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 XGe is systematically increased. For XGe < 10%, the thermopower is n-type (as found in undoped a-Si:H), while for XGe > 25% p-type transport is observed. For films with 10% < XGe < 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.
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We gratefully acknowledge fruitful discussions with B. Shklovskii and J. Chelikowsky. This work was partially supported by NSF Grant DMR-0705675, the NINN Characterization Facility, the Nanofabrication Center, an Xcel Energy grant under RDF Contract #RD3-25, NREL Sub-Contract XEA-9-99012-01, and the University of Minnesota.