Oscillations in the thickness dependences of the Seebeck coefficient in SnTe thin films
E.I. Rogacheva1, O.N. Nashchekina1, Ye.O. Vekhov1, N.A. Kalashnik1, M.S. Dresselhaus2
1National Technical University "Kharkov Polytechnic Institute", 21 Frunze St., Kharkov 61002, Ukraine
2Massachusetts Institute of Technology, 77 Massachusetts Ave., Cambridge, MA 02139, USA
IV-VI-based thermoelectric materials are well known and used both in bulk and thin film states. For thin film applications, it is necessary to take into account size quantization, which can drastically change thermoelectric properties. The objects of this study are SnTe films grown by thermal evaporation in vacuum on KCl substrates. The dependence of the Seebeck coefficient (S) on the SnTe film thickness (d=5-650 nm) was studied at room temperature. Distinct oscillations in the d-dependences were observed. The monotonic component of S decreases with increasing thickness up to d~100 nm and then remains constant. The maximum values of S observed in the SnTe films at small thicknesses exceed those for bulk crystals. The possible causes of a change in the monotonic component of the d-dependences of S (deviation from stoichiometry in SnTe, the dependence of the thermodynamic equilibrium conditions and equilibrium concentration of vacancies on d, etc) were considered.
We attribute the existence of oscillations in the d-dependences of S to size quantization in SnTe thin films. Making a number of simplifying assumptions, we roughly estimated the oscillation period D d. The discrepancy between the theoretical and experimental D d can be explained by the model oversimplification and an insufficient number of experimental data.