Abstract
Small-angle X-ray scattering from GaN nanowires grown on Si(111) is measured in the grazing-incidence geometry and modelled by means of a Monte Carlo simulation that takes into account the orientational distribution of the faceted nanowires and the roughness of their side facets. It is found that the scattering intensity at large wavevectors does not follow Porod's law I(q) ∝ q-4. The intensity depends on the orientation of the side facets with respect to the incident X-ray beam. It is maximum when the scattering vector is directed along a facet normal, reminiscent of surface truncation rod scattering. At large wavevectors q, the scattering intensity is reduced by surface roughness. A root-mean-square roughness of 0.9 nm, which is the height of just 3-4 atomic steps per micrometre-long facet, already gives rise to a strong intensity reduction.
Highlights
GaN nanowires (NWs) form spontaneously in plasma-assisted molecular-beam epitaxy (PA-MBE) on various substrates at elevated temperatures under an excess of N (FernandezGarrido et al, 2009, 2012)
We show that the Grazing-incidence small-angle X-ray scattering (GISAXS) intensity from GaN NWs at large wavevectors depends on the azimuthal orientation of the NW ensemble with respect to the incident X-ray beam
GaN NWs nucleate spontaneously on Si(111) and grow with a substantial disorder with respect to their orientations. Their growth is, epitaxial: the NWs inherit the out-ofplane and in-plane orientations of the substrate. Since these NWs are typically long and thin, the range of orientations of their long axes of 3–5 is sufficient to provide the same average in the small-angle scattering intensity as if they would have all orientations in space
Summary
GaN nanowires (NWs) form spontaneously in plasma-assisted molecular-beam epitaxy (PA-MBE) on various substrates at elevated temperatures under an excess of N (FernandezGarrido et al, 2009, 2012). In contrast with the vapour–liquid– solid (VLS) growth approach followed to synthesize the majority of semiconductor NWs, PA-MBE growth of GaN NWs takes place without a metal particle on the top (Risticet al., 2008). GaN NWs on Si(111), which is the most common substrate, grow in dense ensembles (>$1010 cmÀ2) and initially possess radii of tens of nanometres as well as broad radius and length distributions (Consonni, 2013). As they grow in length, they bundle together (Kaganer et al, 2016a). The top parts gradually attain a hexagonal cross-sectional shape, so that in top-view images some of them may look like a single NW with a larger radius
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