Method for measuring electrical properties of semiconductor quantum dot material based on free carrier absorption

Abstract

The invention relates to a method for measuring electrical properties of a semiconductor quantum dot material based on free carrier absorption. Comprising the following steps that pumping laser and continuous detection laser are sequentially emitted to the same position of a semiconductor sample to be detected, the power of the pumping laser is changed, the concentration of free carriers is subjected to nonlinear change, the nonlinear characteristic of the free carriers is closely related to the characteristics of excitons and carriers in quantum dots, and the concentration of the free carriers is detected. Transmitting light signal data at different powers are collected through a photoelectric detector, and the obtained data are recorded and processed to obtain electrical characteristic parameters of the detected semiconductor sample. The method is not influenced by the response time or bandwidth limitation of the photoelectric detector, the data processing is simple, and the measurement precision is high; the measuring device is simpler, and the equipment cost is low; the method is wide in application range, can be used for measuring quantum dot materials with deep energy level traps, and is particularly suitable for measuring quantum dot materials with more shallow energy level traps.

Description

Technical Field

[0001] This invention relates to the field of semiconductor material property measurement technology, and specifically to a method for measuring the electrical properties of semiconductor quantum dot materials based on free carrier absorption. Background Technology

[0002] Semiconductor quantum dots (QDs) are a type of low-dimensional nanomaterial. Semiconductor colloidal quantum dots prepared by solution methods have the advantages of low preparation cost, tunable bandgap width (such as size control, morphology control, or heterojunction structure control), and multi-exciton effect, making them significant in applications such as visible and infrared detectors, electroluminescent devices, and photovoltaic devices.

[0003] There are two main types of commonly used methods for measuring the electrical properties of quantum dot materials: electrical measurement methods and optical measurement methods.

[0004] Electrical measurement methods mainly include field-effect transistor (FET) methods, time-of-flight (TOF) methods, and Hall effect measurements. Among these, using FETs to study the carrier transport properties of quantum dot materials is widely used; however, the carrier mobility measured by this method is an effective mobility. The TOF method characterizes carrier mobility by measuring the transient displacement current generated after short laser pulse excitation. Electrical measurement methods typically require the fabrication of corresponding device structures, therefore the measurement results are affected by the contact effect between the electrodes and the measured material. Furthermore, the electrode fabrication process itself can cause some damage to the measured material, ultimately affecting the accuracy of the measurement results.

[0005] The patent publication number "CN120142880A" discloses a "Test Method for Electrical Properties of Semiconductors". The measurement process requires multiple measurements of the electrical parameters of semiconductor materials using Hall effect measurement and secondary ion mass spectrometry (SIMS). It can measure the depth distribution of doping elements from a few nanometers to tens of micrometers. However, Hall effect measurement requires the preparation of Hall test results, and secondary ion mass spectrometry requires bombarding the sample with an ion beam, which damages the material being tested.

[0006] Optical measurement methods use optical probes instead of electric currents, which can effectively eliminate the influence of contact effects. Combining transient and spectroscopic characteristics, trap-captured carriers and free carriers with band edges can be studied separately, such as transient absorption (TA), terahertz (THz) and photoluminescence (PL) techniques.

[0007] The patent publication number "CN108827914B" discloses a "Terahertz Transient Absorption Spectroscopic Detection System and Carrier Lifetime Measurement Method." This method excites the sample under test using a pump light source, and then uses a spectral distribution detector to detect the probe light passing through the sample, obtaining the spectral distribution after absorption. After mathematical processing, the non-equilibrium carrier lifetime in the sample is obtained. From the measurement method's principle and system composition, it can be seen that the system requires a tunable femtosecond laser system consisting of a laser oscillator and an optical parametric amplifier, and a spectral distribution detector with a picosecond-level response. The measurement method is complex and the system cost is high.

