Method and system for nondestructive testing of polarity of AlN crystal

Abstract

The invention discloses a method and a system for non-destructively detecting the polarity of an AlN (aluminum nitride) crystal. According to the method, detection is realized based on the phase matching relationship between the polarity of the AlN crystal and a second harmonic (SHG). The AlN crystal is of a non-centrosymmetric wurtzite structure, an SHG signal generated by excitation of an Al polar surface and an N polar surface of the AlN crystal along a c axis has an inherent phase difference with a period of pi, accurate discrimination of the polarity of the AlN crystal is realized based on the phase matching characteristic of the SHG signal, and no physical damage exists in the detection process. When the method is implemented, pulse laser normally enters the surface of the AlN single crystal to be measured, and an SHG signal of the AlN single crystal to be measured is collected and measured; comparing the phase of the to-be-measured signal with the phase of the known Al polar surface standard sample, and if the two phases are consistent, determining that the to-be-measured signal is an Al polar surface; and if the phase difference between the two is 180 degrees (pi), the polar plane is an N polar plane. The method is completely lossless, simple and convenient to operate and high in detection speed, can be used for polarity discrimination of the AlN single crystal and the substrate slice, and is suitable for industrial production detection of an aluminum nitride-based semiconductor material.

Description

Technical Field

[0001] This invention belongs to the field of semiconductor material characterization and non-destructive testing technology, specifically relating to a method and system for non-destructive testing of the polarity of AlN crystals. Background Technology

[0002] Aluminum nitride (AlN), as an important third-generation wide bandgap semiconductor material, has a direct bandgap of up to 6.2 eV, high thermal conductivity, and good piezoelectric and thermoelectric properties. It shows great application prospects in deep ultraviolet optoelectronic devices, high-frequency surface acoustic wave (SAW) and bulk acoustic wave (BAW) filters, high-power electronic devices, and microelectromechanical systems (MEMS).

[0003] AlN crystals belong to a hexagonal wurtzite structure (space group 1). The structure exhibits significant spontaneous polarization and piezoelectric polarization effects along the

[0001] direction. It lacks central inversion symmetry, thus forming two distinct polar surfaces: an aluminum (Al) polar surface (…). (Direction) and nitrogen (N) polar surface (direction) Crystal polarity not only directly affects the chemical stability, surface adsorption energy, and etching characteristics of materials, but also fundamentally determines the band shift, two-dimensional electron gas transport characteristics, and the direction and efficiency of piezoelectric response in AlN-based heterojunctions. It is a core parameter for device design and performance optimization. For example, in GaN / AlN high electron mobility transistors, the correct polarity direction is a prerequisite for forming a high-concentration two-dimensional electron gas; in acoustic wave devices, the polarity direction determines the effectiveness of piezoelectric excitation. Therefore, accurately and reliably identifying the polarity of AlN wafers is an indispensable key step in materials research and device manufacturing.

[0004] Patent CN202110515624.7 discloses a method and apparatus for distinguishing polar surfaces based on the piezoelectric effect of AlN. This method involves applying controllable deformation to a wafer and detecting the polarity of the induced charge generated, thereby determining the crystal polarity. However, AlN crystals are hard and brittle, making them prone to breakage and scrapping during deformation. If the deformation is too small, the induced electrical signal is weak and difficult to collect. Furthermore, the direct contact between the probe and the wafer surface during detection easily creates scratches or pits, damaging the surface smoothness and requiring an additional secondary polishing process to repair the damage.

[0005] Patent CN2021102660873.5 proposes a corrosion differentiation method based on the chemical stability differences of AlN polar surfaces: after treating the wafer surface with a specific ratio of etching solution, the surface that remains smooth and flat is the Al polar surface, while the surface that shows obvious roughness is the N polar surface. This method requires visualizing the polarity difference through the corrosion reaction, and it inevitably causes chemical damage to the wafer surface, requiring a secondary surface treatment to remove corrosion marks.

[0006] In summary, both of the above-mentioned AlN polarity detection methods have significant limitations, failing to balance non-destructive testing with high efficiency, and are ill-suited to the actual needs of non-destructive and rapid testing in industrial production.

