CN115000229B - Dark current suppressed semi-insulating 4H-SiC-based ultraviolet photoelectric detector and preparation method thereof - Google Patents

Abstract

The present invention belongs to the technical field of ultraviolet photodetectors, and specifically relates to a semi-insulating 4H-SiC-based ultraviolet photodetector with dark current suppression and a preparation method thereof, wherein the detector comprises a top electrode layer, a _{semiconductor layer, an Al2O3 layer} and a bottom electrode layer arranged in sequence from top to bottom, wherein the top electrode layer is a semi-transparent metal electrode, the semiconductor layer is a 4H-SiC substrate, and the bottom electrode layer is an opaque metal electrode. The present invention can achieve dark current suppression of the ultraviolet photodetector, so that the weakest detectable optical power of the device reaches the picowatt level, improves the weak light detection performance of the semi-insulating 4H-SiC-based ultraviolet photodetector, and can promote the application of 4H-SiC-based ultraviolet photodetectors in the field of automatic fire extinguishing and explosion suppression.

Description

A semi-insulating 4H-SiC-based ultraviolet photodetector with dark current suppression and its preparation method

Technical Field

The invention belongs to the technical field of ultraviolet photoelectric detectors, and in particular relates to a semi-insulating 4H-SiC-based ultraviolet photoelectric detector with dark current suppression and a preparation method thereof.

Background technique

Ultraviolet photodetectors play a role in sensing weak sparks in modern automatic fire extinguishing and explosion suppression devices, and are of great value in reducing casualties and economic losses caused by the explosion of oil and equipment. After decades of development, the fire extinguishing and explosion suppression system has undergone many improvements from the initial concept to the current large-scale use, and its performance has been steadily improved. However, the ultraviolet photodetectors used in the automatic fire extinguishing and explosion suppression system have always used vacuum photomultiplier tubes (PMTs) working in the day-blind ultraviolet band. Although PMT has advantages such as high sensitivity, large photosensitive area and relatively mature preparation technology, it has inherent disadvantages that are difficult to overcome by vacuum devices, mainly including easy damage, heavy weight, high-voltage operation and high false alarm rate. In the application field of coal mines, the stability and volume of PMT have become bottlenecks hindering the development of its fire extinguishing and explosion suppression technology. Therefore, the development of high-performance solid-state semiconductor ultraviolet detection devices to replace traditional ultraviolet PMTs has always been the mainstream technology evolution direction pursued by the fire extinguishing and explosion suppression industry and many related national economic fields.

Silicon carbide (SiC) UV detectors have been widely used in the fields of UV radiation dose measurement and solar radiation index measurement. The SiC substrate materials available on the market include two crystal forms: 4H-SiC and 6H-SiC. Among them, 4H-SiC has a higher carrier mobility than 6H-SiC and has more advantages in the development of optoelectronic devices. After loading a suitable filter, 4H-SiC UV photodetectors can be used in the field of spark sensing. In the past two decades, UV photodetectors based on 4H-SiC substrates have been widely studied. All reported 4H-SiC photodetectors include PN or PIN diode type, metal-semiconductor-metal (MSM) type, etc. PN or PIN diode type 4H-SiC UV photodetectors with vertical structures have high external quantum efficiency and can detect weak light at the pW level when working in avalanche mode. However, the main functional layers such as P-type 4H-SiC and N-type 4H-SiC in these devices need to be processed by epitaxial methods, and the manufacturing process is relatively complicated, resulting in high device costs. In addition, the photosensitivity surface of 4H-SiC avalanche diode is limited, usually in the size of 100 μm × 100 μm, which limits its application in spark sensing. In comparison, the simpler MSM-type 4H-SiC ultraviolet photodetector has a lower manufacturing cost and is suitable for making devices with large photosensitivity surfaces. It has certain advantages in the development of ultraviolet detectors for the field of automatic fire extinguishing and explosion suppression.

The simplest MSM type 4H-SiC photodetector is obtained by directly making metal interdigitated electrodes on the 4H-SiC substrate, which can be called a horizontal MSM type 4H-SiC photodetector. Most of the horizontal MSM type 4H-SiC photodetectors that have been reported are made by first growing a P-type or N-type SiC film on a SiC substrate and then making electrodes. The additional epitaxial process has caused a significant increase in device costs. et al. directly processed Ni/Au interdigital electrodes on 4H-SiC substrates by photolithography to produce horizontal MSM-type 4H-SiC photodetectors (Reference: Physica Status Solidi (c) 2012, 9, 1680). Under -4V bias, the dark current of the device is as low as 100fA. Under 365nm wavelength and 0.5mW/cm2 light irradiation, the bright current of the device is 10nA, and the bright-dark current ratio is 105. However, the weak light detection capability of the MSM-type 4H-SiC photodetector is insufficient, usually only reaching the μW/cm2 level, which is far lower than the weak light detection capability of ultraviolet vacuum phototubes (about pW/cm2), and cannot meet the needs of the field of automatic fire extinguishing and explosion suppression.