[0008] The patent publication number "CN120253770A" discloses a "Method for Testing Carrier Lifetime by Time-Resolved Photoluminescence with Variable Excitation Intensity." This method uses pulsed laser pumping to obtain the decay curve of non-equilibrium carrier concentration over time in the sample under test. Then, it obtains the carrier SRH (Shockley-Read-Hall) recombination lifetime, radiative recombination lifetime, and Auger recombination lifetime through bi-exponential fitting. From the measurement principle, it can be seen that this method requires recording the signal change over time. The data in the fast response region of the measurement results are easily affected by the pulse width of the pulse excitation and the response speed of the detector. Furthermore, the presence of shallow level traps makes the time decay signal more complex, making this method unsuitable for materials with many shallow level traps. Summary of the Invention

[0009] To address the problems of existing technologies having complex measurement methods and high system costs, making them unsuitable for materials with many shallow energy level traps, this invention provides an optical measurement method for the electrical properties of semiconductor quantum dot materials based on free carrier absorption.

[0010] To address the problems existing in the prior art, the technical solution of this invention is: a method for measuring the electrical properties of semiconductor quantum dot materials based on free carrier absorption, comprising the following steps: Step 1: Irradiate the surface of the semiconductor sample under test with a periodically modulated pump laser. The modulation frequency is denoted as . f ; Step 2: Measure the incident light power using an optical power meter. Combined with the reflection-to-transmission ratio of the beam splitter k The optical power incident on the surface of the semiconductor sample under test was calculated. ; Step 3: Irradiate the surface of the semiconductor sample under test with a continuous probe laser. The irradiated area is the same as the pump laser irradiated area in Step 1. Step 4: Use a photodetector to detect the continuous probe laser passing through the semiconductor sample under test, and record the amplitude of the signal through a lock-in amplifier. Change the optical power of the pump laser incident on the surface of the semiconductor sample under test through an optical intensity modulation device to obtain the amplitude A(P) of the transmitted probe light signal under different optical power P. Step 5: Combine the theoretical model with multi-parameter fitting of the measurement data A(P) to obtain characteristic parameters such as trap concentration, exciton and free carrier recombination lifetime of the tested semiconductor sample: The mathematical expression of the theoretical model is as follows: in: In the formula, A coefficient related to the exciton rate production rate. The rate at which excitons dissociate into free charge carriers. For exciton recombination lifetime, For free carrier recombination lifetime, The interband radiative recombination rate of free carriers. For the trap concentration, The rate at which charge carriers are captured by the trap. The rate at which trapped charge carriers are thermally excited back to the conduction or valence band. It is a proportionality constant. This refers to the DC portion of the free carriers generated after pump light excitation.

[0011] Furthermore, the photon energy of the aforementioned pump laser is greater than the intrinsic semiconductor bandgap of the semiconductor sample being tested.

[0012] Furthermore, the photon energy of the aforementioned probe laser is less than the intrinsic semiconductor bandgap of the semiconductor sample being tested.

[0013] Compared with the prior art, the advantages of the present invention are as follows: 1) This invention provides a new measurement approach. Since the concentration of photoexcited excess free carriers has a nonlinear relationship with the pump light power, and its nonlinear characteristics are closely related to the characteristics of excitons and carriers in quantum dots, when measuring the electrical properties of semiconductor samples (quantum dot materials), the characteristic parameters such as trap concentration, exciton and free carrier recombination lifetime of the semiconductor sample under test can be obtained simply by fitting the amplitude signal of the transmitted probe light signal under different pump light intensities. 2) When measuring the electrical properties of the semiconductor sample under test, this invention is not affected by the response time or bandwidth limitations of the photodetector, and the data processing is simple and the measurement accuracy is high; 3) When measuring the electrical properties of the semiconductor sample under test, the present invention does not require an ultrafast laser source and a fast-response photodetector, making the measurement device simpler and the equipment cost lower.

