Self-assembled nanowires offer the prospect of accurate and scalable device engineering at an atomistic scale for applications in electronics, photonics and biology. However, deterministic nanowire growth and the control of dopant profiles and heterostructures are limited by an incomplete understanding of the role of commonly used catalysts and specifically of their interface dynamics. Although catalytic chemical vapour deposition of nanowires below the eutectic temperature has been demonstrated in many semiconductor-catalyst systems, growth from solid catalysts is still disputed and the overall mechanism is largely unresolved. Here, we present a video-rate environmental transmission electron microscopy study of Si nanowire formation from Pd silicide crystals under disilane exposure. A Si crystal nucleus forms by phase separation, as observed for the liquid Au-Si system, which we use as a comparative benchmark. The dominant coherent Pd silicide/Si growth interface subsequently advances by lateral propagation of ledges, driven by catalytic dissociation of disilane and coupled Pd and Si diffusion. Our results establish an atomistic framework for nanowire assembly from solid catalysts, relevant also to their contact formation.

Epitaxial Si nanowires grown from Au seeds using the vapor-liquid-solid method begin growing normal to the Si(111) substrate atop a tapered base. After a kinetically determined length, the NWs may kink away from [111] to another crystallographic direction. The smallest NWs prefer growth along 110 while larger Si NWs choose either 111 or 112 based on whether growth conditions favor Au-free sidewalls. "Vertical" growth normal to the Si(111) substrate is obtained only for slowly growing NWs with Au-decorated sidewalls. At the fastest growth rates, single-crystal Si NWs smoothly, continuously, and randomly vary their growth directions, producing a morphology that is qualitatively different than highly kinked growth.

Atomic force microscope (AFM) imaging and cross-sectional analysis were used to document the shape evolution of Ge/Si(100) islands, grown by molecular beam epitaxy, as a function of growth conditions. Growth temperatures of 450, 550, 600, and 650 °C with Ge coverages between 3.5 and 14.0 monolayers (ML) were investigated for a deposition rate of 1.4 ML/min. Low coverages produced small hut clusters which then evolved into dome clusters at higher coverages. These dome clusters eventually dislocated after further growth. Higher growth temperatures activated additional pathways for the Ge islands to relieve their strain such as Ge/Si intermixing and the formation of trenches around the islands. Our detailed AFM cross-sectional analysis indicated that dome clusters form several crystal facets in addition to those previously reported.

Ge͞Si͑100͒ island size distributions were monitored for coverages between 3.5 and 14.0 monolayers at growth temperatures from 450 to 600 ± C. Features in these distributions are correlated with characteristic island morphologies. The mean dome cluster size increased and the onset of island dislocation was delayed as the growth temperature increased. At 600 ± C, very large hut clusters are formed. This behavior is attributed to strain-assisted alloying of the Ge clusters. Energy dispersive x-ray analysis confirms Si diffusion into the Ge clusters at 600 ± C. An atomistic elastic model supports the interpretation that alloying is driven by strain energy enhancement near the island perimeters.

Quantitative, nanometer-scale spatial resolution electron energy-loss spectroscopy (EELS) was used to map the composition of coherent islands grown by molecular-beam epitaxy of pure Ge onto Si(100). The Ge concentration XGe decreased, and the Ge/Si interface became more diffuse as the growth temperature increased from 400 to 700 °C. Integrated island volumes measured by atomic force microscopy (AFM) increased linearly with Ge coverage θGe, with slopes greater than 1. This result confirmed that island growth is faster than the Ge deposition rate due to Si interdiffusion. The linearity of the island volume versus θGe curves implied that XGe was independent of island size. XGe measured by EELS and AFM agree well with each other and correctly predicted the minimum dome size observed at each growth temperature.

Trenches formed at Ge/Si(100) island bases become an effective strain-relief mechanism at high growth temperatures. Trenches result from diffusion of the most highly strained material to regions of lower strain. The trench depth self-limits, scaling linearly with island diameter. A simple atomistic model of island elasticity indicates that this self-limiting behavior is of kinetic rather than energetic origin.

The pressure and temperature dependencies for vapor-liquid-solid (VLS) growth of Ge nanostructures on Si using chemical vapor deposition are reported. Gold nanodots self-assembled by evaporation on clean hydrogen-terminated and heated Si substrates are used to seed the liquid eutectic VLS growth. Digermane pressures are varied from 4×10−5 to 1×10−2Torr and substrate temperatures from 400 to 600°C for heteroepitaxial growth on Si(111). Two types of nanostructures are identified, nanowires and nanopillars, with a transition from nanopillar growth to nanowire growth occurring with increasing pressure. Nanowires are characterized by rapid vertical growth, long-aspect-ratio structures, and linear dependence of the growth rate on pressure. At lower pressures a transition to nanopillars is observed; these exhibit both vertical and lateral growth with typical aspect ratios of 1:2. For Si(111) substrates nanowires grow epitaxially with their growth axis along the ⟨111⟩ direction. High-resolution transmission electron microscopy shows that the Ge nanowires are relaxed to their equilibrium lattice spacings a short distance from the Si substrate interface.

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