Transient light current measuring system and measuring method for photoelectric material

Abstract

The invention discloses a transient photocurrent measurement system and measurement method for a photoelectric material. The transient photocurrent measurement system comprises a double-pulse femtosecond laser detection system and a transient photocurrent high-resolution acquisition system, the double-pulse femtosecond laser detection system is used for outputting two beams of time sequence adjustable femtosecond pulse light which are respectively used as pump light and detection light, irradiating the pump light on a photoelectric material, irradiating the detection light on an ultrafast light switch material, and regulating and controlling the optical path difference of the two beams of light reaching a sample; and the transient light current high-resolution acquisition system is used for extracting picosecond-level time resolution of the transient light current of the photoelectric material. The picosecond-level time resolution can be realized, the simplified electrode structure design not only reduces the manufacturing cost and complexity, but also improves the stability and reliability of the system, so that the technology can adapt to more types of materials and application scenes, the device structure is compatible with a traditional method for testing the bandwidth of the device, and the test efficiency is greatly improved. And a convenient research method is provided for establishing a relation between device performance and a working mechanism.

Description

Technical Field

[0001] The present invention relates to the field of photoelectric detection technology, and in particular to a transient photocurrent measurement system and a measurement method for photoelectric materials. Background Art

[0002] The transient photocurrent test system is an analytical tool widely used in the optoelectronic field. It provides researchers with a means to deeply explore key optoelectronic properties such as photoelectric conversion efficiency, carrier dynamics, electronic structure and charge separation efficiency by measuring the transient current changes generated by the material after light excitation. This technology not only plays an important role in the analysis of solar cells, photodetectors and semiconductor materials, but also shows significant potential in material screening and optimization, promoting the rapid evaluation of new optoelectronic materials, and promoting the development of optoelectronic science and technology. It has an important impact on improving the performance of optoelectronic devices, optimizing designs, and developing efficient energy conversion and photocatalytic applications.

[0003] At present, there are two main development paths for transient photocurrent testing technology: the first is the traditional oscilloscope-based photocurrent detection technology, the performance of which is limited by the bandwidth and sampling rate of the oscilloscope. Although the bandwidth of high-performance oscilloscopes on the market can reach tens of GHz and provide a time resolution of hundreds of picoseconds, they are usually expensive and have limitations in the study of photodetectors at the hundred-GHz level due to insufficient time resolution. The second technology is the optical pumping electrical detection technology based on ultrafast optical switches. This technology uses ultrafast optical switches as core components to generate photocurrents by pumping light to excite samples, and uses ultrafast optical switches with a response speed of up to 1ps to measure transient photocurrents. However, existing reports show that the optical pumping electrical detection technology based on ultrafast optical switches has problems such as complex electrode structure and poor applicability. In view of the limitations of existing technologies, it is urgent to develop a new transient photocurrent characterization technology that is both stable and reliable and has wide adaptability. This technology should have a temporal and spatial resolution at the picosecond level, a simple electrode structure, strong applicability, and the ability to verify test results through traditional methods. The development of this technology will help identify and analyze the key factors affecting the performance of optoelectronic devices and provide theoretical and experimental support for the improvement and optimization of device performance. Summary of the invention

[0004] Purpose of the invention: The present invention provides a transient photocurrent measurement system and method for optoelectronic materials, which can accurately capture and analyze the key influencing factors of the performance of optoelectronic devices, thereby promoting the efficient development and performance optimization of optoelectronic materials and devices.

[0005] Technical solution: The transient photocurrent measurement system for optoelectronic materials described in the present invention includes: a dual-pulse femtosecond laser detection system and a transient photocurrent high-resolution acquisition system; the dual-pulse femtosecond laser detection system is used to output two beams of femtosecond pulse light with adjustable timing, which are used as pump light and detection light respectively, and the pump light is irradiated on the optoelectronic material, and the detection light is irradiated on the ultrafast optical switch, and the optical path difference between the two beams of light reaching the sample is adjusted; the transient photocurrent high-resolution acquisition system is used to extract the transient photocurrent of the optoelectronic material with picosecond time resolution.

[0006] Furthermore, the dual-pulse femtosecond laser detection system includes: a femtosecond laser, a beam splitting module, a delay module, a chopper and a beam combining module;

[0007] The femtosecond laser generates laser pulses, which are divided into two pulsed lights by the beam splitting module, namely pump light and detection light;

[0008] The pump light passes through a delay module to control the optical path difference between the pump light and the probe light;

[0009] The pump light then passes through a chopper, which is used to frequency modulate the pump light so that a subsequent signal acquisition system can extract a specific frequency signal;

[0010] The beam combining module is used to combine the pump light and the detection light and then collimate and irradiate them onto the photoelectric material and the ultrafast optical switch respectively.

[0011] Furthermore, the delay module includes a precision electrically controlled translation stage, an optical reflector and a plurality of reflector groups, and can realize the transient characteristic measurement of the photocurrent generated by the photoelectric material by adjusting the time interval between the two pulsed lights.

