Method for controlled thinning of a multilayer van der Waals coating system

Abstract

A method for the controlled production of an atomic monolayer by thinning atomic multiple layers arranged on a substrate, wherein, to determine the number of atomic layers during production, at least one energy spectrum of photons re-emitted by multiple layers after irradiation is recorded, characterized in that, depending on the determined photon energies, thinning parameters are dynamically adjusted according to the progress of the monolayer production, and wherein the thinning is carried out by means of photons or electrically charged particles, and between the pulses of the photons or the electrically charged particles, the processed layer is irradiated with a photon beam of a preferred spectral range for Raman and photoluminescence spectroscopy.

Description

[0001] The invention relates to a method for producing an atomic monolayer according to the preamble of claim 1.

[0002] It is known that some materials whose crystal structures consist of layered systems change their physical properties when they exist as a single monolayer. The best-known example of such a material is graphene. But semiconductor materials such as molybdenum disulfide (MoS₂) or other transition metal dichalcogenides such as WS₂, WSe₂, MoSe₂, etc., also exhibit this behavior.

[0003] While bilayers and all other multilayer systems of these materials, especially MoS2, exhibit an indirect band gap, a monolayer possesses a direct band gap. This direct band gap of monolayers enables the realization of entirely new electronic and optoelectronic components. Due to the associated significant application potential, efforts are being made to produce monolayers of these materials with high reliability and quality.

[0004] Currently, the fabrication of atomic monolayers is uncontrolled. Various methods for producing single-layer MoS₂ coatings are known. These include, for example, mechanical and chemical exfoliation, vapor deposition, and molecular beam epitaxy. With each of these methods, predominantly bilayers or multilayers remain alongside the monolayer, which reduces the quality of the monolayer structure. Another method for producing a monolayer is thinning by laser irradiation. Here, multilayer coatings are irradiated and scanned with a high-intensity laser beam. This laser irradiation gradually reduces the number of multilayer coatings until a single monolayer remains on the substrate. This method is therefore, in principle, well-suited for processing multilayer semiconductor coatings to produce large-area monolayers. In this way, it is possible to create monolayers with a width of approximately 1000 cm.to produce lengths of a few tens of micrometers.

[0005] In known methods, the layer-by-layer thinning of the semiconductor material is measured by a Raman mode A. 1gand monitored via optical contrast. This means that while the laser beam is moved across the multilayers at constant power and step size, Raman spectroscopy is simultaneously performed on the photons emitted from the surface of the multilayers. Adjusting the thinning parameters, such as the laser power, the step size of the laser radiation on the surface, or the spacing of the grid points depending on the achieved thinning, has not yet been carried out. Subsequent control examinations of the processed surfaces or the monolayers reveal a strong variation in the optical activity of the layers. This means that either a bi- or multilayer is present due to insufficient irradiation, or the monolayer has been damaged by excessive irradiation.

[0006] Other methods are known in which the distance between the Raman modes E 1 2g and A1g The Raman modes are evaluated during laser thinning. The spacing of these Raman modes allows for an estimation of the number of layers on the substrate. For example, monolayers of molybdenum disulfide exhibit a mode difference of approximately 19 cm. -1 on, while bilagen show a difference of 21 cm -1 up to 22 cm -1 One problem that arises is that the spacing of the Raman modes between unprocessed monolayers and monolayers after laser thinning differs. The spacing of the Raman modes of a laser-thinned monolayer is approximately 20 cm. 1 This is the difference between unprocessed monolayers and bilayers. Reliable production of large-area monolayers is therefore not possible with known methods. Monolayers of high quality, which is necessary for certain applications, cannot be produced.

[0007] In the scientific paper "Hu, L., Shan, X., Wu, Y. et al. Laser Thinning and Patterning of MoS2 with Layer-by-Layer Precision. Sci Rep 7, 15538 (2017). https: / / doi.org / 10.1038 / s41598-017-15350-4", MoS2 is thinned using a laser. During this process, Raman spectroscopy is used to monitor the number of atomic layers. The laser intensity and the exposure time of the MoS2 are controlled during thinning.

[0008] “Castellos-Gomez [et al.]. Laser-Thinning of MoS2: On demand Generation of a Single-Layer Semiconductor. Nano Letters 20212 12 (6), 3187-3192 [online]. DOI: 10.1021 / nl301164v” reveals how the laser of a Raman spectrometer is used to thin a MoS2 layer.

[0009] “Lin, Z., Ye, X., Han, J. et al. Precise Control of the Number of Layers of Graphene by Picosecond Laser Thinning. Sci Rep 5, 11662 (2015). https: / / doi.org / 10.1038 / srep11662” reveals how graphene can be precisely thinned into thin layers using laser thinning, which are then measured using Raman spectroscopy.

