  Control of the crystallization process is central to developing novel
materials with atomic precision to meet the demands of electronic and quantum
technology applications. Semiconductor nanowires grown by the
vapor-liquid-solid process are a promising material system in which the ability
to form components with structure and composition not achievable in bulk is
well-established. Here we use in situ TEM imaging of GaAs nanowire growth to
understand the processes by which the growth dynamics are connected to the
experimental parameters. We find that two sequential steps in the
crystallization process - nucleation and layer growth - can occur on similar
time scales and can be controlled independently using different growth
parameters. Importantly, the layer growth process contributes significantly to
the growth time for all conditions, and will play a major role in determining
material properties. The results are understood through theoretical simulations
correlating the growth dynamics, liquid droplet and experimental parameters.
