Photo-generated carrier transient transport plotting system and method with high temporal-spatial resolution

Abstract

The invention discloses a photo-generated carrier transient transport plotting system and method with high time-space resolution, and the method comprises the steps: generating two beams of femtosecond laser by a femtosecond laser module, and respectively taking the two beams of femtosecond laser as pumping laser and detection laser; pump laser enters the microscopic module through the pump light frequency doubling module, the delay line module and the first chopping module. The pumping laser enters a first chopping module, the detection laser enters a detection light scanning module through a second chopping module, space scanning control is carried out on the detection laser, then the pumping laser and the detection laser roughly coincide at the microscopic module and then enter a sample photoelectric measurement base module, and the detection laser enters a signal acquisition module after being reflected by a sample. According to the invention, the transient transport process of the current carrier can be represented, the dynamic process of the current carrier can be clarified, key factors influencing the performance of the device can be analyzed, and theoretical and experimental guidance can be provided for the performance improvement of the photoelectric device.

Description

Technical Field

[0001] The present invention relates to the technical field of optical nondestructive testing, and in particular to a system and method for imaging transient transport of photogenerated carriers with high temporal and spatial resolution. Background Art

[0002] Optoelectronic devices are the core components for realizing silicon photonic integration, optical communication and optical computing. With the development of the information age, key core technologies of high-performance optoelectronic devices, such as high-frequency and high-efficiency light sources, high-frequency and low-power modulation, and on-chip high-speed light detection, are in urgent need of breakthroughs. Under the action of external electric fields or current injection, transient carrier transport processes such as carrier injection, migration, interface state gain and defect capture will occur during the operation of optoelectronic devices. Therefore, the key factor affecting the performance of optoelectronic devices is the transport process of photogenerated carriers in optoelectronic devices, especially the transient microscopic transport process. Although the Haynes-Shockley experiment can obtain physical quantities such as the diffusion constant and mobility of photogenerated carriers, these results are derived by characterizing macroscopic quantities such as current or voltage. There are problems such as the inability to directly characterize the transient motion trajectory of photogenerated carriers, low temporal resolution and insufficient spatial resolution, which greatly limits the further understanding of the transport properties of photogenerated carriers in optoelectronic devices. In recent years, although pump-probe technology can be used to measure the microscopic diffusion motion of photogenerated carriers, such as studying the influence of defects on the diffusion motion of photogenerated carriers (Adv. Mater. 2020, 32, 1906540) and the diffusion motion of hot carriers (Adv. Sci. 2022, 9, 2105746), there is still no technical method to directly measure the transient drift motion of photogenerated carriers. Therefore, there is an urgent need for a new characterization technology with high temporal and spatial resolution that can characterize the transient transport process of carriers, clarify its dynamic process, and then analyze the key factors affecting device performance, providing theoretical and experimental guidance for the performance improvement of optoelectronic devices. Summary of the invention

[0003] Purpose of the invention: The present invention provides a system and method for imaging the transient transport of photogenerated carriers with high temporal and spatial resolution, which can characterize the transient transport process of carriers, clarify its dynamic process, and then analyze the key factors affecting device performance, providing theoretical and experimental guidance for improving the performance of optoelectronic devices.

[0004] Technical solution: The high-time-space resolution photogenerated carrier transient transport imaging system described in the present invention includes: a femtosecond laser module, a pump light frequency doubling module, a delay line module, a pump light chopping module, a microscope module, a detection light chopping module, a detection light scanning module, a sample photoelectric measurement base module, and a signal acquisition module; the femtosecond laser module generates two femtosecond lasers, which are used as a pump laser and a detection laser respectively; the pump laser enters the microscope module through the pump light frequency doubling module, the delay line module and the first chopping module; the detection laser enters the detection light scanning module through the second chopping module, and the detection laser is spatially scanned and controlled, and then the pump laser and the detection laser roughly overlap at the microscope module and enter the sample photoelectric measurement base module, wherein the detection laser enters the signal acquisition module after being reflected by the sample.

[0005] Furthermore, the femtosecond laser module includes a femtosecond laser, a beam splitter, a first reflector and an optical parametric oscillator; the femtosecond laser output by the femtosecond laser is divided into two laser beams after passing through the beam splitter, one laser beam enters the pump light frequency doubling module and then enters the pump light path; the other laser beam enters the optical parametric oscillator through the first reflector to generate a laser of a specific wavelength in the visible to near-infrared band, and then enters the detection light path.

