Carrier Mapping in Thermoelectric Materials
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Carrier Mapping in Thermoelectric Materials Georgios S. Polymeris1, Euripides Hatzikraniotis1, Eleni C. Stefanaki1, Eleni Pavlidou1, Theodora Kyratsi2, Konstantinos M. Paraskevopoulos1, Mercouri G. Kanatzidis3 1 Physics Department, Aristotle University of Thessaloniki, GR- 54124, Thessaloniki, Greece 2 Department of Mechanical and Manufacturing Engineering, University of Cyprus, 1678 Nicosia, Cyprus. 2 Department of Chemistry, Northwestern University, 2145 North Sheridan Road, Evanston, IL 60208, U.S.A ABSTRACT The application of micro-fourier transform infrared (FTIR) mapping analysis to thermoelectric materials towards identification of doping inhomogeneities is described. Micro-FTIR, in conjunction with fitting, is used as analytical tool for probing carrier content gradients. The plasmon frequency ȦP2 was studied as potential effective probe for carrier inhomogeneity and consequently doping differentiation based on its dependence of the carrier concentration. The method was applied to PbTe-, PbSe- and Mg2Si- based thermoelectric materials. INTRODUCTION In the past decade several promising bulk thermoelectric materials have been identified and developed, including filled skutterudites [1], complex bismuth tellurides [2], nanostructured lead chalcogenides such as PbTe and PbSe [3-8], magnesium silicides [9] and Zn-Sb alloys [10]. In any of the aforementioned cases, a microscopically homogeneous material has to be assumed in order to acquire a comprehensive understanding between the measured thermoelectric properties, such as the Seebeck coefficient and the microscopic intrinsic features of each material. One of the promising ways of increasing the thermoelectric efficiency is the development of new inhomogeneous materials and structures including nanostructuring, heterostructures, segmented and functionally graded materials [7]. In functionally graded materials with carrier concentration gradient, the thermoelectric properties vary continuously along the length due to a continuous compositional or doping gradient, whereas in segmented materials the thermoelectric properties are changed step-like at the interface where two dissimilar materials are bonded together. Another approach of introducing in-homogeneities is the doping modulation. Usually, thermoelectric materials are heavily doped semiconductors, and a guest element is used to tune the carrier concentration with a reduction of carrier mobility due to a notable ionized impurityelectron scattering [11]. Modulation doped materials are two-phase composites with a matrix-phase of low carrier concentration and heavily doped inclusions used to provide the carriers [12]. Free carrier concentration is thus, a key parameter in thermoelectric materials. Intentional or unintentional phase and doping separations create local variations in the material composition and therefore in thermoelectric parameters. Accurate profiling is consequently becoming increasingly important and a number of profiling techniques are available today for spatial resolution of thermal and elec
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