[0012] Furthermore, the transient photocurrent high-resolution acquisition system includes: a device to be tested and a sample stage, an electrical measurement module, and a lock-in amplifier;

[0013] The device under test and the sample stage use a lead wiring machine to electrically connect the electrodes of the device under test to the electrical measurement base;

[0014] The electrical measurement module applies or tests voltage or current to the sample;

[0015] The phase-locked amplifier inputs the trigger signal of the chopper module to the phase-locked amplifier as a reference signal, and at the same time connects the current output end to the input port of the phase-locked amplifier to collect the current signal with the same frequency as the chopper modulation signal.

[0016] Furthermore, the device to be tested includes an insulating substrate, an ultrafast optical switch material, a photoelectric sample, a current output terminal, a ground terminal and a bias voltage terminal; the insulating substrate is made of insulating high-resistance material, such as sapphire, quartz or a high-resistance silicon wafer with an oxide layer on the surface; the photogenerated carrier lifetime of the ultrafast optical switch material should be within 0.5-2ps, the length and width range is 10μm to 100μm, and the thickness is 0.5μm to 2μm; the length and width range of the photoelectric material is 10μm to 100μm, and the thickness is less than 2μm; the interval between the current output terminal and the ground terminal is 2-20μm, the interval between the ground terminal and the bias voltage terminal is 2-20μm, and the interval between the ground terminal and the bias voltage terminal is 2-20μm.

[0017] Accordingly, a method for measuring transient photocurrent of a photoelectric material comprises the following steps:

[0018] Step 1: Using micro-nano processing technology to manufacture a device under test with a specific electrode configuration;

[0019] Step 2: fix the device under test on the electrical test base of the sample stage, and establish an electrical connection between the device under test and the electrical test base of the sample stage; establish an electrical connection between the test base and the lock-in amplifier and the electrical measurement module;

[0020] Step 3, using a femtosecond laser to generate a femtosecond pulse laser; splitting it into a pump beam and a probe beam through a beam splitter module; irradiating the probe beam to the ultrafast optical switch center of the device under test; using the chopper and delay module to work together to modulate the pump beam to a specific frequency, and using the beam combiner module to allow the pump beam to irradiate the test area of ​​the device under test;

[0021] Step 4: Use the pump beam to act on the device under test to generate transient photocurrent; at the same time, use the detection beam to irradiate the ultrafast optical switch material. A single pulse of the detection beam will create a 1-2ps path between the ground terminal and the current output terminal; use the delay module to accurately control the time difference between the pump beam and the detection beam reaching the sample, and measure the photocurrent passing through the loop at each specific moment before and after the excitation light; draw a curve graph of the transient photocurrent of the device under test changing with time.

[0022] Furthermore, in step 1, the micro-nano processing technology is used to manufacture a device under test with a specific electrode configuration, which specifically includes the following steps:

[0023] Step 11: Use a diamond knife to cut a high-resistance silicon wafer with a 285nm silicon oxide layer into a square with a length and width of about 1 cm as a substrate;

[0024] Step 12: preparing a photoelectric material to be tested with a length and width of about 20 μm; using a high-precision fixed-point transfer platform to transfer the photoelectric material to the center of the substrate;

[0025] Step 13: preparing an ultrafast optical switch material with a length and width ranging from 10 μm to 100 μm and a thickness ranging from 0.5 μm to 2 μm, and using a high-precision fixed-point transfer platform to transfer the ultrafast optical switch material to the vicinity of the optoelectronic material prepared in step 12, ensuring that the center distance between the optoelectronic material and the ultrafast optical switch material is about 120 μm;

[0026] Step 14: Spin-coat a photoresist with a thickness of about 2 μm on the sample prepared in step 13, and then use laser micro-nano processing technology to etch the area at a fixed point. After development, the photoresist in this area is washed off; use thermal vacuum coating technology to evaporate 300 nm of gold on the sample, and then soak the gold-plated sample in acetone solution for more than 10 minutes, and the gold in the photoresist-covered area falls off.

[0027] Further, in step 2, the connection method is to connect the bias voltage terminal to the voltage output terminal of the electrical measurement module, connect the ground terminal to the ground terminal of the electrical measurement module, and connect the current output terminal to the lock-in amplifier, which specifically includes the following steps:

[0028] Step 21, adhere the sample to the electrical measuring base of the sample stage through silver glue, and then use a wire bonding machine to weld each electrode of the sample to the electrical measuring base through wires;

[0029] Step 22, using a BNC transmission line, connect the BNC interface of the electrical measurement base to the electrical measurement module and the phase-locked amplifier; the connection method is: the current output end is connected to the electrical measurement module; the ground end is connected to the ground end of the electrical measurement module;

[0030] Step 23: Use an electrical measurement module to test the resistance of the ultrafast optical switch in the presence and absence of light to ensure that the switch ratio is greater than ten to the fifth power;

[0031] Step 24, using a BNC transmission line, connect the BNC interface of the electrical measurement base to the electrical measurement module and the phase-locked amplifier; the connection method is that the current output end is connected to the phase-locked amplifier; the ground end is connected to the ground end of the electrical module; the bias voltage end is connected to the electrical measurement module;

[0032] Step 25: Use the electrical measurement module to test the volt-ampere characteristic curve of the photoelectric material to ensure that it works properly.

[0033] Furthermore, in step 4, drawing a curve graph of the transient photocurrent of the device under test changing with time specifically includes the following steps:

[0034] Step 41: Start the femtosecond laser and wait for it to be preheated. After ensuring that the laser is running stably, turn on the shutter switch of the laser to make the laser emit laser light and set the wavelength to 800nm.

