2D-Materials

15 Oct 2023

Full Scale Wafer Analysis by Imaging Ellipsometry and Ellipsometric Contrast Micrography (ECM)

S. Funke, P. Braeuninger -Weimer, P. H. Thiesen, S. Hoffmann Imaging Ellipsometry Imaging ellipsometry, combines the power of ellipsometry with optical microscopy. It achieves the highest lateral resolution in the field of ellipsometry and offers a very sensitive imaging technology for thin films. Typical samples range from tiny samples like flakes of 2D materials to inhomogeneous surfaces like some CVD grown samples of 2D materials or terraces at epitaxial Graphene surfaces. The ep4 is the latest microscopic imaging spectroscopic ellipsometer with unique measurement capabilities. It can provide real-time ellipsometric enhanced contrast images, and maps of the ellipsometric angles Δ and Ψ . These data can be transferred into Thickness and the complex refractive indices by computerized optical modeling.     ECM – Contrast Mode   The standard setup of an imaging ellipsometer: The lens system and CCD offer a microscopic view on the sample. With the polarizer (P), compen-sator (C) and analyzer (A) a full control of the state of polarization of the incoming light and light after reflection and interaction with the sample is given. The rotation of P, C, A may change the contrast in the view as it is shown in b and c. The only difference between the two images are the rotational angles of P and A.     Ellipsometry Mode     For the ellipsometry mode contrast images are taken for different rotational angles of P, C, A in order to determine the change in the state of polarization, that is induced by the sample. It is expressed in Δ/Ψ. From the stack of intensity images, micromaps of Δ/Ψ are calculated. The micro-maps can be modeledwith an optical model (layerstack of materials) to thickness and refractive index maps. An example of a thickness map of Graphene on a SiO2 layer is shown on the right.     Optical Properties of MoS2/WSe2     a): The ellipsometric enhanced contrast micrograph and the layer structure used for the final modelling. Conventional microscopy can hardly distinguish between monolayers and substrates. b): An exemplary Psi map at 410 nm with the regions used for extracting Delta and Psi spectra (right) in the range of 400 nm – 800 nm. The resulting dispersions for all regions are shown in c). The measurement and analysis is done simultaneously for all regions ECM Compared to Other Methods   a): The same region of Graphene on 300 nm SiO2 measured by Raman-mapping, Optical Microscopy (OM), Ellipsometry Mode and ECM mode and their measurement time. b): The histogram of each method shows best distinguishing for the ellipsometry mode. The numbers indicate the layer number of Graphene. The higher the distance in-between 2 peaks, the better the method is able to distinguish the layer numbers.     Graphene on Copper a): ECM stitching obtains a large sample overview. False coloring according to layer number as depicted in the histogram. For the overview ECM images are recorded with an motorized XY-stage and stitched seamlessly. The histogram can be used to calculate layer coverage ratios. In figure b) another comparison of OM and ECM is shown. The different regions of stacked 2D materials can be hardly distinguished in the OM (left) image, but very well in the ECM Image on the right.   Graphene on Cu-foil as Grown a): ECM stitching obtains a large sample overview. False coloring according to layer number as depicted in the histogram. For the overview ECM images are recorded with an motorized XY-stage and stitched seamlessly. The histogram can be used to calculate layer coverage ratios. In figure b) another comparison of OM and ECM is shown. The different regions of stacked 2D materials can be hardly distinguished in the OM (left) image, but very well in the ECM Image on the right. Further investigations at the flipped contrasts are done. Figure c) shows a Graphene hexagon with two different regions (flipped and non-flipped contrast). Spectroscopic imaging ellipsometry measurements are done for the Regions of interest (ROI) shown in grey shapes in the Ψ map. The resulting spectra are shown in d). The optical modeling yields the refractive indices for the Copper, Graphene and the Copper-oxide (figure e)). The contrast flip is not induced by the curvature of the foil, but by the partial oxidation of the Copper. Full 4“ Wafer Scan   The figure above shows a 4” native – Silicon wafer, that is partially covered with Graphene monolayer. Conventional method for fast analysis are not able to distinguish Graphene monolayer on native Silicon, but ECM is. The complete scan is done within 47 minutes and reveals defects, contaminations and cracks as small as 10 µm.

