How polytypism in InAs nanowires is affected by the presence of liquid indium during the growth on silicon

The self-assisted growth of vertical InAs nanowires on Si(111) substrates offers the possibil-ity to integrate monolithically the two materials, e.g. for novel transistor architectures, without the risk of contamination by foreign catalysts. However, arsenide nanowires that grow along the [111] crystallographic orientation are prone to wurtzite-zincblende polytypism, making the control of the crystal phase very challenging. In this work, we attempt to describe the dynamic relation between the growth conditions and the structural composition of the nanowires, and to identify potential ways to achieve phase-pure, particularly wurtzite, InAs nanowires.
Using in-situ X-ray scattering and diffraction measurements during the growth by molecular beam epitaxy, we were able to monitor the liquid phase of indium and the crystal structure of the growing nanowires throughout the growth process (Fig. 1). Although we used a much higher flux for arsenic than for indium as it is typically done for InAs nanowires, we directly observed the spontaneous build-up of liquid indium in the beginning of the growth process. Most im-portantly, the presence of liquid indium was associated with the simultaneous nucleation of InAs nanowires predominantly in the wurtzite phase. Since the build-up of liquid indium is driven by the surface diffusion of indium adatoms on the Si substrate under extremely arsenic-rich conditions, only a limited number of liquid indium sites were possible to form on the substrate, while their existence lasted for a limited period of time. In fact, the number and the lifetime of the liquid indium sites were the two parameters that defined the nucleation phase for the nan-owires.
After their nucleation, the nanowires continue to grow in the absence of liquid indium, and with a highly defective wurtzite structure. Numerical simulations based on a Monte Carlo ap-proach were employed to fit the ex-situ diffuse X-ray scattering measurements, showing that the structural degradation of the nanowires is due to the formation of planar stacking faults with their planes perpendicular to the growth direction. The onset of the formation of stacking faults is correlated with the transition from indium- to arsenic-rich conditions on each nanowire shortly after their nucleation.
After all, our study reveals the role of liquid indium in the nucleation and the structural com-position of InAs nanowires that grow on Si(111), implying that pure wurtzite nanowires may be obtained if the growth is performed in the continuous presence of liquid indium, i.e. the vapour-liquid-solid mode.