Gallium oxide crystal defect characterization method and detection system

Abstract

The invention provides a characterization method and a detection system for gallium oxide crystal defects. The characterization method comprises the following steps: independently building a set of ultrafast transient absorption microscopic imaging device; the polarization direction of the detection light is adjusted to be parallel to the c axis of the crystal, and the transient absorption spectrum of the gallium oxide substrate is measured and mainly comprises a defect-state absorption signal; and carrying out transient absorption microscopic imaging on the gallium oxide body defect. As a technology with high time resolution (sub-picosecond) and spatial resolution (sub-micron), the method is suitable for researching the body defect morphology in the gallium oxide substrate and the defect state carrier dynamics process, is expected to provide theoretical guidance for synthesis of the gallium oxide substrate with higher quality, and further promotes high-speed development of the gallium oxide industry.

Description

Technical Field

[0001] The present invention relates to the field of optical detection technology, and in particular to a method for characterizing defects in a gallium oxide crystal and a detection system. Background Art

[0002] β-Gallium oxide, with a bandgap of about 4.69 eV, is a very promising ultra-wide bandgap oxide semiconductor material. It is one of the key materials for the manufacture of future deep ultraviolet optoelectronic devices, high-power devices, high-frequency communication devices, and other devices. It is called the fourth-generation semiconductor. Defects in gallium oxide crystals can greatly affect the performance of devices. Crystal defects in β-gallium oxide include point defects, line defects, surface defects, and body defects, and these defects have not yet been fully understood. Therefore, it is particularly important to develop a method for characterizing crystal defects in β-gallium oxide. Summary of the invention

[0003] The present invention proposes a characterization method and detection system for gallium oxide crystal defects. The purpose is to develop a new ultrafast transient absorption spectroscopy imaging method for measuring gallium oxide substrate body defects, which is used to characterize body defects in β-gallium oxide crystals. This method is used to test body defects in gallium oxide and perform defect imaging, providing guidance for improving the understanding of internal defects in gallium oxide crystals and thus improving the performance of gallium oxide devices.

[0004] The present invention adopts the following technical solution: a gallium oxide crystal defect detection system, comprising:

[0005] The light source module is used to excite valence band electrons and defect state electrons to the conduction band through two-photon excitation, so that the number of electrons in the valence band and defect states inside the band gap is reduced; white light is used as detection light to detect the changes of excited state carriers in real time;

[0006] A pump-probe time-resolved module, used to adjust the polarization direction of the probe light by adjusting the half-wave plate;

[0007] A microscopic imaging module, used to scan the gallium oxide crystal with the detection light after adjusting the polarization direction;

[0008] The detector module is used to test the transient absorption spectrum of gallium oxide crystal defects according to the scanning results, and detect gallium oxide crystal defects according to the spectrum.

[0009] The light source module comprises a femtosecond laser and an optical parametric amplifier which are connected in sequence; the seed light emitted by the femtosecond laser is reflected and sent to a delay line in a pump-detection time resolution module; the optical parametric amplifier emits pump light to a chopper in the pump-detection time resolution module.

[0010] The pump-detection time resolution module includes a delay line, a YAG crystal, and a chopper;

[0011] A YAG crystal and a half-wave plate are arranged in sequence on the output optical path of the delay line, and the half-wave plate is used to change the polarization direction of the detection light;

[0012] The chopper is used to chop the pump light and reduce the repetition rate of the pump light to half of the original, so that the pump light is spatially colinear with the linearly polarized detection light output through the half-wave plate and enters the microscopic imaging module.

[0013] The microscopic imaging module includes a scanning galvanometer, an inverted optical microscope, a reflector, and a filter which are sequentially arranged on the output light path of the pump-probe time resolution module; the scanning galvanometer is used to adjust the positions of the pump light and the probe light so that the scanning area covers the gallium oxide sample; the filter is used to obtain the probe light at a specific wavelength.