[0006] Furthermore, the pump light frequency doubling module includes a first convex lens, a frequency doubling crystal, a second convex lens and a filter, which are used to generate a short-wavelength pump laser. The first convex lens and the second convex lens with the same focal length are in a coaxial confocal relationship, and the confocal point is on the frequency doubling crystal. The pump fundamental frequency light coming out of the beam splitter is focused by the first convex lens to increase the power density, and then reaches the frequency doubling crystal. The second-order nonlinear effect of the frequency doubling crystal is used to generate a doubled frequency pump laser, which is then converted into parallel light by the second convex lens, and finally the original fundamental frequency light is filtered out by the short-pass filter.

[0007] Furthermore, the delay line module includes an optical reflector and an electrically controlled translation stage. After the pump laser enters the optical reflector, it will return along the original path. At the same time, the optical reflector can move linearly along the direction parallel to the optical axis of the pump light along the electrically controlled translation stage, thereby changing the optical path size of the pump light path, realizing the control of the optical path difference between the detection laser pulse and the pump laser pulse, and then adjusting the time difference between them arriving at the sample.

[0008] Furthermore, the pump light chopper module includes a third convex lens, a first optical chopper and a fourth convex lens, which are used to modulate the pump laser and output the modulation frequency; the third convex lens and the fourth convex lens are in a coaxial confocal relationship, and the confocal point is near the first optical chopper. The pump laser, which is parallel light, is reflected to the third convex lens by the second reflector and the third reflector for focusing, and the focus is at the blade of the first optical chopper to reduce the beam diameter and avoid the influence of uneven chopping of a large beam diameter, and then it is transformed back into parallel light by the fourth convex lens.

[0009] Furthermore, the microscopy module includes a long-wave dichroic mirror and an objective lens for microscopic characterization of the sample; the dichroic mirror is used to reflect the shorter wavelength pump laser and transmit the longer wavelength detection laser. The pump laser reflected by the fourth reflector and the detection laser transmitted through the tube lens intersect at the dichroic mirror, and the long-wave dichroic mirror reflects the shorter wavelength pump laser and transmits the longer wavelength detection laser, and then the two laser beams are incident on the objective lens together and reach the sample photoelectric base module.

[0010] Furthermore, the detection light chopping module includes a second optical chopper, and the detection laser generated by the femtosecond laser module enters the detection light chopping module through a fifth reflector to be modulated, and the modulation frequency is output.

[0011] Furthermore, the detection light scanning module includes a galvanometer, a scanning mirror and a barrel lens, which are used to control the detection light beam to scan the detection light point by point in space on the sample surface. The galvanometer is composed of a stepper motor and two axially perpendicular mirrors, which can control the angle of light emission. The telecentric design of the scanning mirror has a flat image plane, which is used for geometric correction of the galvanometer to ensure a high scanning resolution; the scanning mirror and the barrel lens are coaxially confocal to generate collimated parallel light, and then the parallel detection laser passes through the beam splitter and enters the microscope module to reach the sample surface. The deflection angle of the two mirrors in the galvanometer is controlled by the industrial computer, so that the detection laser can perform point-by-point closed-loop scanning and imaging with equal steps on the sample surface.

[0012] Furthermore, the sample photoelectric base module includes a photoelectric measurement base, a three-axis translation stage and an electrical source meter. The photoelectric measurement base is fixed on the three-axis translation stage and is used to place the sample of the device to be tested. At the same time, an electrical connection port is reserved on the base, which can be connected to the electrical source meter to apply bias or gate voltage to the sample.

[0013] Furthermore, the signal acquisition module includes a second beam splitter, a sixth reflector, a filter, a balanced photodetector, a first lock-in amplifier and a second lock-in amplifier; the detection laser reflected by the sample will be reflected by the second beam splitter to the sixth reflector again, then enter the filter to filter out the short-wavelength pump laser, and finally reach the balanced photodetector for light signal acquisition; the input signal of the first lock-in amplifier is the RF output signal of the balanced photodetector, and the reference signal is the modulation frequency output by the pump light chopping module; the input signal of the second lock-in amplifier is the direct output signal of the balanced photodetector, and the reference signal is the modulation frequency output by the detection light chopping module. The first lock-in amplifier and the second lock-in amplifier can respectively acquire the differential reflection signal and the absolute reflection signal of the detection laser.