15 Oct 2023

Imaging Spectroscopic Ellipsometry and Spectral Reflectometry with Ellipsometric Contrast

Sebastian Funke Niklas Reineking Dirk Hönig Two Methods for the Optical Characterization of 2D-Materials. Introduction     Ellipsometry is a sensitive optical method, which has been used for about a hundred years to derive information about surfaces. It makes use of the fact that the polarization state of light may change when the light beam is reflected from a surface. If the surface is covered by a thin film (or a stack of films), the entire optical system of film & substrate influences the change in polarization. It is therefore possible to deduce information about the film properties, especially the film thickness and optical properties. Ellipsometry remains a macroanalysis technique, i.e., the spot size limits resolution. Features smaller than the spot are not resolved. The question is, how to apply ellipsometry to exfoliated 2D materials?     Imaging Ellipsometry   The development of imaging ellipsometry, which combines the power of ellipsometry with microscopy, has overcome this limitation. The enhanced spatial resolution of imaging ellipsometers potentially expands ellipsometry into new areas of microanalysis, microelectronics, and bioanalytics. The ep4 is a microscopic imaging spectroscopic ellipsometer with unique measurement capabilities. It can provide real-time-ellipsometric enhanced contrast images, maps of the ellipsometric angles Psi and Delta, or data for very small regions of interest. Film thickness and the complex refractive indices of the sample are determined by computerized optical modeling.     Setup of an imaging ellipsometer including objective and 2D CCD     Spectroscopic Imaging Ellipsometry on Exfoliated Micro Crystals   Ellipsometric enhanced contrast by optimized polarizer and analyzer angle at selected wavelength and angle of incidence.       Regions of Interest (ROI) concept for ellipsometry on smallest flakes Microscopic mapping of Delta and Psi.         Automatic Localization & Identification of microcrystallites         Spectroscopic reflectometry with ellipsometric contrast for process control applications Here, we present a setup that allows the acquisition of about 1000 spectroscopic spectra per second with a thickness sensitivity- of less than 0.5 nm. This becomes possible by comparing the sample with a similar reference sample. Due to the orientation of the reference (see Fig. 1) it acts as an ideal compensator for all wavelengths. Hence no compensator is required. If the sample is equal to the reference, the outgoing light is linearly polarized and can extinguished with a crossed analyzer. If there’s any difference between the sample and reference, the light becomes elliptically polarized, and the detected light flux increases. In order to maximize the rate of data acquisition, the analyzer is set to an off null-position. During the measurement neither the polarizer - nor the analyzer - angle is changed, so the measurement speed is only limited by the intensity of the light source and the processing speed of the spectrometer.               

15 Oct 2023

Imaging Ellipsometry as a Tool for the Characterization of 2DMaterials on various Substrates

S. Funke, A. Matkovic, P. Braeuninger -Weimer, P. H. Thiesen, S. Hoffmann, A. Diebold, U. Wurstbauer Imaging Ellipsometry Imaging ellipsometry, combines the power of ellipsometry with optical microscopy. It achieves the highest lateral resolution in the field of ellipsometry and offers a very sensitive imaging technology for thin films. Typical samples range from tiny samples like flakes of 2D materials to inhomogeneous surfaces like some CVD grown samples of 2D materials or terraces at epitaxial Graphene surfaces. Setup of an imaging ellipsometer including objective and 2D CCD The ep4 is the latest microscopic imaging spectroscopic ellipsometer with unique measurement capabilities. It can provide real-time ellipsometric enhanced contrast images, and maps of the ellipsometric angles Δ and Ψ . These data can be transferred into Thickness and the complex refractive indices by computerized optical modeling. Imaging spectroscopic ellipsometer nanofilm_ep4     Extracting Spectra of Δ and Ψ   Knife Edge Illumination   - No contribution by backside reflected light will be recorded. - Non destructive , no treatment of the backside of the sample like roughen, darken, needs to be done. Beam cutter in park position (a) and working position (b).      Working principle of a beam cutter   Ellipsometric enhanced contrast micrograph of a MoS2 flake on sapphire.   Optical Properties of MoS2 Imaging ellipsometry allows to measure the optical constants from small flakes of e.g. MoS2. Different regions (number of layers) can be measured simultaneously. (a) Ellipsometric contrast micrograph of MoS2 flake. Cracks and different layers can be seen (b) Raman mapping to ensure layer-numbers (c) Measured Δ and Ψ obtained from microscopic mapping (d) Optical dispersion based on Lorentz- and Tauc-Lorentz oscillators Pseudo dielectrical function of MoS2 Monolayer   Heterostructure MoS2/WSe2 Investigation of heterostructure of MoS2 monolayer and WSe2 monolayer on glass substrate. a) shows the ellipsometric enhanced contrast micrograph and the layer structure used for the final modelling. Conventional microscopy can hardly distinguish between monolayers and substrate. b) shows an exemplary Psi map at 410 nm with the regions used for extracting Delta and Psi spectra (right) in the range of 400 nm – 800 nm. The resulting dispersions for both monolayer regions and the overlapping hetero-structure are shown in c). The results show no evidence of occurring intralayer excition in the overlapping region   Graphene on Copper   Total coverage of Graphene monolayer islands grown on CuFoil. a) shows a Delta map of a Graphene monolayer island, that is directly grown on the copper-foil. Conventional methods don’t work on cu-foil (optical microscopy needs to oxidize the foil) or are not scalable for complete mapping of a foil (SEM; RAMAN). To handle the waviness of the Cu-foil an auto-focus algorithm is applied and the measurement head of the EP4 is accordingly adjusted in height. b) shows a complete mapping of a Cu-foil. A surface coverage of 4 % is measured. The hexagonal Graphene islands are also distinguishable from the foil in the wavy part.   Automatic Flake Search - Localize areas of desired thicknesses or properties. - Localizing e.g. flakes by spectroscopic imaging ellipsometry (SIE) depends less on the substrate than conventional methods. 1st Step: Measuring stacks of micro maps at different wavelengths 2nd Step: Comparison optical model - experimental results 3rd Step: Display pixel that are equal to the desired parameter, e.g. Mono- or Trilayer Method is proven on hBN to look for regions of same thickness and optical properties     Imaging Müller Matrix of BP Imaging Mueller-matrix ellipsometry is used to characterize an anisotropic microflake of black Phosphorus. - a) shows the measured 12 mm-element. The blockoff diagonal elements reveal an anisotropic behaviour for the microflake. Each pixel denotes a measured MM value within a single measurement. - b) shows exemplary rotational steps of the flake during the theta-spectrum. The resulting spectra for the green region is also shown and reveals an inplane anisotropy. - c) displays the measured MM lambda spectrum. The fitted dispersion and orientation of the optical axis for the microflake is shown on the right     Hexagonal Boron Nitride Few layers of hexagonal Boronnitride can be investigated for their spectral dispersion of n. - a) shows a recorded Δ – map at 430 nm on the left, whereas the image on the right displays measured Ψ–values at 390 nm. - Spectroscopic maps are recorded in the range of 360 nm to 800 nm. - b) shows the extracted spectra of Δ and Ψ for the Bilayer hBN and for the substrate at different angle of incidences (AOI). - Differences between substrate and hBN can be seen around 410 nm. - The fitted Cauchydispersion for hBN can be seen in c) and is in good agreement with the literature with a refractive around 2.2    