[0007] AlN crystals belong to the point group, which lacks central inversion symmetry. This is the crystallographic structural basis for AlN crystals to produce nonlinear optical effects such as second harmonic distortion. The basic parameters of the AlN lattice include: lattice constants a (approximately 3.110–3.113 Å), c (approximately 4.978–4.982 Å), and along the... The ratio of cation-anion bond lengths (u, c / a) along the c-axis is approximately 1.6. These structural features result in a permanent dipole moment along the c-axis. The dipole moments of numerous unit cells are aligned uniformly along the polarity axis, macroscopically manifesting as a spontaneous polarization vector along the c-axis. It is known that this spontaneous polarization direction always follows... Direction, such as Figure 1 As shown, since the second-order nonlinear optical coefficient tensor is directly related to the crystal polarity structure, when the nonlinear light is not aligned with the spontaneous polarization direction, the resulting second-order harmonic generation (SHG) signal will undergo a phase change. Specifically, the Al and N polar surfaces have second-order nonlinear coefficients of equal magnitude but opposite sign. This results in the two types of second-order nonlinear polarization intensities being equal in magnitude but opposite in direction under the same fundamental frequency excitation. Consequently, the corresponding second harmonic (SHG) optical fields exhibit a fixed phase difference of 180° on the Al polar plane and the N polar plane, as shown below. Figure 2 As shown, this phase difference provides a clear physical basis for the non-destructive determination of the polarity plane of AlN crystals.

[0008] In second-order nonlinear optical effects, the relationship between the second-order nonlinear polarization intensity and the fundamental frequency photoelectric field satisfies the tensor equation: in, —Second-order nonlinear polarization intensity; —Second-order nonlinear polarizability; —Fundamental frequency photoelectric field intensity.

[0009] Second-order nonlinear polarizability With commonly used nonlinear optical coefficients satisfy: wurtzite-structured AlN crystals exist only in , , (and equivalent to) of And equivalent to of These are the three independent non-zero components. When the fundamental frequency light is incident along the c-axis (crystallurgical direction 3) of the AlN crystal, and considering only the nonlinear response along the c-axis, the polarization intensity equation can be simplified to: at this time, It directly represents the second-order nonlinear optical response capability along the c-axis.

[0010] In AlN crystals, because the polarization direction is perpendicular to the polar surface, the nonlinear optical coefficients of the Al and N polar surfaces are different. Numerical values ​​are equal in magnitude but opposite in sign. That is: Based on the above relationship, it can be concluded that the second-order polarization intensities of the Al polar plane and the N polar plane of the AlN wafer are equal in magnitude but opposite in sign, that is: .

[0011] Due to the fundamental frequency photoelectric field of the second harmonic. Vector and second-order polarization intensity Since the directions are consistent, a reversal of crystal polarity will cause a 180° phase shift in the optical field. In other words, there is a 180° phase difference between the second harmonic signals generated by the Al polar plane and the N polar plane of the AlN crystal.

[0012] Based on this principle, by performing phase calibration on a standard sample with known polarity and comparing it with the phase information of the SHG signal of the crystal under test, the polarity of the AlN crystal can be determined quickly and non-destructively. The detection process is as follows: Figure 3 As shown.

[0013] Currently, the characterization of AlN crystal polarity mainly relies on the following techniques: 1. Chemical Etching Method: This method utilizes the different etching rates of alkaline solutions such as KOH or NaOH on the Al and N polar surfaces. The polarity is determined by observing the morphology of the etching pits using a scanning electron microscope (SEM). This method is destructive, consuming or contaminating the sample, and has poor applicability to thin film samples, making it unsuitable for subsequent device fabrication.

[0014] 2. Transmission electron microscopy (TEM) combined with convergent beam electron diffraction (CBED): Polarity is determined by analyzing the lack of central symmetry in the diffraction pattern. This method is highly accurate and is considered the "gold standard," but it requires complex sample preparation (such as ion thinning to the nanoscale), expensive equipment, highly specialized operation, and extremely low detection efficiency. It completely lacks non-destructive and in-situ detection capabilities.

[0015] 3. Secondary ion mass spectrometry (SIMS): This method determines molecular ion yield by monitoring the difference in molecular ion yield at a specific polar surface under ion sputtering, but it is also destructive.

[0016] In summary, existing mainstream technologies all have obvious limitations: they are either destructive, have high equipment and operating costs, or have complex and time-consuming processes, making it difficult to meet the urgent needs of the modern semiconductor industry for rapid, non-destructive, in-situ, and low-cost polarity detection, especially in application scenarios for online quality monitoring of large batches of AlN epitaxial wafers or devices on production lines.