[0014] A method for characterizing defects in a gallium oxide crystal comprises the following steps:

[0015] Light source modulation step: 1030nm laser in the light source module enters the pump-detection time resolution module to generate 400nm excitation light and 420nm-900nm detection light;

[0016] Defect detection steps: By adjusting the half wave plate of the pump-probe time resolution module, the polarization direction of the probe light is adjusted to test the transient absorption spectrum of the gallium oxide crystal defects.

[0017] Scanning imaging steps: by adjusting the filter to obtain the detection light at a specific wavelength, the scanning galvanometer parameters are adjusted and the number of scanning points is set through the microscopic imaging module, and all pixel positions are scanned to obtain transient absorption imaging, that is, the gallium oxide crystal defect imaging diagram;

[0018] Defect characterization steps: Detect and characterize defects in gallium oxide crystals based on the spectrum.

[0019] The detection light at the specific wavelength is 600-830nm, preferably 700nm.

[0020] The polarization direction of the detection light is parallel to the c-axis of the gallium oxide crystal.

[0021] The detection of gallium oxide crystal defects according to the spectrum comprises the following steps:

[0022] A ΔA signal is obtained according to the spectrum, where ΔA represents the difference between the absorbance of the detection light with pump light excitation and the absorbance of the detection light without pump light excitation;

[0023] When the change in the ΔA value exceeds the intensity range of the set threshold, the corresponding scanned pixel position is the defective part of the gallium oxide crystal and is marked.

[0024] The present invention has the following beneficial effects and advantages:

[0025] In the prior art, the characterization method of gallium oxide crystal defects is to use etching means (such as phosphoric acid etching) to expose the body defects in the form of pits on the crystal surface, and then use optical microscopes, scanning electron microscopes or atomic force microscopes to observe the distribution and morphological characteristics of the pits. However, the traditional etching method will affect the original defects and cause irreversible damage to the crystal. The present invention combines a microscope imaging system based on defect state photon absorption to non-destructively detect body defects in gallium oxide single crystal substrates.

[0026] At present, the research on defects in gallium oxide crystals usually uses transmission electron microscopy to study its defect structure, but it is impossible to study the effect of defects on the dynamics of photogenerated carriers. However, the dynamics of excited state carriers have a great influence on the function of gallium oxide optoelectronic devices. The present invention uses two-photon pump-probe technology to excite gallium oxide semiconductors to an excited state, which can deeply study the excited state relaxation process of carriers, deepen the understanding of defects in gallium oxide crystal structure, and provide theoretical guidance for further improving the performance of gallium oxide devices. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 Schematic diagram of the energy band structure and electronic transition of gallium oxide, where the PIA (photoinduced absorption) signal comes from: defect state absorption ①, ②; intraband absorption of free carriers in the conduction band ③.

[0028] Figure 2 Schematic diagram of the detection system for gallium oxide crystal defects.

[0029] Figure 3 The present invention is a flow chart of the method.

[0030] Figure 4 The time-sharing spectra of the femtosecond transient absorption spectrum in the embodiment are shown in Figure (a) when the polarization direction of the detection light is parallel to the crystal c-axis (θ=0°), and in Figure (b) when the polarization direction of the detection light is parallel to the crystal a-axis (θ=90°).

[0031] Figure 5 Graphs showing gallium oxide carrier dynamics for two different detection light polarization directions in the embodiment.

[0032] Figure 6 These are transient absorption microscopic images of a body defect inside the gallium oxide single crystal in the embodiment; wherein (a) is a defect imaging image at a delay time of 2ps, (b) is a defect imaging image at a delay time of 200ps, (c) is a defect imaging image at a delay time of 1000ps, and (d) is a defect imaging image at a delay time of 3000ps.

