Mobility measuring method, measuring device and application thereof

Abstract

The invention provides a mobility measurement method, a mobility measurement device and application thereof, and the mobility measurement method comprises the following steps: adjusting a substrate with a sample in the measurement device, and enabling the substrate to form an included angle with the center line of the measurement device; pump light and terahertz light obtained by laser beam splitting are converged on the surface of a sample film, and a light spot formed by converging terahertz waves penetrating through a sample and probe light obtained by beam splitting are parallel and collinear and coincide on the surface of a detection substrate, so that terahertz detection is realized; the method comprises the following steps: acquiring energy E0 and delta E of terahertz waves before and after penetrating through a sample, forming an included angle a1 between the terahertz waves and a normal at the sample before the terahertz waves enter the sample, forming an included angle a2 between the terahertz waves and the normal at the sample after the terahertz waves are discharged, and forming an included angle a3 between the terahertz waves and the normal at the sample outlet in the sample, and calculating the photo-induced conductivity delta sigma: # imgabs0 # through a formula, c is the light velocity in vacuum, d is the thickness of the sample film, n1 is the refractive index of terahertz on a sample out-of-plane material, and nsub is the terahertz refractive index of the substrate.

Description

Technical Field

[0001] The present invention relates to the technical field of photoconductivity and carrier mobility calculation, and in particular to a mobility measurement method, a measurement device and applications thereof. Background Art

[0002] As a key parameter of semiconductor materials, carrier mobility profoundly affects device performance, such as switching speed, response time and efficiency. It defines the ease with which carriers can move in a directional manner under the action of an electric field. The higher the mobility, the faster the carrier movement speed, and the better the device performance is generally. Therefore, accurate measurement and understanding of the carrier mobility of materials is crucial for materials science, semiconductor physics, and the design and optimization of optoelectronic devices. Especially in the fields of new materials, such as organic semiconductors, perovskite materials, and two-dimensional materials, the measurement and research of mobility is the focus of attention.

[0003] Traditional mobility measurement methods mainly rely on electrical means, such as Hall effect measurement and time-of-flight (TOF). Hall effect measurement requires the production of a specific electrode structure on the sample, which is a contact measurement that may damage the sample and is difficult to apply to thin film samples or micro-nano structures. The time-of-flight method requires thicker samples and higher voltages, which limits its application in certain material systems. In addition, these traditional methods are usually difficult to measure at high frequencies and cannot capture the carrier transport characteristics of materials under rapidly changing electric fields.

[0004] With the development of ultrafast laser technology, terahertz time-domain spectroscopy (THz-TDS) technology based on ultrafast laser technology has emerged, providing a new non-contact and non-destructive characterization method for the measurement of carrier mobility. THz-TDS technology uses femtosecond laser pulses to excite photoconductive antennas or nonlinear crystals to generate and detect terahertz pulses. Its spectrum range usually covers 0.1THz to several THz, which corresponds to the characteristic scattering time of carriers in many materials. By measuring the time domain waveform of the transmitted or reflected terahertz pulse, the optical parameters such as the complex dielectric constant, refractive index, and absorption coefficient of the material can be obtained, and then the information such as the carrier mobility of the material can be extracted. Compared with traditional electrical measurement methods, THz-TDS technology has many advantages: non-contact measurement avoids damage to the sample; no specific electrode structure is required, which simplifies the sample preparation process; it can be measured in a wide bandwidth, providing richer carrier dynamics information; and it can be combined with optical pumping-terahertz detection (OPTP) technology to study the ultrafast dynamics of photoexcited carriers. However, most of the existing mobility measurement methods based on THz-TDS technology are limited to the measurement geometry of vertical incidence. In this case, the extracted mobility is usually the average mobility in the sample plane. For many materials, especially those with anisotropic structures, such as crystalline materials, oriented polymer films and two-dimensional materials, their carrier mobility varies significantly in different crystal directions or molecular arrangement directions. This anisotropic mobility characteristic is closely related to the microstructure, molecular arrangement, lattice symmetry, etc. of the material, and is of great significance for understanding the carrier transport mechanism of the material and designing high-performance optoelectronic devices. For example, in organic semiconductor materials, the arrangement and orientation of molecules can significantly affect the transport path and efficiency of carriers, resulting in anisotropy of mobility. In two-dimensional materials, due to the influence of layered structure and lattice symmetry, there may also be differences in mobility in different directions in the plane. Therefore, simply measuring the average mobility cannot fully reflect the carrier transport characteristics of the material, and also limits the in-depth understanding of the relationship between material structure and performance. Summary of the invention

[0005] The technical problem to be solved by the present invention is to provide a mobility measurement method, a measurement device and their application, which can realize the measurement of carrier mobility in different directions within a material, and use optical pumping technology to further improve the flexibility and accuracy of the measurement.