[0035] Step 42: Use a reflector in front of the sample stage to modulate the positions of the pump beam and the probe beam reaching the sample, ensuring that the pump beam and the probe beam are irradiated on the optoelectronic material and the ultrafast optical switch material respectively;

[0036] Step 43: Use the electrical measurement module to apply bias voltage to both ends of the photoelectric material, use the reflector to fine-tune the position where the excitation light irradiates the photoelectric material, and find the state where the photocurrent is maximum;

[0037] Step 44: Using the time zero position recorded when building the optical path, move the delay line so that it reaches the vicinity of the position corresponding to time zero; use a computer program to control the delay line to move back and forth at this point, and read the phase-locked amplifier reading in real time to find a position where the transient photocurrent is the largest, and record this position as time zero. If the position is not found, please use the method described below to find the time zero again; move the delay line device so that the two beams of light reach the sample area at the same time. At this time, the position of the delay line device corresponds to the time zero. The method of finding the time zero includes: using the precise measurement of the optical path, respectively measuring the optical path of the two beams of light from the beam splitter at the separation to the sample stage, so as to calculate the time zero when the delay line reaches a certain position; using the second-order nonlinear effect of the frequency doubling crystal (for example, α-phase barium borate crystal (α-BBO)), place α-BBO on the sample stage to ensure that the position of the excitation light on the sample stage remains unchanged when the delay line moves, and then modulate the reflector in front of the sample stage so that the two beams of light completely overlap in space and shine on α-BBO, and then move the delay line back and forth. When the brightness of the doubled frequency light generated by α-BBO is the largest, the position of the delay line is the time zero. The approximate position of the time zero is determined when the optical path is built. The delay line needs to be fine-tuned each time the test is performed to verify the time zero;

[0038] Step 45: Use software to control the delay line to move sequentially within the time range of -10 to 50 ps (0 ps corresponds to time zero, -10 ps corresponds to the time when the excitation light arrives at the sample 10 ps later than the detection light, and 50 ps corresponds to the time when the excitation light arrives at the sample 50 ps earlier than the detection light), with an accuracy of 0.1 ps, and use software to communicate with the phase-locked amplifier to read the transient current value at each moment; plot the obtained data into a graph, and use the experimental data to analyze the response time of the optoelectronic material and study the device bandwidth.

[0039] Beneficial effects: Compared with the prior art, the present invention has the following significant advantages: the present invention can achieve a time resolution of picoseconds, which is crucial for in-depth exploration of ultrafast dynamic processes in optoelectronic devices; the simplified electrode structure design not only reduces manufacturing costs and complexity, but also improves the stability and reliability of the system, making the technology adaptable to a wider variety of materials and application scenarios; the device structure is compatible with traditional methods for testing device bandwidth, providing a convenient research method for building the connection between device performance and working mechanism. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] Figure 1 It is a schematic diagram of the structure of the measurement system of the present invention.

[0041] Figure 2 Schematic diagram of the optical path of the measurement system of the present invention.

[0042] Figure 3 This is a time domain diagram of the transient photocurrent of the graphene photodetector of the present invention.

[0043] Among them, 1. Femtosecond laser; 2. Beam splitting module; 3. Delay module; 4. Chopper; 5. Beam combining module; 6. Sample stage; 7. Electrical measurement module; 8. Phase-locked amplifier. DETAILED DESCRIPTION

[0044] like Figure 2 As shown, a transient photocurrent measurement system for optoelectronic materials includes: a dual-pulse femtosecond laser detection system and a transient photocurrent high-resolution acquisition system; the dual-pulse femtosecond laser detection system is used to output two beams of femtosecond pulse light with adjustable timing, which are used as pump light and detection light respectively, and the pump light is irradiated on the optoelectronic material, and the detection light is irradiated on the ultrafast optical switch material, and the optical path difference between the two beams of light reaching the sample is adjusted; the transient photocurrent high-resolution acquisition system is used to extract the transient photocurrent of the optoelectronic material with picosecond time resolution.

[0045] The dual-pulse femtosecond laser detection system includes: a femtosecond laser, a beam splitting module, a delay module, a chopper and a beam combining module; the femtosecond laser generates laser pulses, which are divided into two pulsed lights, namely pump light and detection light, through the beam splitting module; the pump light passes through the delay module to control the optical path difference between the pump light and the detection light; then the pump light passes through the chopper, which is used to frequency modulate the pump light so that the subsequent signal acquisition system can extract specific frequency signals; the beam combining module is used to combine the pump light and the detection light, and then collimate them on the photoelectric material and the ultrafast optical switch material respectively. The delay module includes a precision electric control displacement stage, an optical reflector and a plurality of reflector groups, which can realize the transient characteristic measurement of the photocurrent generated by the photoelectric material by adjusting the time interval between the two pulsed lights.

[0046] The transient photocurrent high-resolution acquisition system includes: a device to be tested and a sample stage, an electrical measurement module, and a phase-locked amplifier; the device to be tested and the sample stage use a lead wiring machine to electrically connect the electrodes of the device to be tested to the electrical measurement base; the electrical measurement module applies or tests voltage or current to the sample; the phase-locked amplifier inputs the trigger signal of the chopper module to the phase-locked amplifier as a reference signal, and at the same time connects the current output end to the input port of the phase-locked amplifier to collect a current signal with the same frequency as the chopper modulation signal.

