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Raman Spectroscopy of Semiconducting Tin Chalcogenides

SRIVtharith (Tharith Sriv, Sogang University, Graduate School, Department of Physics)

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Raman spectroscopy is recognized as one of the most successful tools used to probe several physical properties of two-dimensional (2D) materials such as mechanical, thermal, electrical, magnetic and mainly optoelectronic properties. In this dissertation, the technique is used to study the optical ph...
Raman spectroscopy is recognized as one of the most successful tools used to probe several physical properties of two-dimensional (2D) materials such as mechanical, thermal, electrical, magnetic and mainly optoelectronic properties. In this dissertation, the technique is used to study the optical phonons in tin chalcogenides such as 2H-SnS2 and SnSe(1–x)Sx (0<=x<=1) semiconductor materials. Low-frequency micro-Raman spectroscopy is first used to investigate the lattice dynamics of mechanically exfoliated few-layer 2H-SnS2. The investigation is done at room temperature in high vacuum condition (~10-6 Torr range) by using four different excitation wavelengths: 632.8, 532, 514.4 and 441.6-nm. The interlayer in-plane (shear) and out-of-plane (breathing) modes show strong dependence on layer thickness resulting in robust criteria for determining few-layer sample thickness by using Raman spectroscopy. In addition, the interlayer in-plane and out-of-plane force constants are estimated by fitting the experimental data using the linear chain model (LCM). The comparison with those of several other layered materials shows that the estimated force constants are relatively smaller. In the case of anisotropic SnSe(1–x)Sx (0<=x<=1), the anisotropy and composition dependence of phonon modes are studied. By using six different excitation wavelengths such as 784.8, 632.8, 532, 514.5, 488 and 441.6-nm in the room temperature low-frequency polarized Raman spectroscopy, significant variation of the polarization dependence of Raman spectra is found as a function of excitation wavelength and sample thickness. Particularly, by comparing with a direct measurement of the crystal axes using electron microscopy, the results show that the Ag2 mode being observed in Raman backscattering geometry can be reliably used to determine the crystal axes in SnSe(1–x)Sx (0<=x<=1). More interestingly, a mixture of ‘one-mode’ and ‘two-mode’ behaviors are found in the SnSe(1–x)Sx semiconductor alloys. In conclusion, the study covered in this dissertation provides important parameters for optoelectronic device fabrications by using tin chalcogenides. Particularly, making unambiguous phonon mode assignments in SnSe(1–x)Sx and establishing their evolution as a function of the composition will benefit the analysis of other compounds such as the solar cell materials Cu2ZnSn(S,Se)4 in which Sn(S,Se) is often found as a secondary phase material that limits the cell performance.