[0017] This invention utilizes the inherent second-order nonlinear optical effect of the non-centrosymmetric structure of AlN crystals to propose an innovative non-destructive polarity determination solution, realizing a simple, intuitive, and completely non-destructive polarity detection method. This has significant scientific and engineering value for promoting fundamental research on AlN materials and their large-scale application in high-performance devices. Summary of the Invention

[0018] To address the shortcomings of existing AlN crystal polarity detection methods, such as destructive nature, expensive equipment, complex operation, or inability to achieve rapid in-situ detection, this invention provides an AlN crystal polarity discrimination method that is reliable in principle, simple to operate, accurate in results, and completely non-destructive.

[0019] To achieve the above objectives, the present invention adopts the following technical solution: A method for non-destructive testing of the polarity of AlN crystals, characterized by comprising the following steps: S1: The Al polarity surface of the AlN standard sample is detected using a second harmonic (SHG) detection device to obtain the reference curve of the second harmonic (SHG) signal of the Al polarity surface.

[0020] S2: Under the same detection environment and detection parameters, the upper surface of the AlN crystal to be tested is tested in the same way to obtain the second harmonic (SHG) signal detection curve of the surface to be tested.

[0021] S3: Extract the phase information of the reference curve and the detection curve respectively, and compare and analyze the phase deviation between the two.

[0022] S4: If the detection curve of the surface to be tested is in phase with the reference curve, the surface to be tested is determined to be an Al polar surface; if the two are 180° (π) out of phase, the surface to be tested is determined to be an N polar surface.

[0023] The second harmonic optical detection conditions are consistent between the Al polar surface of the AlN standard sample and the AlN wafer to be tested.

[0024] Preferably, the pulsed laser is a femtosecond or picosecond laser pulse with a center wavelength in the near-infrared band, such as 1064nm.

[0025] Preferably, the fundamental frequency light is incident normally, that is, the fundamental frequency light is incident perpendicular to the standard c-plane.

[0026] Preferably, the measurement of the second harmonic signal intensity is performed using a spectrometer or a detection system consisting of a photomultiplier tube and a lock-in amplifier.

[0027] Compared with the prior art, the beneficial effects of the embodiments of the present invention are: Completely non-destructive: The detection process is purely optical, without contact or damage to the sample surface. After detection, the sample can be directly used for subsequent processes, achieving truly non-destructive characterization.

[0028] Fast and efficient: Second harmonic detection can be completed in a short time, greatly improving detection efficiency.

[0029] Easy to operate: Based on the inherent nonlinear optical effect of wurtzite AlN, the physical principle is clear, the judgment logic is clear, and the results are objective.

[0030] Highly expandable functionality: It is easy to integrate into the production line to achieve online monitoring, and can be extended to evaluate the polarity or quality of other non-centrosymmetric crystal materials (such as GaN, ZnO, etc.). Attached Figure Description

[0031] Figure 1 This is a schematic diagram of the spontaneous polarization direction of an AlN crystal.

[0032] Figure 2 The curves show the normalized intensity of the SHG on the polar surface of Al / N single crystal AlN as a function of the phase matching angle.

[0033] Figure 3 This is a flowchart of the non-destructive testing process for the polarity of AlN crystals.

[0034] Figure 4 This is a schematic diagram of a device for non-destructive testing of the polarity of AlN crystals according to the present invention. Detailed Implementation

[0035] The technical solution of this patent will be further described in detail and practically in conjunction with the accompanying drawings and specific embodiments. This embodiment covers three core application samples: AlN bulk single crystal, AlN ingot, and AlN substrate wafer. It clarifies key operations such as testing system debugging, error control, and multi-scenario parameter adaptation. All embodiments were completed in a constant temperature, constant humidity, and dust-free (class 1000 cleanliness) experimental environment that meets the testing standards for semiconductor materials. The test results were cross-validated by multiple methods to ensure the accuracy, repeatability, and industrial adaptability of the method of this invention.

