Revealing an elusive metastable wurtzite CuFeS2 and the phase switching between wurtzite and chalcopyrite for thermoelectric thin films

Abstract

As the alternative for the high-performance thermoelectric semiconductors such as GeTe, PbTe, and other compounds with rare and expensive elements, the cost-effective and abundant sulfides have been considered as one of the promising materials and garnered intensive interest. In this study, a rarely reported metastable wurtzite-type phase of CuFeS2 has been successfully obtained as thin film grown on quartz substrates via magnetron sputtering. Subsequent temperature-dependent depositions were performed to establish a phase transition between the metastable wurtzite and the chalcopyrite phase in the temperature range between room temperature and 473 K while maintaining the nominal composition ratio of CuFeS2. The wurtzite metastable analogue can be recovered and stabilized when the temperature reached 573 K and above. Thermoelectric properties were evaluated and constituted the first experimental result of the inorganic CuFeS2 thin film for thermoelectrics. The phase change between the wurtzite and chalcopyrite structure induced a significant effect on the thermoelectric properties associated with the lattice disorder and carrier scattering. The reduced thermal conductivity compared with bulk was demonstrated by the picosecond time-domain thermoreflectance method, which was found to be well correlated with the polycrystal sizes. The work provides new insight into synthesizing and manipulating the metastable wurtzite phase of the ternary I-III-VI for future applications in thermoelectrics, solar cells, and other electronic applications.