached to the SiC substrate surface with the porous side facing down, and then sequentially immersed in three fresh acetone solutions for 10 minutes each to completely dissolve and remove the PMMA support layer, achieving a tight transfer of the AAO template to the SiC surface; from Figure 1 As can be seen from the morphological diagram, the transferred AAO template exhibits a regular periodic porous array distribution on the SiC surface. The pore size is uniform and the template is tightly attached to the substrate, which provides a patterned mask basis for subsequent etching of highly uniform anti-reflection structures.

[0054] Process Route B (Nanoimprinting): Spin-coating a resist onto the SiC surface at a spin speed of 5000 r / min for 30–50 s; then performing hot imprinting (imprinting temperature 100–150℃, imprinting pressure 5–10 MPa, imprinting time 30–60 s) or ultraviolet imprinting (ultraviolet irradiation intensity 100–200 mW / cm²) using a soft or hard stencil with corresponding periodic and raised arrays. 2The curing process involves irradiation for 30–60 seconds, followed by demolding. Then, a slight oxygen plasma descumbing process (50–100W plasma power, 10–30 seconds) is used to form a regular array of polymer mask pores on the SiC surface.

[0055] Process route C (electron beam exposure): Electron beam photoresist is spin-coated onto the SiC surface at a spin speed of 3000–5000 r / min for 30–60 s, followed by pre-baking (temperature 80–100℃, time 30–60 min). Patterning exposure is then performed using electron beam direct writing technology according to the designed cycle and aperture, with an exposure dose of 100–500 μC / cm. 2 After exposure, the mask is baked (at a temperature of 100-120℃ for 10-30 minutes), and then developed with a developer (development time of 1-5 minutes) to obtain a high-precision mask pattern.

[0056] Step 3: Dry etching to form inverted conical nanopores under mask protection

[0057] Under the protection of the patterned mask, the SiC thin film is etched using anisotropic dry etching technology.

[0058] Taking AAO mask combined with ICP etching in process route A as an example: the etching gas is SF6, the ICP power is 300W, the RF power is 30W, and the etching time is 42s. During the etching process, SF6 plasma etches SiC. Under the combined effect of mask hole blocking and anisotropic etching, the etching rate of SiC thin film shows a difference between the top and the bottom.

[0059] After etching, inverted conical nanopores are formed in SiC below the AAO mask aperture, ultimately obtaining an array of inverted conical nanopores with a period of approximately 100 nm, a top diameter of approximately 80 nm, and a depth of approximately 120 nm, with a sidewall tilt angle of approximately 75°. In particular, by precisely controlling the etching time and etching rate, it is ensured that the etching depth of the formed inverted conical nanopores (e.g., 120 nm) is strictly less than the total thickness of the SiC film (e.g., 1 μm), which is only about 1 / 8 of the thickness of the SiC film. This avoids piercing the film and destroying its overall electrical transport channels, ensuring that the electrical performance of the device is not affected.

[0060] Step 4: Remove the mask to obtain the surface antireflection structure.

[0061] If it is an AAO template, immerse the sample in a 5 wt.% phosphoric acid solution at room temperature for 15 min to dissolve and remove the AAO template; if it is a polymer mask, remove it using organic solvents or oxygen plasma ashing (immerse at room temperature for 15 min to dissolve and remove); if it is an electron beam photoresist mask, remove it by immersing in acetone solution for 15 min to dissolve and remove. After removing the mask, wash the sample four times with deionized water for 8 min each time. After washing and drying, the antireflection structure of the inverted conical nanopore array on the SiC surface is obtained; combined with Figure 2 The structural morphology images show that a dense and regularly arranged array of nanopores formed on the surface of the etched SiC film. The pores exhibit a distinct three-dimensional morphology with a wider upper portion and a narrower lower portion, resembling an inverted cone shape, thus verifying the effectiveness of anisotropic etching. Figure 3 The schematic diagram of the fabrication process shown illustrates the cross-sectional state changes of four key process steps: heterogeneous substrate preparation, mask transfer, dry etching, and mask removal. It can be seen intuitively that the inverted conical hole does not penetrate the SiC thin film under the obstruction of the mask.

[0062] Example 2: Application in SiC thin-film ultraviolet photodetectors

[0063] This embodiment illustrates the general integration scheme of the antireflection structure of the present invention in different types of SiC thin-film photodetectors, taking metal-semiconductor-metal (MSM) structure detectors and lateral PIN structure detectors as examples. The present invention is not limited to specific device electrode fabrication processes. Its core lies in deploying the antireflection structure at the light incident interface without covering the electrode area, ensuring that the electrical performance of the device is not affected.