Summary of the invention

The present invention overcomes the deficiencies of the prior art and aims to solve the technical problem of providing a semi-insulating 4H-SiC-based ultraviolet photodetector with dark current suppression and a preparation method thereof, so as to improve the weak light detection performance of the detector.

In order to solve the above technical problems, the technical solution adopted by the present invention is: a semi-insulating 4H-SiC-based ultraviolet photodetector with dark current suppression, comprising a top electrode layer, a semiconductor layer, _{an Al2O3} layer and a bottom electrode layer arranged in sequence from top to bottom, the top electrode layer is a semi-transparent metal electrode, the semiconductor layer is a 4H-SiC substrate, and the bottom electrode layer is an opaque metal electrode.

The thickness of the Al ₂ O ₃ layer is 0.6 nm ± 0.06 nm.

The top electrode layer is made of TiN, and the bottom electrode layer is made of Al.

The thickness of the semiconductor layer is 100-1000 μm, the thickness of the top electrode layer is 15 nm±5 nm, and the thickness of the bottom electrode is 100 nm±20 nm.

The 4H-SiC substrate is semi-insulating and has a resistivity of 1e13 ohm·cm to 1e15 ohm·cm.

The present invention also provides a method for preparing a semi-insulating 4H-SiC-based ultraviolet photodetector with dark current suppression, which is used to prepare the semi-insulating 4H-SiC-based ultraviolet photodetector with dark current suppression, comprising the following steps:

S1, cleaning the 4H-SiC substrate;

S2. Using magnetron sputtering, a top electrode is fabricated on one side of the cleaned 4H-SiC substrate;

S3, using atomic layer deposition method, flip the sample over and make an interface modification layer on the other side of 4H-SiC;

S4. Using magnetron sputtering, a bottom electrode is fabricated on the interface modification layer.

The cleaning of the 4H-SiC substrate includes the following steps:

S101. Use a measuring cylinder to add hydrogen peroxide, ammonia and deionized water in a ratio of 10:10:1 into a polytetrafluoroethylene beaker, then place the 4H-SiC substrate into the solution, cover the beaker mouth with aluminum foil, and soak for more than 20 minutes. Then take out the 4H-SiC substrate and rinse it with clean water to remove the residual solution.

S102, add a nitric acid solution diluted with deionized water in a ratio of 4:1 into another polytetrafluoroethylene beaker, place the 4H-SiC substrate therein, then cover the beaker mouth with aluminum foil, perform ultrasonic treatment for 30 minutes, then take out the 4H-SiC substrate and rinse it with clean water to remove the residual solution;

S103, applying detergent on the surface of the sheet, and repeatedly rubbing and cleaning the 4H-SiC substrate under running water until a concentrated water film is formed on the surface of the 4H-SiC substrate when rinsed with clean water;

S104. Subsequently, the 4H-SiC substrate is placed vertically on a beaker stand in a glass beaker, and deionized water, acetone, and anhydrous ethanol solution are added in sequence, and each is ultrasonically treated for 15 minutes. At this point, the 4H-SiC substrate is cleaned, and the cleaned 4H-SiC substrate is placed in a beaker containing an isopropanol solution for later use.

Step S2 specifically includes the following steps:

S201, installing the top electrode target material to be sputtered on the radio frequency sputtering target head of the magnetron sputtering coating machine;

S202, after laminating and loading a metal mask on one side of the 4H-SiC substrate, load it on a sample holder of a magnetron sputtering coating machine and place it directly above the target material;

S203, close the magnetron sputtering cabin door, open and zero the vacuum gauge, turn on the mechanical pump and pre-pumping valve on the display screen, close the pre-pumping valve when the pressure drops to 30Pa, open the plug valve and the molecular pump, and when the cabin pressure reaches the order of 10-4Pa, turn on the argon ionization valve and the argon channel power supply;

S204, opening the argon magnetic control valve, mechanical valve, and flow meter in sequence, selecting a suitable argon flow rate, and then adjusting the gate valve of the molecular pump to maintain the cavity pressure at 2 Pa;

S205. Turn on the sputtering power supply and adjust the power required for sputtering. After starting, further adjust the pressure through the gate valve so that the sputtering rate reaches the film formation requirement. Pre-sputter for 10 minutes before formal sputtering. When the required film thickness is reached, close the large baffle first, then turn off the sputtering power supply, take out the sample from the coating chamber, and remove the metal mask.

