Raman-inactive interlayer breathing mode testing method for semiconductor thin films

Abstract

The present invention provides a method for testing Raman-inactive interlayer breathing modes of semiconductor slices, which relates to the field of spectroscopy technology. The method comprises: preparing a multilayer two-dimensional semiconductor slice sample on a composite silicon substrate using a transition metal chalcogenide; finding a sample to be tested on the surface of the composite silicon substrate using an optical microscope, wherein the tested area forms a phonon-optical cavity; measuring the low-wavenumber micro-Raman spectrum of the sample in parallel polarization and cross-polarization configurations using a low-wavenumber micro-Raman spectrometer; and subtracting the low-wavenumber micro-Raman spectrum in the cross-polarization configuration from the low-wavenumber micro-Raman spectrum in the parallel polarization configuration to obtain a breathing mode Raman spectrum of the sample, thereby determining the Raman-inactive interlayer breathing modes of the multilayer two-dimensional semiconductor slice sample. The Raman-inactive interlayer breathing modes of the multilayer two-dimensional semiconductor slice sample can be detected by the strong coupling effect between the phonon cavity and the optical cavity of the multilayer two-dimensional semiconductor slice sample on the composite silicon substrate.

Description

Technical Field

[0001] The present invention relates to the field of spectral technology, and in particular to a method for testing a Raman inactive interlayer breathing mode of a semiconductor slice. Background Art

[0002] With the trend toward miniaturization of semiconductor devices, a variety of low-dimensional semiconductor quantum structures have emerged. Two-dimensional semiconductor flakes, primarily transition metal dichalcogenides (MX2), have attracted significant attention due to their rich physical properties, ease of manipulation, and ability to form two-dimensional heterojunctions with sharp interfaces under conditions of lattice mismatch and method incompatibility. A common characteristic of two-dimensional layered materials is that atoms within a layer are covalently coupled, while the layers interact through van der Waals forces. This results in MX2 flakes exhibiting physical properties such as band structures and phonons that vary significantly with the number of layers and interlayer interactions. Phonons are important carriers of heat transport. The two-dimensional nature of MX2 flakes leads to the dominant role of low-wavenumber phonon branch dispersion in thermal diffusion, which determines the thermal management capabilities of MX2 flake materials and devices. This necessitates the detection and manipulation of low-wavenumber interlayer phonons in MX2 flakes. The experimental detection of all low-wavenumber interlayer phonons in MX2 thin films is a prerequisite for studying and regulating the properties of phonons in MX2 thin films, and is of great significance in the application of thermal and electrical transport in semiconductor devices.

[0003] The main technical means of detecting low-wavenumber phonons include low-wavenumber Raman spectroscopy and Brillouin scattering spectroscopy. For a semiconductor device with N layers of MX2 thin films, N layers of MX2 thin films typically have (N-1) low-wavenumber interlayer breathing modes. However, due to the high symmetry of MX2 crystals, half of these low-wavenumber interlayer breathing modes are usually Raman-inactive. Existing detection methods can only detect phonon modes in the Raman-active layer, but cannot detect phonon modes in the Raman-inactive layer. This greatly limits the detection and understanding of low-wavenumber phonons in two-dimensional semiconductors. Summary of the Invention

[0004] The present invention provides a method for testing a Raman inactive interlayer breathing mode of a semiconductor slice, which is used to solve the defect in the prior art that the Raman inactive interlayer breathing mode cannot be directly detected.

[0005] The present invention provides a Raman inactive interlayer breathing mode test of a semiconductor wafer, comprising:

[0006] Using transition metal chalcogenides as semiconductor materials, multilayer two-dimensional semiconductor thin film samples were prepared on a composite silicon substrate;

[0007] Using an optical microscope, a test area of ​​the multi-layer two-dimensional semiconductor thin slice sample is found on the surface of the composite silicon substrate; the test area constitutes a phonon-optical cavity;

[0008] Using a low-wavenumber micro-Raman spectrometer to emit vertically polarized excitation light to the test area and collect vertically polarized scattered light signals to test the low-wavenumber micro-Raman spectrum of the test area in a parallel polarization configuration;

[0009] emitting horizontally polarized excitation light to the test area and collecting vertically polarized scattered light signals using the low-wavenumber micro-Raman spectrometer to measure the low-wavenumber micro-Raman spectrum of the test area in a cross-polarization configuration;

[0010] subtracting the low-wavenumber micro-Raman spectrum under the cross-polarization configuration from the low-wavenumber micro-Raman spectrum under the parallel polarization configuration to obtain a breathing mode Raman spectrum of the multilayer two-dimensional semiconductor thin film sample;

[0011] The Raman inactive interlayer breathing modes of the multilayer two-dimensional semiconductor thin film sample are determined according to the breathing mode Raman spectrum.

