  Semiconductor nanowires are promising material systems for coming of age
nanotechnology. The usage of the vapor solid solid (VSS) route, where the
catalyst used for promoting axial growth of nanowire is a solid, offers certain
advantages compared to the common vapor liquid solid (VLS) route (using liquid
catalyst). The VSS growth of group-IV elemental nanowires have been
investigated by other groups in situ during growth in a transmission electron
microscope (TEM). Though it is known that compound nanowire growth has
different dynamics compared to monoatomic semiconductors, the dynamics of VSS
growth of compound nanowires has not been understood. Here we investigate VSS
growth of compound nanowires by in situ microscopy, using Au-seeded GaAs as a
model system. The growth kinetics and dynamics at the wire-catalyst interface
by ledge-flow is studied and compared for liquid and solid catalysts at similar
growth conditions. Here the temperature and thermal history of the system is
manipulated to control the catalyst phase. In the first experiment discussed
here we reduce the growth temperature in steps to solidify the initially liquid
catalyst, and compare the dynamics between VLS and VSS growth observed at
slightly different temperatures. In the second experiment we exploit thermal
hysteresis of the system to obtain both VLS and VSS at the same temperature.
The VSS growth rate is comparable or slightly slower than VLS growth. Unlike in
the VLS case, during VSS growth we see several occasions where a new layer
starts before the previous layer is completely grown, i.e. multilayer growth.
Understanding the VSS growth mode enables better control of nanowire properties
by widening the range of usable nanowire growth parameters.