[0014] Accordingly, a method for imaging transient transport of photogenerated carriers with high temporal and spatial resolution comprises the following steps:

[0015] Step 1: System initialization configuration: Turn on the system power and start each module; set the target wavelength on the optical parametric oscillator controller according to the sample and measurement requirements; set the appropriate modulation frequency on the pump light chopper module and the detection light chopper module respectively; insert the sample to be measured into the photoelectric measurement base and ensure good contact; at this time, if there is no obvious deviation in the optical path, the pump laser and the detection laser spot should be at the same position on the sample surface;

[0016] Step 2: Configure the parameters of the delay line module, the detection light scanning module and the signal acquisition module on the industrial computer: set the measurement step and range of the delay line module; set the scanning imaging range and step of the galvanometer in the detection light scanning module; set the time constant, phase, sensitivity and other sampling parameters of the first and second lock-in amplifiers in the signal acquisition module;

[0017] Step 3: Set the sample photoelectric measurement base module on the industrial computer to put the sample in an electrical working state;

[0018] Step 4, use the measurement program in the industrial computer to start the delay line module and the detection light scanning module, and the measurement program will automatically perform a mapping characterization of the photogenerated carrier transport process of the sample according to the setting parameters in step 2; the measurement program controls the position of the electric displacement stage in the delay line module, thereby changing the optical path of the detection light path, so that the detection laser pulse is delayed relative to the pump laser pulse to reach the sample surface; every time the electric displacement stage moves to a certain position, the measurement program controls the detection light scanning module to make the detection light perform a point-by-point closed-loop scan with equal steps near the pump light on the sample surface, and then use the signal acquisition module to record the differential reflection signal at different spatial positions of the detection light, so as to obtain the transient distribution of photogenerated carriers of the sample to be tested under an external bias or gate voltage; when the delay line module scans, the transient transport distribution of the photogenerated carriers at each delay time can be obtained, and the transport mapping is completed;

[0019] Step 5: Record and save the collected scanning image data.

[0020] Furthermore, in step 3, a sample photoelectric measurement base module is set on the industrial computer to put the sample in an electrical working state, and the bias value or gate voltage value of the electrical source meter is set according to measurement requirements; the sample external bias, sample current, leakage current, etc. are monitored by the electrical source meter to ensure that the sample remains stable in the electrical working state.

[0021] Beneficial effects: Compared with the prior art, the present invention has the following significant advantages: (1) The present invention can not only measure the transient diffusion process of photogenerated carriers, but also measure the migration movement of photogenerated carriers under the control of external bias or gate voltage during the operation of the photoelectric detection device, and observe the real movement state of photogenerated carriers during the operation of the device; (2) The device operation process can be directly observed, and the microscopic motion trajectory of carriers can be observed in real time and intuitively by optical means; the time resolution is ≤1ps, and the pump detection technology can capture the transient phenomena in the transport process; the spatial resolution is ≤0.1μm, and the distribution of photogenerated carriers at the hundred-nanometer level can be measured, providing detailed information on the microscopic mechanism; (3) The transient transport movement of photogenerated carriers under the working state of the device can be measured, so as to identify the key factors affecting the device performance, and then optimize the design, improve the various indicators of the optoelectronic device, promote the development of new optoelectronic materials and device structures, and enhance the basic research of semiconductor physics, providing a new perspective for the development of related theories. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 It is a schematic diagram of the system structure of the present invention.

[0023] Figure 2 This is a schematic diagram of the structure of the two-dimensional material photoelectric detector of the present invention.

[0024] Figure 3 This is a distribution diagram of the photogenerated carriers of the present invention at different times.

[0025] Figure 4 This is a data diagram of the drift motion of photogenerated carriers in the present invention that changes with time when driven by an electric field.

