Semiconductor Nanowires: Synthesis and Quantum Transport

Semiconductor nanowires are believed to be one of most promising building blocks in nanotechnology. In this dissertation, we report the controlled synthesis and quantum transport in InAs nanowires and topological insulator Bi₂Se₃ nanoribbons, two small band gap semiconductors with important applications in high speed transistor, spintronics, thermoelectric, etc.

First, InAs nanowires and Bi₂Se₃ nanoribbons were synthesized based on the Au nanoparticle catalyzed vapor-liquid-solid mechanism in a chemical vapor deposition system. We first found that small vacuum leakage in the system incorporated oxygen in the InAs nanowires. Such nanowires exhibit low electron mobility (~100 cm²/Vs). Upon improving the system vacuum sealing, we showed that pure InAs nanowires with correct stoichiometry and superior mobility (~1000 cm²/Vs) can be consistently grown. Particularly, we studied the effect of Au nanoparticles’ shape on InAs nanowire growth and found that shaped Au nanoparticles can double the average growth rate compared with spherical ones. We attributed this enhanced growth rate to the better wetting ability of non-melted flat facets in shaped Au nanoparticles.

Secondly, due to the small diameter (<100nm) of nanowires, low temperature electronic transport of the nanowires can be low dimensional and quantum mechanical in nature. For low mobility InAs nanowires, one-dimensional weak localization was observed. The anisotropic suppression of weak localization in these nanowires was studied and attributed to the radial size confinement of time reversed electron diffusion paths. For pure InAs nanowires with high mobility, weak anti-localization was observed due to strong intrinsic spin orbit interaction. We further demonstrated the application of a surrounding electrolyte gate scheme to tune the Rashba spin orbit interaction by six fold within 1 V of gate voltage.

Thirdly, we performed magneto-transport study of nanoribbons of topological insulator materials (Bi₂Se₃, Bi₂Te₃, etc) which have novel two-dimensional helical Dirac surface states with linear dispersion. For the first time, a linear magneto-resistance persisting up to room temperature was discovered in a single Bi₂Se₃ nanoribbon, resembling the quantum linear magneto-resistance of other Dirac materials (graphene, etc). Angular dependent magneto-resistance and Shubnikov-de Haas oscillations indicate a two dimensional transport origin of this linear magneto-resistance.