Growth and thermoelectric transport properties of bismuth-based thin film and multilayer structures.

摘要

Bi/Sb superlattices with different superlattice periods were grown on CdTe(111)B substrates using MBE. No interfacial misfit dislocations were observed in the cross-sectional HRTEM images of the Bi and Sb layers over an area of 0.1 μm2. Even structures with sub-monolayer Sb thicknesses exhibited clear periodicity, confirmed by the existence of the XRD satellite peaks. The electronic structure of superlattices with almost the same average composition could be modified by changing the superlattice period. While long-period samples showed semimetallic behavior, a semimetal-semiconductor transition was observed at short periods. The cross-plane thermal conductivity was also different from the same average composition bulk random alloy. For a 50 Å Bi/50 Å Sb superlattice, it was a factor 2 lower than the literature value for the Bi_0.5Sb_0.5 bulk alloy, which we interpret as arising from an increase in phonon scattering from the superlattice interfaces.; The anisotropy of Bi and BiSb alloy was studied using films, tilted by 19° from the substrate normal, grown on (211)CdTe. In Seebeck and magneto-Seebeck coefficient measurements, we observed a strong in-plane anisotropy, suggesting that films grown with the (00.1) axis tilted from the substrate normal show better in-plane thermoelectric properties than the more typical (00.1) films. In order to obtain high quality Bi films, post annealing just below the melting point of Bi was performed. A 10 μm Bi film grown on CdTe and subsequently annealed showed a large magnetoresistance ratio, 2700 at 5 K and 5 T, compared with 340 for the unannealed sample. The large magnetoresistance ratio of annealed bismuth films can be ascribed to a higher mobility relative to that in unannealed samples.; The well known room temperature thermoelectric materials Bi₂Te ₃ and Sb₂Te₃ were studied. The most stoichiometric sample exhibited high crystallinity, high thermopower and high electron mobility, which may be understood as arising from antisite defects. In Bi_1+xTe _1−x films, we found that the structural and electronic properties are affected by the compositional deviation from stoichiometric Bi₁Te ₁, due to structural disorder by the insertion/removal of the Bi-Bi sequences among the Te-Bi-Te-Bi-Te sequences. Thermopower measurements revealed that, as the composition changes from Te-rich to Bi-rich, the thermopower varies from n-type to p-type, but in a way which is not a simple function of the composition.