[0026] Among them, 1. femtosecond laser; 2. first beam splitter; 3. first reflector; 4. optical parametric oscillator; 5. first convex lens; 6. frequency doubling crystal; 7. second convex lens; 8. filter; 9. reflector; 10. electric translation stage; 11. second reflector; 12. third reflector; 13. third convex lens; 14. first chopper; 15. fourth convex lens; 16. fourth reflector; 17. long-wave pass dichroic mirror; 18. objective lens; 19. photoelectric measurement base; 20. three-axis translation stage; 21. electrical source meter; 22. fifth reflector; 23. second chopper; 24. galvanometer; 25. scanning mirror; 26. barrel lens; 27. second beam splitter; 28. sixth reflector; 29. ​​filter; 30. balanced photodetector; 31. first phase-locked amplifier; 32. second phase-locked amplifier; 33. industrial computer. DETAILED DESCRIPTION

[0027] like Figure 1As shown, a high-time and space resolution photogenerated carrier transient transport imaging system comprises: a femtosecond laser module I, a pump light frequency doubling module II, a delay line module III, a pump light chopping module IV, a microscope module V, a detection light chopping module VI, a detection light scanning module VII, a sample photoelectric measurement base module VIII, and a signal acquisition module IX; the femtosecond laser module I generates two femtosecond lasers, which are used as a pump laser and a detection laser respectively; the pump laser enters the microscope module V through the pump light frequency doubling module II, the delay line module III and the first chopping module IV; the detection laser enters the detection light scanning module VII through the second chopping module VI, and the detection laser is spatially scanned and controlled, and then the pump laser and the detection laser roughly overlap at the microscope module V and enter the sample photoelectric measurement base module VIII, wherein the detection laser enters the signal acquisition module IX after being reflected by the sample.

[0028] Accordingly, a method for imaging transient transport of photogenerated carriers with high temporal and spatial resolution comprises the following steps:

[0029] Step 1, system initialization configuration: turn on the system power and start each module; set the target wavelength on the optical parametric oscillator 4 controller according to the sample and measurement requirements; set the appropriate modulation frequency on the pump light chopping module IV and the detection light chopping module VI respectively; insert the sample to be measured into the photoelectric measurement base 19 and ensure good contact; at this time, if there is no obvious deviation in the optical path, the pump laser and the detection laser should hit the same position on the sample surface;

[0030] Step 2: Parameter configuration of the delay line module III, the detection light scanning module VII and the signal acquisition module IX is performed on the industrial computer 33: the measurement step and range of the delay line module III are set; the scanning imaging range and step of the galvanometer 24 in the detection light scanning module VII are set; the time constant, phase and sensitivity sampling parameters of the first lock-in amplifier 31 and the second lock-in amplifier 32 in the signal acquisition module IX are set;

[0031] Step 3, setting the sample photoelectric measurement base module VIII on the industrial computer 33 to put the sample in an electrical working state;

[0032] Step 4, use the measurement program in the industrial computer 33 to start the delay line module III and the detection light scanning module VII, and the measurement program will automatically perform a mapping characterization of the photogenerated carrier transport process of the sample according to the setting parameters in step 2; the measurement program controls the position of the electric displacement stage 9 in the delay line module III, thereby changing the optical path of the detection light path, so that the detection laser pulse is delayed relative to the pump laser pulse to reach the sample surface; every time the electric displacement stage 9 moves to a certain position, the measurement program controls the detection light scanning module VII, so that the detection light performs a point-by-point closed-loop scan with equal steps near the pump light on the sample surface, and then uses the signal acquisition module IX to record the differential reflection signal at different spatial positions of the detection light, so as to obtain the transient distribution of the photogenerated carriers of the sample to be tested under the external bias or gate voltage; when the delay line module III finishes scanning, the transient transport distribution of the photogenerated carriers at each delay time can be obtained, and the transport mapping is completed;

[0033] Step 5: Record and save the collected scanning image data.

[0034] Embodiment 1:

[0035] Measurement of transient transport of photogenerated carriers in optoelectronic devices driven by built-in electric fields:

[0036] Step 1: Start the femtosecond laser 1 and wait for it to be preheated to ensure that the laser operates stably. Open the shutter switch of the laser 1 to make the laser emit laser light;

[0037] Step 2: setting the target wavelength on the optical parametric oscillator 4 controller and turning on the optical output switch of the optical parametric oscillator 4 to generate a detection laser of the required wavelength;

[0038] Step 3: Place the photoelectric device to be tested on the photoelectric measurement base 19 and ensure good contact. Move the three-axis translation stage so that the excitation light and the detection light are irradiated on the material surface 0.5 μm away from the electrode, that is, the built-in electric field area formed by the contact between the material and the electrode. Optionally, the sample is as follows: Figure 2 The two-dimensional material optoelectronic device shown includes a SiO2 / Si substrate, a two-dimensional semiconductor material WS2, a source electrode, and a drain electrode. A wiring machine is used to connect the gold electrode to the base;