[0006] In order to solve the above technical problems, the technical solutions provided by the present invention are as follows:

[0007] The mobility measurement method comprises the following steps:

[0008] Adjusting the substrate with the sample in the measuring device so that it is set at an angle to the center line of the measuring device;

[0009] The pump light and terahertz light obtained by laser beam splitting converge on the surface of the sample film, and the light spot of the terahertz wave passing through the sample is parallel and collinear with the detection light obtained by beam splitting, and overlaps on the surface of the detection substrate, realizing terahertz detection;

[0010] Obtain the electric field strength E0 and ΔE of the terahertz wave before and after passing through the sample, as well as the angle a1 formed by the terahertz wave with the normal line at the sample before entering the sample, the angle a2 formed by the terahertz wave with the normal line at the sample after exiting the sample, and the angle a3 formed by the terahertz wave inside the sample and the normal line at the exit of the sample, and calculate the photoconductivity Δσ by the formula:

[0011]

[0012] Where ε0 is the dielectric constant in vacuum, c is the speed of light in vacuum, d is the thickness of the sample film, n1 is the refractive index of the material outside the sample surface at the terahertz wave, and n sub is the terahertz refractive index of the substrate.

[0013] Furthermore, the laser beam splitting is specifically to first split into beam one and beam two, beam one is reflected to beam two and then split again into beam three and beam four, beam three and beam four are terahertz excitation light source and detection light source respectively, beam two is pump light excitation light source.

[0014] Furthermore, the terahertz excitation light source obtains terahertz light after passing through the terahertz generation module laser, and the terahertz light generates terahertz light in the nonlinear crystal after passing through a focusing lens in the measuring device.

[0015] Furthermore, after the detection light enters the measuring device via the reflection route, it is reflected and passes through the central hole of the parabolic mirror to be parallel and collinear with the light spot where the terahertz wave converges.

[0016] Furthermore, it also includes a detection light mechanism, in which a delay unit is provided. The delay unit adjusts the optical path to change the time domain position of the detection light detecting the terahertz electric field, and at the same time corrects the light intensity and propagation direction of the detection light before use.

[0017] The present invention also provides a measuring device for the above-mentioned mobility measurement method, comprising a laser transmitter, a beam splitter assembly, a terahertz generating mechanism, a detection light mechanism, a pump light generating mechanism and a measuring mechanism, wherein a sample film is arranged in the measuring mechanism;

[0018] The laser generator is connected to the terahertz generating mechanism, the detection light mechanism and the pump light producing mechanism respectively through the beam splitter assembly;

[0019] It also includes a control mechanism connected to the laser transmitter, the beam splitter assembly, the terahertz generating mechanism, the detection light mechanism and the pump light producing mechanism respectively, and the formula for calculating the photoconductivity is stored in the control mechanism.

[0020] Furthermore, the measuring mechanism comprises a rotating base and a sample rack located on the rotating base, the sample rack is provided with a bearing surface, and the bearing surface is obliquely arranged in the sample rack.

[0021] Furthermore, it also includes an optical auxiliary device, wherein the optical auxiliary component includes a first reflective light component for detection light transmission, a second reflective component for pump light transmission, a third reflective component for terahertz transmission, and a parabolic mirror component.

[0022] The present invention also provides the application of the above mobility measurement method in the mobility of anisotropic materials;

[0023] Or applications in microstructural studies of anisotropic materials, molecular arrangement studies, or lattice symmetry studies.

[0024] Furthermore, the anisotropic material includes a crystalline material, an oriented polymer film and a two-dimensional material.

[0025] The present invention has the following beneficial effects:

[0026] The present invention combines a set of designed corner devices with terahertz time-domain spectroscopy (THz-TDS) technology, which can adjust the propagation angle of terahertz in the sample, and combine with optical pumping technology (OPTP) to achieve photoconductivity measurement at different angles in the material. Using terahertz time-domain spectroscopy (THz-TDS) combined with optical pumping technology, the carrier behavior of the material after excitation can be accurately detected, and the conductivity change in any direction can be captured by adjusting the detection angle. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 A flow chart of the mobility measurement method provided by the present invention;

[0028] Figure 2 A light propagation path diagram in the mobility measurement method provided by the present invention;

[0029] Figure 3 A circuit schematic diagram of a mobility measurement device provided by the present invention;

[0030] Figure 4 The front view of the measuring mechanism provided by the present invention

[0031] Figure 5 It is a structural schematic diagram of the measuring mechanism provided by the present invention;

[0032] Figure 6 A schematic diagram of the structure of the sample rack provided by the present invention;

[0033] Figure 7 The E0 spectra of the organic thin film sample (PM6:Y6) measured in the present invention at different terahertz incident angles;

[0034] Figure 8 ΔE spectra of the organic thin film sample (PM6:Y6) measured in the present invention at different terahertz incident angles;

[0035] In the figure:

[0036] 1. Optical rod; 2. Sample rack; 21. Rack body 1; 22. Rack body 2; 23. Sample stage base; 24. Sample; 3. Carrying surface; 4. Substrate;

[0037] 10. Laser; 20. Pump light generation module; 30. Terahertz generation module; 40. Nonlinear crystal; 50. Detection light module; 60. Thin film beam splitter;

[0038] 101, a first focusing lens; 102, a first reflecting mirror; 103, a second reflecting mirror; 104, a third reflecting mirror; 105, a fourth reflecting mirror; 106, a fifth reflecting mirror; 107, a sixth reflecting mirror;

[0039] 201, a first parabolic mirror; 202, a second parabolic mirror; 203, a third parabolic mirror; 204, a fourth parabolic mirror;

[0040] 301. A first beam splitter; 302. A second beam splitter. DETAILED DESCRIPTION

[0041] The present invention is described in detail below in conjunction with the various embodiments shown in the accompanying drawings, but it should be noted that these embodiments are not limitations of the present invention, and any equivalent transformations or substitutions in functions, methods, or structures made by ordinary technicians in the field based on these embodiments are all within the scope of protection of the present invention.