[0012] According to the Raman inactive interlayer breathing mode test of the semiconductor slice provided by the present invention, the excitation light wavelength used in the low wavenumber micro-Raman spectroscopy test matches the wavelength corresponding to the C exciton energy of the multilayer two-dimensional semiconductor slice sample.

[0013] According to the Raman inactive interlayer breathing mode test of the semiconductor slice provided by the present invention, the Raman inactive interlayer breathing mode of the multi-layer two-dimensional semiconductor slice sample is determined based on the breathing mode Raman spectrum, including:

[0014] Performing Lorentz fitting on the breathing mode Raman spectrum to obtain the peak position of the breathing mode Raman spectrum;

[0015] Comparing the peak position with the prediction results of a preset linear chain model and a bond polarizability model to obtain a Raman inactive interlayer breathing mode of the multilayer two-dimensional semiconductor thin film sample;

[0016] The linear chain model is used to predict the peak position of the interlayer breathing mode of the multilayer two-dimensional semiconductor thin film sample;

[0017] The bond polarizability model is used to predict the intensity distribution of the interlayer breathing mode of the multilayer two-dimensional semiconductor thin film sample.

[0018] According to the Raman inactive interlayer breathing mode test of the semiconductor slice provided by the present invention, the linear chain model is used to calculate the peak position Pos(LB of the jth breathing mode of the multilayer two-dimensional semiconductor slice sample. N,N-j ) is predicted as follows:

[0019]

[0020] Among them, α ⊥is the interlayer breathing coupling force constant of the two-dimensional semiconductor thin film sample, c is the speed of light, μ is the atomic mass per unit area of ​​a single-layer two-dimensional semiconductor thin film in the multi-layer two-dimensional semiconductor thin film sample, and N is the number of layers of the multi-layer two-dimensional semiconductor thin film sample.

[0021] According to the Raman inactive interlayer breathing mode test of semiconductor slices provided by the present invention, the grating resolution of the low-wavenumber Raman spectrometer is less than 0.3 wavenumbers.

[0022] According to the Raman inactive interlayer breathing mode test of the semiconductor slice provided by the present invention, the number of layers of the multi-layer two-dimensional semiconductor slice sample ranges from 10 to 100, and the two-dimensional size is greater than 2 microns.

[0023] According to the inactive interlayer breathing mode test of the semiconductor slice provided by the present invention, the composite silicon substrate includes a silicon dioxide layer and a silicon crystal layer, and the silicon dioxide layer is formed on the silicon crystal layer.

[0024] According to the Raman inactive interlayer breathing mode test of the semiconductor slice provided by the present invention, the Raman spectrum range of the breathing mode is 5 to 50 wave numbers.

[0025] According to the Raman inactive interlayer breathing mode test of the semiconductor slice provided by the present invention, the transition metal chalcogenide compound includes molybdenum disulfide, tungsten disulfide, molybdenum selenide, tungsten diselenide and molybdenum ditelluride.

[0026] According to the Raman inactive interlayer breathing mode test of semiconductor slices provided by the present invention, the multilayer two-dimensional semiconductor slice sample is prepared by any one of micromechanical exfoliation, dry transfer or wet transfer.

[0027] The Raman-inactive interlayer breathing mode test of semiconductor slices provided by the present invention can detect the Raman-inactive interlayer breathing mode by using parallel-polarized and cross-polarized excitation light and the strong coupling effect of the phonon cavity and optical cavity of the multi-layer two-dimensional semiconductor slice sample itself on the composite silicon substrate. The high spatial overlap of the phonon cavity and the optical cavity enhances the coupling strength between the two, allowing the breathing mode between the Raman-inactive layers of the multi-layer two-dimensional semiconductor slice sample to be detected. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] In order to more clearly illustrate the technical solutions in the present invention or the prior art, a brief introduction is given below to the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0029] Figure 1Schematic diagram of phonon-optical cavity coupling for the Raman-inactive interlayer breathing mode testing method of a semiconductor wafer provided by the present invention;

[0030] Figure 2 Schematic diagram of low-wavenumber micro-Raman spectroscopy under parallel polarization and cross-polarization configurations provided by the present invention;

[0031] Figure 3 This is an optical image of a WS2 thin-sheet sample prepared by the micromechanical exfoliation method provided by the present invention;

[0032] Figure 4 Schematic diagram of the phonon frequency of the breathing mode of the 20-layer WS2 thin film sample provided by the present invention;

[0033] Figure 5 It is a schematic diagram comparing the theoretical values ​​and test results of the low-wavenumber Raman spectra of WS2 thin film samples with different numbers of layers provided by the present invention. DETAILED DESCRIPTION

[0034] To make the objectives, technical solutions, and advantages of the present invention more clear, the technical solutions of the present invention will be clearly and completely described below in conjunction with the accompanying drawings. Obviously, the embodiments described are only some of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts shall fall within the scope of protection of the present invention.

