Origin of p-type characteristics in a SnSe single crystal

SnSe single crystals have recently been found to exhibit excellent thermoelectric performance with an extremely high figure of merit (ZT) value of 2.6. Although this high ZT value has attracted considerable attention, the microscopic origin of the p-type characteristics of SnSe is not yet clearly understood. Here, we directly observed and identified intrinsic point defects existing on SnSe via scanning tunneling microscopy (STM) and investigated the effect of defects on the electronic properties using density functional theory (DFT) calculations. Our results demonstrate that the most dominant Sn vacancies move the Fermi energy inside the dispersive valence band and produce extra holes throughout the system. On the other hand, Se vacancies create a nondispersive donor level and generate immobile electrons localized near the vacancy site. Our combined STM/DFT studies show that the p-type characteristics of SnSe originate from extra holes in the dispersive Bloch-like band created by Sn vacancies. We expect that our results provide important information for the development of highly efficient SnSe-based thermoelectric devices.