Abstract
The evolution of the optical phonons in layered semiconductor alloys SnSe(1–x)Sx is studied as a function of the composition by using polarized Raman spectroscopy with six different excitation wavelengths (784.8, 632.8, 532, 514.5, 488, and 441.6 nm). The polarization dependences of the phonon modes are compared with transmission electron diffraction measurements to determine the crystallographic orientation of the samples. Some of the Raman modes show significant variation in their polarization behavior depending on the excitation wavelengths. It is established that the maximum intensity direction of the Ag2 mode of SnSe(1−x)Sx (0 ≤ x ≤ 1) does not depend on the excitation wavelength and corresponds to the armchair direction. It is additionally found that the lower-frequency Raman modes of Ag1, Ag2 and B3g1 in the alloys show the typical one-mode behavior of optical phonons, whereas the higher-frequency modes of B3g2, Ag3 and Ag4 show two-mode behavior.
Highlights
The evolution of the optical phonons in layered semiconductor alloys SnSe(1–x)Sx is studied as a function of the composition by using polarized Raman spectroscopy with six different excitation wavelengths (784.8, 632.8, 532, 514.5, 488, and 441.6 nm)
Tin (II) selenide (SnSe), tin (II) sulfide (SnS) and their alloys are an interesting class of 2D materials for a variety of reasons: their band gap falls between 0.9 eV (SnSe) and 1.5 eV (SnS)[2,3,4], an important range for photovoltaic and optoelectronic a pplications[5,6,7,8,9,10,11,12,13]; due to the in-plane anisotropy of the crystal, the optical and electronic properties depend on the in-plane direction of a device; and unlike a similar material, black phosphorus, they are fairly stable in a ir[13,14,15]
Previous studies did not take into account the excitation wavelength dependence of the polarized Raman signals: because the polarization dependence of the Raman signals in anisotropic 2D materials sometimes depend sensitively on the excitation wavelength[17], polarized Raman spectroscopy has to be compared with a direct determination of the crystallographic direction using electron microscopy
Summary
The evolution of the optical phonons in layered semiconductor alloys SnSe(1–x)Sx is studied as a function of the composition by using polarized Raman spectroscopy with six different excitation wavelengths (784.8, 632.8, 532, 514.5, 488, and 441.6 nm). By comparing the polarization dependence of the phonon modes with the transmission electron microscopy (TEM) measurements, the crystallographic direction was determined without any ambiguity.
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