15 Oct 2023

Automatic Localization of Mono- to NLayers of 2D Materials by Spectroscopic Imaging Ellipsometry

Sebastian Funke   In this application note we present the capability of imaging spectroscopic ellipsometry to automatically localize small flakes down to a few microns in lateral dimension and to distinguish between mono-, bi- or n-layers. The method is based on the pixel-wise comparison of a stack of delta and psi maps with the optical model. In general, it offers the opportunity to identify areas of a sample with defined layer thickness or optical properties. For demonstration purposes, a sample with a graphene flake in-between a number of thicker layers and one with a number of exfoliated Molybdenum disulfide flakes are presented.   Introduction The research of A. Geim and K. Novoselov leveraged the mechanical exfoliation of graphene as a new preparation method [1]. Frindt and Yoffe [1963] prepared 2D flakes of Molybdenum Disulfide in a similar way [2]. The first step is the exfoliation, the second the identification of the 2D material in between flakes of layer- and bulk material areas. The state of the art is the exfoliation on a special SiO2 (300 nm)|Si substrate to enable the localization by the use of a light microscope. This procedure has to be performed manually and the result is strongly related to the skills of the operator. Raman spectra are established as the identification method. Also Raman mapping is an opportunity for the localization, but due to low sensitivity of Raman-scattering, the task is very time consuming. A new focus in 2D material related research is the characterization of physical properties of layered systems related to the thickness as well as the formation of complex stacks made from different 2D materials. By stacking e.g. graphene and MoS2 Monolayers the hybrid structures have promising properties for e.g. optoelectronic devices [3]–[6]. Regarding MoS2 2D materials the variation in thickness seems to be related to the band gap [7]. To summarize a method is needed, that not only allows the localization and identification of monolayers of 2D materials independent of the substrate but also enables the localization and identification of flakes with a well-defined thickness. Figure 1. EP4 with Polarizer arm on the left and Analyzer arm with the optical system at the right. The sample can be placed onto a motorized xy-table. In ellipsometry, the ellipsometric angles△and ψ are related to the thickness and the dispersion function of a layered system. Kravets et al. obtained the optical dispersion of mechanically exfoliated Graphene by spectroscopic ellipsometry [8]. The wavelength spectra or the ellipsometric angles at significant wavelength can be understood as a finger print of the required flake. Spectroscopic imaging Ellipsometry permits measurement of△and ψ on tiny regions with the “Region of Interest” concept. Upon decreasing the regions to the size of a pixel, microscopic maps of△and ψ with a lateral resolution down to 1 µm can be obtained. Matkovic et al. and Wurstbauer et al. showed that imaging ellipsometry is able to characterize the optical properties of small flakes of Graphene [9], [10]. Thermally reduced Graphene was investigated by Jung et al. [11]. Albrektsen et al. reported a thickness map that reveals distinct terraces separated by steps, which have heights corresponding to monolayers or bilayers of graphene [12]. The reported profile show five steps on a distance of 25 µm. In addition, they applied several complementary techniques like optical reflection microscopy, AFM and Raman spectroscopy. The findings of imaging ellipsometry and complementary techniques are in excellent agreement. Imaging Ellipsometry In ellipsometry the ellipsometric angles△and ψ describe the phase shift and the amplitude ratio of the s, and p polarization after the reflection from a sample respectively. To obtain△and ψ a welldefined state of polarization before the reflection from a sample must be provided by a Polarizer (P) and a Compensator. The Analyzer (A) detects the state of polarization after the reflection. By doing nulling ellipsometry the calculation of△and ψ can be directly obtained of the angles of P and A by: In nulling ellipsometry the compensator is a quarter-wave plate, whereas P and A are linear polarizers. The rotation of P polarizes the incoming light in such way, that the reflected beam is linearly polarized. By rotating A to 90° to the linearly polarized light, minimum intensity at the detector will be observed. A detailed description of the theoretical