[0039] Step 4, configuring parameters of the delay line module III, the detection light scanning module VII and the signal acquisition module IX on the industrial computer 33;

[0040] Step 5, controlling the electrical source meter 21 to apply 0V gate voltage and 0V bias voltage to the photoelectric device to be tested, and measuring the drift motion of photogenerated carriers under this condition;

[0041] Step 6, starting the pump light chopping module IV and the detection light chopping module VI;

[0042] Step 7: The electric translation stage 10 in the delay line module III is controlled by the industrial computer 33 to scan, thereby changing the optical path difference between the excitation light and the detection light. When the two beams of light reach the sample surface at the same time, the balanced photodetector 30 can measure the maximum RF signal.

[0043] Step 8, control the detection light scanning module Ⅶ through the industrial computer 33, so that the detection light performs a point-by-point closed-loop scan with equal steps near the pump light on the sample surface, and the scanning step is 100nm. The differential signal of the detection light at different positions is recorded through the signal acquisition module Ⅸ to obtain the distribution of photogenerated carriers at the initial moment after the material is excited.

[0044] Step 9, the position of the electric displacement stage 9 in the delay line module III is controlled by the measurement program of the industrial computer 33, thereby changing the optical path of the detection light path, so that the detection laser pulse is delayed relative to the pump laser pulse to reach the sample surface; every time the electric displacement stage 9 moves to a certain position, the measurement program controls the detection light scanning module VII, so that the detection light performs a point-by-point closed-loop scan with equal steps near the pump light on the sample surface, and then uses the signal acquisition module IX to record the differential reflection signal at different spatial positions of the detection light to obtain the transient distribution of photogenerated carriers of the sample to be tested under the external bias or gate voltage. When the delay line module III finishes scanning, the transient transport distribution of photogenerated carriers at each delay time can be obtained, and the transport image is completed.

[0045] Figure 3 The figure shows the transient distribution of photogenerated carriers at different times in the optoelectronic device. In the absence of an external field, the photogenerated carriers should diffuse uniformly in the original position. However, driven by the electric field, the photogenerated carriers move in a directional manner. Specifically, the maximum values ​​of the photogenerated carriers at different times are at different positions and move in the same direction. This is the transient drift process of the photogenerated carriers. Figure 4 The maximum value of the photogenerated carrier distribution curve changes with time.

[0046] Embodiment 2:

[0047] Measurement of transient transport of photogenerated carriers driven by an external electric field between electrodes.

[0048] Step 1: Start the femtosecond laser 1 and wait for it to be preheated to ensure that the laser operates stably. Open the shutter switch of the laser 1 to make the laser emit laser light;

[0049] Step 2: setting the target wavelength on the optical parametric oscillator 4 controller and turning on the optical output switch of the optical parametric oscillator 4 to generate a detection laser of the required wavelength;

[0050] Step 3: Place the photoelectric device to be tested on the photoelectric measurement base 19 and ensure good contact. Move the three-axis translation stage so that the excitation light and the detection light are irradiated on the surface of the material, that is, the built-in electric field area formed by the contact between the material and the electrode. Optionally, the sample is as follows: Figure 2 The two-dimensional material optoelectronic device shown includes a SiO2 / Si substrate, a two-dimensional semiconductor material WS2, a source electrode, and a drain electrode. A wiring machine is used to connect the gold electrode to the base;

[0051] Step 4, configuring parameters of the delay line module III, the detection light scanning module VII and the signal acquisition module IX on the industrial computer 33;

[0052] Step 5: Control the electrical source meter 21 to apply a 0V gate voltage and a 1V bias voltage to the photoelectric device to be tested, and measure the transport movement of photogenerated carriers under this condition. Further, a 0V gate voltage and a -1V bias voltage may be applied.

[0053] Step 6, starting the pump light chopping module IV and the detection light chopping module VI;

[0054] Step 7: The electric translation stage 10 in the delay line module III is controlled by the industrial computer 33 to scan, thereby changing the optical path difference between the excitation light and the detection light. When the two beams of light reach the sample surface at the same time, the balanced photodetector 30 can measure the maximum RF signal.

[0055] Step 8, control the detection light scanning module Ⅶ through the industrial computer 33, so that the detection light performs a point-by-point closed-loop scan with equal steps near the pump light on the sample surface, and the scanning step is 100nm. The differential signal of the detection light at different positions is recorded through the signal acquisition module Ⅸ to obtain the distribution of photogenerated carriers at the initial moment after the material is excited.

