Craig A. Hoffman

Artificially ordered BiSb alloys: growth and transport properties

We propose a new approach for potentially achieving high-ZT materials: artificially ordered alloy structures. We have prepared artificially ordered Bi/Sb alloys with different (Bi/Sb) periods from 7.7 /spl Aring/ to 55 /spl Aring/ by MBE. Atomic-scale ordering effects on the structural and transport properties have been studied. The formation of an ordered alloy was confirmed by the presence of XRD superlattice satellites. Using electrical resistivity, thermopower, and magneto-transport measurements, we have observed a semimetal-semiconductor transition in an ordered BiSb superlattice. The intentional ordering of the layered Bi-Sb structure has produced a new phase and the measured differences in the electronic spectrum relative to the random alloy are a consequence of its atomic structure.

Structural and thermoelectric properties of MBE-grown doped and undoped BiSb alloy thin films

We have successfully grown BiSb alloy thin films on CdTe(111)B over a wide range of Sb compositions using molecular beam epitaxy. It is well known that small bandgap (/spl sim/18 meV) bulk BiSb alloys are good n-type thermoelements at liquid nitrogen temperature. We have observed that the power factor (S/sup 2//spl sigma/) for MBE-grown 1 /spl mu/m thick BiSb thin films grown on CdTe(111) peak at a significantly higher temperature (250 K) than previous results for the bulk alloy (80 K), possibly due to an enhanced bandgap. For doping experiments we used the group IV(VI) element Sn(Te) as an acceptor(donor). Thermoelectric Power (TEP) and electrical resistivity were studied in the range of temperatures 2-300 K. Doping Sn into the BiSb system causes the TEP to change sign (from negative to positive), and the maximum value of the TEP can be controlled with the Sn dopant concentration.

Bi1-xSbx alloy thin film and superlattice thermoelectrics

We have grown Bi1-xSbx alloy thin films on CdTe(111)B over a wide range of Sb concentrations (0≤x≤0.183) using MBE. We have observed several differences with the bulk system. The 3.5 and 5.1% Sb alloys show semiconducting behavior, and the Sb concentration with the maximum bandgap is shifted to a lower Sb concentration, from 15% in bulk to 9%. The power factor S2/ρ (where S is thermoelectric power(TEP) and ρ electrical resistivity) peaks at a significantly higher temperature (250K) than previously reported for the bulk alloy (80K). The magnetotransport properties of Bi1-x,Sbx thin films (x = 0, 0.09, and 0.16) and Bi/CdTe superlattices have been determined by applying the Quantitative Mobility Spectrum Analysis (QMSA) and multicarrier fitting to the magneticfield- dependent resistivities and Hall coefficients, using algorithms which account for the strong anisotropy of the mobilities. The calculated S values are in good agreement with experimental results. The structural stability of bulk Bi is studied using the local density linear muffin-tin orbital method. It is shown that the internal displacement changes the Bi electronic structure from a metal to a semimetal, in qualitative agreement with a Jones-Peierls-type transition. The total energy is calculated to have a double well dependence on the internal displacement, and to provide a stabilization of the trigonal phase. We show that an increase of the trigonal shear angle leads to a semimetal-semiconductor transition in Bi.