A Study of the Evolution of Laser-Induced Periodic Surface Microstructures on Silicon and Electronic Transport in Composite Carbon Nanotube Networks

This dissertation addresses two main topics, the formation of micron-scale surface structures on single crystal silicon by irradiation with a KrF (248 nm) pulsed excimer laser source, and an investigation into electronic transport through composites of polymer (PMMA) and single-walled carbon nanotubes. It is shown that the spacing between laser-generated surface structures is determined primarily by the melt time and that the crystal orientation of the substrate can determine whether surface structures will form at all and also influence the shape of individual surface structures. During the melt/solidification cycle, the shape of liquid-vapor interface in modified by surface tension, and the shape of the solid-liquid interface is modified by a combination of thermal diffusion and faceting processes. At the last stages of solidification, the two interfaces converge but will be shaped differently resulting in an incongruity and isolated pools of molten material on the solid surface. The shape, and more importantly the final height, that these molten pools assume after they solidify depends the surface atomic density of the advancing solid interface (k_s) compared to the surface atomic density of the liquid (k_l). If k_s > k_l, as in the (110) face, the final height of the solid will be lower than the initial height of the liquid pool and surface structures will not form. If k_s < k_l, then perturbations with the correct periodicity will evolve into surface structures. Micro-cone structures form during irradiation in the presence of an SF₆ atmosphere due to multiple reflections of the incident light on a sufficiently rough surface. Electronic transport studies were performed on nanotube polymer composites using scanning electron microscopy and scanning impedance microscopy. These techniques have been used to map potential distribution within embedded networks and image paths for current flow.