Jeff Drucker

Ledge-flow-controlled catalyst interface dynamics during Si nanowire growth

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.

Microstructural evolution during the heteroepitaxy of Ge on vicinal Si(100)

Microstructural evolution during the initial stages of islanding of Ge on vicinal Si(100) has been studied in situ with nanometer resolution in an ultrahigh‐vacuum scanning transmission electron microscope. Ge is deposited using molecular‐beam‐epitaxy (MBE) techniques on vicinal Si(100) misoriented 1° and 5° toward 〈110〉. For MBE‐type experiments, there is evidence for metastable growth of the Ge intermediate layer to much greater than the equilibrium critical thickness. The layer may grow up to seven monolayers thick before islanding in the Stranski–Krastanov growth mode. The presence of strong adatom sinks significantly alters the growth and size distribution of the islands when the spacing of these sinks is less than an adatom diffusion distance. Studies of the initial stages of islanding in solid‐phase MBE indicate that there is no long‐range adatom diffusion. There is an initial fast transformation from a disordered layer growth, followed by a sluggish growth of islands. We have studied the coarsenin...

Evolution of Ge/Si(100) islands: Island size and temperature dependence

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.

Vapor-liquid-solid growth of germanium nanostructures on silicon

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 elect...

Strain relief via trench formation in Ge/Si(100) islands

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.

Strain mapping in nanowires

A method for obtaining detailed two-dimensional strain maps in nanowires and related nanoscale structures has been developed. The approach relies on a combination of lattice imaging by high-resolution transmission electron microscopy and geometric phase analysis of the resulting micrographs using Fourier transform routines. We demonstrate the method for a germanium nanowire grown epitaxially on Si(111) by obtaining the strain components epsilon(xx), epsilon(yy), epsilon(xy), the mean dilatation, and the rotation of the lattice planes. The resulting strain maps are demonstrated to allow detailed evaluation of the strains and loading on nanowires.

Self-assembling Ge(Si)/Si(100) quantum dots

The morphological evolution of self-assembled epitaxial quantum dots on Si(100) is reviewed. This intensely investigated material system continues to provide fundamental insight guiding the growth of nanostructured electronic materials. Self-assembled quantum dots are faceted, three-dimensional islands which grow atop a planar wetting layer. Pure Ge growth at higher substrate temperatures results in narrower island size distributions but activates additional strain-relief mechanisms which will alter the optical and electronic properties of the dots. Optical and electrical characterization has shown that electrons and holes are confined to different regions of the dot. This results in a spatially indirect, type II recombination mechanism. Emerging device applications which exploit properties of these nanoscale Ge islands are discussed.

Nanometer-scale composition measurements of Ge/Si(100) islands

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.

Biassed secondary electron imaging in a UHV-STEM

Abstract A new dedicated ultra-high-vacuum scanning transmission electron microscope (UHV-STEM) has been developed for the NSF HREM facility at Arizona State University, in conjunction with VG Microscopes Ltd. This instrument is fitted with a specimen preparation chamber in which vacua of better than 5x10-10 mbar can be routinely achieved; the pressure inside the column is better than 10-10 mbar. The initial performance in various imaging modes is reported. Several techniques have been incorporated to obtain information from surfaces. Here we demonstrate the usefulness of biassed secondary electron imaging from both sides of thin transmission samples. In conjunction with conventional STEM imaging and analysis techniques, these methods can be used to correlate surface and subsurface information on samples with complex surface topography. Examples typical of catalytic and semiconductor applications are given.

Spreading of liquid AuSi on vapor-liquid-solid-grown Si nanowires.

Vapor-liquid-solid growth of high-quality Si nanowires relies on the stability of the liquid metal seed. In situ transmission electron microscopy shows that liquid AuSi seed spreads along the sidewalls of Si nanowires for some growth conditions. This liquid thin film phase separates to form solid Au clusters as the nanowire is quenched below the solidus temperature. The length that the liquid film spreads from the seed and its thickness can be explained by considering the spreading thermodynamics of droplets on cylinders.