Fabrication and Electronic Properties of CZTSe Single Crystals

CZTSe single crystals of different compositions were fabricated by solid-state reaction of elements in a sealed ampoule below the melt temperature. About 2-5-mm crystals were achieved without a flux agent, which, when present, can potentially affect defect properties in the material. A broad PL peak is observed with significant luminescence below the bandgap similar to the literature reports of band-tailing from disorder. Cu-poor and near stoichiometric compositions were prepared, and electronic and transport properties were analyzed with Hall and temperature-conductivity measurements. Intragrain measurements showed record hole mobilities for pure CZTSe in excess of 100 cm² /(V · s). Carrier concentrations ranged from 10¹⁶-10¹⁹ and correlated with Cu concentration. Temperature conductivity analysis suggests Mott variable-range hopping, as has been reported in CZTS and other disordered semiconductors, while a metal-insulator transition was seen for high-Cu concentrations. The results of transport and PL measurements are suggestive of a highly disordered material, despite long fabrication times conducive to near-equilibrium bulk conditions.