[0033] Figure 71 is a transient absorption microscopic image of another type of body defect inside the gallium oxide single crystal in the embodiment; (a) is a defect image at a delay time of 10 ps, ​​and (b) is a defect image at a delay time of 500 ps. DETAILED DESCRIPTION

[0034] The present invention is further described in detail below in conjunction with examples and drawings, but the embodiments of the present invention are not limited thereto.

[0035] The present invention proposes a characterization method and detection system for gallium oxide crystal defects, including the following steps: adjusting the polarization direction of the detection light to be parallel to the crystal c-axis, measuring the transient absorption spectrum of the gallium oxide substrate, which mainly includes the defect state absorption signal; and performing transient absorption microscopic imaging of gallium oxide body defects. As a technology with both high temporal resolution (sub-picosecond) and spatial resolution (sub-micron), the present invention is suitable for studying the body defect morphology and defect state carrier dynamics process in gallium oxide substrates, and is expected to provide theoretical guidance for the synthesis of higher quality gallium oxide substrates, and further promote the rapid development of the gallium oxide industry.

[0036] like Figure 3 As shown, a method for characterizing defects in a gallium oxide crystal is as follows:

[0037] Light source modulation step: 1030nm laser in the light source module of the detection system enters the pump-detection time resolution module to generate 400nm excitation light and 420nm-900nm detection light;

[0038] Defect detection steps: adjust the half wave plate of the gallium oxide defect detection system, adjust the polarization direction of the detection light, and test the transient absorption spectrum of the gallium oxide crystal defect.

[0039] Scanning imaging steps: The microscopic imaging module selects a 20x objective lens, adjusts the bandpass filter to obtain detection light at a specific wavelength, adjusts the scanning galvanometer parameters, sets the number of scanning points, and scans all pixel positions to obtain transient absorption imaging, that is, gallium oxide crystal defect imaging.

[0040] The probe light at a specific wavelength is 600-830 nm, preferably 700 nm. The polarization direction of the probe light is parallel to the c-axis of the crystal.

[0041] A gallium oxide crystal defect detection system comprises a light source module, a pump detection time resolution module, a microscopic imaging module and a detector module.

[0042] The light source module includes a femtosecond laser and an optical parametric amplifier;

[0043] The pump-probe time-resolved module includes delay line, YAG crystal, chopper, etc.

[0044] The microscopic imaging module includes an inverted optical microscope, a scanning galvanometer, a scanning lens, a detection light collection component, etc.;

[0045] The detector module includes a spectrometer, a photomultiplier tube and a lock-in amplifier. The photomultiplier tube and the lock-in amplifier are connected via a BNC cable, and the BNC terminal load has an impedance of 250k ohms.

[0046] A half wave plate is placed on the detection light path after the YAG crystal to change the polarization direction of the detection light. The detection light collection component is a bandpass filter to obtain detection light at a specific wavelength.

[0047] The present invention adopts two-photon pump-probe technology and utilizes the polarization dependence of defect states on the probe light to realize the imaging of bulk defects inside gallium oxide. The band gap of gallium oxide is 4.69eV, and 400nm is selected as the excitation light (photon energy is 3.1ev). The valence band electrons and defect state electrons are excited to the conduction band through two-photon excitation technology, so that the number of electrons in the valence band and the defect state inside the band gap is reduced; supercontinuum white light is used as the probe light to detect the changes of excited state carriers in real time. Figure 1 As shown in the figure, the process of detection light detection includes defect state absorption ①, ② and intraband absorption of free carriers in the conduction band ③. Different polarization directions of the detection light can separate these two absorption processes. When the polarization direction of the detection light is parallel to the a-axis of the crystal, the transient absorption signal originates from the absorption of free electrons in the conduction band; when the polarization direction is parallel to the c-axis of the crystal, the transient absorption signal mainly includes defect state absorption.

