An optoelectronic testing method for identifying electronic-grade diamond

Abstract

The present invention relates to the field of diamond identification technology, and specifically to a photoelectric testing method for identifying electronic-grade diamonds. The present invention provides a non-destructive means for identifying electronic-grade diamonds using photoelectric testing. Electrodes are sputtered on the top and bottom of the diamond to be tested to form a device. The external quantum efficiency of the diamond that varies with wavelength is calculated based on the photocurrent signal of the diamond under different incident light wavelengths. The wavelength-varying external quantum efficiency spectrum is then used to obtain the half-maximum width of the spectral response peak of the diamond. Finally, diamonds with a half-maximum width of the spectral response peak less than 15nm are determined to be electronic-grade diamonds. The photoelectric testing method of the present invention can accurately identify whether the quality of electronic-grade diamond single crystals meets the standards. Compared with ICP and SIMS detection methods, it has the advantages of no loss, low cost, and more comprehensive reflection of the diamond quality grade, providing a new and reliable verification method for the field of electronic-grade diamond quality identification.

Description

Technical Field

[0001] The present invention relates to the technical field of diamond identification, and in particular to a photoelectric testing method for identifying electronic-grade diamonds. Background Art

[0002] Diamond, an ultra-wide-bandgap semiconductor, possesses exceptional properties such as high carrier mobility and saturation velocity, a large breakdown electric field, high thermal conductivity, high chemical inertness, and thermal stability, making it an ideal material for power electronic devices. Currently, methods for artificially synthesizing diamond include high-temperature and high-pressure methods and chemical vapor deposition. The quality of the resulting diamond varies greatly depending on the specific growth method and parameters. However, only high-purity, high-quality electronic-grade diamond can meet the application requirements of power electronic devices, necessitating the characterization and identification of the quality of the produced diamond. Specifically, this can be categorized into crystal structure characterization and elemental composition characterization. Currently, the most commonly used method for diamond crystal structure characterization is X-ray diffraction (XRD), which can quantitatively calculate the dislocation density of a single crystal by measuring the rocking curve. Other methods for characterizing the elemental composition of diamond include inductively coupled plasma (ICP) and secondary ion mass spectrometry (SIMS). These methods can quantitatively analyze the impurity elements and their content in diamond with accuracy down to the ppb level. However, these characterization methods can only reflect a certain aspect of diamond properties, and ICP and SIMS also have the disadvantages of being costly and destructive to samples. Therefore, there is an urgent need to develop a new non-destructive detection method that can comprehensively reflect the quality of diamonds.

[0003] The presence of various defects in diamond, such as impurity defects (point defects), line defects, and surface defects, determines its quality. These defects increase carrier scattering, thereby affecting carrier mobility. Furthermore, the defect energy levels formed by these defects can cause recombination or carrier capture, thereby affecting carrier lifetime. Carrier mobility and carrier lifetime jointly determine the photoelectric response characteristics of the material. Based on this, the present invention proposes a new method for identifying electronic-grade diamond materials using photoelectric testing. Summary of the Invention

[0004] To overcome the shortcomings of the above-mentioned prior art, the present invention provides a photoelectric testing method for identifying electronic-grade diamonds. The method calculates the half-width at half-maximum of the response peak of the curve showing the change of external quantum efficiency with the spectrum, and uses the half-width at half-maximum of the response peak to infer the trap state density of the diamond, thereby determining whether the sample being tested is electronic-grade diamond.

[0005] In order to achieve the above object, the technical solution adopted by the present invention is:

[0006] The present invention provides a photoelectric testing method for identifying electronic-grade diamonds, comprising the following steps:

[0007] S1. Sputter a layer of metal on the top and bottom of the diamond single crystal as the positive and negative electrodes to form a diamond device. The top metal is a semi-transparent electrode, and the positive and negative electrodes are connected to the circuit.

[0008] S2. Under vacuum conditions, illuminate the top of the diamond device with a light source having an incident light wavelength of 160 to 280 nm. Use a source meter to output photocurrent signals at different wavelengths and calculate the external quantum efficiency that varies with wavelength. The half-width at half maximum of the diamond's spectral response peak is then obtained from the wavelength-varying external quantum efficiency spectrum. Finally, diamonds with a spectral response peak half-width of less than 15 nm are identified as electronic-grade diamonds.