[0064] 1. Application in thin-film MSM structures

[0065] For thin-film MSM devices, the structure, from bottom to top, consists of a Si substrate, a SiO2 insulating layer, a SiC thin film, and interdigitated electrodes. During fabrication, photolithography of the interdigitated electrodes is first performed on the SiC thin film surface on the heterogeneous substrate. The interdigitated electrodes use a Ni / Au metal layer with a thickness of 100–200 nm. A Ni protective layer (50–100 nm thick) needs to be sputtered first to prevent damage to the electrodes in subsequent processes. Then, the process described in Example 1 is executed to fabricate an inverted conical nanopore antireflection structure with a depth less than the film thickness in the light-receiving area of ​​the exposed SiC thin film between the interdigitated electrodes (in this example, the depth is 120 nm, the period is 100 nm, and the top diameter is 80 nm). Finally, the mask is removed using acetone solution, and the Ni protective layer is removed using dilute nitric acid solution to complete the device fabrication. This structure directly reduces the interface reflection loss between the interdigitated electrodes and enhances the light absorption in the light-receiving area, improving the device responsivity. An application diagram is shown below. Figure 4 As shown in (a) in the figure.

[0066] 2. Application in thin-film lateral PIN structures

[0067] For a thin-film lateral PIN detector, its structure, from bottom to top, consists of a Si substrate, a SiO2 insulating layer, a SiC thin film (laterally distributed P-region, I-region, and N-region), and a metal electrode. During fabrication, after ion implantation doping (Al doping in the P-region and N doping in the N-region) on the ultrathin SiC thin film plane, followed by annealing activation, the etching process of this invention is directly applied to the intrinsic I-region surface at the top of the device, which is not obscured by the metal electrode, to fabricate an inverted conical nanopore antireflection structure with a depth of 120 nm, a period of 100 nm, and a top diameter of 80 nm. Because the etching depth is controlled to be less than the total film thickness, the inverted conical nanopore does not cut off the transport channel for lateral charge carriers. Simultaneously, this inverted conical structure enables large-angle scattering of incident ultraviolet light, causing multiple internal reflections between the finite-thickness SiC thin film and the interface below, maximizing the confinement and coupling of ultraviolet light within the extremely thin depletion region, thereby significantly improving the photoelectric conversion efficiency of the device without increasing the physical thickness of the thin film. An application schematic is shown below. Figure 4 As shown in (b) of the diagram.

[0068] Example 3: Anti-reflection mechanism, FDTD optical simulation and performance verification

[0069] This embodiment aims to clarify the anti-reflection mechanism of the present invention, and to explain how to optimize parameters through a combination of process and simulation, and finally verify its effect through actual measurement.

[0070] Traditional vertical cylindrical cavities can only provide a single and abrupt equivalent refractive index layer, failing to achieve a smooth transition of refractive index, resulting in limited anti-reflection effects and an inability to effectively enhance the light absorption path. In contrast, this invention employs an inverted conical morphology that is wider at the top and narrower at the bottom. The physical mechanism is as follows: First, the inverted conical aperture allows the duty cycle of the SiC material on the horizontal cross-section to gradually increase from the top to the bottom of the interface (smaller duty cycle at the top, larger duty cycle at the bottom). According to the equivalent medium theory, this constructs a continuous gradient layer on the SiC surface where the equivalent refractive index smoothly transitions from air to bulk SiC, effectively reducing the abrupt change in refractive index at the SiC-air interface and greatly suppressing Fresnel reflection on the surface. Second, the inverted conical sidewalls have a specific tilt angle (70°–85°). When vertically incident ultraviolet light hits the sidewalls, it is scattered into oblique light at a large angle, thereby significantly increasing the lateral optical path of photons within the confined SiC thin film, effectively suppressing transmission escape from the bottom interface and improving light absorption.

[0071] The sidewall tilt angle of the inverted conical nanopore is determined by the actual dry etching process and is mainly controlled by parameters such as etching power and etching gas pressure. Therefore, the parameter optimization logic of this invention is as follows: First, through actual etching process experiments, the range of inverted conical tilt angles that can be stably processed is calibrated; then, this tilt angle is used as an inherent constraint condition and input into the FDTD simulation model to further find the optimal period and depth matching to ensure that the optimal anti-reflection and light trapping effects are achieved in the target ultraviolet band.

