Electronic Transport Phenomena in Composite Nanocrystalline/Amorphous and Free-Standing Nanocrystalline Thin Films

Composite materials consisting of nanocrystalline semiconductors embedded within a bulk amorphous semiconductor or an insulator have attracted interest for applications ranging from photovoltaics, thermoelectrics, thin film transistors, particle detectors and electroluminescent devices. These materials combine the best of both worlds – the thin film large area advantages of disordered semiconductors with the superior opto-electronic properties of crystals, and often display electronic properties not observed in either material separately. Using a unique dual-chamber Plasma Enhanced Chemical Vapor Deposition system, we have synthesized nanocrystals of silicon or germanium in a surrounding hydrogenated amorphous silicon (a-Si:H) matrix (a/nc-Si:H). The dark conductivity of n-type doped a/nc-Si:H films displays three distinct conduction mechanisms: thermally activated conduction, multi-phonon hopping and Mott variable range hopping, as the crystal fraction and temperature of these films is varied. Studies of the thermopower of composite films of a-Si:H containing germanium nanocrystals find that transport changes from n-type to p-type as the nc-Ge concentration is increased, with a transition sharper than expected from a standard two-channel model for charge transport. Using the Zabrodskii analysis technique, the conductivity in the nc-Ge/a-Si:H films is described by an anomalous hopping expression, ~ exp[(To/T)k] where k = ¾, suggesting an entirely new conduction mechanism. Similar studies of free-standing nc Si films, deposited without a surrounding matrix, find that the conduction mechanism varies with the film’s exposure to atmosphere. This research done in collaboration with Uwe Kortshagen, C. Blackwell, Y. Adjallah, L. Wienkes, K. Bodurtha, C. Anderson and J. Trask. This work was partially supported by NSF grants NER-DMI-0403887, DMR-0705675, the NINN Characterization Facility, the Xcel Energy grant under RDF contract #RD3-25, NREL XEA-9-99012-01 and the University of Minnesota