Step S3 specifically includes the following steps:

S301, flip the sample coated with the semi-transparent top electrode, and attach a metal mask to the other side of the 4H-SiC substrate for standby use;

S302, turn on the circulating water cooling, inflate and open the hatch, install the trimethylaluminum and water vapor raw material bottles tightly with the manual valve, close the hatch, set the deposition chamber temperature to 150°C through the computer, and after the temperature stabilizes, set the carrier gas flow rate to 30sccm, set the type, time, flow rate, reaction time and cleaning time of each raw material, control the deposition rate to 0.06nm per cycle; set the waiting time to 1 minute, and start pre-deposition for 40 cycles;

S303, after the pre-deposition is completed, the hatch is opened by inflating, and the sample loaded with the metal mask is loaded into the atomic layer deposition chamber, ensuring that the film growth surface faces upward, and formal deposition begins, and the appropriate number of cycles is set according to the required film thickness requirements;

S304. When the deposition film thickness requirement is reached, the deposition is automatically completed. When the temperature of the deposition chamber drops to room temperature, the sample is inflated and taken out.

Step S4 specifically includes the following steps:

S401, installing the bottom electrode target material to be sputtered on the DC sputtering target head of the magnetron sputtering coating machine;

S402, loading the 4H-SiC substrate with the interface modification layer grown on the sample holder of the magnetron sputtering coating machine and placing it just above the target;

S403, close the magnetron sputtering cabin door, open and zero the vacuum gauge, turn on the mechanical pump and pre-pumping valve on the display screen, close the pre-pumping valve when the pressure drops to 30Pa, open the plug valve and the molecular pump, and when the cabin pressure reaches the order of 10-4Pa, turn on the argon ionization valve and the argon channel power supply;

S404, opening the argon magnetic control valve, mechanical valve, and flow meter in sequence, selecting a suitable argon flow rate, and then adjusting the gate valve of the molecular pump to maintain the cavity pressure at 2Pa;

S405. Turn on the DC sputtering power supply and adjust the power required for sputtering. After starting, further adjust the pressure through the gate valve so that the sputtering rate reaches the film formation requirement. After pre-sputtering for 10 minutes, perform formal sputtering. When the required film thickness is reached, close the large baffle first and then the sputtering power supply, take out the sample from the coating chamber, remove the metal mask, and collect the sample.

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

The invention provides a semi-insulating 4H-SiC-based ultraviolet photodetector with dark current suppression and a preparation method thereof. A vertical ultraviolet photodetector is formed by respectively arranging a top electrode and a bottom electrode on both sides of a 4H-SiC semiconductor layer. Moreover, an Al ₂ O ₃ layer is introduced as a modification layer at the interface between the 4H-SiC semiconductor layer and the bottom electrode layer. When the thickness of the Al ₂ O ₃ layer is 0.6 nm, the dark current of the device can be as low as 22 fA. When a vertical ultraviolet photodetector without adding an Al ₂ O ₃ layer is used as a control device, the dark current of the invention can be reduced to about 1/6 of that of the control device. Accordingly, the weak light detection capability of the device is improved. Specifically, the weakest detectable optical power of the detector modified with 0.6 nm thick Al ₂ O ₃ at a wavelength of 375 nm is reduced to about 1/6 of that of the control device, that is, reduced from 35 pW to 6 pW. Therefore, the invention can achieve dark current suppression of the ultraviolet photodetector and improve the weak light detection performance of the semi-insulating 4H-SiC-based ultraviolet photodetector.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG1 is a schematic structural diagram of a semi-insulating 4H-SiC-based ultraviolet photodetector with dark current suppression provided by an embodiment of the present invention, wherein: 1-semiconductor layer, 2-top electrode layer, 3-Al ₂ O ₃ layer, 4-bottom electrode layer;

FIG2 is a current-voltage characteristic curve of a semi-insulating 4H-SiC-based ultraviolet photodetector with dark current suppression provided by an embodiment of the present invention in a dark state, and a dark state current-voltage characteristic curve of a control device (TiN/4H-SiC/Al); the figure also shows the dark state performance of the device when the thickness of the Al ₂ O ₃ layer is 0.3nm, 0.5nm, 0.7nm, and 0.9nm for reference; wherein, when TiN is connected to positive and Al is connected to negative, the applied bias voltage V _a is greater than 0;

3 is a bar chart showing the dark current of the TiN/4H-SiC/Al ₂ O ₃ (x nm)/Al vertical structure SiC ultraviolet photodetector as a function of the thickness x of the Al ₂ O ₃ layer, wherein x=0, 0.3 nm, 0.5 nm, 0.6 nm and 0.7 nm; wherein x=0.6 nm corresponds to the ultraviolet photodetector device provided in the embodiment of the present invention, x=0 corresponds to the control device in the present invention, and x=0.3 nm, 0.5 nm and 0.7 nm correspond to other reference devices in the present invention;