[0072] Taking a SiCOI thin-film device with a SiO2 insulating layer as an example, a three-dimensional FDTD simulation model including a Si substrate / SiO2 thin film / 1μm SiC thin film is established; from Figure 5 As shown in the simulation model diagram, the model consists of a Si substrate, a SiO2 insulating layer, and a 1μm thick SiC film stacked sequentially from bottom to top. An inverted conical nanopore array with a specific tilt angle and depth is constructed on top of the SiC film. A vertically incident plane wave light source is placed above the model, thus accurately reproducing the optical physics operation of the device under limited film thickness. A finite-thickness film model of 1μm is chosen instead of an infinite-thickness material model to realistically reflect the comprehensive optical modulation capabilities of the inverted conical nanopore array under limited film thickness, including suppressing SiC surface reflection, preventing light escape from the bottom interface, and enhancing the optical path within the film. The inverted conical nanopore array adopts an idealized tetragonal shape. The lattice is periodically arranged, exhibiting translational symmetry in the XY plane. To optimize computational efficiency while ensuring accuracy, a minimal repeating unit with periodicity is selected as the computational region in the simulation. Periodic boundary conditions are applied to the simulation region in both the X and Y directions, while a perfect matching layer (PML) is set in the Z-axis incident and exit directions to simulate an infinitely large array structure and a non-reflective boundary environment. Considering the actual morphological limitations of the AAO template etching process, the process tilt angle of the inverted conical nanopore is set between 70° and 85°. Under this physical constraint, the parameter scanning range is set as follows: period 100–500 nm, etching depth 100–500 nm.

[0073] like Figure 6 As shown in the simulated absorption and reflectance spectrum comparison, in the 200–400 nm wavelength range, the preferred inverted conical nanopore array antireflection structure significantly reduces reflectance compared to planar SiC, while its absorption rate is greatly improved due to the light-trapping effect. Specifically, at the center wavelength of 280 nm, the reflectance of this preferred structure drops to only 4.63%, while the absorption rate at the corresponding wavelength increases to 78.97%, which is significantly better than that of planar SiC (reflectance of approximately 14.79% and absorption rate of approximately 65%), demonstrating significant antireflection and absorption enhancement effects.

[0074] The significant decrease in reflectivity and the substantial increase in absorptivity can be further confirmed by the microscopic optical mechanism within the thin film through the steady-state light field distribution; such as Figure 7 As shown, this embodiment compares the cross-sectional optical field distribution of planar SiC and the preferred structure of the present invention at a wavelength of 280 nm; as Figure 7 As shown in (a), the planar SiC thin film without surface structure only exhibits interference fringes parallel to the surface, indicating that photons undergo simple longitudinal transmission and reflection without lateral broadening, and light easily penetrates the bottom of the film and escapes; in contrast, as Figure 7 As shown in (b), after introducing the inverted conical nanopore array, the optical field interference fringes inside the film exhibit significant lateral distortion and overlap, and a local optical field enhancement region is formed on the sidewall and below the inverted conical hole. The optical field simulation results intuitively show that the inverted conical hole can cause vertically incident ultraviolet light to undergo large-angle scattering, prompting photons to undergo multiple internal reflections within the SiC film of finite thickness, thereby increasing the transmission optical path within the film, effectively suppressing light escape from the bottom interface, and improving the light absorption efficiency.

[0075] The reflectance of the actual sample prepared in Example 1 was measured (using a Cary 5000 spectrometer, test band 200–400 nm, perpendicular incidence, test environment: room temperature, air atmosphere); Figure 8 As shown in the comparative graph of the measured reflectance spectra, within the full ultraviolet test band of 200–400 nm, the reflectance curve of the nanoporous structure sample is consistently much lower than that of the planar SiC sample. Specific test results show that the average reflectance of the planar SiC control sample is 15.84%, while the average reflectance of the inverted conical nanoporous structure sample decreases to 7.58% (an overall reduction of 8.26%), and the reflectance at the representative wavelength of 280 nm decreases from 14.79% to 6.89%. The measured results are basically consistent with the simulation results, verifying the effective suppression of ultraviolet reflection on the SiC surface by the inverted conical nanoporous structure, and also confirming the rationality of the anti-reflection mechanism of this invention.

[0076] This invention proposes an inverted conical nanopore array antireflection structure. Through a dual mechanism of continuous gradient equivalent refractive index and light-trapping enhancement absorption, it simultaneously addresses the core pain points of high surface reflection loss and short light absorption paths in ultra-thin active layers of SiC thin-film ultraviolet photodetectors. Compared to traditional vertical aperture and multilayer dielectric film solutions, it significantly improves ultraviolet light absorption and device responsivity without compromising the electrical performance of the active layer. This invention optimizes the ICP etching process for SiC thin films (using only SF6 etching gas) and designs multiple mask fabrication routes, balancing mass production efficiency and fabrication precision. It solves the problems of complex fabrication processes and poor compatibility with semiconductor processes in existing micro / nano antireflection structures. The integration of the antireflection structure into different types of SiC thin-film detectors (MSM, lateral PIN) limits the etching depth to less than 1 / 5 of the SiC thin film thickness, achieving synergistic optimization of optical and electrical performance and broadening the application compatibility of the antireflection structure.