[0042] Existing terahertz technology usually uses vertical incident light to measure photoconductivity in materials, but it is difficult to perform accurate measurements at any angle, which limits the comprehensive understanding of charge transport in different directions inside the material. Figure 1-8 As shown, the integration of the measurement mechanism formed by the corner device and the terahertz detection system in the present invention provides a high-precision, angle-adjustable measurement solution, which can more accurately analyze the transmission characteristics of carriers in the material.

[0043] First, in this embodiment, in order to complete the relevant rotation, a rotatable measuring mechanism with an inclined surface is provided, which is specifically described as follows:

[0044] Refer to the attached Figure 4-6 As shown, the measuring mechanism in this embodiment includes an optical rod 1 and a sample holder 2 located on the optical rod 1, wherein the sample holder 2 is provided with a bearing surface 3, and the bearing surface 3 is obliquely arranged in the sample holder 2. In use, the entire sample holder 2 is rotated by the optical rod 1, and then the sample is arranged in the sample holder 2, and the sample is arranged in the obliquely arranged inclined surface, and the angle is adjusted. At this time, the angle is adjusted only by replacing different inclined surfaces.

[0045] Before actual use, testing is required, and during the testing process, the conditions of the optical path need to be adjusted. Subsequent operations can be carried out only after the optical path is adjusted to an optimal and stable state. Then pump light can be generated, and the pump light generation module should be adjusted for subsequent use.

[0046] In this embodiment, the sample holder 2 is directly mounted on the top of the optical rod 4; specifically, a 15mm*15mm groove is provided in the center of the sample holder 2, with a depth of 3mm, and a sample thickness of 4mm. The substrate 4 at the bottom of the sample 24 is 1mm thick. When placed facing outward, the surface of the sample 24 intersects parallel to the axis of rotation formed by the optical rod 1, ensuring that the positions of the detection light and the pump light remain consistent during the cornering process. In actual use, the overlap of the detection light and the pump light is pre-adjusted. What is guaranteed here is that the position of the laser detection remains unchanged during the cornering process, the sample surface rotates around the axis of rotation (i.e., the center axis of the optical rod 1), and the pump light hits the axis of rotation to minimize the change in the position of the pump light on the sample surface caused by the rotation, and always maintain the center position of the sample. The sample only needs to rotate, and there is no need to adjust the laser position after the setting is completed, so as to ensure the consistency of the level and height. The consistency of the level and height has been adjusted before the rotation.

[0047] During assembly, the screw hole at the bottom of the sample holder 2 is located at the geometric center, and the substrate 4 at the bottom of the sample 24 is fixed by screws to ensure vertical placement. The lower end of the rod is connected to the rotating part and inserted into the fixed base. The rotating part drives the optical rod 4 to rotate freely at 0°-360°.

[0048] For the sample rack 2, in this embodiment, the setting of the fixed angle is suitable for detecting the angle of samples in a larger range. The sample rack 2 includes a rack body 21 and a rack body 22, a sample stage base 23 and a sample 24. The rack body 21 and the rack body 22 are made of quartz material to ensure good terahertz transmission characteristics. The inclined angles of the supporting parts formed inside the rack body 1 and the rack body 2 are consistent, so that the ideal angle detection effect is achieved when the sample is incident vertically.

[0049] In this embodiment, the frame 1 21 and the frame 2 22 are made of the same material, which should maintain good terahertz transmission. The material selected here is quartz, which is consistent with the material of the sample base, greatly simplifying the complexity while providing a good angle detection range. The angles of the bevels of the frame 1 and the frame 2 are consistent, and the sample can be clamped in the middle. There are two depressions at both ends of the frame 1 and two protrusions at both ends of the frame 2, which ensure a stable connection between the two frames while ensuring the fixed angle and position of the sample placement.

[0050] During actual measurement, the terahertz light is incident vertically into the frame 1 21. The angle does not change when incident vertically. It is incident at a certain angle on the inclined sample surface between the support parts, and refracted at the sample. The magnitude of the refraction angle is the actual detection angle. It will then exit at the same angle as the incident angle, and finally exit the frame 2 22 vertically. It is worth noting that the gaps between the first support part and the sample, and between the sample and the second support part should be kept small enough.