Figure 4 shows the current-voltage characteristic curve of a dark current suppressed semi-insulating 4H-SiC-based ultraviolet photodetector in the bright state provided by an embodiment of the present invention, as well as the bright-state current-voltage characteristic curve of the control device TiN/4H-SiC/Al; the figure also shows the bright-state response performance of the device when the thickness of the Al ₂ O ₃ layer is 0.3nm, 0.5nm, 0.7nm, and 0.9nm for reference; wherein, when TiN is connected to the positive and Al is connected to the negative, the applied bias voltage V _a is greater than 0. Lighting conditions: wavelength 375nm, power 0.4mW;

Figure 5 shows the linear dynamic range of a semi-insulating 4H-SiC-based ultraviolet photodetector with dark current suppression and a reference device TiN/4H-SiC/Al under a bias voltage of 20V provided by an embodiment of the present invention, wherein (a) is the reference device, and (b) is the photodetector of an embodiment of the present invention, and the wavelength of the incident light is 375nm.

Detailed ways

In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are part of the embodiments of the present invention, rather than all the embodiments; based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention.

Embodiment 1

As shown in Figure 1, an embodiment of the present invention provides a semi-insulating 4H-SiC-based ultraviolet photodetector with dark current suppression, including a top electrode layer 2, a semiconductor layer 1, an _Al2O3 layer 3 and a bottom electrode layer 4 arranged in sequence from _top to bottom, wherein the top electrode layer is a semi-transparent metal electrode, the semiconductor layer is a 4H-SiC substrate, and the bottom electrode layer is an opaque metal electrode.

Specifically, in this embodiment, the 4H-SiC substrate selected for the semiconductor layer 1 is semi-insulating, weakly n-type, and has a resistivity between 1e13 ohm·cm and 1e15 ohm·cm. Further, in this embodiment, the top electrode is a semi-transparent TiN electrode, which is prepared on one side of the 4H-SiC, the bottom electrode is an opaque Al electrode, and the Al ₂ O ₃ interface modification layer and the opaque Al bottom electrode are prepared on the other side of the 4H-SiC.

Furthermore, in this embodiment, the thickness of the semiconductor layer 1 is 100-1000 μm, the thickness of the top electrode layer 2 is 15 nm±5 nm, and the thickness of the bottom electrode layer 4 is 100 nm±20 nm.

Preferably, in this embodiment, the thickness of the semiconductor layer 1 is 500 μm±20 μm, the thickness of the top electrode layer 2 is 15 nm±1 nm, and the thickness of the bottom electrode layer 4 is 100 nm±5 nm.

Furthermore, in this embodiment, the thickness of the Al ₂ O ₃ layer is 0.6 nm±0.06 nm.

Preferably, in this embodiment, the thickness of the Al ₂ O ₃ layer is 0.6 nm.

The embodiment of the present invention improves the weak light detection performance of the ultraviolet photodetector by introducing an Al ₂ O ₃ layer as an interface modification layer in the ultraviolet photodetector composed of the top electrode, the semiconductor layer and the bottom electrode. Under 375nm wavelength light irradiation, the dark current of the Al ₂ O ₃ modified device is 22fA (20V bias), and the weakest detectable light power is about 6pW; in comparison, under the same test conditions, the control device without interface modification has a dark current of 131fA, and accordingly, the weakest detectable light power is about 35pW.

The reason why the present invention can improve the weak light detection capability of the detector is that the surface of the semi-insulating 4H-SiC substrate is rich in holes, and _the atomic-thick _Al2O3 is rich in electrons. The two interact with each other, greatly weakening the contribution of surface holes to the dark current of the device. Experiments have shown that thinning or thickening the thickness of the _{Al2O3 interface} modification layer will lead to an increase in dark current, affecting the weak light detection capability of the 4H-SiC ultraviolet photodetector.

Embodiment 2

Embodiment 2 of the present invention provides a method for preparing a semi-insulating 4H-SiC-based ultraviolet photodetector with dark current suppression, which is used to prepare a semi-insulating 4H-SiC-based ultraviolet photodetector with dark current suppression described in Embodiment 1. In this embodiment, the materials used are:

4H-SiC substrate, TiN target, Al target, hydrogen peroxide, ammonia, deionized water, nitric acid, detergent, deionized water, acetone, anhydrous ethanol, trimethylaluminum metal mask. The combined dosage and screening criteria are as follows:

4H-SiC substrate: semi-insulating, weakly n-type, with a resistivity of 1e14 ohm·cm, an area of 20mm×20mm, and a thickness of 500μm;

TiN target: solid, copper backing bonded, 99.9% purity;

Al target: solid, copper backing plate bonded, 99.999% purity;

Hydrogen peroxide: H ₂ O ₂ , 3%;

Ammonia: NH ₄ OH:H ₂ O, 25%

Deionized water: H ₂ O 8000mL±50mL;

Nitric acid: HNO ₃ , 68%

Detergent: 2±0.5mL;

Acetone: CH ₃ COCH ₃ 250 mL ± 5 mL;

Anhydrous ethanol: C ₂ H ₅ OH 500mL±5mL;

Trimethylaluminum: C ₃ H ₉ Al 1.0M hexane solution 100mL±5mL;

Metal mask: stainless steel; stripe pattern, hollow width 2mm, spacing 5mm.