[0009] The working principle of the test method of the present invention is as follows: high-quality electronic-grade diamond has fewer defect states, and most carriers will not be trapped and have a shorter lifespan, resulting in a significant decrease in the light response in the short-wave region, so that the spectrum has an extremely narrow half-width. At the same time, since light below 200nm will be strongly absorbed by air, the photoelectric test is carried out under vacuum conditions. When light is incident from the top of the crystal, the photogenerated electrons and holes in the crystal will drift through the single crystal under an external electric field and be collected by electrodes at both ends to output photocurrent. The external quantum efficiency can be calculated based on the photocurrent, and then the spectral response peak half-width is obtained. The narrower the response peak half-width, the smaller the trap state density of the diamond, reflecting the higher the diamond quality. When the spectral response peak half-width is less than 15nm, the diamond can meet the requirements of electronic-grade diamond.

[0010] Preferably, in step S1, a layer of Pt is sputtered on the top of the diamond single crystal as a semi-transparent anode, and a layer of Pt is sputtered on the bottom as a cathode. A glass substrate on which Au metal is evaporated is contacted under the cathode through silver paste. The diamond device consists of a Pt layer, a diamond layer, a Pt layer, and a glass substrate on which Au metal is evaporated, in order from top to bottom.

[0011] Preferably, in step S1, the thickness of Pt sputtered on the top of the diamond layer single crystal is 15-20 nm.

[0012] Preferably, in step S2, the light source is obtained by a continuous ultraviolet light source and a spectrometer, and the wavelength of the incident light varies from 280 nm to 160 nm.

[0013] Preferably, in step S2, the external quantum efficiency is given by the formula Calculated, where I ph is the photocurrent, P is the incident light power, e is the elementary charge, h is the Planck constant, c is the speed of light, and λ is the wavelength of monochromatic light.

[0014] Preferably, in step S2, when the half-maximum width of the spectral response peak of the tested diamond is less than 10 nm, the tested diamond is identified as a high-quality electronic-grade diamond.

[0015] Preferably, after step S2 is completed, the diamond device is immersed in aqua regia to wash away the electrodes.

[0016] Compared with the prior art, the present invention has the following beneficial effects:

[0017] The present invention provides a photoelectric testing method for identifying electronic-grade diamonds. First, metals are sputtered onto the top and bottom of a diamond single crystal as positive and negative electrodes, respectively, to form a diamond device. The top metal serves as a translucent electrode and is connected to a circuit. Then, under vacuum conditions, a light source with an incident light wavelength of 160 to 280 nm is used to illuminate the top of the diamond device. The wavelength-dependent external quantum efficiency of the diamond is calculated based on the photocurrent signal of the diamond at different incident light wavelengths. The wavelength-dependent external quantum efficiency spectrum is then used to obtain the diamond's spectral response peak half-width (FWHM). A narrower FWHM indicates a lower trap state density in the diamond, thus indicating higher diamond quality. When the FWHM is less than 15 nm, the diamond meets the requirements for electronic-grade diamond. Compared to traditional testing methods such as XRD, ICP, and SIMS, the present invention comprehensively reflects diamond quality from the perspective of the photoelectric effect by analyzing carrier mobility and carrier lifetime. Furthermore, the electrodes can be washed away without damaging the diamond. Compared to the twin-crystal swing test and SIMS test, the test results of the present invention are more distinct and easily distinguishable, enabling better identification of electronic-grade diamonds. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 The twin crystal rocking test results of diamonds A, B and C are shown;

[0019] Figure 2 The SIMS test results of diamonds A, B and C are shown;

[0020] Figure 3 Photoelectric test structure for diamond devices;

[0021] Figure 4 The photoelectric test results of diamonds A, B and C are shown below;

[0022] Figure 5 The photoelectric test results of two diamonds grown under the same conditions (CVD) are shown;

[0023] Figure 6 Figure 2: Exciton emission mechanism of diamond A under photoexcitation. (a) Schematic diagram of the main physical processes occurring in diamond A under photoexcitation, (b) PL spectrum of diamond A under 193 nm pulse excitation.