[0035] In the description of the embodiments of the present application, it should be noted that the terms "center", "longitudinal", "lateral", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside", etc., indicating the orientation or positional relationship, are based on the orientation or positional relationship shown in the accompanying drawings, and are only for the convenience of describing the embodiments of the present application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operate in a specific orientation, and therefore cannot be understood as limiting the embodiments of the present application. In addition, the terms "first", "second", and "third" are used for descriptive purposes only and cannot be understood as indicating or implying relative importance.

[0036] In the description of the embodiments of this application, it should be noted that, unless otherwise specified or limited, the terms "connected" and "connection" should be understood in a broad sense. For example, they can refer to fixed connections, detachable connections, or integral connections; they can refer to mechanical connections or electrical connections; they can refer to direct connections or indirect connections through an intermediate medium. Those skilled in the art will understand the specific meanings of the above terms in the embodiments of this application based on the specific circumstances.

[0037] In the embodiments of the present application, unless otherwise expressly specified or limited, a first feature being "above" or "below" a second feature may mean that the first and second features are in direct contact, or that the first and second features are in indirect contact through an intermediate medium. Furthermore, a first feature being "above," "above," and "above" a second feature may mean that the first feature is directly above or obliquely above the second feature, or simply means that the first feature is higher in level than the second feature. A first feature being "below," "below," and "below" a second feature may mean that the first feature is directly below or obliquely below the second feature, or simply means that the first feature is lower in level than the second feature.

[0038] In the description of this specification, the description with reference to the terms "one embodiment", "some embodiments", "example", "specific example", or "some examples" means that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the embodiments of the present application. In this specification, the schematic representations of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described can be combined in any one or more embodiments or examples in a suitable manner. In addition, those skilled in the art can combine and combine different embodiments or examples described in this specification and the features of different embodiments or examples, unless they are contradictory.

[0039] The purpose of the present invention is to provide an effective method for testing the Raman inactive interlayer breathing mode of semiconductor thin slices prepared from transition metal chalcogenides (MX2). The method can not only break through the limitations of traditional Raman selection rules, but also has high accuracy within a certain range of MX2 thin slice layers, and is universal for different multilayer two-dimensional semiconductors, and is suitable for the detection of Raman inactive interlayer breathing modes of a variety of multilayer two-dimensional semiconductor samples.

[0040] Specifically, the embodiment of the present invention provides a method for testing the Raman inactive interlayer breathing mode of a semiconductor wafer, comprising:

[0041] S1, using transition metal chalcogenides as semiconductor materials, to prepare multilayer two-dimensional semiconductor thin film samples on a composite silicon substrate;

[0042] First, a transition metal chalcogenide is selected as the semiconductor material for fabricating a multilayer two-dimensional semiconductor thin slice sample on a composite silicon substrate. Multiple multilayer two-dimensional semiconductor thin slice samples can be fabricated on the same composite silicon substrate, and the number of layers in each of the multiple two-dimensional semiconductor thin slice samples can be the same or different, without specific limitation.

[0043] Composite silicon substrates can improve the laser reflectivity at the interface between multilayer two-dimensional semiconductor slices and substrates.

[0044] S2, using an optical microscope to find a test area of ​​the multi-layer two-dimensional semiconductor thin film sample on the surface of the composite silicon substrate, wherein the test area constitutes a phonon-optical cavity;

[0045] An optical microscope is used to find a test area of ​​a multi-layer two-dimensional semiconductor thin film sample on the surface of a composite silicon substrate. The multi-layer two-dimensional semiconductor thin films in the test area constitute a phonon-optical cavity.

[0046] The multilayer two-dimensional semiconductor thin film sample on the composite silicon substrate is not only a phonon cavity for the interlayer breathing phonon vibration, but also an optical cavity for the incident excitation light and scattered light. The spatial coupling between the two leads to a strong coupling effect between the phonon cavity and the optical cavity. Figure 1 The coupling diagram of the phonon cavity and the optical cavity is shown in Figure 1 In the embodiment, a multilayer two-dimensional semiconductor thin slice sample 2 prepared by using a transition metal chalcogenide is formed on a composite silicon substrate 1. An optical microscope is used to find the multilayer two-dimensional semiconductor thin slice sample area to be tested on the surface of the composite silicon substrate, namely, test area 2. The multilayer two-dimensional semiconductor thin slice sample in test area 2 forms a phonon-optical cavity in which the incident excitation light and scattered light are coupled with the interlayer breathing mode in the sample, which is used to test the Raman inactive interlayer breathing mode of the multilayer two-dimensional semiconductor thin slice sample. The coupling principle of the phonon-optical cavity can be referred to Figure 1 Part 2-1 is a partially enlarged schematic diagram of the test area 2.