[0014] 4) It has a wide range of applications. It can be used not only for the measurement of semiconductor samples with deep energy level traps, but also for the measurement of semiconductor samples with many shallow energy level traps. Attached Figure Description

[0015] Figure 1 This is a schematic diagram of the measuring device of the present invention; Figure 2 The curves showing the variation of free carrier absorption signal amplitude with pump light power at different trap concentrations, calculated according to the theory of this invention; Figure 3 The figures show the measurement results (symbols) of the free carrier absorption signal amplitude and the multi-parameter fitting results (solid lines) obtained according to the present invention for different pump light powers. The attached figures are labeled as follows: In the diagram: 1-Function generator; 2-Pump laser source; 3-Continuous probe laser source; 4-Adjustable attenuator; 5-First beam splitter; 6-Optical power meter; 7-First focusing lens; 8-Plane mirror; 9-Second focusing lens; 10-Second beam splitter; 11-Semiconductor quantum dot material under test; 12-Third focusing lens; 13-Bandpass filter; 14-Photodetector; 15-Lock-in amplifier; 16-Computer. Detailed Implementation

[0016] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0017] The design principle of this invention is: A pump laser with photon energy greater than the semiconductor bandgap is focused and irradiates the surface of the semiconductor sample under test. The sample absorbs the laser, generating excitons. Some of these excitons dissociate into free carriers under the influence of quantum dot defects and thermal excitation. These free carriers recombine through various mechanisms, including defect recombination and interband recombination, thus affecting their concentration. When a continuous-wave probe laser with photon energy lower than the semiconductor bandgap is incident on the same location on the semiconductor sample, the absorption of the probe laser by the free carriers reduces the intensity of the probe beam after it passes through the sample. Changing the pump laser power causes a nonlinear change in the free carrier concentration, and this nonlinearity is closely related to the characteristics of excitons and carriers in the quantum dot. Transmitted light signal data at different powers are collected using a photodetector. The data is recorded and processed to obtain the electrical characteristic parameters of the semiconductor sample under test.

[0018] The measuring device used in this invention is a common free carrier absorption device, see [link to relevant documentation]. Figure 1 The system includes a function generator 1, a pump laser source 2, a continuous probe laser source 3, an adjustable attenuator 4, a first beam splitter 5, a first focusing lens 7, a plane mirror 8, a second beam splitter 10, a semiconductor quantum dot material under test 11, a third focusing lens 12, a bandpass filter 13, a photodetector 14, a lock-in amplifier 15, and a computer 16. To implement the method of this invention, an optical power meter 6 is installed at the transmitted beam position of the first beam splitter 5.

[0019] The function generator 1 can output a periodic electrical signal to periodically modulate the output light intensity of the pump laser source 2. The pump laser source 2 is a semiconductor bulk laser with a wavelength of 405nm. The photon energy generated by this pump laser is 3.06eV, which is greater than the bandgap of the semiconductor quantum dot being measured (<1.4eV). The continuous probe laser source 3 uses a continuous semiconductor laser with a wavelength of 1560nm. The optical power meter 6 has a wavelength range of 190nm - 20μm and a power measurement range of 0.1mW - 5W. The photodetector 14 is an InGaAs near-infrared photodetector with a detection wavelength range of 800-1700nm.