[0010] The present invention is therefore based on the objective of creating a method for the controlled production of an atomic monolayer, by which a monolayer with a high degree of homogeneity can be produced.

[0011] A method for solving this problem comprises the measures of claim 1. Accordingly, it is provided that during the thinning of the material, at least one energy spectrum of photons, which are re-emitted by the multiple layers after irradiation, is recorded. Depending on the determined photon energies, the thinning parameters are then dynamically adjusted according to the progress of the monolayer fabrication. The thinning is carried out using photons or electrically charged particles, and between the pulses of the photons or electrically charged particles, the processed layer is irradiated with a photon beam of a preferred spectral range for Raman and photoluminescence spectroscopy. By adjusting the laser thinning parameters to the progress of the fabrication, a particularly homogeneous monolayer can be produced on the substrate.The energy spectrum of the emitted photons allows conclusions to be drawn about how many atomic layers remain on the substrate. As soon as only a single layer remains, this is detectable by the photon energy spectra. The surface, or a specific area or pixel of the multilayers, is then thinned or processed until a single layer is identified based on the photon energy spectra. As the process approaches a single layer, the process parameters are dynamically adjusted to minimize the probability of the remaining layer being destroyed.

[0012] In particular, according to the invention, photons, preferably a laser or a laser diode, or electrically charged particles, especially electrons or ions, can be used for thinning, wherein, depending on the determined photon energies, an energy and / or an intensity and / or a pulse rate and / or a pulse duration and / or a step size of the thinning or the spacing of the raster points on the layer of a photon beam or a beam of electrically charged particles is varied as a thinning parameter. The energy transfer of the photons or the electrons or the ions to the individual layers causes them to be removed layer by layer. Due to the covalent bonds between the atoms of a layer, it must be subjected to at least an energy with an intensity sufficient to dissociate the bond. For example, it is provided that a laser with a wavelength of 325 nm is used.The wavelength can also be smaller or larger, for example, 244 nm, 406 nm, or 650 nm. However, it is intended to use wavelengths that are also suitable for generating the photon energy spectra. Depending on the determined photon energies, the aforementioned dilution parameters must be changed individually or in combination. The extent to which the dilution parameters need to be adjusted can be determined by a control unit or software. The control data for the adjustment is then transmitted to the corresponding apparatus for producing the monolayer. The advantage of using charged particles is that even nanostructured structures can be produced on the substrate or on the multiple layers; whereas the use of lasers limits the size of the structures that can be produced to a few hundred nm.The aforementioned thinning parameters are adjusted in such a way and for as long as necessary until the entire target area on the substrate consists of only an atomic monolayer.

[0013] Preferably, for thinning, a region of the multilayers can be scanned stepwise several times in a raster by the photon or particle beam, with photon energy spectra being recorded during this process. While the spot sizes of the laser beams are several hundred nm, the spot size of a particle beam is significantly smaller. Accordingly, the various beams must be moved across the multilayers or the substrate more frequently to thin the entire area. Alternatively, however, the laser spot or the focus of the particle beam can remain at a point until the photoluminescence and / or Raman signals have reached predefined intensities. Only then is the next raster point approached.

[0014] Furthermore, according to the invention, it can be provided that during the production of the monolayer, a temporal evolution and / or a spacing of several Raman modes, preferably of the E, can be determined from the photon energy spectra over several measurements or thinning steps. 1g - and / or the E 1 2g - and / or the A 1g - and / or the B 1 2g - and / or the MoO xThe Raman mode and / or the Si mode and / or second-order modes are recorded from the processed layer, and the number of layers on the substrate is determined based on the intensities of the Raman modes and / or the spacing between the modes. In particular, determining the spacing and / or intensities of at least some of the aforementioned modes can lead to a very reliable result regarding the number of layers on the substrate. The intensity of some modes increases with decreasing layer count, while the intensities of others decrease. Similarly, some modes shift relative to each other with decreasing layer count. By observing the temporal evolution of at least some of the aforementioned modes during thinning, it is possible to identify a monolayer very reliably. The substrate's Raman mode, in particular, has a distinct characteristic, so that it is immediately apparent when the monolayer has also been thinned.After the multilayers are exposed to a photon beam, electron beam, or ion beam for thinning, at least one photon energy spectrum is recorded to determine the temporal evolution of the Raman modes. For sufficient statistical analysis, several photon energy spectra can be recorded and evaluated per thinning step.

[0015] Furthermore, it can be provided that during the fabrication of the monolayer, a time evolution of a photoluminescence spectrum, particularly between 1.0 eV and 3.0 eV, preferably between 1.5 eV and 2.5 eV, is recorded from the photon energy spectra of the processed layer during several measurements or thinning steps, and the number of layers on the substrate is determined based on the measured energy-selective photon intensities. Because the monolayer exhibits a direct band gap, the achievement of a monolayer is readily apparent from the photoluminescence spectrum. As long as a multilayer remains on the substrate, the individual layers exhibit an indirect band gap, resulting in a significantly different photoluminescence spectrum than that of a monolayer. In photoluminescence spectroscopy, photon energies are detected and evaluated across a sufficiently large spectral range, for example, from 500 nm to 1000 nm.