[0024] Figure 7 Figure 3: Study of the photoexcitation carrier mechanism of diamond A, including (a) the photoexcitation excess carrier generation rate G in diamond A, (b) the spatial distribution of steady-state carriers, and (c) the experimental EQE and simplified calculated EQE of diamond A at three selected wavelengths.

[0025] Figure 8 Figure 3: Exciton emission mechanism of diamond C under photoexcitation. (a) Schematic diagram of the main physical processes that may occur in diamonds B and C under photoexcitation, (b) PL spectra of diamonds A, B, and C under 193nm pulse excitation, and (c) voltage signals of diamonds A, B, and C under 93nm pulse excitation.

[0026] Figure 9 Study on the carrier mechanism of photoexcitation in diamond C, including (a) the photoexcited excess carrier generation rate G in diamond C, (b) the spatial distribution of steady-state carriers, and (c) the experimental EQE and simplified calculated EQE of diamond C marked with three selected wavelengths. DETAILED DESCRIPTION

[0027] The following is a further description of specific embodiments of the present invention. It should be noted that the description of these embodiments is intended to facilitate understanding of the present invention and does not constitute a limitation of the present invention. In addition, the technical features involved in the various embodiments of the present invention described below may be combined with each other as long as they do not conflict with each other.

[0028] The experimental methods in the following examples are conventional methods unless otherwise specified, and the experimental materials used in the following examples are commercially available unless otherwise specified.

[0029] Example 1 Establishment of a Photoelectric Identification Method for Electronic-Grade Diamonds and Study of Its Feasibility Mechanism

[0030] This example uses three different diamond single crystals for comparative testing to further verify the effectiveness of the present invention. These three diamond single crystals are designated Diamond A, Diamond B, and Diamond C. Diamonds A and B have a thickness of 680 μm, while Diamond C has a thickness of 780 μm. Diamonds A and C are grown by chemical vapor deposition (CVD), while Diamond B is grown by high-temperature and high-pressure deposition. The preparation methods are described in the literature (doi.org / 10.1016 / B978-0-444-63303-3.00017-1).

[0031] 1. Conventional methods for testing diamonds

[0032] First, the three diamonds were characterized using conventional methods, including twin-crystal rocking tests and SIMS tests. The half-height widths of the rocking curves of diamond A, B, and C (004) planes were 28.9 arcsec, 34.9 arcsec, and 39.3 arcsec, respectively. Figure 1 ), which reflects the lower dislocation density of diamond A. The SIMS test results are analyzed as follows Figure 2 As shown in the figure, the contents of elements and chemical bonds are basically the same. The above results do not clearly distinguish the quality of the three types of diamonds.

[0033] 2. Preparation of diamond devices

[0034] First, a thin layer of Pt (17 nm) was sputtered on the top of diamond single crystals A, B and C as a semi-transparent anode, and a Pt layer was sputtered on the bottom as a cathode. The cathode was contacted with a glass substrate with Au metal evaporated through silver glue to obtain diamond devices A, B and C. The diamond device consists of a Pt layer, a diamond layer, a Pt layer and a glass substrate with Au metal evaporated from top to bottom (see Figure 3 Connect the positive and negative electrodes to the probes and connect them to the circuit, then read the electrical output signal using a source meter. When light is incident from the top of the crystal, the photogenerated electrons and holes within the crystal drift through the single crystal under the influence of the applied electric field and are collected by the electrodes at both ends, generating a photocurrent.

[0035] 3. Photoelectric testing of diamond

[0036] Since light below 200nm is strongly absorbed by air, the photoelectric test is carried out under vacuum conditions. Diamond devices A, B and C are placed in a vacuum chamber for testing. Monochromatic light in the range of 160 to 280nm is incident vertically from the top of the diamond device. The monochromatic light is obtained by a deuterium lamp and a monochromatic spectrometer. The wavelength of the incident light changes from 280nm to 160nm, and the corresponding photocurrent signal is read by the source meter. According to the formula The external quantum efficiency (EQE) that varies with wavelength can be calculated, where I ph is the photocurrent, P is the incident light power, e is the elementary charge, h is the Planck constant, c is the speed of light, and λ is the wavelength of monochromatic light. After the test is completed, the diamond device is immersed in aqua regia to wash away the metal electrode without causing damage to the sample.

