J. L. Taraci

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...

Discrete magnetic microfluidics

We present a method to move and control drops of water on superhydrophobic surfaces using magnetic fields. Small water drops (volume of 5–35μl) that contain fractions of paramagnetic particles as low as 0.1% in weight can be moved at relatively high speed (7cm∕s) by displacing a permanent magnet placed below the surface. Coalescence of two drops has been demonstrated by moving a drop that contains paramagnetic particles towards an aqueous drop that was previously pinned to a surface defect. This approach to microfluidics has the advantages of faster and more flexible control over drop movement.

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.

Simple chemical routes to diamond-cubic germanium–tin alloys

We report the development of a simple chemical route to growing Ge1−xSnx semiconductors using ultrahigh-vacuum chemical vapor deposition and the molecular precursor (Ph)SnD3 as the source of Sn atoms. Thin films were deposited on oxidized and oxide-free Si by reactions of (Ph)SnD3 with Ge2H6 at 350 °C. The composition, microstructure, and bonding properties of the films were characterized by Rutherford backscattering, high-resolution analytical electron microscopy, and Raman spectroscopy. As-deposited Ge1−xSnx on oxidized Si displayed good crystallinity which improved significantly by annealing at 400 °C. High-resolution electron microscopy and diffraction indicated a diamond-cubic structure with lattice constants intermediate to those of Ge and α-Sn. As-deposited Ge1−xSnx on pure Si was monocrystalline and epitaxial. Nanoprobe analysis in plan view and cross section revealed that the as-deposited and annealed materials were homogeneous with good chemical purity. The Raman spectra showed bands correspondi...

Nanopillar growth mode by vapor-liquid-solid epitaxy

We report epitaxial growth of Ge nanopillars (NPs) on Si(100) by vapor-liquid-solid (VLS) growth from digermane. This growth morphology is characterized by short, low-aspect-ratio pillars and is markedly different from the long, narrow nanowires (NWs) previously reported for VLS growth. The NP growth mode occurs at low digermane pressures. It is attributed to surface-diffusion-induced lateral growth in combination with an insufficient Ge concentration gradient in the AuGe eutectic to catalyze NW growth. High resolution electron microscopy confirms that the NPs are epitaxial with the Si (100) substrate and are fully relaxed and strain free.

Optical Vibrational and Structural Properties of Ge1−xSn x alloys by UHV-CVD

Abstract : UHV-CVD growth based on a deuterium stabilized Sn hydride and digermane produces Ge-Sn alloys with tunable bandgaps. The Ge(1-x)Sn(x) (x=2-20 at.%) alloys are deposited on Si (100) and exhibit superior crystallinity and thermal stability compared with MBE grown films. Composition, crystal and electronic structure, and optical and vibrational properties are characterized by RBS, low energy SIMS, high resolution electron microscopy TEM, x-ray diffraction, as well as Raman and IR spectroscopies. TEM studies reveal epitaxial films with lattice constants between those of Ge and Sn. X-ray diffraction shows well-defined (004) peaks and rocking curves indicate a tightly aligned spread of the crystal mosaics. Resonance Raman indicate a E1 bandgap reduction relative to Ge, consistent with a decrease of the E2 critical point observed in spectroscopic ellipsometry. IR transmission spectra indicate an increase in absorption with increasing Sn content consistent with a decrease of the direct bandgap.