Thesis presented May 06, 2009

Abstract:
The goal of this thesis was to better understand and control nanowire growth by the Vapour Liquid Solid method, by means of structural, chemical and electronic characterization of nanowires using transmission electron microscopy. A first subject of study was the behavior of the gold catalyst particle. We have shown that control can be obtained over gold diffusion from the catalyst particle over the nanowire sidewall, by changing the growth parameters, which in turn influence the nanowire sidewall properties. The quantity of absorbed silane at the nanowire sidewall seems to be the important parameter, that depends both on silane partial pressure and temperature and also an effect of nanowire size was observed. A second topic was the crystallographic properties of the wires. The growth direction and sidewall faceting were studied. In general the crystalline quality of wires grown with a gold catalyst using silane was high. If other catalyst materials or precursor gasses were used more defects were observed. Especially in nanowires grown with a Ni catalyst and with a gold catalyst in combination with TMA, many vertical twin defects are present, as observed in an FIB prepared wire section sample. Therefore the different grains can superpose in the TEM image/DP, which can be falsely interpreted as a wurtzite hexagonal crystal structure. Clear evidence shows that in the observed nanowires no hexagonal phase (with long range order) was present, but a high twin defect density can mimic a hexagonal structure on the length of a few atomic planes. A third theme was active dopant detection using off-axis electron holography. We show for the first time doping contrast in thin (60 nm) nanowires with axially modulated doping. Using off axis electron holography the electrical properties of nanowires with axially modulated doping were studied, with the goal to determine the active dopant concentration. We show that active dopant detection is possible in nanostructures such as nanowires. Doping concentrations as low as 10¹⁸ at. cm^-3 can be detected with respect to intrinsic silicon in nanowires as thin as 60 nm. Potential simulations have been performed to better understand the experimental results. Comparison of experiments and simulations shows that the surface properties of these nanostructures, and more specifically the amount of charge present at or near the surface, are extremely important for the potential distribution in the structure. The estimated amount of charge present at the wire-oxide interface is -1x10¹² e.c. (electron charges) cm^-2, which is in excellent agreement with the amount of charge estimated from transport measurements.

Keywords:
nanowires, silicon, holography, defects, doping, gold diffusion, dewetting

On-line thesis.