Thermopower of nanocrystalline germanium/hydrogenated amorphous silicon composite thin films

Thin films consisting of hydrogenated amorphous silicon (a-Si: H), in which germanium nanocrystals (nc-Ge) are embedded, have been synthesized using a dual-chamber co-deposition system. The thermopower and conductivity are studied as the germanium crystal fraction X-Ge is systematically increased. For X-Ge < 10%, the thermopower is n-type (as found in undoped a-Si: H), while for X-Ge > 25% p-type transport is observed. For films with 10% < X-Ge < 25%, the thermopower shifts from n-type to p-type as the temperature is lowered from 450K to 350 K. The n-type to p-type transition is sharper than expected from a standard two-channel parallel conduction model for charge transport. (C) 2013 AIP Publishing LLC.