[0020] Based on the above design principle, the present invention provides a method for measuring the electrical properties of semiconductor quantum dot materials based on free carrier absorption, comprising the following steps in sequence: Step 1: Irradiate the surface of the semiconductor sample under test (quantum dot material 11 in this embodiment) with the laser emitted from the periodically modulated pump laser source 2. The modulation frequency is denoted as . f : The periodic electrical signal output from function generator 1 is connected to the trigger terminal of the laser. The periodic electrical signal controls the pump laser to periodically output laser light. Then, a portion of the laser light emitted from the periodically modulated pump laser source 2 is irradiated onto the surface of the semiconductor quantum dot material 11 under test to excite and generate excitons and free carriers. The photon energy of the periodically modulated pump laser is greater than the bandgap of the semiconductor quantum dot material 11 under test. The modulation frequency is denoted as [missing information]. f ; Step 2: Measure the incident optical power on the optical power meter 6 using optical power meter 6. Combined with the reflection-to-transmission ratio of the beam splitter k The optical power incident on the surface of the tested semiconductor quantum dot material 11 was calculated. The detection wavelength and power range of the optical power meter 6 cover the wavelength and output power of the pump laser; Step 3: Irradiate the surface of the semiconductor quantum dot material 11 sample with the laser emitted by the continuous probe laser source 3. The irradiated area is the same as the pump laser irradiated area in Step 1. The photon energy of the continuous probe laser is less than the intrinsic semiconductor bandgap of the semiconductor quantum dot material 11. Step 4: Obtain different optical powers P The amplitude A(P) of the transmitted probe light signal: The tested semiconductor quantum dot material 11 sample generates excitons at the irradiated site due to the absorption of pump light energy. Subsequently, some of the excitons dissociate into free carriers under the influence of quantum dot defects and thermal excitation. Due to the diffusion and recombination of the carriers, a quasi-steady free carrier distribution is formed. The intensity of the transmitted continuous probe laser decreases due to the absorption of excess free carriers. A photodetector 14 is used to detect the probe laser transmitted through the tested semiconductor quantum dot material 11 sample, and the amplitude of the signal is recorded by a lock-in amplifier 15. The optical power of the pump laser incident on the surface of the tested semiconductor quantum dot material 11 is changed by an optical intensity modulation device (specifically, an adjustable attenuator 4 in this embodiment), and the amplitude A(P) of the transmitted probe light signal under different optical powers P is obtained. Step 5: Combine the theoretical model with multi-parameter fitting of the measurement data A(P) to obtain characteristic parameters such as trap concentration, exciton and free carrier recombination lifetime of the tested semiconductor quantum dot material 11: The mathematical expression of the theoretical model is as follows: in: In the formula, A coefficient related to the exciton rate production rate. The rate at which excitons dissociate into free charge carriers. fThe modulation frequency of the pump laser, which is periodically modulated in intensity as set in step one, For exciton recombination lifetime, For free carrier recombination lifetime, The interband radiative recombination rate of free carriers. For the trap concentration, The rate at which charge carriers are captured by the trap. The rate at which trapped charge carriers are thermally excited back to the conduction or valence band. It is a proportionality constant. This refers to the DC portion of the free carriers generated after pump light excitation. The specific measurement steps of this invention, in conjunction with the aforementioned device, are as follows: 1. Fix the semiconductor quantum dot material 11 to be tested on the sample holder between the second beam splitter 10 and the bandpass filter 13, and turn on the computer 16, function generator 1, pump laser source 2, continuous probe laser source 3, optical power meter 6, photodetector 14, and lock-in amplifier 15 in sequence. 2. Adjust the pump laser source 2, the first beam splitter 5, the first focusing lens 7 and the second beam splitter 10 so that the pump laser emitted by the pump laser source 2 is focused on the probed area on the surface of the semiconductor quantum dot material 11 under test. 3. The pump laser power transmitted through the first beam splitter 5 is measured using an optical power meter 6. The power is then determined based on the reflection-to-transmission ratio of the first beam splitter 5. k The optical power incident on the surface of the tested semiconductor quantum dot material 11 was calculated. ; 4. Turn on the continuous detection laser source 3, adjust the plane mirror 8 and the second focusing lens 9 to focus the laser emitted by the continuous detection laser source 3 onto the same detection position on the surface of the semiconductor quantum dot material 11 being tested as in step two. 5. A photodetector 14 is used to detect the continuous-wave laser passing through the semiconductor quantum dot material 11 under test, and the amplitude of the signal is recorded by a lock-in amplifier 15. The optical power of the pump laser incident on the surface of the semiconductor quantum dot material 11 is changed by an adjustable attenuator 4 to obtain different optical powers. P The amplitude A(P) of the transmitted probe light signal under the current; 6. By combining the theoretical model with multi-parameter fitting of the measurement data A(P), the characteristic parameters such as trap concentration, exciton and free carrier recombination lifetime of the tested semiconductor quantum dot material 11 are obtained.