[0051] Optionally, after selecting the angle, the entire measurement mechanism needs to be placed. If a small-angle angular resolution cornering device is used, it is necessary to ensure that the sample is placed at the waist of the terahertz beam to ensure the horizontal incidence of the terahertz beam here. This can be achieved by adjusting the front and back position of the horizontal plane of the translation stage. At the same time, the vertical intersection of the rotation axis and the terahertz incident light must be met. In order to ensure that the detection and excitation areas remain consistent during the cornering process, this can be achieved by adjusting the left and right position of the horizontal plane of the translation stage. At the same time, adjust the vertical position of the sample placement to ensure that the excitation position is located at the geometric center of the sample plane.

[0052] In this embodiment, the inclined surface in the sample holder 2 is selected to have a fixed angle, and the angle of the sample 24 is set according to a preset angle. At this time, the placement angle of the sample 24 can be adjusted by adjusting the inclined surface.

[0053] In actual use, if a fixed-angle corner device is used, it is necessary to adjust the position of the combined block cube (i.e., frame 1 and frame 2) and place it at the beam waist position where the terahertz light converges. At the same time, it is necessary to adjust the angle of the block to ensure the vertical incidence of the terahertz light. The entire combined structure is fixed to ensure a tight connection between each part.

[0054] Next, the mobility measurement method in this embodiment is introduced as follows:

[0055] The present invention combines a set of designed corner devices with terahertz time-domain spectroscopy (THz-TDS) technology, which can adjust the propagation angle of terahertz in the sample, and combine with optical pumping technology (OPTP) to achieve photoconductivity measurement at different angles in the material. Using terahertz time-domain spectroscopy (THz-TDS) combined with optical pumping technology, the carrier behavior of the material after excitation can be accurately detected, and the conductivity change in any direction can be captured by adjusting the detection angle.

[0056] Refer to the attached Figure 2-3 As shown, the measuring device in the present invention specifically utilizes the idea of ​​light path splitting, terahertz, pump light, and detection light generation, and then the laser emits a pulse light, which forms light beam one and light beam two after the first beam splitting, and light beam two is used to generate pump light. After beam splitting, light beam one forms excitation light and detection light, wherein the excitation light generates terahertz pulses through a nonlinear crystal. The terahertz generated light and the pump light coincide on the surface of the sample 24, and a turning device can be used at the position of the sample 24 to realize the detection of different internal angles of the sample. The terahertz pulse is focused to the sample through a parabolic mirror, and is colinear with the detection light at the detection crystal, and a terahertz signal is obtained in combination with the refractive index change caused by the terahertz. The pump light is generated from the secondary optical parametric amplification technology. After obtaining the pump light of the required wavelength and light intensity, it is collimated and emitted using a telescope system. The data processing module uses the modified photoconductance calculation formula to calculate the conductivity, and then applies the Drude-Smith model to fit the conductivity under DC conditions, calculates the carrier mobility, and fits the changes in the carrier transport properties after light excitation in the material.

[0057] The integrated terahertz measurement system of the present invention can perform efficient mobility measurement, especially the charge transfer characteristics of the material in the vertical and horizontal directions, by combining with the optical pumping technology, and realize a full-scale analysis of the charge transfer characteristics inside the material.

[0058] In this embodiment, the Drude-Smith model is used to fit the conductivity data and analyze the scattering behavior and mobility of carriers. The detailed steps are as follows:

[0059] 1. Laser emission and beam splitting:

[0060] The first thing to do is to excite the light source and separate the light beam. Turn on the laser 10, and the laser 10 emits a pulsed laser. Optionally, the light is separated into two beams at the first beam splitter 301, forming beam one and beam two. After a series of reflections, beam one reaches the second beam splitter 302, where the first beam is again divided into beam three and beam four. Beam three serves as the terahertz excitation light source, and beam four serves as the detection light source. The two beams come from the same laser source, so they are highly consistent in frequency and phase, so that they can be matched at the detection crystal, and the splitting ratio is set to 9:1.

[0061] In this embodiment, the second light beam arrives at the pump light generating module 20, which generates pump light of a required specific wavelength and meeting a certain power requirement through optical parametric amplification technology, and can be used for optical excitation after being collimated by a telescope system.

[0062] Furthermore, the light beam 3 arrives at the terahertz generation module 30, where it needs to be adjusted in terms of optical path, light spot, and propagation direction before being projected onto the first focusing lens 101, and converged onto the nonlinear crystal 40, where the nonlinear effect is used to generate terahertz. A plastic baffle is then placed to isolate the excitation light and transmit the terahertz light to avoid the influence of the excitation light leakage.

[0063] In this embodiment, a single beam splitter can only split a beam of light into two beams. Even if it can be split into three beams at once, it is achieved by combining two beam splitters. The actual process is the same.

[0064] The purpose of the allocation here is to maximize the application of laser power. The detection light does not require a high-power laser, but the excitation light does, so the splitting operation is performed with a relatively large splitting ratio.

[0065] 2. Light processing adjustment after beam splitting:

[0066] In this embodiment, the terahertz generator can be used as a modular system. The specific adjustment method is to adjust the position of the excitation light, adjust the excitation light intensity, the position of the excitation light on the terahertz generator crystal, and the rotation angle of the crystal. At this time, the crystal is a nonlinear GaP crystal. If adjustment is required, the position, excitation light intensity, and the position of the crystal are adjusted in a loop, and then the crystal angle is rotated.

