Charge transport in nanocrystalline germanium/hydrogenated amorphous silicon mixed-phase thin films

Abstract

ABSTRACTMixed phase 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. Raman spectroscopy and x-ray diffraction measurements confirm the presence of 4 - 4.5 nm diameter nc-Ge homogenously embedded within the a-Si:H matrix. The conductivity and thermopower are studied as the germanium crystal fraction XGe is systematically increased. For XGe < 10%, the thermopower is n-type (as in undoped a-Si:H) while for XGe > 25% p-type transport is observed. For films with 10 < XGe < 25% the thermopower shifts from p-type to n-type as the temperature is increased. This transition is faster than expected from a standard two-channel model for charge transport.