background on ellipso-metry is given in [13]–[15]. Figure 3. An image of monolayer graphene located in a 5x5 xy-pattern. Pixel within a△and ψ range of ±2.5 and a threshold of 0.85 are displayed green. Figure 4. ͘ (top):△plot of monolayer (red) and trilayer (orange) of Graphene over the wavelength range as obtained from the EP4Model (bottom): ψ plot of monolayer (red) and trilayer (orange) of Graphene over the wavelength range. For imaging ellipsometry the reflected beam will be guided through a microscopical system through the Analyzer to the detector. Therefore one can obtain△and ψ at every pixel on the detector for the sample area that is in the field of view of the optical system. By changing the wavelength of the monochromatic incoming light, spectroscopic imaging ellipsometry (SIE) can be done. All following measurements are performed with an EP4 from Accurion, Göttingen as seen in Figure [1].   Automatic Localization of Small Flakes With a motorized xy-table the EP4 is able to automatic pattern a sample. Therefore the user only needs to set the points at the edge of the sample and choose the pattern style. The device will measure with defined parameters maps at every discrete point of the xy-pattern. To locate flakes or areas with a defined thickness, spectroscopic maps at every position are taken. These spectroscopic maps,△and ψ for selected wavelengths, at one position in the pattern will be called a mapstack. For a 5x5 pattern with 4 wavelengths in the measurement parameters 25 mapstacks are obtained. A graphical illustration is shown in Figure 2. For the automatic localization the dispersion of the sought-after material must be known. In the EP4Model the△and ψ values will be calculated for the desired thickness. For a monolayer of Graphene / MoS2 a thickness of 0.36nm / 0.63 nm respectively are known. The program Search for Thickness takes the ȴͬψ values as reference for the search. In an iterative process the values for△and ψ for every pixel at each wavelength in one mapstack will be compared to the data obtained Figure 5. Results for the flakesearch of trilayer of graphene on 5x5 pattern with a△and ψ range of ±2.2 and a threshold of 0.7. from the EP4Model. A ȴͬψ plot for a trilayer (orange) and a monolayer of Graphene on a silicon dioxide-silicon substrate is shown in Figure 4. By defining a threshold noisy data can be compensated, especially for small flakes close to the lateral resolution limit of down to 1 µm. The threshold defines how many percent of the△and ψ values of one pixel compared over the wavelengths in each mapstack need to be in the given range of△and ψ from the model. Once the pixels that are in the range and in the threshold of the Model values are found, a clustersearch must be done. The clustersearch provides a function to find only the   Figure 6. Stitched maps with a pixel for a flake of MoS2 with a thickness of a monolayer. The pixel was found with a threshold of 0.85 and a△and ψ range of ±2.2. regions where found pixels are stacked together, e.g. in small flakes or areas of the desired thickness. The user can choose a cluster size of pixel x pixel and another threshold. The algorithm filters the mapstacks and gives out the positions where for each filtercluster is given. So the threshold describes the occurrence of found pixel in the given cluster   Results The result for a flakesearch for 5x5 pattern with 4 wavelengths from 470 nm - 510 nm is shown in Figure 3. The algorithm clearly identifies the monolayer area of the Graphene flake on the silicon-silicon dioxide layer (green pixels). All green pixels in the stitched result window are in the range with the threshold of the modeldata. The flakesearch algorithm finds a flake in mapstack 21 and 22, where the flake is located in the middle of the patterned area. It neglects e.g. dust particles, edge effects may have the same△and ψ variation as the desired layer. For the same measurement a flake search for a trilayer of Graphene was done. The result is shown in Figure 5. The region of the trilayer can be identified and located next to the area of the found monolayer of Graphene. The Graphene flake has a dimension of ca. 100 µm x 20 µm. Figure 6 demonstrates the capability to locate flakes with a size down to 10 µm. The size of desired flakes or areas is limited to the lateral resolution and described by the clustersearch parameters. For the 3x3 pattern a monolayer flake of MoS2 is found in a mapstack in the middle of the stitched image as seen in Figure 6. The ultraobjective is used to obtain a pattern for the MoS2 flake. The ultraobjective is a Scheimpflug corrected optical system that provides an overall focused image. When using the ultraobjective a focus scan to obtain allover focused maps is no longer needed. That increases the speed of the measurement. The overall time for the 3x3 pattern with the ultraobjective was 20 min while the scanned area at an angle of incidence of 53° is around 1mm x 1mm.   