Thermoelectric performance of B/sub 4/C/B/sub 9/C heterostructures

Abstract

The electronic properties of bulk materials are altered when they are incorporated into quantum wells. Two-dimensional quantum wells have been synthesized by alternating layers of B/sub 4/C and B/sub 9/C. Such nanostructures are being investigated as candidate thermoelectric materials for high figures of merit (Z) at high temperature. The predicted enhancement is attributed to the confined motion of charge carriers in the two dimensions and separating them from the ion donors. This combination of materials is of interest since B/sub x/C alloys are the preferred thermoelectric materials for high temperature applications because they should exhibit little diffusion. Molecular beam epitaxy (MBE) technique has been used to prepare these multilayer films. Films have been deposited on single-crystal silicon substrates. The films have been characterized with X-ray Photoelectron Spectroscopy (XPS) and Auger spectroscopy. The thermoelectric properties of these films have also been determined over a broad range of temperatures from 4.2 K to 1200 K. When these same films were annealed at 1,050/spl deg/C for 25 hours, their /spl alpha//sup 2///spl rho/ values increased dramatically. The /spl alpha//sup 2///spl rho/ values for these P type films were more than a factor of 10 to 30 times higher than bulk P type SiGe. Attempts to measure thermal conductivity are being pursued. The performance of the MBE films have been systematically compared with bulk materials. Preliminary thermoelectric measurements of the multilayer structures, lead us to believe that significant gains in the thermoelectric figure of merit (Z) may be possible with this approach.