[0047] The device to be tested comprises an insulating substrate, an ultrafast optical switch material, a photoelectric material, a current output terminal, a ground terminal and a bias voltage terminal; the insulating substrate is made of an insulating high-resistance material, such as sapphire, quartz or a high-resistance silicon wafer with an oxide layer on the surface; the ultrafast optical switch material is low-temperature gallium arsenide, which is gallium arsenide grown by a molecular beam epitaxy method under the condition of 250-350° C. The photogenerated carrier lifetime of the low-temperature gallium arsenide is 0.5-2ps, the length and width of the low-temperature gallium arsenide small cube ranges from 10μm to 100μm, and the thickness is 0.5μm to 2μm; the photoelectric material is graphene, the length and width of the graphene ranges from 10μm to 100μm, and the thickness is less than 2μm; the interval between the current output terminal and the ground terminal is 2-20μm, the interval between the ground terminal and the bias voltage terminal is 2-20μm, and the interval between the ground terminal and the bias voltage terminal is 2-20μm.

[0048] Accordingly, a method for measuring transient photocurrent of a photoelectric material comprises the following steps:

[0049] Step 1: Using micro-nano processing technology to manufacture a device under test with a specific electrode configuration;

[0050] Step 2: fix the device under test on the electrical test base of the sample stage, and establish an electrical connection between the device under test and the electrical test base of the sample stage; establish an electrical connection between the test base and the lock-in amplifier and the electrical measurement module;

[0051] Step 3, using a femtosecond laser to generate a femtosecond pulse laser; splitting it into a pump beam and a probe beam through a beam splitter module; irradiating the probe beam to the low-temperature gallium arsenide center of the device under test; using the chopper and delay module to work together to modulate the pump beam to a specific frequency, and using the beam combiner module to allow the pump beam to irradiate the test area of ​​the device under test;

[0052] Step 4: Use the pump beam to act on the device under test to generate transient photocurrent. At the same time, use the detection beam to irradiate the low-temperature gallium arsenide. A single pulse of the detection beam will create a 1-2ps path between the ground terminal and the current output terminal. Use the delay module to accurately control the time difference between the pump beam and the detection beam reaching the sample, and measure the photocurrent passing through the loop at each specific moment before and after the excitation light. Draw a curve of the transient photocurrent of the device under test changing with time.

[0053] In step 1, the micro-nano processing technology is used to manufacture a device under test with a specific electrode configuration, which specifically includes the following steps:

[0054] Step 11: grow low-temperature gallium arsenide / aluminum arsenide on a gallium arsenide substrate using a molecular beam epitaxy method, with the thickness of the three components being 2 μm, 0.5 μm, and 625 μm from top to bottom; prepare a corrosion-resistant photoresist with a length and width of about 100 μm and a thickness of about 2 μm on the surface of the low-temperature gallium arsenide using a laser micro-nano processing process; selectively wet-etch the low-temperature gallium arsenide / aluminum arsenide using a hydrogen peroxide / citric acid mixed solution and a dilute hydrochloric acid solution, respectively, and wash off the photoresist with acetone to obtain small low-temperature gallium arsenide cubes with a length and width of about 100 μm, which are loosely stacked on the gallium arsenide substrate;

[0055] Step 12: Use a diamond knife to cut a high-resistance silicon wafer with a 285nm silicon oxide layer into a square with a length and width of about 1 cm as a substrate;

[0056] Step 13: Using a mechanical peeling method, tear off the graphene with dimethylsiloxane (PDMS), and observe under a microscope to find the graphene with a length and width of about 20 μm; using a high-precision fixed-point transfer platform, transfer the graphene material to the center of the substrate;

[0057] Step 14: Use dimethylsiloxane (PDMS) to adhere the low-temperature gallium arsenide small cubes deposited on the gallium arsenide substrate, and use a high-precision fixed-point transfer platform to transfer the low-temperature gallium arsenide small cubes to the vicinity of the graphene prepared in step 13, ensuring that the center distance between the graphene and the low-temperature gallium arsenide small cubes is about 120 μm;

[0058] Step 15: Spin-coat a photoresist with a thickness of about 2 μm on the sample prepared in step 14, and then use laser micro-nano processing technology to etch the area at a fixed point. After development, the photoresist in this area is washed off; use thermal vacuum coating technology to evaporate 300 nm of gold on the sample, and then soak the gold-plated sample in acetone solution for more than 10 minutes, and the gold in the photoresist-covered area falls off.

