Structural, opto-electrical, and band-edge properties of full-series multilayer SnS1-xSex (01) compounds with strong in-plane anisotropy

Two-dimensional (2D) semiconductors with black phosphorus (BP) structure have recently received considerable attention on the realization of polarized optoelectronic device, axial-dependent carrier transport, and asymmetric 2D electronic and energy device owing to the strong in-plane anisotropy presented in the van der Waal plane. In this work, multilayered SnS1-xSex chalcogenides of x = 0, 0.2, 0.4, 0.5, 0.6, 0.8 and x =1 were grown by chemical vapor transport and then exfoliated on a SiO2/Si substrate for further optical characterization. Polarized micro-Raman experiment and theoretical Raman-mode calculation simultaneously demonstrate strong in-plane anisotropy with the maximum Raman intensity of the armchair (AM) chain vibration mode (Ag) shows mutual orthogonality to the zigzag (ZZ) chain-oriented mode (B1g) in the full-series multilayer SnS1-xSex (0 < x < 1). For the intermediate compositions between x = 0.2 and x = 0.8, a crucial physical mechanism as symmetry breaking has been proposed to account for the increased number of separated Ag and B1g modes with different energies as compared to those detected for the binary compounds of orthorhombic SnS and SnSe. For the band-edge anisotropy, the bandgap value of ZZ polarized direction is shown to be lower than that of the AM direction for each of the SnS1-xSex compounds evidenced by polarized-thermoreflectance experiments and first-principles calculations. (c) 2023 The Author(s). Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Automated summary

Two-dimensional semiconductors based on black phosphorus (BP) structure have attracted significant attention recently due to their strong in-plane anisotropy and potential applications in polarized optoelectronic devices, axial-dependent carrier transport, and asymmetric 2D electronic and energy devices. In this study, multilayered SnS1-xSex chalcogenides were grown by chemical vapor transport and exfoliated on a SiO2/Si substrate for optical characterization. The results showed strong in-plane anisotropy, with the maximum Raman intensity of the AM chain vibration mode (Ag) being mutually orthogonal to the ZZ chain-oriented mode (B1g) in the full-series multilayer SnS1-xSex compounds. A physical mechanism of symmetry breaking was proposed to explain the increased number of separated Ag and B1g modes with different energies for intermediate compositions compared to binary compounds of SnS and SnSe. Additionally, the bandgap value of the ZZ polarized direction was found to be lower than that of the AM direction for each of the SnS1-xSex compounds.