[0047] S3, using a low-wavenumber micro-Raman spectrometer to emit vertically polarized excitation light to the test area and collect vertically polarized scattered light signals to measure the low-wavenumber micro-Raman spectrum of the test area in a parallel polarization configuration;

[0048] The low-wavenumber Raman micro-spectrometer is equipped with an incident light path and a scattered light path. The incident light path is equipped with a polarizer and a half-glass slide, whose applicable wavelength range includes a specific wavelength. The scattered light path is equipped with an analyzer, whose applicable wavelength range includes a specific wavelength. By adjusting the position and / or angle of the polarizer, a horizontally polarized light path corresponding to vertically polarized excitation light and vertically scattered light, and a cross-polarized light path corresponding to horizontally polarized excitation light and vertically scattered light can be configured. This allows the low-wavenumber Raman micro-spectroscopy of Area 2 to be measured in both parallel and cross-polarized configurations.

[0049] When testing test area 2, a low-wavenumber micro-Raman spectrometer was used to emit vertically polarized excitation light into test area 2 and collect vertically polarized scattered light signals, thereby measuring the low-wavenumber micro-Raman spectrum of test area 2 in a parallel polarization configuration. Specifically, by adjusting the focus knob of the low-wavenumber micro-Raman spectrometer's objective lens, the first-order Raman signal of the transition metal chalcogenide with a maximum intensity at approximately 420 wavenumbers was found. Under this focusing condition, the low-wavenumber micro-Raman spectrum was measured in a parallel polarization configuration. Parallel polarization configuration refers to the polarization direction of the incident excitation light being parallel to the polarization direction of the scattered light.

[0050] S4, using the low-wavenumber micro-Raman spectrometer to emit horizontally polarized excitation light to the test area and collect vertically polarized scattered light signals to measure the low-wavenumber micro-Raman spectrum of the test area in a cross-polarization configuration;

[0051] A low-wavenumber micro-Raman spectrometer was used to emit horizontally polarized excitation light into test area 2 and collect vertically polarized scattered light signals to measure the low-wavenumber micro-Raman spectrum of test area 2 in a cross-polarization configuration. Specifically, half of the slide in the incident light path was rotated 45 degrees, and the low-wavenumber micro-Raman spectrum was measured in a cross-polarization configuration. Cross-polarization configuration means that the polarization direction of the incident excitation light is perpendicular to the polarization direction of the scattered light.

[0052] In one embodiment, a multilayer two-dimensional semiconductor thin film sample prepared using tungsten disulfide WS2 as a semiconductor material has low-wavenumber micro-Raman spectra under parallel polarization configuration and low-wavenumber micro-Raman spectra under cross polarization configuration as shown in FIG. Figure 2 As shown. Figure 2 In the figure, LB is the peak position of the breathing mode, and S is the peak position of the shear mode.

[0053] S5, subtracting the low-wavenumber micro-Raman spectrum in the cross-polarization configuration from the low-wavenumber micro-Raman spectrum in the parallel polarization configuration to obtain a breathing mode Raman spectrum of the multilayer two-dimensional semiconductor thin film sample;

[0054] S6, determining a Raman-inactive interlayer breathing mode of the multi-layer two-dimensional semiconductor thin film sample according to the breathing mode Raman spectrum.

[0055] The low-wavenumber micro-Raman spectrum under the parallel polarization configuration is subtracted from the low-wavenumber micro-Raman spectrum under the cross-polarization configuration to obtain the breathing mode Raman spectrum of the multilayer two-dimensional semiconductor thin film sample. The breathing mode Raman spectrum includes the Raman active interlayer breathing mode Raman spectrum and the Raman inactive interlayer breathing mode Raman spectrum of the multilayer two-dimensional semiconductor thin film sample.

[0056] Based on the obtained breathing mode Raman spectrum, the Raman inactive interlayer breathing mode of the multilayer two-dimensional semiconductor thin film sample can be determined. Transition metal chalcogenide crystals have high symmetry. For common multilayer two-dimensional semiconductor thin films, samples with an even number of layers belong to D 3d Symmetry, samples with an odd number of layers belong to D 3h Symmetry, odd-numbered breathing pattern (LB N,N-i , i=1,3,5,…) is the Raman active layer, and the even-numbered breathing modes (i.e. LB N,N-i , i=2,4,6,…) is a Raman inactive layer.