[0067] In this embodiment, the light beam 4 arrives at the detection light module 50, which includes a delay line module, which can change the time domain position of the terahertz electric field detected by the detection light by adjusting the optical path, and at the same time, the light intensity and propagation direction of the detection light are corrected and then put into use. In this embodiment, the pulse width of the detection light is at the fs level, and a ps-level terahertz pulse is detected. The change in the time domain detection of terahertz light can be achieved by changing the optical path of the detection light and changing the overlapping position of the detection light and the terahertz light at the detection crystal. For example, if the position of the detection crystal remains unchanged, the terahertz optical path does not change, and the optical path of the detection light increases, it takes longer for the detection light to reach the detection crystal, and the terahertz time domain that it can detect is at a later position than before the change.

[0068] In the present invention, the excitation light used in the system is an adjustable excitation light realized by using optical parametric amplification technology, which utilizes the nonlinear effect of pulsed laser hitting nonlinear crystal to generate new pulsed excitation light. Therefore, two beam splittings are performed to generate three beams.

[0069] In this embodiment, the adjusted detection light is injected into the testing device through the first reflector 102 and the second reflector 103, and the third reflector 104 is used to make it parallel and collinear with the light spot collected by the terahertz through the small hole in the center of the first parabolic mirror 201, and overlap on the surface of the detection crystal to achieve terahertz detection. If the hole is not smooth, a convex lens can be added to reduce the beam, and the convex lens is set between the third reflector 104 and the parabolic mirror.

[0070] In this embodiment, a detector module 70 is also provided, which is provided on the side of the first parabolic mirror 201, and mainly detects the change of the terahertz transmission electric field intensity. The detector module detects the change of the detection light intensity; at the same time, the terahertz and the detection light overlap at the nonlinear crystal, and the terahertz electric field affects the detection light intensity. We can judge the change of the terahertz electric field intensity by detecting the change of the detection light intensity.

[0071] In this embodiment, the generated terahertz light is collected by the second parabolic mirror 202 and then horizontally transmitted to the third parabolic mirror 203, where it is converged again and propagated to the fourth parabolic mirror 204. The generated pump light is reflected back and forth by the fourth reflector 105, the fifth reflector 106, and the sixth reflector 107 in sequence to the thin film beam splitter 60, which allows the terahertz light to pass through while reflecting the pump light of a certain light intensity, and can meet the requirements of parallelism and collinearity of the pump light and the terahertz light, as well as the collimation of the pump light.

[0072] In this embodiment, the terahertz light and pump light converged on the sample surface should satisfy that the pump spot is slightly larger than the terahertz spot, because the optical power distribution of the two satisfies the Gaussian type, and the optical power is unstable at the edge of the spot, which brings errors to the experimental test. The terahertz light passing through the sample hits the first parabolic mirror 201 after passing through the fourth parabolic mirror 204, and converges on the detection crystal. The convergence position can be adjusted by adjusting the horizontal and vertical inclination angles of the first parabolic mirror 201.

[0073] 3. Measurement:

[0074] During the test, E0 is tested first, and then the changes in the light-induced terahertz spectrum are measured. In the process of measuring E0, it is necessary to adjust the terahertz electric field intensity to ensure that the collected signal intensity is strong enough, which will be very beneficial to the experimental test. The electric field intensity of the terahertz generated is mainly adjusted by changing the power of the excitation light, the position of the sample surface, the crystal phase at the position of the sample, and the angle of the excitation light hitting the sample surface. At the same time, the collection also has a very large impact on the experimental test. It determines the degree of utilization of the generated terahertz light intensity. The collection is mainly achieved by adjusting the parallelism, collinearity, and coincidence of the detection light and the terahertz light. The position and crystal phase of the detection crystal also have an important influence on the collection. Through this series of adjustments, it is adjusted to a position where the maximum terahertz signal can be measured to test E0. After the test, the state is kept stable, and the pump light can be put in for ΔE testing. In this embodiment, E0 is the terahertz transmission electric field when the light is not excited, combined with ΔE is the change in the terahertz transmission electric field when the light is excited, and then the two are used to calculate the change in the conductivity of the material after light excitation.

[0075] Before conducting a ΔE test, it is first necessary to adjust the overlap of the pump light and the terahertz light. After adjusting the optimal terahertz position, the terahertz position is kept unchanged, and the incident angle of the pump light is adjusted to meet the parallel and collinear conditions. This is mainly achieved through the thin film beam splitter and the sixth reflector 107. The optimal overlap position satisfies that the pump light spot is larger than the terahertz light spot, and the measured light-induced terahertz electric field change is the largest. This position is the optimal overlap position.