[0016] Another advantageous embodiment of the present invention provides that both Raman spectra and photoluminescence spectra are recorded during the fabrication of the monolayer to determine the number of layers. By recording two different types of spectra, which capture different physical effects, the precision of the layer number determination during the thinning process can be improved. For MoS₂, in particular, a very reliable result regarding the number of monolayers can be obtained with high temporal resolution and a laser with a wavelength of 325 nm. For other materials, other wavelengths or energies are more suitable. The preferred laser energies for Raman spectroscopy of various materials are well known from the literature.The use of wavelengths in the visual spectral range is preferable, as the measurement setup is much easier to handle and cheaper.

[0017] Preferably, the Raman spectra and / or photoluminescence spectra can be recorded, particularly using a beam splitter, during or concurrently with laser thinning, wherein the photons detected for the Raman and / or photoluminescence spectra result from an interaction of the thinning laser with the layers. The detected photons from which the Raman and / or photoluminescence spectra are generated thus result directly from the thinning process. This means that only one radiation source is required for the fabrication of the monolayer and the simultaneous monitoring of the fabrication process. Since the Raman and photoluminescence spectra lie in different wavelength or spectral ranges, two different photon detectors must be used for energy-selective measurement.

[0018] It can also be provided that the monolayer is made of transition metal dichalcogenides, preferably MoS2, WS2, WSe2, heterostructures of different 2D materials, 2D insulators, especially BN, or 2D semimetals such as graphene. Furthermore, it is conceivable that other materials could be produced in a controlled manner using the method described here.

[0019] Another aspect of the present invention may consist in the fact that the Raman and / or photoluminescence spectra are automatically analyzed during the dilution process and, depending on the result of the evaluation, the dilution parameters are automatically adjusted to produce a predetermined structure with an atomic monolayer.

[0020] The following are some spectra that particularly clearly show the formation of a monolayer during thinning: Fig. 1. Representation of the temporal evolution of Raman modes during a thinning process, Fig. 2. Representation of integrated intensities of the Raman modes from Fig. 1, and Fig. 3 A representation of a photoluminescence spectrum during a laser thinning process.

[0021] According to the invention, for the fabrication of an atomic monolayer, a Raman spectrum and / or a photoluminescence spectrum of photons emitted from the surface of the multilayer or monolayer is recorded during a thinning process. In this process, thinning is achieved using photons, i.e., a laser, or by bombarding the surface with charged particles, in particular electrons or ions. When using a laser, the photons emitted by the multilayer or monolayer can be used to generate a Raman spectrum or a photoluminescence spectrum. Alternatively, it is also conceivable that, in addition to the laser or the particle stream, the processed surface is illuminated alternately or simultaneously with the aforementioned beams. The spectral range used for spectroscopy can be ultraviolet, visible, or infrared light.

[0022] The inventive method provides for controlling or adjusting the parameters for thinning the multiple layers based on the temporal development of resonances, characteristic curves, or physical properties of the material, such that a high-quality monolayer, i.e., without a multiple layer or without any layer at all, is produced for the desired structure. Fig. Figure 1 shows the corresponding Raman intensities versus a Raman shift for various thinning steps, namely the 1st, 27th, and 38th thinning steps. The first measurement, i.e., the first thinning step of the multiple layers, shows a value in the range of 200 cm. -1 up to 600 cm -1 Five distinct peaks appear. These peaks are the four Raman modes E. 1g , E 1 2g , A 1g and B 12gof MoS2 as well as a Raman mode of the silicon substrate. In the area around 800 cm -1 Further second-order modes are visible. As the multilayers are thinned, i.e., with further measurements, the intensities of the individual modes and their spacing change. By the 38th measurement, i.e., the final thinning step, the semiconductor's Raman modes have almost completely disappeared, and the substrate peak has become very dominant. This indicates that the MoS₂ layers have been completely thinned from the silicon substrate.

[0023] In the Fig. 2 are for each of the in Fig. Figure 1 shows the integrated intensities of each identifiable Raman mode for each thinning step. The transitions from a triple layer to a bilayer and from a bilayer to a monolayer can be identified from the course of the integrated intensities of the individual Raman modes. While the intensities of some modes decrease significantly at the 28th measurement, the intensity of the silicon mode increases. If this change in intensity is detected during the thinning process, the thinning process is terminated for that area on the substrate. If further areas exist on the substrate where this transition to the monolayer has not yet been detected, the thinning process should be continued in these areas.

