Thermoelectric Properties of PbSe Epitaxial Thin Films and PbSe/EuS Heterostructures
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Thermoelectric Properties of PbSe Epitaxial Thin Films and PbSe/EuS Heterostructures Mildred S. Dresselhaus1, Gene Dresselhaus1, Elena I. Rogacheva, Tatyana V. Tavrina, Sergey N. Grigorov, Konstantin A. Nasedkin, Valentine V. Volobuev, Alexander Yu. Sipatov, National Technical University “Kharkov Polytechnic Institute”, Kharkov, UKRAINE 1 MIT, Dept of Physics, Cambridge, MA, USA; ABSTRACT Systematic investigations were performed of the thickness dependences of the thermoelectric properties of PbSe thin films, freshly prepared and exposed to air at room temperature. It is shown that oxidation leads to a sharp change in the thermoelectric properties of the PbSe films including a change in the sign of the dominant carrier type from n-type to p- type at d ≤ 80 nm. Using a two carrier model for thin films (d < 50 nm) and a two-layer model for thick films (d > 50 nm) allows us to give a satisfactory qualitative interpretation of the observed experimental dependences of the thermoelectric properties on the film thickness. INTRODUCTION The IV-VI lead chalcogenide compounds are traditionally regarded as prospective thermoelectric materials [1,2]. The interest in these materials was further stimulated by the experimental evidence [3-9] confirming theoretical predictions [10-12] regarding the possibility of a significant enhancement in the thermoelectric figure of merit in low-dimensional structures. To realize the potential of low-dimensional structures for thermoelectric applications, it is necessary to overcome a number of problems connected with the preparation of high-quality structures. Since all real semiconductor devices are exposed to the influence of atmospheric oxygen, the absorption of oxygen, which acts as an acceptor, is an important factor affecting device properties. This is very much the case for lead chalcogenides and devices based on these materials [13-17]. In [18] we reported the d dependences of the thermoelectric properties in PbTe and PbS thin films. It was shown that the oxidation of the films in air at 300 K leads to a sign inversion of the carrier type from n to p type in films with d ≤ 125 nm and d ≤ 110 nm for PbTe and PbS, respectively. The presently available results on the oxidation of PbSe films [19-25] are mainly for n- and p-type PbSe films with d ~ 100 - 200 nm. However, the most radical changes in the properties can be expected for films with d comparable with the oxidized layer thickness. The goal of the present work is to study the d - dependences of the thermoelectric properties of freshly prepared PbSe epitaxial films at room temperature. EXPERIMENTAL Monocrystalline thin films of PbSe (d = 4-200 nm) were prepared by the thermal evaporation of n-PbSe single crystals in vacuum 10-6 Pa and their subsequent deposition onto (001) KCl surfaces at 570 K. d was monitored with a calibrated quartz resonator. Layers of EuS were prepared by electron-beam evaporation of EuS. The microstructure and crystal structure of the samples were studied by transmission electron microscopy (TEM) and electron d
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