[0076] After adjusting the light spot overlap, the E0 and ΔE terahertz spectra can be tested successively. During the test, it is necessary to measure E0 and ΔE at different angles, which involves the change of angle. After each angle change, the above optical path adjustment, terahertz collection, and light spot overlap steps are required, because the propagation path of terahertz has changed, and the original position is no longer the optimal position or optimal state. The detection of E0 and ΔE are both obtained through the detection module. The difference lies in the different modulation methods. If modulation one is selected, the detector detects the difference in the presence or absence of terahertz, and what is detected is E0, the change in the terahertz electric field when the light is not excited; if modulation two is selected, the detector detects the difference in the presence or absence of pump light, and the detected ΔE is the difference in the presence or absence of pump light during light excitation, that is, E0 when there is pump minus E when there is no pump. 0。

[0077] In this embodiment, carrier mobility is a property of the substance itself, representing the charge transport performance of the material. Carrier mobility multiplied by carrier density equals photoconductivity.

[0078] The actual measured photoconductivity is divided by the excited carrier density to finally get the mobility. The vector decomposition here is the vertical and horizontal vector decomposition of the terahertz at a certain propagation angle inside the sample, that is, the vector decomposition is performed using the trigonometric function formula.

[0079] The horizontal and vertical mobilities are calculated by decomposing the mobility vector of the terahertz electric field. The relationship between them is given by the following formula:

[0080]

[0081] where φ is the fraction of electron-hole pairs generated per absorbed photon (between 0 and 1), and μ ∥ is the carrier mobility in the sample plane, μ ⊥ is the carrier mobility out of the sample plane and a3 is the propagation angle in the sample.

[0082] In this embodiment, it is necessary to measure the absorbed pump light power before testing. This is because different incident angles affect the reflection and transmission of the pump light. The optical power probe is installed on a rotatable ring outside the rotating table. The normal of the photosensitive plane of the power probe intersects perpendicularly with the rotating axis of the rotating table to ensure that the reflected and transmitted light hits the power probe vertically. The reflected and transmitted light powers at different rotation angles are measured to obtain their absorbed light powers, which are used as subsequent data.

[0083] Figure 7-8 The E0 and ΔE spectra of the organic blend film PM6:Y6 annealed at 200° at incident angles of 0° and 50° collected by the data processing module of the embodiment of the present invention. The data processing module of the present invention adopts the Labview program to receive spectral data through a network cable, and calculates and displays the spectral data according to different conditions. For non-vertical incidence, the refractive index at the sample, the refractive index of the material at the sample incident end, the refractive index of the material at the sample output end, and the incident angle of the terahertz are taken into account for any angle of the horizontally polarized terahertz incident light, and the photoconductivity calculation formula that has been used well is re-derived. The revised photoconductivity calculation formula is:

[0084]

[0085] Where ε0 is the dielectric constant in vacuum, c is the speed of light in vacuum, d is the thickness of the sample film, a1 is the angle of the terahertz outside the sample surface (the angle with the normal of the reflection interface), n1 is the refractive index of the terahertz material outside the sample surface, and n sub is the terahertz refractive index of the sample substrate 4, a2 is the terahertz propagation angle in the substrate (the angle with the normal line of the reflection interface), and a3 is the terahertz propagation angle in the sample (the angle with the normal line of the reflection interface).

[0086] It can be clearly seen that the values ​​of ΔE / E0 are very different at different incident angles, which means that the photoconductivity values ​​measured at different angles within the sample are different.

[0087] After calculating the photoconductivity under DC conditions based on the photoconductivity model, we can calculate its mobility, and then analyze it in combination with the internal structure of the sample. If the mobility components exist in the horizontal and vertical directions, we can perform vector decomposition on it. The mobility in a certain direction is vector decomposed to decompose the mobility components in the vertical and horizontal directions, and the decomposition satisfies the vector synthesis formula.

[0088] Refer to the attached Figure 7 and 8 By comparison, the mobility at different incident angles is obtained after DS fitting, and the vector decomposition of the mobility in the horizontal and vertical directions is performed respectively, and the mobility is fitted to obtain the size of the mobility in different directions.

[0089] In summary, the adjustable angle device provided by the present invention is designed to achieve accurate and stable control of the incident angle of terahertz light. The device includes a sample holder, an optical rod, a rotating part and a fixed base. The structural design allows the sample holder to rotate freely in the range of 0° to 360°, so that the sample can be accurately positioned at different angles during terahertz detection. The fixed-angle angle device can provide detection angles within the light range. The groove in the center of the sample holder ensures the stability of the sample placement, so that the sample surface remains parallel and staggered with the rotating axis during the rotation operation, avoiding the optical path deviation caused by angle changes.

[0090] When in use, the operator only needs to set the desired angle by rotating the component, and then slightly adjust the phone in the terahertz optical path, without repositioning other optical components, thus simplifying the adjustment process and improving experimental efficiency. This design ensures efficient detection of terahertz waves at different incident angles, and is particularly suitable for experimental needs to study the anisotropic optoelectronic properties of materials. Compared with traditional devices, this adjustable angle device has the advantages of high precision, easy operation and wide angle coverage, and can maintain measurement consistency and improve data reliability under complex experimental conditions.