[0037] The EQE spectrum that varies with wavelength is shown in Figure 4 As shown, the half-height widths of the spectral response peaks of diamonds A, B and C are 8nm, 31nm and 52nm respectively, which indicates that diamond A has the smallest trap state density and thus has the highest quality. When the half-height width of the spectral response peak of the measured diamond is less than 10nm, the diamond is identified as a high-quality electronic-grade diamond. Therefore, diamond A is a high-quality electronic-grade diamond, and the quality of diamonds B and C decreases in turn.

[0038] 4. Verification of photoelectric testing methods for diamond

[0039] In order to evaluate the accuracy of this method and avoid accidental factors, the above photoelectric test was performed on two diamonds grown under the same conditions (CVD). It can be found that the EQE spectra of both diamonds have a narrow peak near the absorption edge, with the center at about 228nm, and the half-maximum width of the EQE is only about 8nm ( Figure 5 The consistency of the test results of two identical diamonds proves the reliability of this test method.

[0040] 5. Mechanism explanation of photoelectric testing method

[0041] The proposed photoelectric measurement method is explained as follows: First, in high-quality electronic-grade diamond, due to its fewer defect states, most carriers are not trapped, resulting in a short lifetime. In the low absorption coefficient region, photogenerated carriers can be excited throughout the crystal, where they are collected by electrodes under the action of an electric field. As the wavelength decreases, the absorption coefficient gradually increases, leading to a significant increase in the photoresponse. After reaching the peak response, as the excitation wavelength continues to decrease, the generation of photogenerated carriers becomes increasingly concentrated on the crystal surface. At this point, they are primarily annihilated through radiative recombination and cannot be transmitted to the back surface of the crystal for collection by the electrodes. This results in a significant decrease in the photoresponse in the short-wavelength region, resulting in an extremely narrow half-maximum width at half maximum (FWHM). In contrast, in non-electronic-grade diamond, the abundance of trap energy states allows carriers to be trapped, resulting in a longer lifetime. Even under short-wavelength excitation, carriers are generated close to the surface, but can diffuse into the crystal due to the low radiative recombination. Furthermore, these longer-lived carriers can be collected by the back electrode under the action of an electric field, thus avoiding a decrease in the photoresponse and resulting in a larger FWHM spectrum.

[0042] Specifically, the derivation process is given by taking diamond A and diamond C as examples:

[0043] For diamond A, the main physical processes occurring in the photoelectric test include: absorbing photons (with energy greater than the band gap) to excite electrons from the valence band to the conduction band or higher energy excited states (which can relax to the bottom of the conduction band through processes such as phonon emission), thereby generating freely mobile electrons in the conduction band and leaving freely mobile holes in the valence band. These can cause photoconductivity signals under the action of the electric field (process 1); at the same time, radiative recombination can also occur through localized energy states in the band gap (process 2) ( Figure 6 a). Figure 6b shows the luminescence spectrum of diamond A under 193 nm pulsed light excitation, where intense exciton-assisted free exciton emission is observed. The spectral line with a certain width includes the participation processes of different phonons. This result proves that exciton emission is the dominant recombination process under photoexcitation.