[0036] I. Construction and Calibration of General Testing System The core of the detection system of this invention consists of a laser source, an optical focusing and guiding component, a signal collection component, a signal detection and processing unit, and a high-precision sample stage. All embodiments are based on this system. After the system is built, the optical path and detection unit need to be calibrated first. The specific steps are as follows: Laser optical path calibration: After the pulsed laser emitted from the laser source is guided by the reflector and focusing objective, the coaxiality of the optical path is adjusted by the laser collimator to ensure that the laser is incident on the center of the sample stage with an incident angle deviation of ≤0.1°; the focal length of the focusing objective is adjusted so that the laser spot is focused on the surface of the sample stage, and the spot diameter is controlled within the range of 2~5μm according to the sample type.

[0037] Detector unit calibration: If a fiber optic spectrometer is used as the detection component, first perform dark current calibration and wavelength calibration on the spectrometer to eliminate background noise interference with the signal; if a photomultiplier tube + lock-in amplifier combination is used, adjust the reference frequency of the lock-in amplifier to synchronize with the laser repetition frequency, and adjust the amplification factor to [value missing]. This ensures that the second harmonic (SHG) signal can be stably acquired.

[0038] Sample stage calibration: The high-precision three-dimensional electric displacement stage is zero-point reset to verify its repeatability (±0.1μm) and flatness, ensuring that the detection surface is strictly perpendicular to the laser incident direction after the sample is fixed.

[0039] System general configuration reference: The laser source is a femtosecond / picosecond pulsed laser with a center wavelength in the near-infrared band (1040nm / 1064nm); the signal collection unit is equipped with a short-pass filter (cutoff wavelength 800nm) + a band-pass filter (matching the SHG signal wavelength, such as 520nm / 532nm) to completely filter out the fundamental frequency light; the signal processing unit is a computer equipped with dedicated data acquisition and phase analysis software, which can automatically extract the phase characteristic values ​​of the SHG signal curve.

[0040] Example 1 This embodiment focuses on polarity detection of an unknown polarity AlN bulk single crystal with a standard c-plane and a crystal plane deflection angle ≤0.5°. A femtosecond laser and fiber optic spectrometer combination is used for detection. The specific steps are as follows: Sample pretreatment: The detection surfaces of the known polarity AlN bulk single crystal standard and the unknown polarity test single crystal sample were ultrasonically cleaned with anhydrous ethanol for 5 min to remove surface dust, oil and other impurities. After drying with nitrogen, they were ready for use to avoid surface contaminants interfering with the generation and collection of SHG signals.

[0041] Phase calibration of known polarity standard: (1) Fix the Al polarity surface of the AlN bulk single crystal standard upwards at the center of the high-precision sample stage, ensuring that the detection surface is perpendicular to the laser incident direction; (2) Turn on the laser source and set the parameters: pulse width 150fs, repetition frequency 80MHz, center wavelength 1040nm, laser power 50mW, and control the laser to be perpendicularly incident to the center of the Al polarity surface of the standard; (3) Start the fiber optic spectrometer, collect the SHG signal (wavelength 520nm) generated by the standard, repeat the collection 5 times, remove outliers and take the average value to generate the Al polarity surface SHG signal reference curve, extract and store the phase reference value of the curve. , as a reference standard for polarity determination.

[0042] Polarity detection of the single crystal sample to be tested (1) Keep the laser parameters, optical path configuration and detection unit parameters completely unchanged, and fix the AlN single crystal sample to be tested at the same position on the sample stage; (2) Collect the SHG signal at the center of the detection surface of the sample to be tested in the same way as the standard sample calibration, and repeat the collection 5 times to generate the SHG signal detection curve and extract the phase measurement value. (3) Using phase analysis software in a computer, compare and Phase deviation: if The surface to be tested is determined to be an Al polar surface; if The surface to be tested is determined to be an N-polar surface.

[0043] Verification of test results: The single crystal test surface of the test object was verified by the traditional chemical etching method (4 mol / L KOH solution, etching at 100℃ for 5 min). The morphology of the etching pits was observed by scanning electron microscopy (SEM): the Al polar surface was smooth after etching with no obvious etching pits; the N polar surface showed regular hexagonal etching pits. The verification results were completely consistent with the test conclusion of this invention.