[0091] The present invention also has the following advantages:

[0092] High-precision angle adjustment: The corner device can accurately control the terahertz incident light at any angle, reduce human errors, and accurately measure the photoconductivity in all directions of the material.

[0093] Comprehensive conductivity analysis: Capable of capturing conductivity differences in any direction in the material by adjusting the angle, which is particularly suitable for the study of anisotropic materials.

[0094] Wide range of application areas: This technology can be applied to organic semiconductors, composite materials, polycrystalline materials, etc., and is suitable for research and development in photovoltaics, sensors, electronic devices and other fields.

[0095] Again, about the application.

[0096] The present invention is used in the material PM6:Y6 blend film (organic semiconductor): through this technology, the photoconductivity of the organic semiconductor at different annealing temperatures and different incident angles is successfully measured, indicating that the arrangement of molecules in the vertical and horizontal directions and the size of the domains of the material at different annealing temperatures have an important influence on the charge mobility, and the charge mobility at a specific angle is significantly higher than that in other directions, which provides an important basis for subsequent device optimization.

[0097] The present invention is used in the material CsPbI3 (perovskite material): the anisotropic conductive properties of polycrystalline materials are measured using this technology, and the influence of different grain orientations on carrier transport is discovered.

[0098] Finally, the calculation formula in this embodiment is deduced.

[0099] The conductivity calculation formula obtained in this embodiment is specifically derived by directly changing the incident angle of the pump light. Figure 3 and the plane wave propagation formula), we can conclude that:

[0100] By simplifying the plane wave propagation formula in the model, the time-dependent form of the electric or magnetic field for a plane wave propagating in a homogeneous medium can be expressed as:

[0101] E(r,t)=E0e i(k·r-wt)

[0102] In the above formula, r is the spatial coordinate (position vector), k is the wave vector, which represents the wave propagation direction and wavelength information; w is the angular frequency, w=2πf; f is the frequency; E0 is the electric field amplitude.

[0103] Among them, the last one is to take E sub 、E sample ratio, so we only focus on the difference in light wave propagation in the two cases. This difference is caused by the fact that when the light wave propagates to the sample, E sub The sample position has only the substrate and no film sample attached, while E sample The sample position has a substrate 4 and a sample film (i.e., sample 24) attached. Specifically, one part is reflected in the interface transmission coefficient t, and the other part is reflected in the medium refractive index, i.e., d and n. First, a suggested mathematical expression is used to express the difference in the electric field of these two propagation situations, i.e., E sub 、E sam. Then substitute the specific formula, the so-called transmission coefficient is obtained by the Fresnel formula for horizontally polarized light, and the light wave phase is obtained by the plane wave propagation formula. After the expression is completed, it is necessary to perform calculation simplification. The following series of expressions are all calculation processes.

[0104]

[0105] Where ΔL is the thickness difference between the substrate of sam and the substrate of ref (sam-ref); d is the sample thickness; t represents the transmission coefficient, t 13 Represents the change in the amplitude of the light wave during propagation from medium 1 to medium 3.

[0106] P represents phase change, and P1 represents the phase change of the light wave during the propagation of the light in medium 1. P(d / cosa1) d / cosa1 in brackets represents the propagation distance of the light wave in the medium, and P = exp(-iwnd / c).

[0107]

[0108] α=ωd / c

[0109]

[0110] E sam With E sub According to the basic light wave propagation formula, the final light wave electric field is obtained by multiplying the transmission coefficient on each medium surface. The light wave here refers to any electromagnetic wave (including pump light, terahertz light, here refers to terahertz light). sam and E sub Both come from the first step. There is no connection between the two. They are just derived from their respective representations, and the ratio of the two is used for the next calculation. The very long formula is the calculation link, that is, the Fresnel formula is brought into the representation of the first step, which can be regarded as the expansion of a single parameter transmission coefficient t. The next step is the simplification process, omitting some α high-order infinitesimals, simplifying and omitting some small-influence components to obtain the final photoelectric conductivity formula.

[0111]

[0112] The Fresnel formula is used to describe the reflection and refraction coefficients of the interface. sam and E sub Expand. Fresnel formula:

[0113]

[0114] The expanded expression is:

[0115]

[0116] Simplified to:

[0117]

[0118] When inα is much smaller than 1, the Taylor expansion formula is used: the numerator is expanded to first order and the denominator is expanded to

[0119] First level:

[0120]

[0121] After simplification, we get:

[0122]

[0123] The formula for photoelectric conductivity in the film is:

[0124]

[0125] Here, the conductivity expression formula is used to convert the existing parameter n 2 It is related to the objective function conductivity σ, similar to expressing the y value based on the x value. Through the previous formula, n 2 Solving inversely from the previous formula:

[0126]

[0127] Before excitation, it is n3, a3, after excitation, it becomes n4, a4, then Δn=n4 2 -n3 2

[0128] The last two terms are fixed values.

[0129] When ΔE is much smaller than E0, approximate calculation can be performed.