[0077] The above embodiments are merely illustrative of the principles and effects of the present invention and are not intended to limit the invention. Any person skilled in the art can modify or alter the above embodiments without departing from the spirit and scope of the present invention. Therefore, all equivalent modifications or alterations made by those skilled in the art without departing from the spirit and technical concept disclosed in the present invention should still be covered by the claims of the present invention.

Claims

1. A SiC thin film surface inverted conical nanopore antireflection structure, characterized in that, The device includes a SiC thin film and a periodic inverted conical nanopore array located on the surface of the SiC thin film. The inverted conical nanopore array is arranged perpendicular to the surface of the SiC thin film and penetrates only the surface layer of the SiC thin film. The etching depth is less than the total thickness of the SiC thin film, thus preserving continuous electrical transport channels inside the SiC thin film. The inverted conical nanopore array has a geometric morphology that is wider at the top and narrower at the bottom. Its structural parameters include spatial period, top diameter, and etching depth. The configuration of the values ​​of the structural parameters enables the antireflection structure to form an equivalent refractive index layer that is continuously and gradually changes along the normal direction on the surface of the SiC thin film, so as to suppress interface reflection in the target ultraviolet band.

2. The anti-reflection structure with inverted conical nanopores on the surface of a SiC thin film according to claim 1, characterized in that, The specific range of structural parameters for the inverted conical nanopore array is as follows: spatial period 100–500 nm, top diameter 1–100 nm, and etching depth 100–500 nm.

3. The anti-reflection structure with inverted conical nanopores on the surface of a SiC thin film according to claim 1, characterized in that, The inverted conical nanopore array has a sidewall tilt angle of 70° to 85°, the SiC film is a single-crystal SiC film with a thickness of 0.5 to 2 μm, and the etching depth of the inverted conical nanopore array is less than 1 / 5 of the thickness of the SiC film.

4. A method for preparing an antireflection structure with inverted conical nanopores on the surface of a SiC thin film according to any one of claims 1 to 3, characterized in that, Includes the following steps: S1: Perform surface cleaning and pretreatment on the substrate containing SiC thin film to remove oil, oxide layer and impurities from the substrate surface; S2: Transfer or prepare a patterned mask on the surface of a SiC thin film, wherein the pattern of the mask is consistent with the pattern of the target inverted conical nanopore array; S3: Using a mask as a barrier layer, anisotropic dry etching technology is used to etch the SiC thin film. By adjusting the etching parameters, the etching rate at the top of the film is made greater than the etching rate at the bottom, forming an inverted conical nanopore array that is wider at the top and narrower at the bottom. The etching depth is controlled to be less than the thickness of the SiC thin film. S4: Remove the mask, clean and dry the sample to obtain the anti-reflection structure of the inverted conical nanopore array on the SiC surface.

5. The preparation method according to claim 4, characterized in that, In step S2, the technique used to form the patterned mask is AAO template transfer technology, nanoimprint technology, or electron beam exposure technology.

6. The preparation method according to claim 4, characterized in that, In step S3, the dry etching technique is reactive ion etching (RIE) or inductively coupled plasma etching (ICP).

7. The preparation method according to claim 4, characterized in that, The dry etching technique is inductively coupled plasma etching, with SF6 as the etching gas; the ICP power is 200-400W, the RF power is 20-50W, the etching gas pressure is 10-50mTorr, and the etching time is 30-60s; the etching parameters and etching time are matched to form an inverted conical morphology with a sidewall tilt angle of 70°-85°.

8. The application of an inverted conical nanopore antireflection structure on the surface of a SiC thin film in a SiC thin film ultraviolet photodetector, characterized in that, An anti-reflection structure of an inverted conical nanopore array on the SiC surface, as described in any one of claims 1 to 3, is disposed on the surface of the light-receiving region of the light incident path of a SiC thin-film ultraviolet photodetector, without covering the electrode region of the detector; the thickness of the SiC thin film is greater than the etching depth of the inverted conical nanopore array, ensuring that the nanopores do not disrupt the lateral carrier transport channels of the thin film.

9. The application according to claim 8, characterized in that, The SiC thin-film ultraviolet photodetector comprises, from bottom to top, at least a heterogeneous substrate, an insulating layer, and a SiC thin film located on the heterogeneous substrate or the insulating layer; the periodic inverted conical nanopore array is disposed on the exposed light-receiving area of ​​the SiC thin film surface that is not blocked by metal electrodes.

10. The application according to claim 8, characterized in that, The SiC thin-film ultraviolet photodetector is an MSM structure detector or a lateral PIN structure detector.

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