Conclusion This application note shows the capability of spectroscopic imaging ellipsometry to automatic localize small flakes of Mono- to N-Layers of 2D materials. The method is indepedent from the substrate as well as from the experience of the user. It is also fast and provides information on the layer number with increased accuracy by using spectroscopic data. The algorithm is able to differentiate between a monolayer, bilayer and trilayer of graphene. Imaging ellipsometry also allows the localization of Molybdenum disulphide flakes with a size down to 10 µm. A Scheimpflug corrected ultraobjective provides a speed increase in the recording of the pattern. However, the application is not limited to the localization of flakes of 2D materials. It is expandable to a wide range of thicknesses up to several microns of known transparent materials on known sample structures.   References [1] K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, Y. Zhang, S. V. Dubonos, I. V. Grigorieva, and A. A. Firsov, “Electric Field Effect in Atomically Thin Carbon Films,” Science, vol. 306, no. 5696, pp. 666–669, Oct. 2004. [2] R. F. Frindt and A. D. Yoffe, “Physical properties of layer structures: optical properties and photoconductivity of thin crystals of molybdenum disulphide,” Proc. R. Soc. Lond. Ser. Math. Phys. Sci., vol. 273, no. 1352, pp. 69–83, 1963. [3] G.-H. Lee, Y.-J. Yu, X. Cui, N. Petrone, C.- H. Lee, M. S. Choi, D.-Y. Lee, C. Lee, W. J. Yoo, K. Watanabe, T. Taniguchi, C. Nuckolls, P. Kim, and J. Hone, “Flexible and Transparent MoS 2 Field-Effect Transistors on Hexagonal Boron Nitride-Graphene Heterostructures,” ACS Nano, vol. 7, no. 9, pp. 7931–7936, Sep. 2013. [4] L. Britnell, R. M. Ribeiro, A. Eckmann, R. Jalil, B. D. Belle, A. Mishchenko, Y.-J. Kim, R. V. Gorbachev, T. Georgiou, S. V. Morozov, and others, “Strong light-matter interactions in heterostructures of atomically thin films,” Science, vol. 340, no. 6138, pp. 1311–1314, 2013. [5] W. Choi, M. Y. Cho, A. Konar, J. H. Lee, G.- B. Cha, S. C. Hong, S. Kim, J. Kim, D. Jena, J. Joo, and others, “High-detectivity multilayer MoS2 phototransistors with spectral response from ultraviolet to infrared͕͟Ěǀ͘ Mater., vol. 24, no. 43, pp. 5832–5836, 2012. [6] K. Roy, M. Padmanabhan, S. Goswami, T. P. Sai, G. Ramalingam, S. Raghavan, and A. Ghosh, “A nearly relaxation-free optoelectronic memory from ultra-thin grapheneMoS2 binary hybrids,” ArXiv Prepr. ArXiv13091455, 2013. Text [7] J. Qi, X. Li, X. Qian, and J. Feng, “Bandgap engineering of rippled MoS2 monolayer under external electric field,” Appl. Phys. Lett., vol. 102, no. 17, p. 173112, 2013. [8] V. G. Kravets, A. N. Grigorenko, R. R. Nair, P. Blake, S. Anissimova, K. S. Novoselov, and A. K. Geim, “Spectroscopic ellipsometry of graphene and an exciton shifted van Hove peak in absorption,” Phys. Rev. B, vol. 81, no. 15, p. 155413, 2010. [9] U. Wurstbauer, C. Röling, U. Wurstbauer, W. Wegscheider, M. Vaupel, P. H. Thiesen, and D. Weiss, “Imaging ellipsometry of graphene,” Appl. Phys. Lett., vol. 97, no. 23, p. 231901, 2010. [10]10] A. Matkovié, A. Beltaos, M. Milicevic, U. Ralevic, B. Vasic, D. Jovanovic, and R. Gajic, "Spectroscopic imaging ellipsometry land Fano resonance modeling of graphene," J. Appl. Phys., vol. 112, no. 12, p. 123523, 2012. [11] I. Jung, M. Vaupel, M. Pelton, R. Piner, D. A. Dikin, S. Stankovich, J. An, and R. S. Ruoff, "Characterization of Thermally Reduced Graphene Oxide by Imaging ¡Ellipsometry," J. Phys. Chem. C, vol. 112, no. 23, pp. 8499-8506, Jun. 2008. [12] O. Albrektsen, R. L. Eriksen, S. M. Novikov, D. Schall, M. Karl, S. I. Bozhevolnyi, and A. C. Simonsen, “High resolution imaging of few-layer graphene,” J. Appl. Phys., vol. 111, no. 6, p. 064305, 2012. [13] H. Fujiwara, Spectroscopic ellipsometry: principles and applications. Chichester, England; Hoboken, NJ: John Wiley & Sons, 2007. [14] R. M. A. Azzam and N. M. Bashara, Ellipsometry and polarized light. Amsterdam; New York: North-Holland : Sole distributors for the USA and Canada, Elsevier Science Pub. Co., 1987. [15] H. G. Tompkins and E. A. Irene, Handbook of ellipsometry. Norwich, NY; Heidelberg, Germany: William Andrew Pub. ; Springer, 2005.

