Hole localization in thermoelectric half-Heusler (Zr0.5Hf0.5)Co(Sb1-xSnx) thin films

The (Ti, Zr, Hf)Co(Sb1-xSnx) material class has recently come into focus as an attractive p-type high-temperature thermoelectric material. This study experimentally demonstrates that homogeneous, highly textured (Zr0.5Hf0.5)Co(Sb1-xSnx) thin films can be grown on single crystalline MgO. By varying the sputter power, samples with both positive and negative Seebeck coefficient can be grown. The underlying reason for the sign change is the segregation of Sn nano-inclusions, which lower the effective doping of the half-Heusler matrix. Similarly the Hall constant also switches sign at low temperatures, which is modeled assuming semi-metal behavior and low temperature hole localization in an acceptor band. Both resistivity and Hall constant are well described by this model.