[0044] The steady-state distribution of carriers in the crystal under photoexcitation directly affects the photoconductivity output, which is jointly determined by the photoexcitation process, diffusion process, and recombination process. As the photon energy increases, the excess electron-hole pairs generated by photoexcitation are more concentrated near the surface. As Figure 7 shown in a, the spatial distribution of the photoexcitation excess carrier generation rate G in the sample decays exponentially. According to the Beer-Lambert law, that is, G ∝ αe -αx (the thickness d of diamond A is 680 μm), where α is the absorption coefficient of the material for light with wavelength λ, and e is the natural constant. The three characteristic wavelengths selected are 228 nm, 225 nm, and 210 nm, and the corresponding EQE is as Figure 7 shown in c. The light at 228 nm decays more slowly with depth, can penetrate the crystal, and generate photocarriers inside the crystal. As the wavelength decreases, the generation of photoexcited carriers is more concentrated on the surface. Especially, the light at 210 nm cannot penetrate the crystal due to strong absorption on the surface. Obviously, the generation of photocarriers has an uneven distribution inside the crystal, so they need to diffuse into the crystal. In addition, all radiative and non-radiative recombination transition processes will lead to the annihilation of carriers and thus reduce the carrier concentration. Considering the influence of surface recombination, taking electrons as an example, the steady-state distribution of photoexcited carriers can be expressed as:

[0045]

[0046] where J is a constant related to the incident light flux and reflectivity; D e is the electron diffusion coefficient related to the mobility; s is the surface recombination velocity, which is proportional to the excess carrier concentration on the surface; α is the absorption coefficient of the material for light with wavelength λ; d is 680 μm; 0 < x < d. Thus, the spatial distribution of carriers in the crystal under different wavelength excitations can be obtained ( Figure 7 b). As the wavelength decreases, the carrier concentration in the crystal gradually decreases, and the maximum value of the whole concentration moves towards the surface. In addition, the strong absorption near the surface will cause stronger radiative recombination, thus significantly increasing the surface recombination velocity. This result can qualitatively explain the narrowband output of the device based on diamond A. Specifically, on this basis, it is assumed that diamond A is a perfect crystal, that is, the influence of all defect energy levels is ignored. And it is considered that the surface recombination velocity is very large, so that sd / D e >> 1. Under the condition of αd >> 1, the photoconductivity can be expressed as:

[0047]

[0048] Where A is the illuminated area; J is a constant related to the incident light flux and reflectivity; μ n and μ p are the electron and hole mobilities, respectively; τ n and τ p are the electron and hole lifetimes, respectively; L D is the bipolar diffusion length, which depends on μ n τ n and μ p τ p ; α is the absorption coefficient of the material for light with a wavelength of λ; d is 680μm; s is the surface recombination velocity. From this formula, it can be seen that the change of ΔG with the excitation wavelength mainly depends on This factor, where L D , D e Both d and d can be considered as constants. Obviously, the absorption coefficient and surface recombination velocity will increase with decreasing wavelength, resulting in a significant decrease in photoconductivity. The experimental EQE and the simplified EQE calculated by the above formula vary with wavelength as shown in the figure below. Figure 7 As shown in Figure c, the half-height width of the response peak of diamond A under the assumption of a perfect crystal is smaller than that of the experiment. It should be noted that the simplified calculation here is to illustrate the trend of change, and its absolute value is meaningless.

[0049] For diamond C, the main physical processes that occur in the crystal under light excitation include the processes (1) and (2) mentioned above, as well as the donor-acceptor radiative recombination transition process (3) and the capture process of carriers by defect energy levels (1') ( Figure 8 a). Process (3) is the same as process (2) in that radiative recombination leads to the annihilation of carriers. However, process (1') increases the lifetime of carriers and thus increases the photoconductive signal, that is, it contributes to the output of electrical signals. The radiative recombination process is measured by the photoluminescence spectrum, and the measurement results are as follows: Figure 8 As shown in Figure b, compared with diamond A, the exciton emission intensity (FEs emission) of diamonds B and C decreases in turn. In addition, diamond B also has defect emission (defect emission) in the visible light range, proving the existence of process (3). The carrier lifetime is measured by the transient excitation voltage response of the device under an electric field. The voltage data is shown in Figure 2. Figure 8 As shown in Figure c, as expected, Diamond A has the shortest carrier lifetime, confirming the rapid annihilation of carriers through radiative recombination. Diamond C, on the other hand, has the longest lifetime, more than ten times that of Diamond A, demonstrating that the larger trap state density in Diamond C leads to significant carrier capture within the crystal.

