Temperature and Thickness Dependences of Thermoelectric Properties of PbS/EuS Bilayers
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Temperature and Thickness Dependences of Thermoelectric Properties of PbS/EuS Bilayers Elena I. Rogacheva, Sergey N. Grigorov, Tatyana V. Tavrina, Olga N. Nashchekina, Yegor O. Vekhov, Alexander Yu. Sipatov, Valentine V. Volobuev, Mildred S. Dresselhaus1 National Technical University “Kharkov Polytechnic Institute”, Kharkov, UKRAINE; 1 MIT, Dept of Physics, Cambridge, MA, USA ABSTRACT A non-monotonic character of the dependences of the thermoelectric properties of PbS/EuS/(001)KCl heterostructures on the PbS layer thickness d (d = 2 – 200 nm) was detected. Pronounced extrema at d ~ 15 nm and less distinct extrema at d ~ 30 nm and d ~ 100 nm were observed. It is suggested that the complex character of the dependences is caused by the competition between percolation phenomena and size quantization. The critical exponent for the electrical conductivity (t = 1.6 ± 0.15) is determined. INTRODUCTION Theoretical predictions [1,2] and subsequent experimental evidence [3-5] for a significant increase in the thermoelectric efficiency in superlattices (SL’s) as the quantum well (QW) width decreases have attracted attention to a detailed study of the thermoelectric phenomena taking place in nanosized thin films. Extrema in the dependences of the thermoelectric properties on the thickness d of PbTe films grown on (001) KCl and covered with EuS were reported at d ~ 50 nm [6], and these extrema were attributed to a percolation transition from an island-like film to a continuous film and processes of self-organization stimulated by this transition. In this connection a question arises about the universality of the observed phenomenon. The goal of the present work is to study the effect of varying the PbS layer thickness d at a fixed thickness of the EuS layer on the thermoelectric properties (electrical conductivity σ, the Seebeck coefficient S, the Hall coefficient RH, charge carrier mobility µ, and thermoelectric power factor P = S2σ) of PbS/EuS/(001) KCl bilayers. This work was stimulated by the following: i) A significant increase in P is observed [7] upon decreasing the PbS well width in PbS/EuS/(001)KCl SL’s; ii) It was established [8] that EuS cover-layer of 25-30 nm completely protects films from oxidation. It is convenient to use EuS because this layer makes practically no contribution to the electrical conductivity; iii) PbS is often used as a buffer layer when growing IV-VI SL’s on (001) KCl substrates and it is necessary to know the role of this layer in determining the transport properties of the SL’s. EXPERIMENTAL DETAILS PbS/EuS bilayers were prepared by thermal evaporation of PbS, electron-beam evaporation of EuS in vacuum (10-5-10-6) Pa and deposition onto (001) KCl substrates at (570 ± 10) K. Thicknesses of PbS layers were varied in the range of d = 2-200 nm, and the EuS layer thickness was 25-30 nm. The epitaxial growth and structure of the samples were studied by transmission electron microscopy (TEM). RH and σ were measured in the temperature range 77 to 300 K and G8.19.1
in a magnetic field of 0.8 T, using a
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