[0044] Example 2 This embodiment focuses on polarity detection for AlN ingots with unknown polarity that have been ground and have a crystal plane deflection angle ≤0.5°. Since the ingot sample has a larger detection area, multiple detection points are needed to ensure comprehensive polarity determination. A femtosecond laser + fiber optic spectrometer detection combination is used, and the specific steps are as follows: Sample pretreatment: The detection surface of the AlN crystal ingot was ultrasonically cleaned with anhydrous ethanol for 10 minutes, dried with nitrogen, and the crystal ingot was fixed on the sample stage with a high-temperature resistant clamp free of adhesive residue to avoid crystal ingot displacement causing optical path deviation.

[0045] Standard sample phase calibration: Reproduce the phase calibration steps of the AlN bulk single crystal standard sample in Example 1, and obtain and store the phase reference value of the Al polar surface SHG signal. .

[0046] Polarity detection of the ingot sample to be tested: (1) Keep all detection parameters consistent with the standard sample calibration, and select 5 detection points on the detection surface of the AlN ingot (1 point in the center + 4 points around the perimeter, arranged in a cross shape, with a point spacing ≥ 5 mm); (2) Control the sample stage to move and collect SHG signals at the 5 detection points in sequence. Each point is collected 3 times, and the phase measurement value of each point is extracted. (3) Compare the phase values ​​at each detection point with... Deviation: If the phase deviation of all detection points satisfies The detection surface of the crystal ingot is determined to be the Al polar surface; if the phase deviation of all detection points satisfies If the phase deviation is inconsistent at some points, it is determined that there are polar domains on the crystal ingot detection surface, and the abnormal area needs to be marked.

[0047] Verification of test results: Chemical etching was performed on the five test points on the test surface of the crystal ingot. The etching results were consistent with the polarity determination conclusions of each point, proving that this method can realize the full-domain polarity detection of crystal ingot samples.

[0048] Example 3 This embodiment focuses on polarity detection for AlN substrates (epitaxy substrates, thickness 200~500μm) of unknown polarity produced in industrial applications. It meets the rapid detection requirements of industrial production lines and employs a detection combination of picosecond laser, photomultiplier tube, and lock-in amplifier to improve signal detection speed and sensitivity. The specific steps are as follows: Sample pretreatment: The AlN substrate standard and the substrate to be tested were ultrasonically cleaned with semiconductor-specific cleaning solution to remove residual epitaxial precursors on the surface. After being dried with nitrogen, they were fixed with a vacuum adsorption sample stage (to avoid contact damage to the substrate surface by the fixture).

[0049] Phase calibration of known polarity standard: (1) Place the Al polarity surface of the AlN substrate standard sample upwards, vacuum adsorb and fix it on the sample stage, turn on the laser source, and set the parameters: pulse width 10ps, repetition frequency 100MHz, center wavelength 1064nm, laser power 30mW; (2) Start the photomultiplier tube and lock-in amplifier, collect the SHG signal (wavelength 532nm) of the Al polarity surface of the standard sample, repeat the acquisition 3 times, and extract the phase reference value. And store it; the entire calibration process takes ≤1 minute.

[0050] Rapid polarity detection of the substrate under test (1) Keep the detection parameters constant, vacuum adsorb the AlN substrate under test onto the sample stage, collect the SHG signal at the center of the detection surface, the time for a single acquisition is ≤10s, and extract the measured phase value. (2) Automatic comparison and The phase deviation is determined according to the judgment criteria in Example 1, and the polarity determination result is quickly output.

[0051] Verification of test results: The substrate was verified using the piezoelectric effect method (low deformation, avoiding substrate breakage) to detect the polarity of the induced charge. The results were consistent with the test conclusions of this invention, and the substrate test surface showed no physical / chemical damage and could be directly used for subsequent epitaxial processes.

[0052] II. Key Points of System Debugging and Error Control Phase extraction error control: The phase extraction of the SHG signal curve adopts the Fourier transform method. The signal curve is denoised and fitted by software before the phase value is extracted to avoid phase deviation caused by background noise. Multiple acquisitions are performed for each detection and the average value is taken to reduce random errors.

[0053] Laser parameter adaptation: For AlN samples of different thicknesses, the laser power can be finely adjusted (20~100mW): for thin substrates (≤200μm), low power (20~30mW) is selected to avoid laser damage to the sample; for bulk single crystals / ingots, medium to high power (50~100mW) is selected to improve the SHG signal intensity.

[0054] Optical path deviation correction: If optical path deviation occurs during the detection process, it is calibrated in real time by a laser collimator to ensure that the laser is incident normally and the incident angle deviation is controlled within 0.1° to avoid phase shift caused by oblique incidence.

