Anomalous Hopping and Multi-Phonon Hopping Charge Transport in Amorphous Silicon-Germanium Alloy Thin Films

Hydrogenated amorphous Si and Ge alloys (a-Si_xGe_1-x:H) are particularly interesting for device applications such as solar cells, due to the addition of Ge to the a-Si:H network allows for the tuning of the optical gap. We describe studies that find that the electronic mobility gap model traditionally employed to describe charge transport in a-Si:H is not the best description for a-Si_xGe_1-x:H alloys. Pure a-Si:H films exhibit anomalous hopping conduction, while the conductivity of a-Ge:H is best fit by a power-law temperature dependence, characteristic of multi-phonon hopping. Corresponding measurements of the thermopower find that conduction is n-type for the purely a-Si:H and a-Ge:H samples but that the Seebeck coefficient exhibits a transition from n-type to p-type as a function of Ge content and temperature. This change in sign may be indicative of the cross-over in conduction mechanism. These findings are interpreted in terms of charge transport either through a density of exponential bandtail states or hopping via dangling bond defects, that does not involve the traditional Mott mobility gap model of highly disordered materials.