Controlled formation of non-equilibrium crystal structures is one of the most important challenges in crystal growth. Catalytically grown nanowires are ideal systems for studying the fundamental physics of phase selection, and could lead to new electronic applications based on the engineering of crystal phases. Here we image gallium arsenide (GaAs) nanowires during growth as they switch between phases as a result of varying growth conditions. We find clear differences between the growth dynamics of the phases, including differences in interface morphology, step flow and catalyst geometry. We explain these differences, and the phase selection, using a model that relates the catalyst volume, the contact angle at the trijunction (the point at which solid, liquid and vapour meet) and the nucleation site of each new layer of GaAs. This model allows us to predict the conditions under which each phase should be observed, and use these predictions to design GaAs heterostructures. These results could apply to phase selection in other nanowire systems.

We have investigated a large set of individual wurtzite/zinc-blende GaAs heterojunction nanowires using transmission electron microscopy (TEM), photoluminescence (PL), and cathodoluminescence (CL). In several cases we have combined PL and TEM on the same wire. We find a fairly deep emission that shows a strong blueshift with increasing excitation power density. By using a variety of experiments we show that this emission is due to recombination across a heterojunction with a type II band alignment. The data give a valence band offset of about 100 meV, in excellent agreement with theoretical predictions. Spatially resolved CL data on heterojunction nanowires and PL data on pure wurtzite nanowires show that the band gap of wurtzite GaAs is very close to the band gap of zinc-blende GaAs.

Polytype nanodots are arguably the simplest nanodots than can be made, but their technological control was, up to now, challenging. We have developed a technique to produce nanowires containing exactly one polytype nanodot in GaAs with thickness control. These nanodots have been investigated by photoluminescence, which has been cross-correlated with transmission electron microscopy. We find that short (4–20 nm) zincblende GaAs segments/dots in wurtzite GaAs confine electrons and that the inverse system confines holes. By varying the thickness of the nanodots we find strong quantum confinement effects which allows us to extract the effective mass of the carriers. The holes at the top of the valence band have an effective mass of approximately 0.45 m₀ in wurtzite GaAs. The thinnest wurtzite nanodot corresponds to a twin plane in zincblende GaAs and gives efficient photoluminescence. It binds an exciton with a binding energy of roughly 50 meV, including central cell corrections.