SummaryControlled formation of non-equilibrium crystal structures is one of the most important challenges in crystal growth. Catalytically-grown nanowires provide an ideal system for studying the fundamental physics of phase selection, while also offering the potential for novel electronic applications based on crystal polytype engineering. Here we image GaAs nanowires during growth as they are switched between polytypes by varying growth conditions. We find striking differences between the growth dynamics of the polytypes, including differences in interface morphology, step flow, and catalyst geometry. We explain the differences, and the phase selection, through a model that relates the catalyst volume, contact angle at the trijunction, and nucleation site of each new layer. This allows us to predict the conditions under which each phase should be preferred, and use these predictions to design GaAs heterostructures. We suggest that these results may apply to phase selection in other nanowire systems.

III-V-based nanowires usually exhibit random mixtures of wurtzite (WZ) and zinc blende (ZB) crystal structure, and pure crystal phase wires represent the exception rather than the rule. In this work, the effective group V hydride flow was the only growth parameter which was changed during MOVPE growth to promote transitions from WZ to ZB and from ZB to WZ. Our technique works in the same way for all investigated III-Vs (GaP, GaAs, InP, and InAs), with low group V flow for WZ and high group V flow for ZB conditions. This strongly suggests a common underlying mechanism. It displays to our best knowledge the simplest changes of the growth condition to control the nanowire crystal structure. The inherent reduction of growth variables is a crucial requirement for the interpretation in the frame of existing understanding of polytypism in III-V nanowires. We show that the change in surface energetics of the vapor-liquid-solid system at the vapor-liquid and liquid-solid interface is likely to control the crystal structure in our nanowires.

Using scanning tunneling microscopy and spectroscopy we study the atomic scale geometry and electronic structure of GaAs nanowires exhibiting controlled axial stacking of wurtzite (Wz) and zinc blende (Zb) crystal segments. We find that the nonpolar low-index surfaces {110}, {101[overline]0}, and {112[overline]0} are unreconstructed, unpinned, and without states in the band gap region. Direct comparison between Wz and Zb GaAs reveal a type-II band alignment and a Wz GaAs band gap of 1.52 eV.

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.

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 processnucleation and layer growthcan 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.A central challenge in crystal growth is to understand the dynamic and transient processes underlying the nucleation and growth steps. The ability to independently control these two steps would greatly expand the potential to design the structure, morphology and properties of the resulting material. Understanding the steps in crystallization is particularly important in

Semiconductor nanowires offer the opportunity to incorporate novel structures and functionality into electronic and optoelectronic devices. A clear understanding of the nanowire growth mechanism is essential for well-controlled growth of structures with desired properties, but the understanding is currently limited by a lack of empirical measurements of important parameters during growth, such as catalyst particle composition. However, this is difficult to accurately determine by investigating post-growth. We report direct measurement of the catalyst composition of individual gold seeded GaAs nanowires inside an electron microscope as they grow. The Ga content in the catalyst during growth increased with both temperature and Ga precursor flux. A direct comparison of the calculated phase diagrams of the Au-Ga-As ternary system to the measured catalyst composition not only lets us estimate the As content in the catalyst but also indicates the relevance of phase diagrams to understanding nanowire growth.

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Non-tapered vertically straight Ga(x)In(1-x)P nanowires were grown in a compositional range from Ga(0.2)In(0.8)P to pure GaP in particle-assisted mode by controlling the trimethylindium, trimethylgallium and hydrogen chloride flows in metal-organic vapor phase epitaxy. X-ray energy dispersive spectroscopy in transmission electron microscopy revealed homogeneous radial material composition in single nanowires, whereas variations in the material composition were found along the nanowires. High-resolution x-ray diffraction indicates a variation of the material composition on the order of about 19% measuring an entire sample area, i.e., including edge effects during growth. The non-capped nanowires emit room temperature photoluminescence strongly in the energy range of 1.43-2.16 eV, correlated with the bandgap expected from the material composition.