Electrical properties of single GaAs, Bi₂S₃ and Ge nanowires
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Miniaturization, continued scaling and cost reduction in microelectronic devices are goals being actively pursued in research. As a result, semiconducting nanowires, due to their size, unique properties and potential manufacturability, are being considered as “building blocks” for future electronic and photonic devices. Nanowires could function as either the active device element, such as the conductive channel in a transistor or the light emitting element in a light emitting diode, or as the inter connections between devices on a chip. The growth and implementation of nanowire technologies hinge on high quality synthetic techniques, advanced microfabrication techniques to interface the wires with the outside world, and a fundamental understanding of their properties. Their electrical transport properties in particular have important implications on device performance. Three different semiconductor nanowire materials were studied. Electrical transport was measured as a function of temperature through individual solution-grown GaAs nanowires. The current-voltage (IV) curves were nonlinear at low temperature, becoming increasingly nonlinear with decreasing temperature. The current vii density, J, scaled with voltage and followed relationship, +1 ∝ l J V . This scaling is consistent with space charge limited currents. The characteristic trap energies estimated from the IV data were found to vary from wire to wire, ranging from 0.024 to 0.11 eV below the band edge. In the low bias region of the IV curves, where the curves were ohmic, the activation energy (related to the Fermi energy) was calculated and found to be shifted towards the band edge, consistent with either the presence of impurities or charged surface states. Bi2S3 nanowire bundles were investigated. The nanowires were synthesized using a solventless reaction involving a single-source bismuth thiolate precursor and stabilizing organic ligands. For electrical testing, the nanowire bundles were dispersed in solution and drop cast onto a substrate with gold contact pads patterned by electron beam lithography techniques (EBL). Electrical connections were made by depositing platinum interconnect lines between the nanowires and the gold pads by focused ion beam (FIB) chemical vapor deposition. Current-voltage (IV) curves were measured under nitrogen as a function of temperature. The data revealed activated transport that followed a MeyerNeldel relationship. Annealing under vacuum decreased the nanowire resistance by nearly four orders of magnitude. The annealed nanowires followed an inverse MeyerNeldel relationship. Illumination with UV light increased the current and air exposure decreases the current under constant applied bias. Solution-grown single-crystal Ge nanowires were studied as the conductive channels in field effect devices. Nanowires were deposited on a Si substrate, contacted with Pt wires using focused ion beam (FIB) chemical vapor deposition and gated with TaN or Au with ZrO2 dielectric. The Ge nanowire conductivity increased with negative gate potentials, characteristic of a p-type semiconductor. The Ge nanowires were photoconductive with higher conductivity when illuminated with UV light. Temperature viii dependent and field dependent measurements revealed that the carrier mobility increased with increasing temperature, indicating that transport is probably dominated by hopping. The effect of a second back gate on the nanowire conductivity was explored and found to provide an additional parameter for tuning the current response.