[0059] In step 2, the connection method is to connect the bias voltage terminal to the voltage output terminal of the electrical measurement module, connect the ground terminal to the ground terminal of the electrical measurement module, and connect the current output terminal to the lock-in amplifier, which specifically includes the following steps:

[0060] Step 21, adhere the sample to the electrical measuring base of the sample stage through silver glue, and then use a wire bonding machine to weld each electrode of the sample to the electrical measuring base through wires;

[0061] Step 22, using a BNC transmission line, connect the BNC interface of the electrical measurement base to the electrical measurement module and the phase-locked amplifier; the connection method is: the current output end is connected to the electrical measurement module; the ground end is connected to the ground end of the electrical measurement module;

[0062] Step 23, using an electrical measurement module, testing the resistance of the low-temperature gallium arsenide in the presence and absence of light to ensure that the switching ratio is greater than ten to the fifth power;

[0063] Step 24, using a BNC transmission line, connect the BNC interface of the electrical measurement base to the electrical measurement module and the phase-locked amplifier; the connection method is that the current output end is connected to the phase-locked amplifier; the ground end is connected to the ground end of the electrical module; the bias voltage end is connected to the electrical measurement module;

[0064] Step 25: Use the electrical measurement module to test the volt-ampere characteristic curve of the graphene device under a bias voltage of -0.5V to 0.5V to ensure that the resistance of the graphene is below the kilo-ohm level.

[0065] In step 4, drawing a curve graph of the transient photocurrent of the device under test changing with time specifically includes the following steps:

[0066] Step 41: Start the femtosecond laser and wait for it to be preheated. After ensuring that the laser is running stably, turn on the shutter switch of the laser to make the laser emit laser light and set the wavelength to 800nm.

[0067] Step 42: Use a reflector in front of the sample stage to modulate the positions of the pump beam and the probe beam reaching the sample, ensuring that the pump beam and the probe beam are irradiated on the graphene and the low-temperature gallium arsenide, respectively;

[0068] Step 43: Use the electrical measurement module to apply a 0.1V bias voltage to both ends of the graphene, use the reflector to fine-tune the position where the excitation light irradiates the graphene, and find the state where the graphene photocurrent is the largest;

[0069] Step 44: Using the time zero position recorded when building the optical path, move the delay line so that it reaches the vicinity of the position corresponding to time zero; use a computer program to control the delay line to move back and forth at this point, and read the phase-locked amplifier reading in real time to find a position where the transient photocurrent is the largest, and record this position as time zero. If the position is not found, please use the method described below to find the time zero again; move the delay line device so that the two beams of light reach the sample area at the same time. At this time, the position of the delay line device corresponds to the time zero. The method of finding the time zero includes: using the precise measurement of the optical path, respectively measuring the optical path of the two beams of light from the beam splitter at the separation to the sample stage, so as to calculate the time zero when the delay line reaches a certain position; using the second-order nonlinear effect of the frequency doubling crystal (for example, α-phase barium borate crystal (α-BBO)), place α-BBO on the sample stage to ensure that the position of the excitation light on the sample stage remains unchanged when the delay line moves, and then modulate the reflector in front of the sample stage so that the two beams of light completely overlap in space and shine on α-BBO, and then move the delay line back and forth. When the brightness of the doubled frequency light generated by α-BBO is the largest, the position of the delay line is the time zero. The approximate position of the time zero is determined when the optical path is built. The delay line needs to be fine-tuned each time the test is performed to verify the time zero;

[0070] Step 45: Use software to control the delay line to move in sequence within the time range of -10 to 50 ps (0 ps corresponds to time zero, -10 ps corresponds to the time when the excitation light reaches the sample 10 ps later than the detection light, and 50 ps corresponds to the time when the excitation light reaches the sample 50 ps earlier than the detection light) with an accuracy of 0.1 ps, and use software to communicate with the phase-locked amplifier to read the transient current value at each moment; plot the obtained data into a graph to obtain Figure 3 The experimental results shown can be used to analyze the response time of the graphene device and study the device bandwidth.

[0071] like Figure 1 As shown, an innovative electrode structure for a transient photocurrent measurement system is disclosed, which mainly includes: a bias voltage terminal connected to a voltage output terminal of an electrical measurement module; a ground terminal connected to a ground terminal of the electrical measurement module; a current output terminal connected to a lock-in amplifier; Figure 1 The solid black line frame in the middle represents low-temperature GaAs, the dashed line frame represents the measured sample, and the dotted line frame represents the high-frequency probe area.

[0072] When only the pump beam is irradiated to the sample and the probe beam is not irradiated to the low-temperature gallium arsenide: the resistance of the low-temperature gallium arsenide is greater than 10 10 Ω, the circuit between the current output terminal and the ground terminal is open, and the photocurrent generated by the measured sample all flows out through the ground terminal.

[0073] When the pump beam irradiates the sample and the probe light irradiates the low-temperature gallium arsenide, the resistance of the low-temperature gallium arsenide is less than 10 2 Ω, a path is formed between the ground terminal and the current output terminal. When the excitation light irradiates the sample, part of the photocurrent generated by the measured sample will flow from the measured sample through the low-temperature gallium arsenide into the phase-locked amplifier (current output terminal) and then be collected by the phase-locked amplifier. At this time, the magnitude of the transient photocurrent at time zero is measured.

[0074] When the pump beam irradiates the sample, the probe light irradiates the low-temperature gallium arsenide after a delay of n ps: the resistance of the low-temperature gallium arsenide is less than 10 2 Ω, a path is formed between the ground terminal and the current output terminal. After the excitation light irradiates the sample, at an interval of n ps, part of the photocurrent generated by the measured sample will flow from the measured sample through the low-temperature gallium arsenide into the phase-locked amplifier (current output terminal) and then be collected by the phase-locked amplifier. The magnitude of the transient photocurrent at n ps is measured.

