Thermoelectric Properties of RhSb 3 Crystals and Thin Films
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ABSTRACT RhSb 3 belongs to a class of solids called skutterudites which were recently identified as promising novel thermoelectric materials. Our intent was to explore thin film growth of RhSb 3 and to assess the thermoelectric properties of such films. RhSb 3 films were prepared by electron-beam deposition on silicon and sapphire substrates. Thermopower, electrical resistivity and the Hall effect were measured over the temperature range from 2K to 300K. The data are compared to measurements on single crystals of RhSb 3. INTRODUCTION Skutterudite compounds, the name originating from a town in Norway where mineral deposits of these compounds were first mined, were recently identified as promising thermoelectric materials [1,2]. The skutterudites are compounds of the form AB 3 where A is a metal such as Co, Rh, or Ir, and B is a pnicogen such as Sb, As, or P. The compounds crystallize with a cubic structure consisting of 32 atoms per unit cell (space group IM3). The pnictide atoms constitute four-member rings located at the center of the cubes formed by the metal atom [3]. The compounds show very high hole mobilities and large thermopower, the key ingredients for a good thermoelectric material. A relatively large thermal conductivity which is a distinct disadvantage from the perspective of thermoelectricity can be effectively suppressed by filling the voids and forming the so-called filled skutterudite structure [4,5]. The resulting figure of merit of filled skutterudites appears to be at least as high [6] as the figure of merit of the current state-of-the-art thermolectric materials. Low temperature transport measurements on a related compound CoSb 3 have been made recently [7]. RhSb 3 is another member of the skutterudite family and we are interested in exploring its thin film growth because the recent theoretical predictions point to a potential of obtaining very high figures of merit for 2-dimensional structures such as quantum wells [8]. The growth of high quality skutterudite films is a prerequisite for fabrication of quantum well structures in this material system. EXPERIMENT Films of RhSb 3 were grown in a UHV-compatible dual e-gun growth chamber. Antimony (99.9999% purity) and rhodium (99.9%) slugs were co-evaporated on either
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Mat. Res. Soc. Symp. Proc. Vol. 452 01997 Materials Research Society
Si(lll) or A12 0 3 (110) substrates. Evaporation rates were controlled by two Inficon controllers and were typically set at 0.4A/s for Rh and 2.4A/s for Sb. The vacuum, normally on the order of lx10 9 torr before the deposition, rises to 5x10-7 torr during the deposition, mostly on account of a high vapor pressure of antimony. X-ray measurements confirmed the resulting structure to be RhSb 3. Single crystals of RhSb 3, against which we compare our film results, were prepared at the Jet Propalsion Laboratory in Pasadena by the vertical gradient freeze technique [9]. Resistivity, thermopower and Hall effect measurements were made with the aid of a steady-state method in a liquid helium cryostat. Typically one e
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