15 Oct 2023

Hetero-structure Mos2/Wse2 Investigated By Imaging Ellipsometry

SAMPLE AND SETUP:   2D-materials often show superior properties compared to their bulk or few-layer materials. The stacking of different mono layered materials do even promise more interesting combinations. MoS2 and WSe2 show strong excitonic dominated behaviour. In theory, the stacking shows the opportunity of intra-layer excitons. Imaging ellipsometry with the nanofilm_EP4 offers highest lateral ellipsometric resolution down to 1 µm. It combines the sensitivity for thickness and refractive indices measurements of ellipsometry with the benefits of magnification and lateral resolution from optical microscopy. Spectroscopic dispersions can be obtained by varying the wavelength.   MEASUREMENT:   The mechanically exfoliated flakes of MoS2 and WSe2 are stacked, that their mono layered regions overlap. The unique feature knife-edge illumination of the nanofilm_EP4 is used to avoid disturbing backside reflections from the thin, transparent glass substrate. The Ψ map on the left (λ = 450 nm) shows differentiated contrast for both monolayer regions and the hetero-structure. The overlapping area is 2 µm x 6 µm sized. The 20x objective offers high lateral ellipsometric resolution within a field of view of 150 µm x 150 µm. The wavelength is varied from 400 nm to 800 nm. All regions inside the field of view are measured simultaneously. RESULTS - Spectroscopic Δ, Ψ values/maps for MoS2, WSe2 and the overlapping hetero-structure within one single measurement - Refractive indices for each regions independently

15 Oct 2023

Graphene on Copperfoil Investigated by Imaging Ellipsometry

SAMPLE AND SETUP: To cover the increasing demand of high-quality Graphene, CVD grown Graphene upon a Cu-foil substrate seems to be a very promising way. However, traditional methods of quality and quantity control, like optical microscopy, SEM, Raman, either do not work or are too slow on the Cu-foil directly. Imaging ellipsometry with the QDQRILOPB(3 offers highest lateral ellipsometric resolution down to 1 µm. It combines the sensitivity for thickness and refractive indices measurements of ellipsometry with the benefits of magnification and lateral resolution from conventional optical microscopy.   MEASUREMENT: The shown ǻ map on the left clearly distinguishes the hexagonal structure of the grown Graphene from the Cu-foil. The shown field of view is measured within one single measurement. By moving the sample, a pattern of the sample can be recorded and stitched automatically by the QDQRILOPB(3. The field of view obtained with the 5x objective was 1 mm X 1.4 mm at an angle of incidence of 60°. A pattern of 1.5 cm X 1 cm on the Cu-foil was done within 2 hours and allowed the identification of structures down to 5 µm.   RESULTS: - Graphene crystals can be clearly distinguished from Cu-foil on the stitched ǻ map at 390 nm - Surface coverage of Graphene - Folds of Graphene induced by the different temperature coefficients during the cooling down can be seen       Hetero-Structure MoS2 /WSe2 INVESTIGATED BY IMAGING ELLIPSOMETRY   SAMPLE AND SETUP: 2D-materials often show superior properties compared to their bulk or few-layer materials. The stacking of different mono layered materials do even promise more interesting combinations. MoS2 and WSe2 show strong excitonic dominated behaviour. In theory, the stacking shows the opportunity of intra-layer excitons. Imaging ellipsometry with the QDQRILOPB(3 offers highest lateral ellipsometric resolution down to 1 µm. It combines the sensitivity for thickness and refractive indices measurements of ellipsometry with the benefits of magnification and lateral resolution from optical microscopy. Spectroscopic dispersions can be obtained by varying the wavelength.   MEASUREMENT: The mechanically exfoliated flakes of MoS2 and WSe2 are stacked, that their mono layered regions overlap. The unique feature knife-edge illumination of the QDQRILOPB(3is used to avoid disturbing backside reflections from the thin, transparent glass substrate. The Ȍ map on the left (Ȝ = 450 nm) shows differentiated contrast for both monolayer regions and the hetero-structure. The overlapping area is 2 µm x 6 µm sized. The 20x objective offers high lateral ellipsometric resolution within a field of view of 150 µm x 150 µm. The wavelength is varied from 400 nm to 800 nm. All regions inside the field of view are measured simultaneously.   RESULTS:     - Spectroscopic ǻ Ȍvalues/maps for MoS2, WSe2 and the overlapping hetero-structure within one single measurement - Refractive indices for each regions independently                

