Germanium nanowire controlled synthesis, alignment, and field-effect-transistor characteristics

Much excitement has been generated recently about semiconductor nanowires (NW) for future high performance electronics. Single crystal NWs with diameters ranging from 5nm to 50nm can be chemically synthesized with relative ease; thus allowing the opportunity for bottom-up fabrication of NW-based integrated circuits. Furthermore, the cylindrical symmetry of NWs offers an advantage over top-down lithographically patterned circuits for the realization of surround-gated structures to minimize short channel effects. As continued reductions in length scales are pushing the fundamental limitations of silicon, germanium (Ge) has gained renewed interest as a material for future electronics owing to its high hole mobility. Combining both the structural symmetry of NWs and the high mobility of Ge, GeNW-based devices have the potential to address future device scaling limitations.; In this thesis dissertation, I review the electrical properties and discuss the first direct gate capacitance measurements of various GeNW-based field effect transistors (FETs). Single crystalline GeNWs were synthesized via the vapor-liquid-solid mechanism at 275°C from gold nanoparticles. Several methods were developed to control the registry, orientation, and pitch of GeNW arrays, including an e-beam lithography method to form arrays of gold nanoparticles followed by 100% yield synthesis and a flow alignment method to deposit thin films of aligned nanowires with controlled density. Various geometries (back-gated, top-gate, surround-gated) of depletion-mode GeNW FETs with Schottky source-drain metal contacts were fabricated and analyzed. C-V curves of disk capacitors fabricated on planar Ge substrates treated with various nitridation and silicon interlayer deposition processes were analyzed to reduce hysteresis and the density of interface states. A novel method for measuring atto-farads of capacitance was developed and used to measure gate capacitance in individual top-gated and surround-gated GeNW FETs. This direct measurement enabled the first accurate evaluation of hole mobility ∼400cm 2/Vs in GeNWs. 2D finite element simulations with carefully measured oxide thicknesses and dielectric constants shed light into the validity of using such approximations in mobility calculations. Optimized surround-gated GeNW FETs exhibited high saturation current and capacitance per unit length with subthreshold slope ∼100mV/dec, and could potentially be one component of future high mobility nanowire integrated circuits.