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 450 K 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.