[0057] Furthermore, determining the Raman inactive interlayer breathing mode of the multilayer two-dimensional semiconductor thin film sample based on the obtained breathing mode Raman spectrum specifically includes:

[0058] S61, performing Lorentz fitting on the breathing mode Raman spectrum to obtain the peak position of the breathing mode Raman spectrum;

[0059] S62, comparing the peak position with the prediction results of a preset linear chain model and a bond polarizability model to obtain a Raman inactive interlayer breathing mode of the multilayer two-dimensional semiconductor thin film sample;

[0060] The linear chain model is used to predict the peak position of the interlayer breathing mode of the multilayer two-dimensional semiconductor thin film sample;

[0061] The bond polarizability model is used to predict the intensity distribution of the breathing mode between active layers of the multilayer two-dimensional semiconductor thin film sample.

[0062] The breathing mode Raman spectra of the multilayer two-dimensional semiconductor flakes obtained are microscopic Raman spectra in the 5-50 wavenumber range. The peak positions of the breathing modes (LB modes) are determined by Lorentz fitting. The peak positions of the breathing modes are compared with the predictions of the pre-set linear chain model and the bond polarizability model to identify the Raman-inactive interlayer breathing modes of the multilayer two-dimensional semiconductor flake samples.

[0063] The preset linear chain model and bond polarizability model are derived based on prior data on breathing modes in multilayer two-dimensional semiconductor flake samples. The linear chain model can be used to predict the peak positions of all interlayer breathing modes in multilayer two-dimensional semiconductor flake samples, while the bond polarizability model can be used to predict the intensity distribution of all interlayer breathing modes in multilayer two-dimensional semiconductor flake samples. The linear chain model's predictions include the peak positions of both Raman-active and Raman-inactive interlayer breathing modes in multilayer two-dimensional semiconductor flake samples.

[0064] Furthermore, the linear chain model is used to calculate the peak position Pos(LB of the jth breathing mode of the multilayer two-dimensional semiconductor thin film sample. N,N-j) is predicted as follows:

[0065]

[0066] Among them, α ⊥ is the interlayer breathing coupling force constant of the two-dimensional semiconductor thin film sample, c is the speed of light, μ is the atomic mass per unit area of ​​a single-layer two-dimensional semiconductor thin film in the multi-layer two-dimensional semiconductor thin film sample, and N is the number of layers of the multi-layer two-dimensional semiconductor thin film sample. For WS2, α ⊥ =8.82×10 19 N·m -3 , μ=4.67×10 -6 kg·m -2 .

[0067] The atomic displacement of the i-th layer of the two-dimensional semiconductor thin film sample is The normalized Raman intensity of the jth breathing mode is in is the Bose-Einstein distribution function, T is the thermodynamic temperature, k B is the Boltzmann constant.

[0068] In some embodiments, the composite silicon substrate includes a silicon dioxide layer (SiO2) and a silicon crystal (Si) layer, and the SiO2 layer is formed on the Si layer. A transition metal chalcogenide is used as a semiconductor material to prepare a multilayer two-dimensional semiconductor thin film sample on a composite silicon substrate (SiO2 / Si substrate). The transition metal chalcogenide includes molybdenum disulfide MoS2, tungsten disulfide WS2, molybdenum selenide MoSe2, tungsten diselenide WSe2 and molybdenum ditelluride MoTe2. Methods for preparing multilayer two-dimensional semiconductor thin film samples include but are not limited to micromechanical exfoliation, dry or wet transfer. Figure 3 , Figure 3 This is an optical image of a multilayer two-dimensional semiconductor thin film sample prepared by micromechanical exfoliation using WS2 as the semiconductor material. Figure 3 The part corresponding to 3-1 shows a two-dimensional semiconductor thin film sample with 20 layers and a lateral size greater than 15 microns, and the part corresponding to 3-2 shows a two-dimensional semiconductor thin film sample with 38 layers and a lateral size greater than 15 microns.

[0069] Furthermore, the multilayer two-dimensional semiconductor thin slice samples to be tested have a number of layers ranging from 10 to 100, and a two-dimensional dimension greater than 2 microns. The excitation wavelength used in low-wavenumber micro-Raman spectroscopy testing matches the wavelength corresponding to the C exciton energy of the multilayer two-dimensional semiconductor thin slice sample. Specifically, based on the transfer matrix method and the coupling effect between the phonon cavity and the optical cavity, the backscattered signal of the interlayer breathing mode corresponding to the excitation wavelength is calculated within the thin slice of a specific number of MX2 thin slices on a composite silicon substrate. Within the backscattered signal, the amplitude ratio of the Raman-inactive interlayer breathing mode component to the nearby Raman-active interlayer breathing mode component is calculated. The number of MX2 thin slices corresponding to the extreme value of the amplitude ratio is determined, and a corresponding relationship between the amplitude ratio and the number of MX2 thin slice layers is established. In actual testing, the excitation light wavelength is selected to match the wavelength corresponding to the C exciton energy of the multilayer two-dimensional semiconductor thin slice sample based on the selected semiconductor material and the number of layers of the two-dimensional semiconductor thin slice sample to be tested.