This embodiment specifically includes the following steps:

S1. Cleaning the 4H-SiC substrate.

In step S1, the method for cleaning the 4H-SiC substrate is:

S101. Use a measuring cylinder to add hydrogen peroxide, ammonia and deionized water in a ratio of 10:10:1 into a polytetrafluoroethylene beaker, then place the 4H-SiC substrate into the solution, cover the beaker mouth with aluminum foil, and soak for more than 20 minutes. Then take out the 4H-SiC substrate and rinse it with clean water to remove the residual solution.

S102, add a nitric acid solution diluted with deionized water in a ratio of 4:1 into another polytetrafluoroethylene beaker, place the 4H-SiC substrate therein, then cover the beaker mouth with aluminum foil, perform ultrasonic treatment for 30 minutes, then take out the 4H-SiC substrate and rinse it with clean water to remove the residual solution;

S103, apply detergent on the surface of the thin slice, and repeatedly rub and clean the 4H-SiC substrate under running water until a concentrated water film is formed on the surface of the 4H-SiC substrate when rinsed with clean water.

S104, then, the 4H-SiC substrate is vertically placed on a beaker stand, placed in a glass beaker, and deionized water, acetone, and anhydrous ethanol solution are added in sequence, and ultrasonicated for 15 minutes each. At this point, the 4H-SiC substrate is cleaned, and the cleaned 4H-SiC substrate is placed in a beaker containing an isopropanol solution for standby use.

S2, using magnetron sputtering to make a semi-transparent top electrode on one side of the cleaned 4H-SiC substrate. In step S2, the method for making the semi-transparent top electrode is:

S201, installing the TiN target material to be sputtered on the radio frequency sputtering target head of the magnetron sputtering coating machine.

S202. A metal mask is attached to one side of the 4H-SiC substrate, and then it is loaded on the sample holder of the magnetron sputtering coating machine. At this time, the film growth surface is the side on which the metal mask is loaded, and this side faces downward. The sample holder is adjusted so that the 4H-SiC substrate is directly above the target material.

S203. Close the door of the magnetron sputtering chamber, open and zero the vacuum gauge, turn on the mechanical pump and pre-pumping valve on the display screen, close the pre-pumping valve when the pressure drops to 30 Pa, open the plug valve and molecular pump, and when the chamber pressure reaches 10 ^-4 Pa, turn on the argon ionization valve and argon channel power supply.

S204, open the argon magnetic control valve, mechanical valve, and flow meter in sequence, select a suitable argon flow rate, and then adjust the gate valve of the molecular pump to maintain the cavity pressure at 2Pa.

S205, turn on the sputtering power supply, adjust the power required for sputtering, and after ignition, further adjust the pressure through the gate valve so that the sputtering rate reaches the film formation requirement. Pre-sputter for 10 minutes before formal sputtering. When the required film thickness (15nm) is reached, close the large baffle first, then turn off the sputtering power supply, take out the sample from the coating chamber, and remove the metal mask.

S3. Using atomic layer deposition technology, on the basis of the device with the top electrode made, the sample is flipped over to make an Al ₂ O ₃ layer on the other side of the 4H-SiC.

S301. Turn over the sample coated with the semi-transparent top electrode, and attach a metal mask to the other side of the 4H-SiC substrate for use, paying attention to protecting the film layer that has been prepared.

S302, turn on the circulating water cooling, inflate and open the hatch, install the trimethylaluminum and water vapor raw material bottles tightly with the manual valve, close the hatch, set the deposition chamber temperature to 150°C through the computer, and after the temperature stabilizes, set the carrier gas flow rate to 30sccm, set the type, time, flow rate, reaction time and cleaning time of each raw material, and control the deposition rate to 0.06nm per cycle. Set the waiting time to 1 minute and start pre-deposition for 40 cycles.

S303. After the pre-deposition is completed, the hatch is opened by inflating, and the sample loaded with the metal mask is loaded into the atomic layer deposition chamber, ensuring that the film growth surface faces upward, and formal deposition begins. The appropriate number of cycles is set to achieve the required film thickness requirement (0.6nm).

S304, when the deposition film thickness requirement is reached, the deposition is automatically completed. When the temperature of the deposition chamber drops to room temperature, the sample is taken out by inflating, and the metal mask is not removed, and the next step is prepared. After that, the instrument is evacuated, the manual valve is closed, and all the residual raw materials in the pipeline are emptied. Then, the air is inflated to atmospheric pressure, the vacuum pump is turned off, the heating is stopped, and the temperature drops to room temperature, and the power switch of the equipment is turned off.