15 Oct 2023

Characterization of Graphene through Imaging Spectroscopic Ellipsometry

Introduction Graphene consists of a sp²-hybridized atomic carbon crystal. Since it was first reported by Novoselov et al. (2004) it has been of increasing interest, because of unique physical properties and promising applications. First Graphene samples were fabricated by micro mechanical cleavage of graphite. Currently, technical processes have been established to produce graphene on large areas. To achieve a better control of thickness and properties , a characterization in the micrometer scale is required. The characterization should include the thickness and the number of layers, respectively. Also the optical properties are essential. Imaging Ellipsometry is a fast and easy way to characterize localized exfoliated graphene Delta map Air | Graphene | SiO2 | Si      Imaging ellipsometry Ellipsometry is a sensitive optical method, which has been used for about a hundred years to derive information about surfaces. It makes use of the fact that the polarization state of light may change when the light beam is reflected from a surface. If the surface is covered by a thin film (or a stack of films), the entire optical system of film & substrate influences the change in polarization. It is therefore possible to deduce information about the film properties, especially the film thickness. The name ”ellipsometry” implies that it has to do something with an ”ellipse”: The elliptical state of polarization, where the electrical field vector travels along an ellipse when observed at a fixed point in space, is the most general state of polarization The basic components of an ellipsometer are: a light source, some optical components to modify the polarization and a detector. By using imaging technology, one can extend the classical ellipsometer to a new form of visualization tool or a microscope with extreme sensitivity to thin films. Figure 1. State of polarization of a light beam reflected at a surface   Figure 2. Principle of nulling ellipsometry.   Figure 3. Setup of an imaging ellipsometer.   Maps with the highest lateral resolution Delta and Psi maps are based on a number of images taken at changing orientations of the polarizer and analyzer (Fig. 4 a). In general, the ellipsometric angles were obtained by analyzing the minimum in grayscale distribution for each pixel (Fig. 4b). Albrektsen et al. (2012) characterized few layer graphene by different imaging methods and showed clearly the capability of imaging ellipsometry to distinguish between single, double and few layer graphene. Matković et al. calculated based on Delta maps at different wavelength the thickness of the graphene layer and a water interlayer independent from each other. Figure 4. Scheme of recording a map. Figure 5. Psi map of a graphene flake.       Ellipsometric enhanced contrast Based on the nulling concept, the optical setup of an imaging ellipsometers offers the option to set the optical components in a way to enhance the contrast of the micrograph. Matković published micrographs of graphene flakes on different SiO2|Si substrates. Wurstbauer et al. (2010) published ellipsometric contrast micrographs of few-layer graphene on a GaAs substrate. The contrast was optimized for the graphene layer such that the adhesive tape residues vanish, while in a second image the contrast of the immediate vicinity is enhanced.   Figure 6. Ellipsometric contrast micrograph of graphene flakes.   New: Imaging NIR-ellipsometry Based on new developments, spectra with higher spectral resolution are available in the NIR up to 1700 nm. For wavelength above 1000 nm an InGaAs cooled FPA detector was included. Delta- and Psi maps of a graphene flake are recorded and show significant different behavior than the substrate. The technique is promising to obtain information about Drude adsorption with a lateral resolution of few micro meters. It will improve the understanding of the dielectrical properties of graphene and of the graphene substrate interactions.   Figure 7. Technical drawing of setup with NIR camera. Figure 8. Delta– and Psi-maps of a graphene flake at selected wavelength in the NIR.   Spectra at the smallest regions of interest (ROI) One unique feature of imaging ellipsometry is the option of regions of interest. Unlike conventional ellipsometers that illuminate a single microspot and collect data for that one spot at a time, Imaging ellipsometers imaging the illuminated area onto a 2D array detector for a million or more simultaneous measurements. This facilitates ROI by enabling pixel selection. The first wavelength spectra measured by the use of a ROI on single layers graphene were published by Wurstbauer et al. Matković et al. identified the Fano resonace based on spectra, measured with the ROI concept.    Figure 9. Wavelength spectra of Delta and Psi at different angle of incident. The new nanofilm_ep4 The nanofilm_ep4 is based on a new frame for more free space and rigidity. It has an integrated Z-lift with higher accuracy; a new EP4 Align Sensor including the detection of the z-position and a new Instrument Alignment (instead of sample alignment) that enables the alignment on large samples like troughs, production line, etc.. Other improvements involve the new modular software concept including the optical modeling, the instrument control, and the data studio. The new “Ultra_Objective”-Imaging optics offers fully focused field-ofview and fast mapping in real time. Imaging spectroscopic ellipsometry is improved with a continuously tunable, high throughput monochromator and a new light source (laser-driven Xe plasma, ca. 4 times more light for better SNR) for improved precision. The setup offers new possibilities for the technical integration of other techniques like AFM, QCM, Terahertz spectroscopy or even customized solutions.   Literature Matković et al. (2012) Spectroscopic imaging ellipsometry and Fano resonance modeling of graphene. J. Appl. Phys. 112, 123523 Albrektsen et al. (2012) High resolution imaging of few-layer graphene. J. Appl. Phys. 111, 064305 ellipsometry: Wurstbauer et al. (2010) Imaging ellipsometry of graphene. Appl. Phys. Lett. 97, 231901    