[0070] Taking the preparation of WS2 thin film samples on SiO2 / Si substrate with WS2 as the semiconductor material as an example, an optical microscope is used to find the multi-layer two-dimensional semiconductor thin film sample area to be tested on the surface of the SiO2 / Si substrate, and a laser light source with a wavelength matching the wavelength corresponding to the C exciton energy of the multi-layer two-dimensional semiconductor thin film to be tested is selected. A low-wavenumber micro-Raman spectrometer is used to test the micro-Raman spectrum of the multi-layer two-dimensional semiconductor thin film on the SiO2 / Si substrate in the wavenumber range of 5 to 50 under parallel polarization and cross-polarization scattering configurations.

[0071] In some embodiments, the grating resolution of the low-wavenumber micro-Raman spectrometer used in the test is less than 0.3 wavenumbers; the laser power irradiated on the sample surface during the test is less than 0.3 milliwatts to avoid the heating effect of the laser on the sample; the parallel polarization scattering configuration refers to the polarization direction of the incident excitation light being parallel to the polarization direction of the scattered light, and usually the polarization direction of the incident excitation light is specified as vertical polarization, and the polarization direction of the scattered light is also specified as vertical polarization; the cross-polarization scattering configuration refers to the polarization direction of the incident excitation light being perpendicular to the polarization direction of the scattered light, and usually the polarization direction of the incident excitation light is specified as horizontal polarization direction, and the polarization direction of the scattered light is specified as vertical polarization direction.

[0072] Under the excitation of the excitation light, the WS2 thin film produces an interlayer breathing vibration mode with a wave function extending to all layers. The vibration direction is perpendicular to the two-dimensional plane, and the vibration direction and amplitude are:

[0073]

[0074] t is the thickness of a single layer of two-dimensional semiconductor material, m is the layer index, N is the total number of layers in the two-dimensional semiconductor slice, and j is the phonon mode index. Since silicon compounds have lower phonon impedance than conventional two-dimensional semiconductor materials, the phonon impedance mismatch between the two is approximately -1, that is, the interlayer breathing vibration of the multilayer two-dimensional semiconductor slice is localized in the two-dimensional semiconductor slice and will not be transmitted to the composite silicon substrate. In addition, for the jth interlayer vibration phonon,

[0075]

[0076] Interlayer breathing mode phonons form standing wave vibrations in multilayer two-dimensional semiconductor thin films, which are similar to the phonon vibrations in an open phonon cavity. Therefore, multilayer two-dimensional semiconductor thin films are phonon cavities for interlayer breathing vibrations.

[0077] In addition, the incident photons will simultaneously excite electron-hole pairs in multi-layer two-dimensional semiconductor films, forming C excitons localized in each layer. The interaction between the standing wave-like interlayer breathing phonon vibrations and a series of localized C exciton states can be described by deformation potential, thereby generating scattered photons.

[0078] Because of the significant refractive index differences between the air-2D semiconductor slice and the 2D semiconductor slice-composite silicon substrate interfaces, both incident and scattered light are reflected and refracted at these interfaces. This causes the forward and reverse light waves to coherently form within the multilayer 2D semiconductor slice, generating optical standing waves localized within the slice. Therefore, the multilayer 2D semiconductor slice also functions as an optical cavity for both incident and scattered light.

[0079] The direct overlap of the phonon cavity and the optical cavity in space leads to strong coupling between the phonon cavity and the optical cavity, which directly leads to spatially modulated photon-electron interaction and electron-phonon interaction, which are respectively determined by the incident light, the scattered light electric field distribution and the interlayer breathing phonon vibration displacement vector. The coherent superposition of the spatially modulated incident photon-electron interaction, electron-phonon interaction and scattered photon-electron interaction periodically modulates the Raman intensity of each interlayer phonon mode. The relative intensity of the odd and even branches of the interlayer breathing phonon depends on the reflectivity of the two-dimensional semiconductor thin film-composite silicon substrate, as well as the scattering -Incident light The wave vector difference and the interlayer phonon wave vector (q j ), is the complex refractive index of WS2. If Δk=k s -k i and Δq=q j -q j-1If the reflectivity of the two-dimensional semiconductor wafer-composite silicon substrate is higher, the relative intensities of the odd- and even-numbered interlayer phonons approach unity.

[0080] For WS2, the C exciton energy is about 2.7eV, corresponding to an excitation light wavelength of 460 nanometers. Therefore, an excitation light with a wavelength of 488 nanometers can be used to resonantly excite the WS2 thin film sample and measure the WS2 Raman spectral signal at 5 to 50 wavenumbers under parallel polarization and cross-polarization scattering configurations. The specific steps are as follows:

[0081] First, a polarizer with an applicable wavelength range including 488 nanometers and half a glass slide are placed on the incident light path, where the polarization direction of the polarizer is vertical and the fast axis of the half glass slide is located at 0 degrees of correction. At the same time, a polarizer with an applicable wavelength range including 488 nanometers is placed on the scattered light path, and the polarization direction is vertical.