S4. Using magnetron sputtering, a bottom electrode is fabricated on the interface modification layer. The specific method for fabricating the opaque bottom electrode is as follows:

S401, installing the Al target material to be sputtered on the DC sputtering target head of the magnetron sputtering coating machine.

S402, load the 4H-SiC substrate with the interface modification layer grown on the sample tray of the magnetron sputtering coating machine. At this time, the film growth surface is the side with the Al ₂ O ₃ interface modification layer added, and this side faces downward. Adjust the sample tray so that the 4H-SiC substrate is located directly above the target.

S403. Close the door of the magnetron sputtering chamber, open and zero the vacuum gauge, turn on the mechanical pump and pre-pumping valve on the display screen, close the pre-pumping valve when the pressure drops to 30Pa, open the plug valve and molecular pump, and when the chamber pressure reaches 10-4Pa, turn on the argon ionization valve and argon channel power supply.

S404, open the argon magnetic control valve, mechanical valve, and flow meter in sequence, select a suitable argon flow rate, and then adjust the gate valve of the molecular pump to maintain the cavity pressure at 2Pa.

S405, turn on the DC sputtering power supply, adjust the power required for sputtering, and after starting, further adjust the pressure through the gate valve so that the sputtering rate reaches the film formation requirement, and pre-sputter for 10 minutes. Finally, perform formal sputtering to reach the required film thickness (100nm), first close the large baffle and then turn off the sputtering power supply, take out the sample from the coating chamber, remove the metal mask, and collect the sample.

Detection, analysis and characterization: The performance of the prepared 4H-SiC electrical detector was detected, analyzed and characterized.

The current-voltage characteristic curve of the device in the dark state was measured using the Aglient B1500 high-precision digital source meter; the current-voltage characteristic curve of the 4H-SiC UV photodetector in the bright state was measured using the Thorlabs 375nm LED and Aglient B1500. The light power irradiated to the effective area of the device was adjusted using an attenuator, and the bright-state current-voltage characteristic curve under different light powers was tested, based on which the linear dynamic range diagram of the device was drawn.

Conclusion: The dark current-voltage characteristic curve and _{the bright current-voltage characteristic curve of the vertical structure TiN/4H-SiC/Al 2 O 3 ( xnm )} /Al ultraviolet photodetector and the control device (i.e., TiN/4H-SiC/Al without Al ₂ O ₃ layer) after interface modification by different thickness of Al 2 O 3 layer were analyzed, and the linear dynamic range performance diagram of the embodiment device TiN/4H-SiC/Al ₂ O ₃ (0.6nm)/Al and the control device TiN/4H-SiC/Al were compared. Figure 2 shows the current-voltage characteristic curve of the 4H-SiC ultraviolet photodetector containing different thickness of Al ₂ O ₃ interface modification layer and the control device TiN/4H-SiC/Al under forward bias. As can be seen from Figure 2, the TiN/4H-SiC/Al ₂ O ₃ (0.6nm)/Al device has the lowest dark current.

Next, the specific dark current values of the devices with Al ₂ O ₃ layer thickness x=0, 0.3nm, 0.5nm, 0.6nm and 0.7nm at 20V bias were plotted, as shown in Figure 3. By comparison, it can be seen that when the thickness of the introduced Al ₂ O ₃ is thinner, the dark current of the device shows obvious degradation compared with the control device TiN/4H-SiC/Al. Specifically, the dark current of the control device is 131fA, and the dark current of the device after the introduction of 0.3nm thick Al ₂ O ₃ is as high as 2.95pA. Afterwards, as the thickness of Al ₂ O ₃ increases, the dark current performance gradually improves, and the dark current of the 0.5nm thick Al ₂ O ₃ device is lower than that of the 0.3nm thick Al ₂ O ₃ device, which is 1.74pA. When the thickness of Al ₂ O ₃ increases to 0.6nm, the dark current of the device is optimal, as low as 22fA. However, when the thickness of Al ₂ O ₃ is further increased to 0.7 nm, the dark current begins to degrade again, increasing to 184 fA. When the thickness of Al ₂ O ₃ is further increased to 0.9 nm, the dark current of the device degrades very seriously, increasing to 13 pA, as shown in FIG2 .

Figure 4 shows the bright current-voltage characteristic curves of 4H-SiC UV photodetectors with different thicknesses of Al ₂ O ₃ interface modification layers and control devices without Al ₂ O ₃ under forward bias, with the incident light wavelength of 375nm (the light power received in the effective area of the device is 0.4mW). It can be seen from Figure 4 that the addition of the Al ₂ O ₃ interface modification layer has a relatively weak effect on the bright current of the device, and the bright current of the device under 20V bias falls within the range of 0.4μA to 1.2μA.