[0050] Figure 9 a shows the spatial distribution of the photoexcitation excess carrier generation rate G in diamond C (the thickness of diamond C is 780μm). The typical photoexcitation wavelengths are 220nm and 205nm. The corresponding EQE is as follows: Figure 9 c. Obviously, the distribution of excess carrier generation rate under the two wavelength excitations is similar, which is caused by the similar absorption coefficient. Based on the PL spectrum and transient excitation voltage response of diamond C, it can be known that the photogenerated carriers contribute more to process (1') rather than process (2). Therefore, a simplified case can be considered, that is, ignoring the surface recombination of diamond, in which case formula (1) can be simplified to The steady-state distribution of photogenerated carriers obtained is as follows: Figure 9 b. The carrier distribution under 205nm light excitation is similar to that under 220nm light excitation, but the overall carrier concentration of the former is slightly greater than that of the latter, which is consistent with the results shown by EQE. Under the premise of ignoring the surface recombination effect, the photoconductivity formula (2) of the device is simplified to The EQE calculated based on this formula is as follows Figure 9 As shown in Figure c, it shows a trend consistent with the experimentally measured E QE.

[0051] In summary, the above derivation demonstrates that photoelectric testing of diamonds is an effective method for determining diamond quality. The method provided by this invention demonstrates that higher-quality diamonds exhibit narrower half-maximum widths at their spectral response, providing a new and reliable verification method for diamond quality identification.

[0052] The embodiments of the present invention are described in detail above, but the present invention is not limited to the described embodiments. It is apparent to those skilled in the art that various changes, modifications, substitutions, and variations of these embodiments may be made without departing from the principles and spirit of the present invention, and the changes still fall within the scope of protection of the present invention.

Claims

1. A photoelectric testing method for identifying electronic grade diamonds, characterized in that: First, the wavelength-dependent external quantum efficiency of diamond is calculated based on the photocurrent signal of the diamond at different incident light wavelengths. Then, the wavelength-dependent external quantum efficiency spectrum is used to obtain the half-maximum width of the diamond's spectral response peak. Finally, diamonds with a spectral response peak half-maximum width less than 15nm are identified as electronic-grade diamonds. The photoelectric testing method comprises the following steps: S1. Sputter a layer of metal on the top and bottom of the diamond single crystal as the positive and negative electrodes to form a diamond device. The top metal is a semi-transparent electrode, and the positive and negative electrodes are connected to the circuit. S2. Irradiate the top of the diamond device with a light source having an incident light wavelength of 160 to 280 nm under vacuum conditions, use a source meter to output photocurrent signals at different wavelengths, and calculate the external quantum efficiency that varies with wavelength. The half-width at half maximum of the diamond's spectral response peak is then obtained from the wavelength-varying external quantum efficiency spectrum. Diamonds with a half-width at half maximum of a spectral response peak less than 15 nm are identified as electronic-grade diamonds. In step S1, a layer of Pt is sputtered on the top of the diamond single crystal as a semi-transparent anode, and Pt is sputtered on the bottom as a cathode. A glass substrate on which Au metal is evaporated is contacted below the cathode through silver paste. The diamond device is composed of a Pt layer, a diamond layer, a Pt layer, and a glass substrate on which Au metal is evaporated from top to bottom. In step S2, the light source is obtained by a continuous ultraviolet light source and a spectrometer, and the wavelength of the incident light changes from 280 nm to 160 nm.

2. The photoelectric testing method for identifying electronic grade diamonds according to claim 1, characterized in that: In the step S1, the thickness of the metal sputtered on the top of the diamond layer single crystal is 15 to 20 nm.

3. The photoelectric testing method for identifying electronic grade diamonds according to claim 1, characterized in that: The external quantum efficiency is given by the formula Calculated, where I ph is the photocurrent, P is the incident light power, e is the elementary charge, h is the Planck constant, c is the speed of light, and λ is the wavelength of monochromatic light.

4. The photoelectric testing method for identifying electronic grade diamonds according to claim 1, characterized in that: Diamonds with a spectral response peak half-width less than 10 nm are judged to be high-quality electronic-grade diamonds.

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