[0056] Step 9, the position of the electric displacement stage 9 in the delay line module III is controlled by the measurement program of the industrial computer 33, so as to change the optical path of the detection light path, so that the detection laser pulse is delayed relative to the pump laser pulse to reach the sample surface; every time the electric displacement stage 9 moves to a certain position, the measurement program controls the detection light scanning module VII, so that the detection light performs a point-by-point closed-loop scan with equal steps near the pump light on the sample surface, and then uses the signal acquisition module IX to record the differential reflection signal at different spatial positions of the detection light to obtain the transient distribution of photogenerated carriers of the sample to be tested under the external bias or gate voltage. At the end of the scanning of the delay line module III, the transient transport distribution of the photogenerated carriers at each delay time can be obtained, and the transport image is completed. When the external electric field is the same as the built-in electric field, the migration of the photogenerated carriers is promoted. When the external electric field is opposite to the built-in electric field, the migration of the photogenerated carriers is suppressed.

[0057] Embodiment 3:

[0058] Measurement of transient transport process of photogenerated carriers under gate voltage regulation.

[0059] Step 1: Start the femtosecond laser 1 and wait for it to be preheated to ensure that the laser operates stably. Open the shutter switch of the laser 1 to make the laser emit laser light;

[0060] Step 2: setting the target wavelength on the optical parametric oscillator 4 controller and turning on the optical output switch of the optical parametric oscillator 4 to generate a detection laser of the required wavelength;

[0061] Step 3: Place the photoelectric device to be tested on the photoelectric measurement base 19 and ensure good contact. Move the three-axis translation stage so that the excitation light and the detection light are irradiated on the surface of the material, that is, the built-in electric field area formed by the contact between the material and the electrode. Optionally, the sample is as follows: Figure 2 The two-dimensional material optoelectronic device shown includes a SiO2 / Si substrate, a two-dimensional semiconductor material WS2, a source electrode, and a drain electrode. A wiring machine is used to connect the gold electrode to the base;

[0062] Step 4, configuring parameters of the delay line module III, the detection light scanning module VII and the signal acquisition module IX on the industrial computer 33;

[0063] Step 5: Control the electrical source meter 21 to apply -60V gate voltage and 0V bias voltage to the photoelectric device to be tested, and measure the drift motion of photogenerated carriers under this condition. Further, +60V gate voltage and 0V bias voltage can be applied, and the measurement under different bias voltages described in Example 1 can be carried out under different gate voltage conditions.

[0064] Step 6, starting the pump light chopping module IV and the detection light chopping module VI;

[0065] Step 7: The electric translation stage 10 in the delay line module III is controlled by the industrial computer 33 to scan, thereby changing the optical path difference between the excitation light and the detection light. When the two beams of light reach the sample surface at the same time, the balanced photodetector 30 can measure the maximum RF signal.

[0066] Step 8, control the detection light scanning module Ⅶ through the industrial computer 33, so that the detection light performs a point-by-point closed-loop scan with equal steps near the pump light on the sample surface, and the scanning step is 100nm. The differential signal of the detection light at different positions is recorded through the signal acquisition module Ⅸ to obtain the distribution of photogenerated carriers at the initial moment after the material is excited.

[0067] Step 9, the position of the electric displacement stage 9 in the delay line module III is controlled by the measurement program of the industrial computer 33, so as to change the optical path of the detection light path, so that the detection laser pulse is delayed relative to the pump laser pulse to reach the sample surface; every time the electric displacement stage 9 moves to a certain position, the measurement program controls the detection light scanning module VII, so that the detection light performs a point-by-point closed-loop scan with equal steps near the pump light on the sample surface, and then uses the signal acquisition module IX to record the differential reflection signal at different spatial positions of the detection light, so as to obtain the transient distribution of photogenerated carriers of the sample to be tested under the external bias or gate voltage. At the end of the scanning of the delay line module III, the transient transport distribution of the photogenerated carriers at each delay time can be obtained, and the transport image can be completed. The gate voltage can regulate the type of excess carriers in the material and the direction of the built-in electric field between the material and the electrode, thereby regulating the speed and direction of the migration of the photogenerated carriers.