[0082] Secondly, by adjusting the focusing knob of the low-wavenumber microscope Raman objective lens, the WS2 first-order Raman signal with the maximum intensity at around 420 wavenumbers was found, and under this focusing condition, the WS2 Raman spectrum signal with 5 to 50 wavenumbers under the parallel polarization configuration was tested.

[0083] Then, half of the glass slide on the incident light path was rotated 45 degrees, and the WS2 Raman spectrum signal with a wave number of 5 to 50 was obtained under the cross-polarization configuration.

[0084] The breathing mode Raman spectrum of the two-dimensional semiconductor thin film sample was obtained by subtracting the low-wavenumber micro-Raman spectrum under the cross-polarization configuration from the low-wavenumber micro-Raman spectrum under the parallel polarization configuration, and the peak position of the breathing mode was obtained by Lorentz fitting.

[0085] like Figure 4 As shown, Figure 4 Yes Figure 3 Schematic diagram of the phonon frequency of the breathing mode of the 20-layer WS2 flake sample shown in, Figure 4 From left to right in the figure are the prediction results of the peak position of the interlayer breathing mode of the 20-layer WS2 thin film sample by the linear chain model, the test results of the interlayer breathing mode of the 20-layer WS2 thin film sample, and the theoretical results of the active interlayer breathing mode of the 20-layer WS2 thin film sample. Figure 4 Comparing the prediction results of the linear chain model in the leftmost image, it is found that the peak position of the interlayer breathing mode phonon of the 20-layer WS2 thin film sample obtained by the test corresponds to the model prediction results. In addition, in addition to observing the original Raman active odd-numbered interlayer breathing phonon vibration (such as Figure 4 As shown in the rightmost figure, the original theoretical prediction of Raman inactive even-branch interlayer breathing phonons LB20,20-j (j = 2, 4, 6, ...) can also be detected. Therefore, by selecting an excitation light with a wavelength that matches the wavelength corresponding to the C exciton energy to resonantly excite a multilayer two-dimensional semiconductor thin film sample on a composite silicon substrate to enhance the phonon cavity-optical cavity coupling effect, it is possible to test and obtain Raman-inactive interlayer breathing modes.

[0086] With the development of van der Waals heterojunction and related device technologies, most two-dimensional semiconductor devices usually contain dozens of layers of semiconductor materials such as MX2. However, current research on low-wavenumber interlayer phonons in MX2 thin films is usually limited to less than 10 layers, which further limits the improvement of thermal and electrical transport performance of two-dimensional semiconductor devices.

[0087] In order to further verify the role of phonon cavity-optical cavity coupling effect in testing Raman inactive interlayer phonon modes, WS2 thin film samples with different numbers of layers (such as 2 layers, 15 layers and 44 layers) can also be prepared on SiO2 / Si composite silicon substrates, and the phonon frequency of the low-wavenumber Raman spectrum of each sample under excitation light can be tested. The test results are as follows Figure 5 As shown, Figure 5 The low-wavenumber Raman spectra of each sample under 488 nm excitation light and the low-wavenumber Raman spectra of 44-layer WS2 thin film sample under 458 nm excitation light are shown. ex is the wavelength of the excitation light. Figure 5 In the low-wavenumber Raman spectrum diagram of WS2 flake samples with the same number of layers, the upper curve corresponds to the theoretical value, and the lower curve corresponds to the actual test result. Figure 5 The test results shown in the figure show that only the Raman active interlayer phonon mode can be detected in the 2-layer WS2 thin film sample, and a Raman inactive even-branch phonon mode LB can be detected in the 15-layer WS2 thin film sample. 15,13 . As the number of layers increases further, Δk is approximately equal to Δq, and all Raman forbidden even-branch interlayer phonon modes can be detected. This effect is more significant when the wavelength of the excitation light is close to the wavelength corresponding to the C exciton energy of the WS2 thin film sample. This illustrates that the detection of Raman-inactive interlayer breathing modes can be achieved based on the phonon-optical cavity coupling effect. At the same time, it also illustrates that the Raman-inactive interlayer breathing mode test method for semiconductor thin film samples provided by the present invention is suitable for the characterization of Raman-inactive interlayer breathing modes of various multi-layer two-dimensional semiconductor thin film samples.

[0088] The present invention provides a method for testing Raman-inactive interlayer breathing modes in semiconductor wafers. The experimental testing method is simple, widely applicable, and highly accurate. First, the strong coupling effect between the phonon cavity and optical cavity of a multilayer two-dimensional semiconductor wafer sample on a SiO2 / Si composite silicon substrate is utilized to characterize the Raman-inactive interlayer phonon modes. The complete spatial overlap of the phonon and optical cavities enhances the coupling strength between them. This testing method does not require additional microcavity preparation processes and is simple and easy to perform.