[0075] Based on the above mechanism, the transient photocurrent magnitude at different times after the sample is photoexcited can be collected, and the more photocurrent flows from the ground end to the current output end, the stronger the collected signal. Figure 1 The symmetrical electrode shown in the figure has an electrode impedance at the photoelectric material area that is equal to the electrode impedance at the low-temperature gallium arsenide, thereby maximizing the effective transmission power. At the same time, the center distance between the low-temperature gallium arsenide and the measured sample is as small as possible (120μm is the minimum distance to avoid overlap between the pump beam and the detection beam), which can reduce the loss of photocurrent flowing from the ground end to the current output end. On the other hand, the length of the ground end is as long as possible because when the photocurrent propagates in the horizontal direction of the ground end, there will be partial current signal reflection when it reaches the left and right ends. Extending the electrode helps to make the reflected signal attenuate and then disappear during the propagation process. Transient photocurrent test is performed on the graphene photodetector using a transient photocurrent measurement system.

[0076] Preparation with Figure 1 The preparation process of the device to be tested of the structure shown includes: using a two-dimensional material dry transfer process, transferring the low-temperature gallium arsenide sample with a length and width range of 10μm to 100μm and a thickness of 0.5μm to 2μm on a high-resistance silicon substrate with a 285nm silicon oxide layer and a length and width of about 1cm; and transferring the photoelectric material with a length and width range of 10μm to 100μm and a thickness of less than 2μm, and keeping the center distance between the low-temperature gallium arsenide and the tested sample at 120μm; using an electrode preparation process to prepare Figure 1 The shape and thickness of gold are 100nm-300nm as electrodes.

[0077] The preparation process of low-temperature gallium arsenide of suitable size includes: growing low-temperature gallium arsenide / aluminum arsenide on a gallium arsenide substrate by molecular beam epitaxy (the thickness of the three components from top to bottom is 0.5μm-2μm, 0.5μm-2μm, and about 625μm, respectively); and preparing a corrosion-resistant photoresist with a length and width of about 100μm and a thickness of about 2μm on the surface of the low-temperature gallium arsenide by laser micro-nano processing technology; wet selectively etching the low-temperature gallium arsenide / aluminum arsenide by hydrogen peroxide / citric acid mixed solution and dilute hydrochloric acid solution, respectively, and washing off the photoresist with acetone to obtain low-temperature gallium arsenide blocks with a length and width of about 100μm, which are loosely stacked on the gallium arsenide substrate; and transferring the small gallium arsenide blocks to the substrate by dry transfer technology.

[0078] The preparation process of the measured material of suitable size includes: using dry and wet transfer processes to first prepare the measured material of suitable size onto a solid material with suitable viscosity and hardness (such as dimethylsiloxane (PDMS)); then using a two-dimensional material transfer platform to transfer the measured material to the position mentioned above on the high-resistance silicon substrate of the 285nm silicon oxide layer.

[0079] The process of preparing the metal electrode includes: firstly, spin coating a photoresist with a thickness of about 2 μm on the sample, and then etching the sample using laser micro-nano processing technology. Figure 1 The photoresist in the area shown in FIG. 1 is washed off after development; using the thermal vacuum coating technology, 100nm-300nm of gold is evaporated on the processed sample, and then the gold-plated sample is immersed in an acetone solution for about 10 minutes to form the following Figure 1 Electrodes of the shape shown.

[0080] The electrical connection method between the device to be tested and the electrical test base of the sample stage and the electrical connection method between the electrical test base and the phase-locked amplifier and the electrical measurement module include: firstly adhering the sample to the electrical test base of the sample stage by silver glue, and then using a wire bonding machine to weld each electrode of the sample to the electrical test base by wires; using a BNC transmission line to connect the BNC interface of the electrical test base to the electrical measurement module and the phase-locked amplifier; the connection method is: the current output end is connected to the phase-locked amplifier; the ground end is connected to the ground end of the electrical measurement module; the bias voltage end is connected to the electrical measurement module.

[0081] The method for testing the time difference between the pump light beam and the detection light beam arriving at the sample area includes: first, it is necessary to find the position of the delay line device when the two light beams arrive at the sample area at the same time. This moment can be called the equal optical path point or the time zero point. The method for finding the time zero point includes:

[0082] By using the precise measurement of the optical path, the optical path of the two beams of light from the beam splitter at the separation point to the sample stage is measured respectively, so as to calculate the time zero when the delay line reaches a certain position; by using the second-order nonlinear effect of the frequency doubling crystal (for example, α-phase barium borate crystal (α-BBO) is selected), the α-BBO is placed on the sample stage to ensure that when the delay line moves, the position of the excitation light on the sample stage remains unchanged, and then the reflector in front of the sample stage is modulated to make the two beams of light completely overlap in space and shine on the α-BBO, and then the delay line is moved back and forth. When the brightness of the doubled frequency light generated by the α-BBO is the largest, the position of the delay line is the time zero;

[0083] After finding the time zero, record the delay line position corresponding to the time zero, calculate the change in optical path difference caused by the movement of the delay line device, and divide the optical path difference by the speed of light to get the time difference caused by the delay line device moving a certain distance.