15 Oct 2023

Spectroscopic Ellipsometry and Optical Constants of MoS2 Flakes

  E. Parzinger, S. Funke, B. Miller, P. Thiesen, A. W. Holleitner, and U. Wurstbauer Introduction Transition Metal Dichalcogenides, e.g. MoS2 mono- and fewlayers are atomistic thin semiconductors with a band gap in the visible range featuring extremely high light absorption efficiency as well as strong excitonic properties making the material of great interest for both fundamental research as well as for optoelectronic applications. Since the optical properties of MoS2 strongly dependent on the number of layers, a layer selective knowledge of the optical constants is demanding. Refractive index (n) and extinction parameter (k) are fundamental to describe the light-matter interaction of the material.   We report on the imaging intensity and spectroscopic ellipsometry (SE) on mechanically exfoliated MoS2 flakes transferred to Si/SiO2 and sapphire substrates. The outstanding lateral resolution of ~1µm [1] enables the layer selective extraction of the optical constants from SE and the study of their lateral uniformity. Contrary to reflectance spectra [2], SE is sensitive not only to the in-plane but also to the out-of-plane component of the optical constants. We have extracted both components of the optical constants using an anisotropic model. Number of layers, as well as intrinsic doping density and lateral homogeneity have been studied by complementary µ-Raman spectroscopy. Imaging ellipsometry set-up   (Imaging) Ellipsometry: non-destructive method to determine optical properties with high lateral resolution. Change in the polarization of the reflected light is measured.           PCSA configuration => beam linear polarized after reflection => Intensity-minimum measured by 90° positioned analyzer (‘nulling condition’) The Knife edge at the top, the flake in the backside-reflection free area and the shadow of the flake in the backside reflection area. Imaging ellipsometry (IE) – exfoliated MoS2 on SiO2 Optical microscopy image, imaging ellipsometry contrast map and monochromatic reflectivity map of aMoS2 flake on Si/SiO2. Mono, bi, triand multilayer regions can be distinguished from all methods with a high lateral resolution of less than 1 µm. Contrast in the region of band gap weak for MoS2 on SiO2 => Spectroscopic ellipsometry and modeling of ellipsometric angles to determine optical constants is difficult and requires different substrate (e.g. sapphire)     Raman spectroscopy of A1g and E1 2g mode is used to determine the number of layers.   Spectroscopic ellipsometry – exfoliated MoS2 on sapphire     Lateral variation of dielectric function     • Lateral variation of MSE mainly dictated by spectra range between B and C excitons (2.2 eV – 2.7 eV). • Energy of A1g phonon mode sensitive probe for changes in charge carrier density [4]: Mode energy continuously decreases towards sample edge by about ΔE  0.5cm-1 corresponding to change in charge carrier density Δn> 21012 cm-2. • No spatial correlation between change in Raman modes and variations in MSE ⇒ MSE not dependent on change in charge carrier density. • Variation in MSE might be caused by altered van der Waals interact between MoS2 & substrate. Anisotropic dielectric function   • Levenberg-Marquardt fit based on Fresnel coefficients for multilayered films and Tauc-Lorentz approach [3] assuming anisotropic dielectric functions in-plane (x-y plane) and out-of-plane (z direction). • Dielectric function in visible range dominated by x-y component, however finite contribution in zdirection around the band gap for 1L as well as for 3L. • For 1L: peak of ε2 in z-direction at slightly larger energy compared to x-y component. • For 3L :peak of ε2 in z-direction at slightly lower energy compared to x-y component. => Transitions in z-direction are assigned to indirect transitions close to Γ-point of the band structure opposite to the direct A, B and C excitonic transitions observable in x-y plane. References [1] U. Wurstbauer, et al. Appl. Phys. Lett. 97, 231901 (2010). [2] Y. Li, A. et al. Phys. Rev. B 90, 205422 (2014). [3] EP4 Model from Accurion GmbH, Göttingen. [4] B. Miller, et al. Appl. Phys. Lett. 106, 122103 (2015). [5] S. Funke et al. (2015). [6] E. Parzinger et al. (2015) Acknowledgements We acknowledge support by the DFG excellence cluster “Nanosystems Initiative Munich” (NIM), the Center for NanoScience (CeNS) in Munich and BaCaTec.

26 Sep 2023

Microscopic Characterization of 2D Materials by Spectroscopic Imaging Ellipsometry

maging ellipsometry, combines the power of ellipsometry with optical microscopy. It achieves the highest lateral resolution in the field of ellipsometry and offers a very sensitive imaging technology for thin films. Typical samples range from tiny samples like flakes of 2D materials to inhomogeneous surfaces like some CWD samples of 2D materials or terraces at Epitaxial Graphene surfaces.