Electronic properties of microstructured surfaces: Photoemission for pulsed electron beams in a microdiode; and pressure effects on resistance in silicon nanowires

Abstract

Numerical simulations done at the Nanophysics Center at Reykjavík University indicate that under constant illumination from a sufficiently energetic monochromatic light source, a space-charge limited photoemission can cause current modulation in a vacuum microdiode. The period of this modulation can be tuned by the potential applied to the diode, and is comparable to the transit time of electrons across the diode gap that can be as small as hundreds of femtoseconds. This thesis presents work on gallium-arsenide-germanium photocathodes that were fabricated and tested as a possible component for such a tunable microdiode oscillator.

A second topic of this thesis is the investigation of photoemission in microdiodes from laser pulses that are shorter than the characteristic transit time of the diode. This was done using a molecular dynamics code developed at RU. The transition from source-limited emission to space-charge limited emission is studied and compared to commonly used models. It is also shown how to obtain optimal brightness for such an electron pulse, an important issue for time resolved electron microscopy, free-electron lasers and other applications.

Lastly, random pattern silicon nanowires were fabricated and studied with regard to piezoresistance, with pressure-sensing applications in mind. They were found to exhibit resistance-dependant behavior both under isostatic and uniaxial pressure.