In summary, the present invention substantially reduces the dark current of the device by introducing a _{0.6nm Al2O3} interface modification layer while basically maintaining the bright current of the device. Since the energy level of the semi-insulating 4H-SiC is the Fermi level clamped by the surface state, the charge injection barrier is high, and the dark current caused by the intrinsic carriers of the semi-insulating _4H -SiC is also very low, the dark current of the device mainly comes from the hole carriers existing on the surface. There is a certain concentration of negative charges in _Al2O3 , which can just interact with the surface holes of 4H-SiC. Only an appropriate amount of negative charges can completely offset the surface holes, thereby weakening the contribution of the surface holes to the dark current of the device. From the device, it can be seen that the target control accuracy is at the atomic level and sub-nm scale, so it is very necessary to use atomic layer deposition technology.

Figure 5 shows the linear dynamic range performance of the TiN/4H-SiC/Al ₂ O ₃ (0.6nm)/Al ultraviolet photodetector and the control device TiN/4H-SiC/Al at a wavelength of 375nm. As can be seen from Figure 5, the weak light detection capability of the device is effectively improved after the dark current of the device decreases due to the addition of the Al ₂ O ₃ interface modification layer. The weakest detectable light intensity of the Al ₂ O ₃ modified device reaches 6pW, which is 1/6 of the control device (the control device is 35pW). Correspondingly, the linear dynamic range of the Al ₂ O ₃ modified device is widened to 168dB, while that of the control device is only 155dB.

In summary, the present invention has developed a semi-insulating 4H-SiC-based ultraviolet photodetector with dark current suppression. By respectively setting a top electrode and a bottom electrode on both sides of the semiconductor layer, a vertically structured 4H-SiC ultraviolet photodetector is formed. Moreover, by introducing an atomically thick Al ₂ O ₃ layer as a modification layer at the interface between the 4H-SiC and the bottom electrode, when the thickness of the Al ₂ O ₃ layer is 0.6nm, the dark current of the device can be as low as 22fA, that is, the present invention can reduce the weakest detectable optical power at a wavelength of 375nm to 6pW. Therefore, the present invention can suppress the dark current of the ultraviolet photodetector, so that the weakest detectable optical power of the device reaches the picowatt level, and the weak light detection performance of the semi-insulating 4H-SiC-based ultraviolet photodetector is improved. The application process of 4H-SiC-based ultraviolet photodetectors in the field of automatic fire extinguishing and explosion suppression can be promoted.

Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or replace some or all of the technical features therein by equivalents. However, these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims (7)