[0024] In addition to the Raman spectra, the invention provides for the recording of a photoluminescence spectrum of the processed layers. Fig. Figure 3 shows a photoluminescence spectrum in the range of 1.2 eV to 2.4 eV for each measurement or thinning step. The intensities of the individual measured photon energies for each measurement or thinning step are listed logarithmically in the graph. The range around 1.8 eV to 1.9 eV around the 28th measurement is particularly noticeable. The semiconductors described here, as a multilayer system, exhibit an indirect band gap. The monolayer, however, exhibits a direct band gap. As soon as the thinning of the multilayers results in a monolayer, the photoluminescence increases abruptly by 1.8 eV due to the formation of the direct band gap, while at the same time the intensity for the indirect transition at lower photon energies (approx. 1.4 eV) disappears. This is precisely what is shown in Fig.3. Thus, recording a photoluminescence spectrum also proves to be a very good indicator of achieving a monolayer on the substrate. Since not all of the semiconductor materials mentioned here are accessible to Raman spectroscopy or photoluminescence spectroscopy, the combination of both methods, or the possibility of using at least one of them, proves to be particularly advantageous. This feedback of the manufacturing process through the recording of photon energies allows for the precise and highly reliable fabrication of a structure consisting of a monolayer of a specific semiconductor material.

Claims

[1] Method for the controlled production of an atomic monolayer by thinning atomic multiple layers arranged on a substrate, wherein, in order to determine the number of atomic layers during production, at least one energy spectrum of photons re-emitted by multiple layers after irradiation thereof is recorded, characterized by , that depending on the determined photon energies, thinning parameters are dynamically adjusted according to the progress of the production of the monolayer, and wherein the thinning is carried out by means of photons or electrically charged particles and between the pulses of the photons or the electrically charged particles the processed layer is subjected to a photon beam of a preferred spectral range for Raman and photoluminescence spectroscopy. [2] Method for the controlled production of an atomic monolayer according to claim 1, characterized by, that photons, preferably a laser or a laser diode, or electrically charged particles, in particular electrons or ions, are used for the thinning, wherein, depending on the determined photon energies, an energy and / or an intensity and / or a pulse rate and / or a pulse duration and / or a step size of the thinning on the position of a photon beam or a beam of electrically charged particles is changed as a thinning parameter. [3] Method for the controlled production of an atomic monolayer according to claim 1 or 2, characterized by , that to thin out the image, an area, in particular grid points or surfaces, of the multiple layers is scanned stepwise several times in a grid by the photon or particle beam and photon energy spectra are recorded during or afterwards. [4] Method for the controlled production of an atomic monolayer according to any one of the preceding claims, characterized by, that during the production process, a temporal evolution and / or a spacing of several Raman modes, preferably of the E, is observed from the photon energy spectra over several measurements or dilution steps. 1g - and / or the E 1 2g - and / or the A1 g - and / or the B 1 2g - and / or the MoO x -mode and / or a Si mode and / or second-order modes are recorded from the processed layer, and the number of layers on the substrate is determined based on the intensities of the Raman modes and / or the spacing of the modes. [5] Method for the controlled production of an atomic monolayer according to any one of the preceding claims, characterized by, that during the manufacturing process, a temporal evolution of a photoluminescence spectrum, in particular between 1.0 eV and 3.0 eV, preferably between 1.5 eV and 2.5 eV, is recorded from the photon energy spectra of the processed layer over several measurements or thinning steps, and the number of layers on the substrate is determined based on the measured energy-selective photon intensities. [6] Method for the controlled production of an atomic monolayer according to any one of the preceding claims, characterized by , that both Raman spectra and photoluminescence spectra are recorded and evaluated to determine the number of layers. [7] Method for the controlled production of an atomic monolayer according to any one of the preceding claims, characterized by, that the Raman spectra and / or the photoluminescence spectra are recorded, in particular by means of a beam splitter, during thinning using a laser, wherein the photons detected for the Raman and / or photoluminescence spectra result from an interaction of the laser for thinning with the layers. [8] Method for the controlled production of an atomic monolayer according to any one of the preceding claims, characterized by that two different measuring devices, preferably spectrometers, are used for recording the Raman spectra and / or the photoluminescence spectra. [9] Method for the controlled production of an atomic monolayer according to any one of the preceding claims, characterized by that the monolayer is made of transition metal dichalcogenides, preferably MoS2, WS2, WSe2, of heterostructures of different 2D materials, 2D insulators, in particular BN, or 2D semimetals such as graphene. [10] Method for the controlled production of an atomic monolayer according to any one of the preceding claims, characterized by that the Raman and / or photoluminescence spectra are automatically analyzed during dilution and, depending on the result of the evaluation, the dilution parameters are automatically adjusted to produce a predetermined structure with an atomic monolayer.

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