Dielectric Function and Critical Points of SnS 0.52 Se 0.48 in the Temperature Range from 27 to 350 K
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Dielectric Function and Critical Points of SnS0.52 Se0.48 in the Temperature Range from 27 to 350 K Tae Jung Kim
∗
Center for Converging Humanities, Kyung Hee University, Seoul 02447, Korea and Department of Physics, Kyung Hee University, Seoul 02447, Korea
Xuan Au Nguyen, Bogyu Kim, Kyujin Kim, Wonjun Lee and Young Dong Kim
†
Department of Physics, Kyung Hee University, Seoul 02447, Korea
Van Long Le and Hoang Tung Nguyen Department of Physics, Kyung Hee University, Seoul 02447, Korea and Institute of Materials Science, Vietnam Academy of Science and Technology, Hanoi 100000, Vietnam (Received 21 July 2020; accepted 27 July 2020) We report the dielectric function ε = ε1 + iε2 and the critical point (CP) energies of the orthorhombic SnS0.52 Se0.48 alloy for the zigzag (a) and the armchair (b) directions in the spectral energy range from 0.74 to 6.42 eV and the temperature range from 27 to 350 K using spectroscopic ellipsometry. The CP energies were determined from the 2nd derivative spectra by using the standard analytic method. We observed sharpenings and blue-shifts of the CPs with decreasing temperature. We found thirteen and twelve CPs for the a- and the b-directions, respectively, at the lowest temperature while only eight and nine CPs were detected at room temperature. Also, an exciton effect was observed only in the armchair direction at low temperatures. The temperature dependences of the CP energies were determined by fitting the data to a phenomenological expression that contain the Bose-Einstein statistical factor and the temperature coefficient to describe the electron-phonon interaction. Keywords: Bulk SnS0.52 Se0.48 , Dielectric function, Critical point, Spectroscopic ellipsometry DOI: 10.3938/jkps.77.981
I. INTRODUCTION
Recently, IV-VI material groups have been actively studied due to their superior electronic and photonic properties [1–3]. Among them, the binary compounds SnSe and SnS have an orthorhombic crystal structure [4] and have attracted great attention because of their potential in photovoltaics [5, 6], thermoelectric devices [7], and tunable valleytronics [8]. The SnSSe ternary compound has been considered to be an important material because band gap tuning is possible by changing the composition of S in SnSx Se1−x [5,6]. The half mixed crystals of SnS0.5 Se0.5 have the same orthorhombic structure as SnS and SnSe [5] and their optical and electronic properties have been studied widely [4, 5, 7]. The complex dielectric function ε = ε1 + iε2 is especially useful for gaining insight into the electronic bandgap structure needed to characterize device performance [9]. Also, a ∗ E-mail: † E-mail:
[email protected] [email protected]
pISSN:0374-4884/eISSN:1976-8524
knowledge of ε at various temperatures is required to optimize properties for extreme environmental applications [10,11]. However, no systemitical studies on the temperature dependent dielectric function of this ternary alloy have been reported so far. Here, we report dielectric function data for singlecrystal biaxial SnS0.52 Se0.48 in
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