Processing and applications of nanowire electronics

Semiconductor nanowires are potential components for the next generation of electronic devices. Their use in electronic devices offers several advantages and enables the realization of functionalities previously hard to achieve. For example, these can be assembled onto cheap substrates such as plastics for high performance flexible electronics. This thesis discusses processing and applications of silicon nanowires in two such areas - flexible electronics and bioelectronics.;Despite the facile solution-based assembly of nanowires on flexible substrates, challenges such as removal of high-temperature steps like doping, from nanowire device processing present an obstacle in attaining the highest performance achievable from these devices. For the reproducible and reliable deployment of silicon nanowires in electronics, it is vital that post-assembly doping techniques are developed to ensure low-resistance contacts and fabricating more elaborate device structures. However, most post-assembly doping techniques involve high temperature processing steps and hence are incompatible with temperature sensitive materials such as plastics. Our approach to overcoming this problem entails using ion implantation for introducing dopants followed by laser annealing for crystallization of the implant damage and activation of the implanted dopants. Ion implantation is a mature, precise and reproducible technology for doping semiconductors. Laser annealing is shown to be a versatile and efficient method for dopant activation in silicon nanowires as well as removal of the damage introduced by high dose implantation. These two processes in combination form a basis for a post-growth, post-assembly doping scheme completely compatible with temperature-sensitive glass and plastic substrates. The optical properties of silicon nanowires were found to be strongly polarization dependent and the impact of this exhibited polarization anisotropy of nanowires on laser annealing was examined. In situ electrical monitoring of the laser activation process and ex situ structural characterization were used to investigate the interaction of polarized visible radiation with the nanowires and the laser annealing process. The laser annealing process was compared with thermal annealing of the implanted nanowires, and it was found that the laser annealing process could achieve higher activation of the implanted dopants in the silicon nanowires. Four-probe electrical resistivity measurements were performed to extract active carrier concentrations after laser activation of the implanted dopants. Nanowire thin-film transistors fabricated using this approach showed improved yield and device performance compared to thermal annealed due to reduced parasitic resistances. This was manifested in higher drive currents as well as reduction of the unsaturated operating range of thermally annealed nanowire transistors.;In addition to processing the silicon nanowires, the application of ultra-fast pulsed lasers in diagnostics of nanowire electronic properties was demonstrated. We have used femtosecond laser radiation coupled with conductive atomic force microscopy to excite photoelectronic emission across the silicon nanowire oxide. Utilizing this method we studied the nanoscale spatial variations of the dielectric properties of silicon nanowire oxides which are otherwise difficult to probe. Both similarities and differences were revealed in the dielectric properties of oxides on silicon nanowires compared to planar silicon oxides. The interface barrier to electron transit from the semiconductor to the dielectric and the threshold electric field for current flow were found to be similar to planar oxides on silicon wafers, however the lowest currents measured were non-uniformly distributed. This observation and current-voltage data for transport through nanowire oxides suggest a non-uniform and probably higher trap density in the nanowire oxides.;Finally, the use of nanowires as components of hybrid bioelectronic devices by integrating them with lipid bilayers was explored. The nanowires acted as templates for the assembly of lipid molecules into conformal bilayers on the silicon nanowire surfaces. The vesicle fusion technique was used for this purpose. The formation of lipid bilayers on both suspended nanowires and substrate-bound nanowire devices was verified by scanning confocal microscopy. Fluorescence recovery after photobleaching experiments confirmed that the bilayers were fluid and exhibited diffusion coefficients comparable to lipid bilayers formed on flat silicon substrates. Electric characterization of the lipid bilayers by cyclic voltammetry revealed that the lipid coatings block ion transport to nanowire surfaces and shield the nanowires from charged species in solution. The potential of this hybrid bio-nanostructure as a platform for sensing and bioelectronics is shown by the incorporation of different ion-transporting membrane peptides and proteins in the shielding bilayers to achieve different functionalities such as voltage and ligand-gated ionic transport to silicon nanowires.