1. A semi-insulating 4H-SiC-based ultraviolet photodetector with dark current suppression, characterized in that it comprises a top electrode layer, a _{semiconductor} layer, an _Al2O3 layer and a bottom electrode layer arranged in sequence from top to bottom, the top electrode layer is a semi-transparent metal electrode, the semiconductor layer is a 4H-SiC substrate, the bottom electrode layer is an opaque metal electrode, the top electrode layer material is TiN, and the bottom electrode layer is Al; the thickness of the _{Al2O3 layer} is 0.6 nm±0.06nm; the thickness of the semiconductor layer is 100~1000μm, the thickness of the top electrode layer is 15 nm±5nm, and the thickness of the bottom electrode is 100 nm±20nm. 2. A semi-insulating 4H-SiC-based ultraviolet photodetector with dark current suppression according to claim 1, characterized in that the 4H-SiC substrate is semi-insulating and has a resistivity of 1e13ohm•cm~1e15ohm•cm. 3. A method for preparing a semi-insulating 4H-SiC-based ultraviolet photodetector with dark current suppression, characterized in that the method for preparing the semi-insulating 4H-SiC-based ultraviolet photodetector with dark current suppression according to claim 1 comprises the following steps: S1, cleaning the 4H-SiC substrate; S2. Using magnetron sputtering, a top electrode is fabricated on one side of the cleaned 4H-SiC substrate; S3, using atomic layer deposition method, flip the sample over and make an interface modification layer on the other side of 4H-SiC; S4. Using magnetron sputtering, a bottom electrode is fabricated on the interface modification layer. 4. The method for preparing a semi-insulating 4H-SiC-based ultraviolet photodetector with dark current suppression according to claim 3, characterized in that the cleaning of the 4H-SiC substrate comprises the following steps: S101. Use a measuring cylinder to add hydrogen peroxide, ammonia and deionized water in a ratio of 10:10:1 into a polytetrafluoroethylene beaker, then place the 4H-SiC substrate into the solution, cover the beaker mouth with aluminum foil, and soak for more than 20 minutes. Then take out the 4H-SiC substrate and rinse it with clean water to remove the residual solution. S102. Add a nitric acid solution diluted with deionized water in a ratio of 4:1 into another polytetrafluoroethylene beaker, place the 4H-SiC substrate therein, then cover the beaker mouth with aluminum foil, perform ultrasonic treatment for 30 min, then take out the 4H-SiC substrate and rinse it with clean water to remove the residual solution; S103, applying detergent on the surface of the sheet, and repeatedly rubbing and cleaning the 4H-SiC substrate under running water until a uniform water film is formed on the surface of the 4H-SiC substrate when rinsed with clean water; S104. Subsequently, the 4H-SiC substrate is placed vertically on a beaker stand in a glass beaker, and deionized water, acetone, and anhydrous ethanol solution are added in sequence, and each is ultrasonically treated for 15 minutes. At this point, the 4H-SiC substrate is cleaned, and the cleaned 4H-SiC substrate is placed in a beaker containing an isopropanol solution for later use. 5. The method for preparing a semi-insulating 4H-SiC-based ultraviolet photodetector with dark current suppression according to claim 3, characterized in that step S2 specifically comprises the following steps: S201, installing the top electrode target material to be sputtered on the radio frequency sputtering target head of the magnetron sputtering coating machine; S202, after laminating and loading a metal mask on one side of the 4H-SiC substrate, load it on a sample holder of a magnetron sputtering coating machine and place it directly above the target material; S203, close the magnetron sputtering cabin door, open and zero the vacuum gauge, turn on the mechanical pump and pre-pumping valve on the display screen, close the pre-pumping valve when the pressure drops to 30 Pa, open the plug valve and the molecular pump, and when the cabin pressure reaches the order of 10-4 Pa, turn on the argon ionization valve and the argon channel power supply; S204, opening the argon magnetic control valve, mechanical valve, and flow meter in sequence, selecting a suitable argon flow rate, and then adjusting the gate valve of the molecular pump to maintain the cavity pressure at 2 Pa; S205. Turn on the sputtering power supply and adjust the power required for sputtering. After starting, further adjust the pressure through the gate valve so that the sputtering rate reaches the film formation requirement. Pre-sputter for 10 minutes before formal sputtering. When the required film thickness is reached, close the large baffle first, then turn off the sputtering power supply, take out the sample from the coating chamber, and remove the metal mask. 6. The method for preparing a semi-insulating 4H-SiC-based ultraviolet photodetector with dark current suppression according to claim 3, characterized in that step S3 specifically comprises the following steps: S301, flip the sample coated with the semi-transparent top electrode, and attach a metal mask to the other side of the 4H-SiC substrate for standby use; S302, turn on the circulating water cooling, inflate and open the hatch, install the trimethylaluminum and water vapor raw material bottles tightly with the manual valve, close the hatch, set the deposition chamber temperature to 150°C through the computer, and after the temperature stabilizes, set the carrier gas flow rate to 30sccm, set the type, time, flow rate, reaction time and cleaning time of each raw material, control the deposition rate to 0.06nm per cycle; set the waiting time to 1 minute, and start pre-deposition for 40 cycles; S303, after the pre-deposition is completed, the hatch is opened by inflating, and the sample loaded with the metal mask is loaded into the atomic layer deposition chamber, ensuring that the film growth surface faces upward, and formal deposition begins, and the appropriate number of cycles is set according to the required film thickness requirements; S304. When the deposition film thickness requirement is reached, the deposition is automatically completed. When the temperature of the deposition chamber drops to room temperature, the sample is inflated and taken out. 7. The method for preparing a semi-insulating 4H-SiC-based ultraviolet photodetector with dark current suppression according to claim 3, characterized in that step S4 specifically comprises the following steps: S401, installing the bottom electrode target material to be sputtered on the DC sputtering target head of the magnetron sputtering coating machine; S402, loading the 4H-SiC substrate with the interface modification layer grown on the sample holder of the magnetron sputtering coating machine and placing it just above the target; S403, close the magnetron sputtering cabin door, open and zero the vacuum gauge, turn on the mechanical pump and pre-pumping valve on the display screen, close the pre-pumping valve when the pressure drops to 30 Pa, open the plug valve and molecular pump, and when the cabin pressure reaches 10-4 Pa, turn on the argon ionization valve and argon channel power supply; S404, open the argon magnetic control valve, mechanical valve, and flow meter in sequence, select a suitable argon flow rate, and then adjust the gate valve of the molecular pump to maintain the cavity pressure at 2 Pa; S405. Turn on the DC sputtering power supply and adjust the power required for sputtering. After starting, further adjust the pressure through the gate valve so that the sputtering rate reaches the film formation requirement. After pre-sputtering for 10 minutes, perform formal sputtering. When the required film thickness is reached, close the large baffle first and then the sputtering power supply, take out the sample from the coating chamber, remove the metal mask, and collect the sample.