[0084] The software is used to control the delay line and thus the current test moment, and the software is used to communicate with the phase-locked amplifier to read the transient current value at each moment; the obtained data is plotted into a graph and the response speed of the transient photocurrent of the device is analyzed.

[0085] The software is used to control the delay line and thus the current test moment, and the software is used to communicate with the phase-locked amplifier to read the transient current value at each moment; the obtained data is plotted into a graph to obtain the transient photocurrent of the device, and the working mechanism of the device is analyzed.

Claims

1. A transient photocurrent measurement system for photoelectric materials, characterized in that: include: Dual-pulse femtosecond laser detection system and transient photocurrent high-resolution acquisition system; the dual-pulse femtosecond laser detection system is used to output two beams of timing-adjustable femtosecond pulse light, which serve as pump light and detection light respectively, irradiate the pump light on the optoelectronic material, irradiate the detection light on the ultrafast optical switch material, and regulate the optical path difference between the two beams of light reaching the sample; the transient photocurrent high-resolution acquisition system is used to extract the transient photocurrent of the optoelectronic material with picosecond time resolution.

2. The transient photocurrent measurement system for optoelectronic materials according to claim 1, characterized in that: The double-pulse femtosecond laser detection system comprises: a femtosecond laser (1), a beam splitting module (2), a delay module (3), a chopper (4) and a beam combining module (5); the femtosecond laser (1) generates a laser pulse, which is divided into two beams of pulse light through the beam splitting module (2), namely a pump light and a detection light; the pump light passes through the delay module (3) to control the optical path difference between the pump light and the detection light; then the pump light passes through the chopper (4), which is used to perform frequency modulation on the pump light so that a subsequent signal acquisition system can extract a specific frequency signal; the beam combining module (5) is used to combine the pump light and the detection light, and then collimate and irradiate them onto a photoelectric material and an ultrafast optical switch material respectively.

3. The transient photocurrent measurement system for optoelectronic materials according to claim 2, characterized in that: The delay module (3) comprises a precision electric-controlled displacement stage, an optical reflector and a plurality of reflector groups, and realizes the transient characteristic measurement of the photocurrent generated by the photoelectric material by adjusting the time interval between two beams of pulsed light.

4. The transient photocurrent measurement system for optoelectronic materials according to claim 1, characterized in that: The transient photocurrent high-resolution acquisition system comprises: a device to be tested and a sample stage (6), an electrical measurement module (7), and a phase-locked amplifier (8); the device to be tested and the sample stage (6) use a lead wiring machine to electrically connect the electrodes of the device to be tested to an electrical measurement base; the electrical measurement module (7) applies or tests a voltage or current to the sample; the phase-locked amplifier (8) inputs a trigger signal of a chopper module to the phase-locked amplifier as a reference signal, and simultaneously connects a current output end to an input port of the phase-locked amplifier to acquire a current signal having the same frequency as the chopper modulation signal.

5. The transient photocurrent measurement system for optoelectronic materials according to claim 4, characterized in that: The device to be tested includes an insulating substrate, an ultrafast optical switch material, a photoelectric material, a current output terminal, a ground terminal and a bias voltage terminal; the insulating substrate is made of insulating high-resistance material, such as sapphire, quartz or a high-resistance silicon wafer with an oxide layer on the surface; the photogenerated carrier lifetime of the optical switch material is 0.5-2ps, the length and width of the optical switch material ranges from 10μm to 100μm, and the thickness is 0.5μm to 2μm; the length and width of the photoelectric material ranges from 10μm to 100μm, and the thickness is less than 2μm; the interval between the current output terminal and the ground terminal is 2-20μm, the interval between the ground terminal and the bias voltage terminal is 2-20μm, and the interval between the ground terminal and the bias voltage terminal is 2-20μm.

6. A measurement method applied to the transient photocurrent measurement system for photoelectric materials as claimed in claim 1, characterized in that: The steps include: Step 1: Using micro-nano processing technology to manufacture a device under test with a specific electrode configuration; Step 2, fix the device under test on the electrical test base of the sample stage (6), and establish an electrical connection between the device under test and the electrical test base of the sample stage (6); establish an electrical connection between the test base and the lock-in amplifier (8) and the electrical measurement module (7); Step 3, using a femtosecond laser (1) to generate a femtosecond pulse laser; using a beam splitting module (2) to split the laser into a pump beam and a probe beam; irradiating the probe beam onto the center of the ultrafast optical switch material of the device to be tested; using a delay module (3) and a chopper (4) to coordinately modulate the pump beam at a specific frequency, and using a beam combining module (5) to allow the pump beam to irradiate the region of the optoelectronic material to be tested; Step 4: Use a pump beam to act on the device under test to generate a transient photocurrent; at the same time, use a detection beam to irradiate the ultrafast optical switch material, and a single pulse of the detection beam will cause a 1-2ps path to occur between the ground terminal and the current output terminal; use a delay module (3) to accurately control the time difference between the pump beam and the detection beam reaching the sample, and measure the photocurrent passing through the loop at each specific moment before and after the excitation light is excited; and draw a curve graph of the transient photocurrent of the device under test changing with time.

