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0886-F03-10.1

Preparation of [(Bi2Te3)x(HfTe2)]y Superlattices Benjamin A. Schmid, Fred R. Harris, and David C. Johnson† Department of Chemistry and Materials Science Institute, University of Oregon, Eugene, Oregon 97403 [email protected]†

Abstract The synthesis of a number of (Bi2Te3)x(HfTe2)y superlattices was accomplished using the modulated elemental reactants (MER) method. The samples were characterized using X-ray reflectivity and X-ray diffraction in the as deposited (amorphous) state and as a function of annealing temperature and time. Introduction Some of the most exciting developments in thermoelectric materials has been the discovery of enhanced performance of superlattice materials. Venkatasubramaninan et al. have reported a p-type superlattice of Bi2Te3/Sb2Te3 with a ZT of ~2.4 and a n-type Bi2Te3/Bi2Te2.83Se0.17 superlattice with a ZT of ~1.4.1 Boettner et al. have reported high thermoelectric efficiencies for lead telluride doped superlattices.2 In both cases, the largest contribution to the increased ZT stems from a reduction in thermal conductivity relative to that measured for isocompositional alloys. The most accepted rational is that this increase in ZT stems from the ability of superlattices to scatter phonons due to their mass differences between components.3-5 The ability of superlattices to scatter phonons due to mass differences has yet to be experimentally explored due to the lack of isostructural analogs of the superlattices studied, where the components vary significantly in mass. In the Bi2Te3/Sb2Te3, for example, no other compounds exist that are isostructural to Bi2Te3 or Sb2Te3 to prepare analogous superlattices. We have recently reported the synthesis of [(Bi2Te3)x(TiTe2)y] superlattices and reported measurements of promising electrical properties in these new materials.6,7 This system provides an opportunity to study the effect of structural and mass changes on thermal conductivity because an isostructural series of compounds [(Bi2Te3)x(TiTe2)y], [(Bi2Te3)x(ZrTe2)y], and [(Bi2Te3)x(HfTe2)y] could potentially be prepared. Here we report the results of our initial investigation of the synthesis of [(Bi2Te3)x(HfTe2)y]. Experimental Samples were prepared using the Modulated Elemental Reactants (MER) method. This method involves thermally evaporating metals onto an ambient temperature substrate under high vacuum (~5 x 10-7 Torr). Initial experiments involved varying the relative amount of each component deposited to determine what deposit conditions are required to obtain the desired atomic ratios required to form the two binary compounds that make up the title compounds. Compositions were determined using electron probe microanalysis (EPMA). The next challenge was to determine the absolute thickness of the binary components in each sequence of elements required to form one Te-Bi-Te-Bi-

0886-F03-10.2

Te or Te-Hf-Te layer on annealing. A fixed number of Hf-Te repeats was deposited while varying the number of Bi-Te repeats. The resulting modulation lengths of the as deposited reactant