[0048] Embodiment 1:

[0049] Schematic diagram of the detection system for gallium oxide crystal defects Figure 2 As shown. The detection system mainly consists of three components: a femtosecond laser system (1030nm, 100kHz, 100μj per pulse) produced by Hangzhou Yili Technology Co., Ltd., an optical parametric amplifier (OPA) and a microscope imaging system. The 1030nm fundamental frequency light is divided into two parts by a beam splitter, 90% of which enters the optical parametric amplifier (OPA) to tune and produce excitation light of different wavelengths; the remaining 10% passes through the delay line system and is focused on the white light crystal to produce white light, and the wavelength range of white light is 420nm-900nm. The pump light is halved in frequency after passing through the chopper, and the pump light and the detection light are colinearly passed through a long-wave dichroic mirror and enter the two-dimensional scanning imaging system based on the galvanometer. The combined pump light and detection light are focused on the sample surface through a 20x objective lens. After passing through the sample, the detection light is focused by the objective lens and collected by the PMT and converted into an electrical signal. The electrical signal is sent to the phase-locked amplifier for analysis and finally forms transient absorption imaging data. The delay time between the pump light and the detection light is achieved by the movement of the delay line. For the acquisition of transient absorption spectra, we send the probe light into a spectrometer for detection.

[0050] Gallium oxide single crystal transient absorption spectrum test: The sample is a Sn-doped (010) crystal face gallium oxide single crystal substrate. Two-photon excitation is used to excite electrons to the conduction band. The pump light wavelength is 400nm, and the detection light wavelength range is 420nm-900nm. A half-wave plate is placed on the detection light path to change the polarization direction of the detection light. When the polarization direction of the detection light is adjusted to be parallel to the a-axis of the crystal, the transient absorption spectrum is tested; when the polarization direction of the detection light is adjusted to be parallel to the c-axis of the crystal, the transient absorption spectrum is tested.

[0051] Gallium oxide single crystal defect transient absorption spectroscopy imaging: The detection light after passing through the sample is focused by the objective lens, collected by the PMT and converted into an electrical signal. The electrical signal is sent to the phase-locked amplifier for analysis and ultimately forms transient absorption imaging data.

[0052] Data acquisition: Use Labview to write a data acquisition program, which realizes functions such as laser scanning, transient absorption spectroscopy and transient absorption imaging acquisition by controlling devices such as delay lines, timing cards and scanning galvanometers, and saves the experimental data in the computer.

[0053] Embodiment 2:

[0054] The excitation light is 400nm, and the detection light uses supercontinuum white light. The polarization direction of the detection light is adjusted by rotating the 1 / 2 wave plate to collect the transient absorption spectrum of the sample. The results are as follows: Figure 4 shown. Figure 4 (a) is the time-sharing spectrum when the polarization direction of the detection light is parallel to the c-axis of the crystal, which is a wide PIA (photoinduced absorption) signal of 550-800nm, reflecting the absorption process of the defect state. Figure 4 (b) is the time-sharing spectrum when the polarization direction of the detection light is parallel to the a-axis of the crystal. There are broad PIA positive signals and bleaching signals at the same time, which mainly reflect the intra-band absorption of free electrons in the CB (conduction band).

[0055] 4. The carrier dynamics curves of two different polarization directions were collected. Figure 5 As shown, the detection wavelength is 700nm. The dynamics of the two different polarization directions are obviously different. When the polarization direction of the detection light is along the a-axis of the crystal, the change of the ΔA signal originates from the change of the free electron population in the conduction band; when the polarization direction of the detection light is along the c-axis of the crystal, the intensity of the ΔA signal gradually increases in the first 700ps, which is attributed to the defect state absorption of the gallium oxide crystal. Among them, ΔA represents the difference between the absorbance of the detection light with pump light excitation and the absorbance of the detection light without pump light excitation.

[0056] 5. Collect the signal at 610-630nm and detect it with PMT (photomultiplier tube). The imaging result of one type of body defect is as follows: Figure 6 As shown, the defect absorption imaging at 2ps, 200ps, 1000ps and 3000ps is shown respectively. The defects at different times are all polygonal in shape, and the defect shape remains unchanged.

