Atomic step flow in a limited area: in situ observation of GaAs nanowire growth in a transmission electron microscope
Molecular beams of Ga and As4 are implemented in an aberration-corrected transmission electron microscope. GaAs nanowires are grown in situ from Au catalyst particles. Real-time observation gives access to their morphological and structural parameters while growing and the formation of atomic planes at the catalyst-nanowire interface can be examined. We use various conditions which can result in solid or liquid catalyst particle. Surprisingly, the two cases lead to comparable growth rates (about 0.3 ml/s). For liquid catalyst, the contact angle of the droplet evolves rapidly with the V/III vapor flux ratio. At contact angles around 120°, the atomic plane stacking switches from hexagonal to cubic, with a transition region of mixed crystal phases. In agreement with recently reported results1, but using a different growth technique and higher growth rates, we observe that the formation mechanisms of the two crystal phases differ singularly. Namely, hexagonal monolayers grow by slow and continuous step flow on a flat nanowire top facet; cubic monolayers appear incrementally and concomitantly with a truncation of the nanowire top facet. At low temperature, lateral growth on the sidewall facets is observed. It proceeds by step flow of one or more lateral monolayers along the nanowire axis direction. The steps are momentarily stopped and can accumulate at stacking faults present in the nanowire core.