Electronic Transport in Nanostructured and Amorphous Semiconductor Thin Films

University Of Minnesota-Twin Cities, Minneapolis MN

Investigators

Abstract

Non-Technical Description: Composite materials consisting of nanocrystals within a thin-film amorphous matrix can exhibit novel properties not characteristic of either individual constituent. These composite structures, combining the advantages of amorphous semiconductors with the unique properties of nanocrystals may enable new electronic and photovoltaic applications. The goal of this project is to study the electronic properties of free-standing nanocrystalline films and amorphous/nanocrystalline composites as the nanocrystal properties are systematically varied. In order to elucidate the mechanism underlying the unusual conductivity in this type of material, recently discovered by the Principle Investigator, a variety of amorphous semiconductors and amorphous/nanocrystalline composites, synthesized under a broad range of conditions are investigated, testing various conduction models. A graduate student and undergraduate researchers work together on this project, learning a variety of materials synthesis and characterization techniques. The research is integrated with various outreach activities, promoting the importance of advanced materials research to a wide audience. Technical Description: The goal of the research project is to elucidate the charge transport mechanisms in free-standing nanocrystalline films; composite materials of amorphous semiconductors containing nanocrystalline inclusions; and pure amorphous semiconductors. Samples are fabricated by employing an advanced dual-chamber co-deposition system, developed at the University of Minnesota, that enables growth of various composite materials of a nanocrystal Si or Ge embedded in a thin-film matrix of hydrogenated amorphous silicon or germanium, or an insulating silicon nitride. The material properties are evaluated by using structural (transmission electron microscopy, Fourier transform infra-red spectroscopy, and Raman spectroscopy), optical (transmission and constant photocurrent techniques) and electrical characterization techniques. The charge transport in nanostructured films is investigated by using 1/f noise spectroscopy that is highly sensitive to filamentary conduction processes, as well as thermopower and Hall-effect measurements, in order to explore the mechanisms underlying the recently reported anomalous hopping conductivity.