Enhancing thermopower and hole mobility in bulk p-type half-Heuslers using full-Heusler nanostructures

Abstract

The concept of band structure engineering near the Fermi level through atomic-scale alteration of a bulk semiconductor crystal structure using coherently embedded intrinsic semiconducting quantum dots provides a unique opportunity to manipulate the transport behavior of the existing ensembles of carriers within the semiconducting matrix. Here we show that in situ growth of coherent nanometer-scale full-Heusler quantum dots (fH-QDs) within the p-type Ti_0.5Hf_0.5CoSb_0.9Sn_0.1 half-Heusler (hH) matrix induces a drastic decrease of the effective hole density within the hH/fH-QD nanocomposites at 300 K followed by a sharp increase with rising temperature. This behavior is associated with the formation of staggered heterojunctions with a valence band (VB) offset energy, ΔE at the hH/fH phase boundaries. The energy barrier (ΔE) discriminates existing holes with respect to their energy by trapping low energy (LE) holes, while promoting the transport of high energy (HE) holes through the VB of the fH-QDs. This “hole culling” results in surprisingly large increases in the mobility and the effective mass of HE holes contributing to electronic conduction. The simultaneous reduction in the density and the increase in the effective mass of holes resulted in large enhancements of the thermopower whereas; the increase in the mobility minimizes the drop in the electrical conductivity.