[0021] See Figure 2 This is a specific result obtained from simulation of the amplitude of the free carrier absorption signal under different pump light powers according to the theoretical model of the present invention. The parameters of the tested semiconductor quantum dot material 11 are set as follows: coefficient 1×1023 m -3 s -1 W -1 The rate at which excitons dissociate into free carriers 1×10 6 s -1 The modulation angular frequency of the pump beam Modulation frequency f =1kHz, exciton recombination lifetime The free carrier recombination lifetime is 50 ns. The interband radiative recombination rate of free carriers is 50 ns. 1×10- 6 m 3 s -1 Trap concentration 1×10 14 cm -3 Up to 1×10 16 cm -3 The rate at which charge carriers are captured by traps 1×10 -12 m 3 s -1 The rate at which carriers trapped in the trap are thermally excited back to the conduction or valence band. 3×10 5 s -1 proportionality constant The value is 1.

[0022] Figure 3 The relationship between the amplitude of the free carrier absorption signal and the pump light power incident on the surface of the tested semiconductor quantum dot material 11, and the corresponding multi-parameter fitting results are presented. The fitted exciton recombination lifetime, free carrier recombination lifetime, exciton dissociation rate, interband radiative recombination rate, trap concentration, rate at which carriers are trapped, and rate at which trapped carriers are thermally excited back to the conduction band or valence band are 80 ns, 84 ns, and 1.36 × 10⁻⁶, respectively. 6 s -1 4.08×10 -6 m 3 s -1 1.32×10 15 cm -3 2.10×10 -12 m 3 s -1 and 1.11×10 7 s -1 By comparing the results with the experimental results, it can be seen that the results obtained by the method of the present invention are in complete agreement with the experimental data. Therefore, the electrical parameters of the semiconductor quantum dot material 11 under test can be rapidly measured by this method.

[0023] The above description is merely a preferred embodiment of the present invention and is not intended to limit the scope of protection of the present invention. Various modifications and improvements made by those skilled in the art to the technical solutions of the present invention without departing from the spirit of the present invention should fall within the scope of protection defined by the claims of the present invention.

Claims

1. A method for measuring electrical characteristics of a semiconductor quantum dot material based on free carrier absorption, the method comprising: The method comprises the following steps in sequence: ​ Step one, irradiate the surface of the semiconductor sample to be measured with a pump laser with intensity periodic modulation, and the modulation frequency is denoted as f ; Step two, measure the incident optical power with an optical power meter , the reflectance of the beamsplitter k Calculate the optical power incident on the surface of the semiconductor sample under test ; Step three, irradiating a continuous probe laser to the surface of the measured semiconductor sample, and the light irradiation area is the same as the pump laser irradiation area in step one; Step four, the continuous probe laser which transmits through the measured semiconductor sample is detected by a photoelectric detector, and the amplitude of the signal is recorded by a lock-in amplifier, the optical power of the pump laser which is incident to the surface of the measured semiconductor sample is changed by the light intensity adjusting device, and the amplitudes of the transmission probe light signals under different optical powers are obtained P );​ Step five, combining a theoretical model to perform multi-parameter fitting on the measurement data A(P) to obtain characteristic parameters of the measured semiconductor sample, such as trap concentration, exciton and free carrier recombination lifetime, and the like: The mathematical expression of the theoretical model is Wherein: wherein is a coefficient related to the exciton generation rate, is the rate of exciton dissociation into free carriers, is the step is the exciton recombination lifetime, is the free carrier recombination lifetime, is the interband radiative recombination rate of free carriers, is the trap concentration, is the rate of carrier capture by traps, is the rate of thermally excited free carriers from traps back into the conduction or valence band, is a proportionality constant, is the direct current part of the free carriers generated after excitation by the pump light.

2. The method for measuring electrical characteristics of a free carrier absorption based semiconductor quantum dot material according to claim 1, wherein: The photon energy of the pump laser is greater than the intrinsic semiconductor band gap width of the measured semiconductor sample. 3.The method for measuring electrical characteristics of a free-carrier absorption based semiconductor quantum dot material according to claim 1, characterized in that: The photon energy of the probe laser is less than the intrinsic semiconductor band gap width of the measured semiconductor sample.

Images