7. The method for measuring transient photocurrent of photoelectric materials according to claim 6, characterized in that: In step 1, the micro-nano processing technology is used to manufacture a device under test with a specific electrode configuration, which specifically includes the following steps: Step 11: Use a diamond knife to cut a high-resistance silicon wafer with a 285nm silicon oxide layer into a square with a length and width of about 1 cm as a substrate; Step 12: preparing a photoelectric material to be tested with a length and width of about 20 μm; using a high-precision fixed-point transfer platform to transfer the photoelectric material to the center of the substrate; Step 13: preparing an ultrafast optical switch material with a length and width ranging from 10 μm to 100 μm and a thickness ranging from 0.5 μm to 2 μm, and using a high-precision fixed-point transfer platform to transfer the ultrafast optical switch material to the vicinity of the optoelectronic material prepared in step 12, ensuring that the center distance between the optoelectronic material and the ultrafast optical switch material is about 120 μm; Step 14: Spin-coat a photoresist with a thickness of about 2 μm on the sample prepared in step 13, and then use laser micro-nano processing technology to etch the area at a fixed point. After development, the photoresist in this area is washed off; use thermal vacuum coating technology to evaporate 300 nm of gold on the sample, and then soak the gold-plated sample in acetone solution for more than 10 minutes, and the gold in the photoresist-covered area falls off.

8. The method for measuring transient photocurrent of photoelectric materials according to claim 6, characterized in that: In step 2, the connection method is to connect the bias voltage terminal to the voltage output terminal of the electrical measurement module (7), connect the ground terminal to the ground terminal of the electrical measurement module (7), and connect the current output terminal to the lock-in amplifier (8), which specifically includes the following steps: Step 21, adhere the sample to the electrical measurement base of the sample stage (6) by means of silver glue, and then use a wire bonding machine to weld each electrode of the sample to the electrical measurement base by means of wires; Step 22, using a BNC transmission line, connecting the BNC interface of the electrical measurement base to the electrical measurement module (7) and the lock-in amplifier (8); connecting the current output end to the electrical measurement module (7), and connecting the ground end to the ground end of the electrical measurement module (7); Step 23, using the electrical measurement module (7), testing the resistance of the ultrafast optical switch material in the presence and absence of light to ensure that the switch ratio is greater than ten to the fifth power; Step 24, using a BNC transmission line, connecting the BNC interface of the electrical measurement base to the electrical measurement module (7) and the phase-locked amplifier (8); the connection method is that the current output end is connected to the phase-locked amplifier (8), the ground end is connected to the ground end of the electrical measurement module (7), and the bias voltage end is connected to the electrical measurement module (7); Step 25: Use the electrical measurement module (7) to test the volt-ampere characteristic curve of the photoelectric material to ensure that it works properly.

9. The method for measuring transient photocurrent of photoelectric materials according to claim 6, characterized in that: In step 4, drawing a curve graph of the transient photocurrent of the device under test changing with time specifically includes the following steps: Step 41: Start the femtosecond laser and wait for it to be preheated. After ensuring that the laser is running stably, turn on the shutter switch of the laser to make the laser emit laser light and set the wavelength to 800nm. Step 42: Use the reflector in front of the sample stage (6) to modulate the positions of the pump beam and the probe beam reaching the sample, ensuring that the pump beam and the probe beam are irradiated on the optoelectronic material and the ultrafast optical switch material respectively; Step 43: using the electrical measurement module (7) to apply bias voltage to both ends of the photoelectric material, using a reflector to fine-tune the position where the excitation light irradiates the photoelectric material, and finding the state where the photocurrent is maximum; Step 44: using the time zero point position recorded when the optical path is constructed, move the delay line so that it reaches the vicinity of the position corresponding to the time zero point; control the delay line to move back and forth at the point, and read the reading of the phase-locked amplifier (8) in real time, find a position where the transient photocurrent is the largest, and record the position as the time zero point; if the time zero point is not found, find the time zero point again, move the delay line device, so that the two beams of light arrive at the sample area at the same time, and the position of the delay line device at this time corresponds to the time zero point; the approximate position of the time zero point is determined when the optical path is constructed, and the delay line needs to be fine-tuned each time the test is performed to verify the time zero point; Step 45: Control the delay line to move sequentially within the time range of -10 to 50 ps with an accuracy of 0.1 ps, communicate with the phase-locked amplifier (8), and read the transient current value at each moment; plot the obtained data into a graph, and use the experimental data to analyze the response time of the optoelectronic material and study its bandwidth.

10. The method for measuring transient photocurrent of a photoelectric material according to claim 9, characterized in that: In step 44, the time zero point is found, and the optical path of the two beams of light from the beam splitter at the separation point to the sample stage is measured respectively by using the precise measurement of the optical path, so as to calculate the time zero point when the delay line arrives at a certain position; using the frequency doubling crystal, the second-order nonlinear effect of the α-phase barium borate crystal α-BBO is selected, and the α-BBO is placed on the sample stage to ensure that when the delay line moves, the position of the excitation light on the sample stage remains unchanged, and then the reflector in front of the sample stage is modulated to make the two beams of light completely overlap in space and shine on the α-BBO, and then the delay line is moved back and forth. When the brightness of the doubled frequency light generated by the α-BBO is the largest, the position of the delay line is the time zero point.

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