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Séminaire nanostructures et systèmes quantiques

Electron Transport in Semiconducting Nanowires

Jia Grace Lu

Department of Physics and Astronomy
Department of Electrical Engineering
University of Southern California, Los Angeles, CA 90089, USA

Charge transport properties of low dimensional systems are of profound interest due to the quantum mechanical phenomena that occur when their sizes reduce to nanometer scales. I will present measurements on both wide bandgap (ZnO) and narrow bandgap (InN) nanowires. The nanowires are synthesized via catalytic chemical vapor deposition method. The as-grown nanowires are configured as field effect transistors and measured in variable temperature, magnetic field, or light illumination.
In the case of ZnO nanowires, at temperatures T > 50 K, conduction shows Arrhenius behavior and can be expressed as σ exp(-Ea/kT), where Ea 60 meV is the activation energy, attributed to the shallow donor levels below the conduction band edge. On the other hand, for lower temperatures, variable range hopping model governs the transport, with conductivity following 3D Mott model, σ exp(-AT-1/4). Furthermore, the ZnO nanowires have been doped with extrinsic elements. They exhibit semiconducting behavior under dark and 632.8 nm laser illumination. A pronounced semiconductor-to-metal transition is observed under 254 nm UV irradiation, as a consequence of the reduction of electron mobility arising from the drastically enhanced Coulomb interactions. In addition, there exist two reproducible resistance valleys at 220 K and 320 K upon UV irradiation. Combined with photoluminescence spectra, this phenomenon is determined to originate from the trapping and detrapping processes of shallow defect levels. Among the versatile applications of ZnO material as field effect transistors and chemical sensors, photovoltaic devices. Similarly, for InN nanowires, the conduction at low temperature reveals an insulating behavior, which is governed by three dimensional Mott variable range hopping mechanism. With rising temperature, a distinct semiconductor-to-metal transition is observed around 80 K. In addition, the nanowire exhibits negative magnetoresistance under both parallel and perpendicular fields, due to the suppression of the interference of the electron wavefunctions as magnetic field increases. Furthermore, we present a field direction asymmetry effect on the magnetoresistance due to the distinct anisotropy in the conduction channel.