Scanning Microscopy


We have studied the oxidation of a liquid gallium surface with a high spatial resolution scanning ion microprobe. A 40 keV focused gallium ion beam, extracted from a liquid metal ion source, was employed, first, to sputter clean a 40 x 40 μm2 area on a drop of liquid gallium, in a ultra high vacuum (UHV) specimen chamber. It was then used to monitor the oxide growth by secondary ion mass spectrometry imaging microanalysis while the chamber was gradually back-filled with oxygen. In the initial stages, gallium oxide grew in a dendritic pattern from the edge of the cleaned area where oxide preexisted. Gradually the oxide layer grew in thickness and covered the entire area leaving only small islands and channels uncovered. Computer simulations based on aggregation of two dimensional random walkers (or diffusion limited aggregation) show similar dendritic patterns in the initial stages of growth. The similarity is also reflected by their comparable fractal dimensions. For the final stages, qualitative discrepancies between the experimental and simulated patterns are discernible.

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