[0130]

[0131] Substitution And it is approximately believed that

[0132]

[0133] Substitution

[0134]

[0135] Discard all terms of second order and above

[0136]

[0137] The final formula is obtained according to the photoconductivity calculation formula:

[0138]

[0139] The approximate conditions of the above formula are:

[0140] 1. The phase difference of the THz pulse caused by the sample is very small, that is, the film thickness is very thin and much smaller than the THz wavelength (μm-mm level)

[0141] 2. The sample has very little absorption of THz, and ΔE is much smaller than E0

[0142] 3. The substrate is transparent to THz and does not absorb THz, such as quartz

[0143] 4. The angular deviation difference caused by the sample before and after excitation is very small, that is, sina3 / cosa4≈tana3

[0144] In this embodiment, since the reflection angle and the incident angle are the same and the transmitted light is mainly tested, the reflected light is not guided separately.

[0145] It will be apparent to those skilled in the art that the invention is not limited to the details of the exemplary embodiments described above and that the invention can be implemented in other specific forms without departing from the spirit or essential features of the invention. Therefore, the embodiments should be considered exemplary and non-limiting in all respects, and the scope of the invention is defined by the appended claims rather than the foregoing description, and it is intended that all variations falling within the meaning and scope of the equivalent elements of the claims be included in the invention. Any reference numeral in a claim should not be considered as limiting the claim to which it relates.

[0146] In addition, it should be understood that although the present specification is described according to implementation modes, not every implementation mode contains only one independent technical solution. This description of the specification is only for the sake of clarity. Those skilled in the art should regard the specification as a whole. The technical solutions in each embodiment may also be appropriately combined to form other implementation modes that can be understood by those skilled in the art.

Claims

1. A method for measuring mobility, characterized in that: The following steps are included: Adjusting the substrate with the sample in the measuring device so that it is set at an angle to the center line of the measuring device; The pump light and terahertz light obtained by laser beam splitting converge on the surface of the sample film, and the light spot of the terahertz wave passing through the sample is parallel and collinear with the detection light obtained by beam splitting, and overlaps on the surface of the detection substrate, realizing terahertz detection; Obtain the electric field strength E0 and ΔE of the terahertz wave before and after passing through the sample, as well as the angle a1 formed by the terahertz wave with the normal line at the sample before entering the sample, the angle a2 formed by the terahertz wave with the normal line at the sample after exiting the sample, and the angle a3 formed by the terahertz wave inside the sample and the normal line at the exit of the sample, and calculate the photoconductivity Δσ by the formula: Where ε0 is the dielectric constant in vacuum, c is the speed of light in vacuum, d is the thickness of the sample film, n1 is the refractive index of the material outside the sample surface at the terahertz wave, and n sub is the terahertz refractive index of the substrate.

2. The mobility measurement method according to claim 1, characterized in that: The laser beam splitting is specifically to first split into beam one and beam two, beam one is reflected to beam two and then split again into beam three and beam four, beam three and beam four are respectively a terahertz excitation light source and a detection light source, and beam two is a pump light excitation light source.

3. The mobility measurement method according to claim 2, characterized in that: The terahertz excitation light source obtains terahertz light after passing through the laser of the terahertz generation module, and the terahertz light generates terahertz light in the nonlinear crystal after passing through the focusing lens in the measuring device.

4. The mobility measurement method according to claim 1, characterized in that: The detection light enters the measuring device through the reflection route, and then passes through the central hole of the parabolic mirror after reflection to be parallel and collinear with the light spot collected by the terahertz wave.

5. The mobility measurement method according to claim 1, characterized in that: It also includes a detection light mechanism, which has a delay unit. The delay unit adjusts the optical path to change the time domain position of the detection light detecting the terahertz electric field, and at the same time corrects the intensity and propagation direction of the detection light before use.

6. A measuring device for the mobility measurement method according to any one of claims 1 to 5, characterized in that: It includes a laser transmitter, a beam splitter assembly, a terahertz generating mechanism, a detection light mechanism, a pump light producing mechanism and a measuring mechanism, wherein a sample film is arranged in the measuring mechanism; The laser generator is connected to the terahertz generating mechanism, the detection light mechanism and the pump light producing mechanism respectively through the beam splitter assembly; It also includes a control mechanism connected to the laser transmitter, the beam splitter assembly, the terahertz generating mechanism, the detection light mechanism and the pump light producing mechanism respectively, and the formula for calculating the photoconductivity is stored in the control mechanism.

7. The measuring device according to claim 6, characterized in that The measuring mechanism comprises a rotating base and a sample rack located on the rotating base. The sample rack is provided with a bearing surface, and the bearing surface is obliquely arranged in the sample rack.

8. The measuring device according to claim 6, characterized in that It also includes an optical auxiliary device, wherein the optical auxiliary component includes a first reflective light component for detection light transmission, a second reflective component for pump light transmission, a third reflective component for terahertz light transmission, and a parabolic mirror component.

9. Application of the mobility measurement method according to any one of claims 1 to 5 in the mobility of anisotropic materials; Or applications in microstructural studies of anisotropic materials, molecular arrangement studies, or lattice symmetry studies.

10. The use according to claim 9, characterized in that: The anisotropic materials include crystalline materials, oriented polymer films and two-dimensional materials.

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