Claims

1. A high-temporal-resolution photogenerated carrier transient transport imaging system, characterized in that: include: Femtosecond laser module (I), pump light frequency doubling module (II), delay line module (III), pump light chopping module (IV), microscope module (V), detection light chopping module (VI), detection light scanning module (VII), sample photoelectric measurement base module (VIII), signal acquisition module (IX); the femtosecond laser module (I) generates two femtosecond lasers, which are used as pump laser and detection laser respectively; the pump laser enters the microscope module (V) through the pump light frequency doubling module (II), the delay line module (III) and the first chopping module (IV); the detection laser enters the detection light scanning module (VII) through the second chopping module (VI), and the detection laser is spatially scanned point by point. Subsequently, the pump laser and the detection laser are roughly overlapped at the microscope module (V) and then enter the sample photoelectric measurement base module (VIII), wherein the detection laser enters the signal acquisition module (IX) after being reflected by the sample.

2. The high-temporal-resolution photogenerated carrier transient transport imaging system according to claim 1, characterized in that: The femtosecond laser module (I) comprises a femtosecond laser (1), a first beam splitter (2), a first reflector (3) and an optical parametric oscillator (4); the femtosecond laser output by the femtosecond laser (1) is divided into two laser beams after passing through the first beam splitter (2), one laser beam enters the pump light frequency doubling module (II) and then enters the pump light path; the other laser beam enters the optical parametric oscillator (4) through the first reflector (3) to generate a laser beam of a specific wavelength in the visible to near-infrared band, and then enters the detection light path.

3. The high-temporal-resolution photogenerated carrier transient transport imaging system according to claim 1, characterized in that: The pump light frequency doubling module (II) comprises a first convex lens (5), a frequency doubling crystal (6), a second convex lens (7) and a filter (8), and is used to generate a short-wavelength pump laser. The first convex lens (5) and the second convex lens (7) with the same focal length are in a coaxial confocal relationship, and the confocal point is on the frequency doubling crystal (6). The pump fundamental frequency light from the beam splitter (2) is focused by the first convex lens (5) to increase the power density, and then reaches the frequency doubling crystal (6). The second-order nonlinear effect of the frequency doubling crystal is used to generate a doubled frequency pump laser, which is then converted into parallel light by the second convex lens (7), and finally filtered out by the short-pass filter (8).

4. The high temporal and spatial resolution photogenerated carrier transient transport imaging system according to claim 1, characterized in that: Delay line module (III), after the pump laser enters the optical reflector (9), it will return along the original path. At the same time, the optical reflector (9) moves linearly along the direction parallel to the optical axis of the pump light along with the electric control displacement stage (10), thereby changing the optical path length of the pump light path, realizing the control of the optical path difference between the detection laser pulse and the pump laser pulse, and then adjusting the time difference between them arriving at the sample.

5. The high-temporal-resolution photogenerated carrier transient transport imaging system according to claim 1, characterized in that: The pump light chopping module (IV) comprises a third convex lens (13), a first optical chopper (14) and a fourth convex lens (15), and is used for modulating the pump laser and outputting the modulation frequency; the third convex lens (13) and the fourth convex lens (15) are in a coaxial confocal relationship, and the confocal point is near the first optical chopper (14); the pump laser, which is parallel light, is focused by the third convex lens (13) and reaches the blade of the first optical chopper (14) to reduce the beam diameter and avoid the influence of uneven chopping of a large beam diameter, and then is converted back into parallel light by the fourth convex lens (15).

6. The high-temporal-resolution photogenerated carrier transient transport imaging system according to claim 1, characterized in that: The microscopy module (V) includes a long-wave dichroic mirror (17) and an objective lens (18), and is used for microscopic characterization of samples; the pump laser and the detection laser intersect at the dichroic mirror (17), the long-wave dichroic mirror (17) reflects the shorter-wavelength pump laser and transmits the longer-wavelength detection laser, and then the two laser beams are incident on the objective lens (18) together and reach the sample photoelectric base module (VIII).

