Xianwei Zhao

Growth control of GaAs nanowires using pulsed laser deposition with arsenic over-pressure

Using pulsed laser ablation with arsenic over-pressure, the growth conditions for GaAs nanowires have been systematically investigated. The single-crystal structure and geometry of the nanowires have been characterized for various growth conditions. Arsenic over-pressure with As2 molecules was introduced into the system by thermal decomposition of polycrystalline GaAs to control the stoichiometry and shape of the nanowires during growth. GaAs nanowires exhibit a variety of geometries under varying arsenic over-pressures, which can be understood by different growth processes via a vapor?liquid?solid mechanism. Without As2 over-pressure, branched growth of GaAs with uncontrollable size and geometry was observed due to the decomposition of GaAs nanowires, producing metallic Ga which serves as catalysts for the branched growth of GaAs on the nanowire surfaces. Under optimal As2 over-pressure, single-crystal GaAs nanowires with uniform diameter, small diameter distribution, length over 20??m and thin surface oxide layer were obtained and used for I?V characterization.

Defect states and disorder in charge transport in semiconductor nanowires

We present a comprehensive investigation into disorder-mediated charge transport in InP nanowires in the statistical doping regime. At zero gate voltage, transport is well described by the space charge limited current model and hopping transport, but positive gate voltage (electron accumulation) reveals a previously unexplored regime of nanowire charge transport that is not well described by existing theory. The ability to continuously tune between these regimes provides guidance for the extension of existing models and directly informs the design of next-generation nanoscale electronic devices.

Comprehensive control of optical polarization anisotropy in semiconducting nanowires

The demonstration of strong photoluminescence polarization anisotropy in semiconducting nanowires embodies both technological promise and scientific challenge. Here, we present progress on both fronts through the study of the photoluminescence polarization anisotropy of randomly oriented nanowire ensembles in materials without/with crystalline anisotropy, small/wide bandgap, and both III-V/II-VI chemistry (InP/ZnO nanowires, respectively). Comprehensive control of the polarization anisotropy is realized by dielectric matching with conformally deposited Ta2O5 (dielectric ratios of 9.6:4.41 and 4.0:4.41 for InP and ZnO, respectively). After dielectric matching, the polarization anisotropy of the nanowire ensembles is reduced by 86% for InP:Ta2O5 and 84% for ZnO:Ta2O5.

Synthesis of epitaxial silicon nanowires on Si(111) substrates using ultrahigh vacuum magnetron sputtering

Epitaxial silicon nanowires on Si(111) surface and nonepitaxial silicon nanowires on thermally oxidized silicon substrates have been synthesized using ultrahigh vacuum sputtering. Silicon nanowires with noncylindrical shape and diameter of 200–300nm were obtained on thermally oxidized Si substrates using Au(15A)∕Si(8A)∕Au(15A) trilayer as catalysts. Cylindrical silicon nanowires with diameter between 40 and 100nm and length up to 8μm were synthesized using epitaxial growth on Si(111) substrates. Sputter provides an alternative fabrication technique for silicon nanowire synthesis in ultrahigh vacuum environment.