[0055] III. Precautions for Testing Testing environment: The entire process is carried out in a clean environment of Class 1000 or above, with constant temperature (25±2℃) and constant humidity (40±5%RH) to avoid dust and temperature changes that may cause optical path drift or sample surface contamination.

[0056] Sample fixation: Select the appropriate fixation method according to the sample type: use adhesive-free fixtures for bulk single crystals / ingots, and use vacuum adsorption stages for substrates to ensure that the sample detection surface is strictly perpendicular to the laser incident direction, without warping or offset.

[0057] Filter replacement: After the laser center wavelength is changed, the bandpass filter must be replaced simultaneously to ensure that the center wavelength of the filter is completely matched with the SHG signal wavelength, so that the fundamental frequency light is completely filtered out.

[0058] Standard sample reuse: When testing AlN samples of the same batch and specifications, the same standard sample can be used for phase calibration. The standard sample needs to be calibrated regularly (once every 3 months) to ensure the accuracy of the phase reference value.

[0059] IV. Method Extension and Adaptation Those skilled in the art can adjust the detection parameters based on the principles of this invention to suit the detection needs of AlN samples with different surface conditions and substrates in the following ways, and all adjustments fall within the protection scope of this invention: Optical path configuration adjustment: For AlN samples with rough surfaces, the reflective SHG signal collection was changed to a transmissive collection to improve signal collection efficiency; Detection method adjustment: For samples with trace SHG signals, the fiber optic spectrometer was replaced with a high-sensitivity photomultiplier tube + lock-in amplifier combination to enhance the signal detection capability; Scanning strategy adjustment: For large-size AlN samples, control the high-precision sample stage to perform surface scanning, realize the detection of polarity distribution in the entire sample area, and generate a polarity distribution heat map; Laser parameter adjustment: According to the sample requirements, change the laser center wavelength (e.g., 800nm, 1064nm), pulse width (femtosecond / picosecond), and repetition frequency, as long as the laser parameters of the standard sample and the sample to be tested are consistent.

[0060] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for non-destructive testing of the polarity of AlN crystals, characterized in that, Includes the following steps: Using AlN crystals of known polarity as standard samples, the phase information of the second harmonic (SHG) signal of its Al polar surface was calibrated under pulsed laser normal incidence conditions. Obtain the AlN crystal to be tested; Under the same conditions as the calibration steps and detection parameters, the pulsed laser is vertically irradiated onto the surface of the AlN crystal to be tested; The second harmonic signal generated by the surface under test is collected and measured to obtain its phase information; The measured SHG signal phase is compared with the SHG signal phase reference value of the polar surface of the standard sample Al; If the phase of the SHG signal of the surface to be tested is consistent with the phase of the SHG signal of the Al polar surface of the standard sample, then the surface is determined to be an Al polar surface. If the phase of the SHG signal on the surface to be tested differs from the phase of the SHG signal on the polar surface of the standard sample Al by 180° (π), then the surface is determined to be an N polar surface.

2. The method according to claim 1, characterized in that, The AlN crystal is either a bulk AlN single crystal or an AlN substrate.

3. The method according to claim 1, characterized in that, The wavelength of the pulsed laser is located in the near-infrared band.

4. The method according to claim 1 or 3, characterized in that, The pulsed laser is a femtosecond laser pulse or a picosecond laser pulse.

5. The method according to claim 1, characterized in that, The second harmonic signal is collected and measured, specifically using a spectrometer or photodetector in conjunction with a lock-in amplifier.

6. A system for non-destructive testing of the polarity of AlN crystals, for implementing the method according to any one of claims 1-5, characterized in that, include: A laser source used to generate pulsed laser light; An optical focusing and guiding component is used to guide the pulsed laser and vertically irradiate the surface to be tested of the AlN crystal under test; A signal collection component is used to collect the second harmonic signal generated by the surface to be detected. The signal detection and processing unit is used to acquire and measure the second harmonic signal, compare the phase of the SHG signal of the detection surface of the AlN crystal under test with the phase of the SHG signal of the polar surface of the standard Al, and output the polarity determination result based on the comparison result.

7. The system according to claim 6, characterized in that, The signal detection and processing unit includes a spectrometer, or a combination of a photomultiplier tube and a lock-in amplifier.

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