[0057] Embodiment 3:

[0058] Figure 7 Transient absorption imaging of another bulk defect in gallium oxide crystal at 10ps and 500ps is demonstrated.

Claims

1. A gallium oxide crystal defect detection system, characterized in that: include: A light source module is used to excite valence band electrons and defect state electrons to the conduction band through two-photon excitation, so that the number of electrons in the valence band and defect states inside the band gap is reduced; White light is used as the probe light to detect the changes of excited state carriers in real time; A pump-probe time-resolved module, used to adjust the polarization direction of the probe light by adjusting the half-wave plate; A microscopic imaging module, used to scan the gallium oxide crystal with the detection light after adjusting the polarization direction; The detector module is used to test the transient absorption spectrum of gallium oxide crystal defects according to the scanning results, and detect gallium oxide crystal defects according to the spectrum.

2. A gallium oxide crystal defect detection system according to claim 1, characterized in that: The light source module comprises a femtosecond laser and an optical parametric amplifier which are connected in sequence; the seed light emitted by the femtosecond laser is reflected and sent to a delay line in a pump-detection time resolution module; the optical parametric amplifier emits pump light to a chopper in the pump-detection time resolution module.

3. The gallium oxide crystal defect detection system according to claim 1, characterized in that: The pump-detection time resolution module includes a delay line, a YAG crystal, and a chopper; A YAG crystal and a half-wave plate are arranged in sequence on the output optical path of the delay line, and the half-wave plate is used to change the polarization direction of the detection light; The chopper is used to chop the pump light and reduce the repetition rate of the pump light to half of the original, so that the pump light is spatially colinear with the linearly polarized detection light output through the half-wave plate and enters the microscopic imaging module.

4. The gallium oxide crystal defect detection system according to claim 1, characterized in that: The microscopic imaging module includes a scanning galvanometer, an inverted optical microscope, a reflector, and a filter which are sequentially arranged on the output light path of the pump-probe time resolution module; the scanning galvanometer is used to adjust the positions of the pump light and the probe light so that the scanning area covers the gallium oxide sample; the filter is used to obtain the probe light at a specific wavelength.

5. A method for characterizing defects in gallium oxide crystals, characterized in that: The following steps are involved: Light source modulation step: 1030nm laser in the light source module enters the pump-detection time resolution module to generate 400nm excitation light and 420nm-900nm detection light; Defect detection steps: By adjusting the half wave plate of the pump-probe time resolution module, the polarization direction of the probe light is adjusted to test the transient absorption spectrum of the gallium oxide crystal defects. Scanning imaging steps: by adjusting the filter to obtain the detection light at a specific wavelength, the scanning galvanometer parameters are adjusted and the number of scanning points is set through the microscopic imaging module, and all pixel positions are scanned to obtain transient absorption imaging, that is, the gallium oxide crystal defect imaging diagram; Defect characterization steps: Detect and characterize defects in gallium oxide crystals based on the spectrum.

6. The method for characterizing defects of gallium oxide crystals according to claim 5, characterized in that: The detection light at the specific wavelength is 600-830nm, preferably 700nm.

7. The method for characterizing defects of gallium oxide crystals according to claim 5, characterized in that: The polarization direction of the detection light is parallel to the c-axis of the gallium oxide crystal.

8. The method for characterizing defects of gallium oxide crystals according to claim 5, characterized in that: The detection of gallium oxide crystal defects according to the spectrum comprises the following steps: A ΔA signal is obtained according to the spectrum, where ΔA represents the difference between the absorbance of the detection light with pump light excitation and the absorbance of the detection light without pump light excitation; When the change in the ΔA value exceeds the intensity range of the set threshold, the corresponding scanned pixel position is the defective part of the gallium oxide crystal and is marked.

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