N- and P-type SiGe/Si superlattice coolers

SiGe is a good thermoelectric material for high temperature applications. In this paper the fabrication and characterization of single-element SiGe/Si superlattice coolers of both n- and p-type devices are described for room temperature applications. Superlattice structures were used to enhance the device performance by reducing the thermal conductivity between the hot and the cold junctions, and by providing selective removal of hot carriers through thermionic emission. The structure of the samples consisted of a 3 μm thick symmetrically strained Si_0.7Ge_0.3/Si superlattice grown on a buffer layer designed so that the in-plane lattice constant is approximately that of relaxed Si_0.9Ge_0.1. Cooling by 1.7 K for n-type device and by 1.9 K for p-type device at room temperature was measured, corresponding to cooling power densities of hundreds of watts per square centimeter. The results show that the packaged devices of both n and p coolers can work together in similar optimal conditions. This paves the road to fabricate n- and p-type superlattice coolers in an array format electrically in series and thermally in parallel, similar to conventional thermoelectric devices, and thus achieve large cooling capacities with relatively small currents