7. The high-temporal-resolution photogenerated carrier transient transport imaging system according to claim 1, characterized in that: The detection light scanning module (VII) comprises a galvanometer (24), a scanning mirror (25) and a lens barrel lens (26), and is used for controlling the detection light beam so as to make the detection light perform spatial point-by-point scanning on the sample surface; the galvanometer (24) is composed of a stepping motor and two axially perpendicular reflectors to control the light emission angle; the scanning mirror (25) adopts a telecentric design and has a flat image plane, which is used for geometric correction of the galvanometer (24) to ensure a high scanning resolution; the scanning mirror (25) and the lens barrel lens are coaxially confocal, thereby generating collimated parallel light, and then the parallel detection laser passes through a beam splitter (27) and enters the microscope module (V) to reach the sample surface, and the deflection angles of the two reflectors in the galvanometer (24) are controlled by an industrial computer (33) so that the detection laser performs point-by-point closed-loop scanning and imaging with equal steps on the sample surface.

8. The high-temporal-resolution photogenerated carrier transient transport imaging system according to claim 1, characterized in that: The sample photoelectric base module (VIII) comprises a photoelectric measurement base (19), a three-axis displacement stage (20) and an electrical source meter (21). The photoelectric measurement base (19) is fixed on the three-axis displacement stage (20) and is used to place a sample of a device to be measured. An electrical connection port is reserved on the base and is connected to the electrical source meter (21) to apply a bias voltage or a gate voltage to the sample.

9. The high-temporal-resolution photogenerated carrier transient transport imaging system according to claim 1, characterized in that: The signal acquisition module (IX) comprises a second beam splitter (27), a sixth reflector (28), a filter (29), a balanced photodetector (30), a first lock-in amplifier (31) and a second lock-in amplifier (32); the detection laser reflected by the sample will be reflected by the second beam splitter (27) to the sixth reflector (28) again, then enter the filter (29) to filter out the short-wavelength pump laser, and finally reach the balanced photodetector (30) for light signal acquisition; the input signal of the first lock-in amplifier (31) is the radio frequency output signal of the balanced photodetector (30), and the reference signal is the modulation frequency output by the pump light chopping module (IV); the input signal of the second lock-in amplifier (32) is the direct output signal of the balanced photodetector (30), and the reference signal is the modulation frequency output by the detection light chopping module (IV); the first lock-in amplifier (31) and the second lock-in amplifier (32) respectively acquire the differential reflection signal and the absolute reflection signal of the detection laser.

10. A method based on the high temporal and spatial resolution photogenerated carrier transient transport imaging system as claimed in claim 1, characterized in that: The steps include: Step 1: System initialization configuration: Turn on the system power and start each module; set the target wavelength on the optical parametric oscillator (4) controller according to the sample and measurement requirements; Setting appropriate modulation frequencies on the pump light chopping module (IV) and the detection light chopping module (VI) respectively; inserting the sample to be measured into the photoelectric measurement base (19) and ensuring good contact; At this time, if there is no obvious deviation in the optical path, the spots of the pump laser and the detection laser should be at the same position on the sample surface; Step 2, configuring the parameters of the delay line module (III), the detection light scanning module (VII) and the signal acquisition module (IX) on the industrial computer (33): setting the measurement step and range of the delay line module (III); setting the scanning imaging range and step of the galvanometer (24) in the detection light scanning module (VII); setting the time constant, phase and sensitivity sampling parameters of the first lock-in amplifier 31 and the second lock-in amplifier 32 in the signal acquisition module (IX); Step 3, setting a sample photoelectric measurement base module (VIII) on the industrial computer (33) to put the sample in an electrical working state; setting the bias value or gate voltage value of the electrical source meter (21) according to measurement requirements; monitoring the sample external bias voltage, sample current, leakage current, etc. through the electrical source meter (21) to ensure that the sample is still stable in the electrical working state; Step 4, start the delay line module (III) and the detection light scanning module (VII), and perform a mapping characterization of the photogenerated carrier transport process of the sample according to the setting parameters in step 2; control the position of the electric displacement stage (9) in the delay line module (III), thereby changing the optical path of the detection light path, so that the detection laser pulse is delayed relative to the pump laser pulse to reach the sample surface; each time the electric displacement stage (9) moves to a certain position, control the detection light scanning module (VII) to make the detection light perform a point-by-point closed-loop scan with equal steps near the pump light on the sample surface, and then use the signal acquisition module (IX) to record the differential reflection signal at different spatial positions of the detection light to obtain the transient distribution of the photogenerated carriers of the sample to be tested under the external bias or gate voltage; when the delay line module (III) finishes scanning, the transient transport distribution of the photogenerated carriers at each delay time is obtained, and the transport mapping is completed; Step 5: Record and save the collected scanning image data.

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