In situ investigation of GaAs nanowire growth

The growth of nanowires using a catalyst is vaguely understood in terms of the vapor-liquid-solid (VLS) mechanism.1 Accordingly, the reactants supplied in the vapor phase dissolves in a metallic catalyst (often) liquid, supersaturates and eventually precipitates layer-by-layer as a solid nanowire. However, the current mechanistic understanding of this process is insufficient for the level of atomic scale control required for technological applications. In situ studies of nanowire growth can provide valuable insights in aspects such as the layer growth kinetics, chemical nature of the catalyst, catalyst-NW interface dynamics and polytypism. Layer growth dynamics and catalyst composition studied in situ will be discussed in this talk. 

GaAs nanowires with Au nanoparticles as catalyst were grown in a Hitachi HF3300S aberration-corrected environmental TEM connected to an MOCVD system. We studied the composition of the Au-Ga-As catalyst in situ during nanowire growth by energy dispersive X-ray spectroscopy (EDX) as a function of two important growth parameters – temperature and V/III ratio.2 We do not measure any significant quantity of As in the catalyst by EDX, agreeing with theoretical predictions.3 

Growth of nanowires have two important parts – the nucleation of each layer at the catalyst-nanowire interface followed by lateral growth of the nucleus to a full bilayer. Most theoretical models assume that layer completion is instantaneous1. We investigated the bilayer growth time as well as the waiting time between successive layers (‘incubation time’)4 and found that even at precursor partial pressures similar to a typical ex situ MOCVD, the time scales of both these steps are comparable. Layer completion time decreases with increasing As-precursor flow, indicating that it is mainly limited by the As availability; which is not surprising due to the low As concentration in the catalyst particle. Except for very low Ga-precursor fluxes, the layer-completion time is independent of Ga flux. However, the incubation time between bilayers significantly decreases with increasing Ga precursor flux. This trend was also observed in stochastic simulations based on nucleation modelling. Controlling the layer growth and nucleation independently will aid in engineering nanowire properties like doping and impurity incorporation. 

References:
1.	Wagner, R. S. & Ellis, W. C. Applied Physics Letters 4, 89–90 (1964).
2.	Maliakkal, C. B. et al. Nature Communications 10, 1–9 (2019).
3.	Mårtensson, E. K. et al. J. Nano Letters 19, 1197–1203 (2019).
4.	Maliakkal, C. B. et al. ArXiv190508225 Cond-Mat Physics (2019).