[0089] Furthermore, by using excitation light with a wavelength matching the energy of the C excitons in the two-dimensional semiconductor thin slice sample to resonantly excite the sample, the Raman intensity is significantly enhanced under resonant excitation, making it easier to detect. Furthermore, since the excitons of conventional two-dimensional semiconductor thin slice samples are in the visible light range, a standard low-wavenumber micro-Raman spectroscopy system with visible light can meet the testing requirements, making it universally applicable.

[0090] Furthermore, the strong coupling effect between phonon cavities and optical cavities is common in multilayer two-dimensional semiconductor wafers and is suitable for the characterization of phonon modes between Raman-inactive layers of different two-dimensional semiconductors.

[0091] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some of the technical features therein. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the various embodiments of the present invention.

Claims

1. A method for testing Raman-inactive interlayer breathing modes of semiconductor wafers, characterized in that: include: Using transition metal chalcogenides as semiconductor materials, multilayer two-dimensional semiconductor thin film samples were prepared on a composite silicon substrate; Using an optical microscope, finding a test area of ​​the multi-layer two-dimensional semiconductor thin film sample on the surface of the composite silicon substrate; The test area constitutes a phonon-optical cavity; Using a low-wavenumber micro-Raman spectrometer to emit vertically polarized excitation light to the test area and collect vertically polarized scattered light signals to test the low-wavenumber micro-Raman spectrum of the test area in a parallel polarization configuration; The low-wavenumber micro-Raman spectrometer is used to emit horizontally polarized excitation light to the test area and collect vertically polarized scattered light signals to test the low-wavenumber micro-Raman spectrum of the test area in a cross-polarization configuration; the excitation light wavelength used in the low-wavenumber micro-Raman spectrum test matches the wavelength corresponding to the C exciton energy of the multilayer two-dimensional semiconductor thin film sample; subtracting the low-wavenumber micro-Raman spectrum under the cross-polarization configuration from the low-wavenumber micro-Raman spectrum under the parallel polarization configuration to obtain a breathing mode Raman spectrum of the multilayer two-dimensional semiconductor thin film sample; Performing Lorentz fitting on the breathing mode Raman spectrum to obtain the peak position of the breathing mode Raman spectrum; Comparing the peak position with the prediction results of a preset linear chain model and a bond polarizability model to obtain a Raman inactive interlayer breathing mode of the multilayer two-dimensional semiconductor thin film sample; The linear chain model is used to predict the peak position of the interlayer breathing mode of the multilayer two-dimensional semiconductor thin film sample; The bond polarizability model is used to predict the intensity distribution of the interlayer breathing mode of the multilayer two-dimensional semiconductor thin film sample.

2. The method for testing the Raman inactive interlayer breathing mode of a semiconductor wafer according to claim 1, wherein: The linear chain model is used to describe the first j The peak position of the breathing pattern The prediction results are: ； in, is the interlayer breathing coupling force constant of the two-dimensional semiconductor thin film sample, is the speed of light, is the atomic mass per unit area of ​​a single-layer two-dimensional semiconductor thin film in the multi-layer two-dimensional semiconductor thin film sample, N is the number of layers of the multi-layer two-dimensional semiconductor thin film sample.

3. The method for testing the Raman inactive interlayer breathing mode of a semiconductor wafer according to claim 1, wherein: The grating resolution of the low-wavenumber Raman spectrometer is less than 0.3 wavenumbers.

4. The method for testing the Raman inactive interlayer breathing mode of a semiconductor wafer according to claim 1, wherein: The number of layers of the multi-layer two-dimensional semiconductor thin film sample ranges from 10 to 100, and the two-dimensional size is greater than 2 microns.

5. The method for testing the Raman inactive interlayer breathing mode of a semiconductor wafer according to claim 1, wherein: The composite silicon substrate includes a silicon dioxide layer and a silicon crystal layer, wherein the silicon dioxide layer is formed on the silicon crystal layer.

6. The method for testing the Raman inactive interlayer breathing mode of a semiconductor wafer according to claim 1, wherein: The Raman spectrum of the breathing mode ranges from 5 to 50 wave numbers.

7. The method for testing the Raman inactive interlayer breathing mode of a semiconductor wafer according to claim 1, wherein: The transition metal chalcogenide compounds include molybdenum disulfide, tungsten disulfide, molybdenum selenide, tungsten diselenide and molybdenum ditelluride.

8. The method for testing the Raman inactive interlayer breathing mode of a semiconductor wafer according to claim 1, wherein: The multilayer two-dimensional semiconductor thin film sample is prepared by using any one of micromechanical peeling, dry transfer or wet transfer.

Images