Person:

Spaepen, Frans

The capillary part of the chemical potential is calculated for three cases: a single anisotropic grain, a thin film with columnar grains, and a polycrystalline textured binary multilayer. A proposal is made for partioning the contribution arising from the interfacial free energy between the two species.

We demonstrate liquid‐metal‐mediatedhomoepitaxialcrystal growth of Ge on Ge(111) at temperatures in the range of 400–450 °C. Crystal growth proceeds by diffusion of Ge through a liquid layer, followed by precipitation onto the substrate by the vapor‐liquid‐solid mechanism. The liquid‐metal phase at the interface is a Au‐Ge alloy formed by initial deposition of a thin Au layer above the eutectic temperature. Ge vapor is provided by a molecular‐beam evaporator. The resulting films revealed high‐crystalline quality by in situ high‐energy ion scattering and channeling analysis and ex situ by cross‐sectional transmission electron microscopy.

Face-centered cubic single crystals of (σ=1.55 μm) diameter hard-sphere silica colloidal particles were prepared by sedimentation onto (100) and (110) oriented templates. The crystals had a wide interface with the overlaying liquid that was parallel to the template. The location of the interface was determined by confocal microscopic location of the particles, followed by identification of the crystalline and liquid phases by a bond-orientation order parameter. Fluctuations in the height of the interface about its average position were recorded for several hundred configurations. The interfacial stiffness (γ̃) was determined from the slope of the inverse squared Fourier components of the height profile vs the square of the wave number, according to the continuum capillary fluctuation method. The offset of the fit from the origin could quantitatively be accounted for by gravitational damping of the fluctuations. For the (100) interface, (γ̃ =(1.3±0.3)k_{B}T/σ^{2}); for the (110) interface, (γ̃ =(1.0±0.2)k_{B}T/σ^{2}). The interfacial stiffness of both interfaces was found to be isotropic in the plane. This is surprising for the (110), where crystallography predicts twofold symmetry. Sedimentation onto a (111) template yielded a randomly stacked hexagonal crystal with isotropic (γ̃ =0.66k_{B}T/σ^{2}). This value, however, is less reliable than the two others due to imperfections in the crystal.

We performed a series of deformation experiments on a monodisperse, hard-sphere colloidal glass while simultaneously following the 3D trajectories of roughly 50,000 individual particles with a confocal microscope. In each experiment, we deformed the glass in pure shear at a constant strain rate (1 − 5 × 10−5s−1) to maximum macroscopic strains (5 − 10%), then reversed the deformation at the same rate to return to zero macroscopic strain. We also measured 3D particle trajectories in an identically-prepared quiescent glass in which the macroscopic strain was always zero. We find that shear transformation zones exist and are active in both sheared and quiescent colloidal glasses, revealed by a distinctive four-fold signature in spatial autocorrelations of the local shear strain. With increasing shear, the population of local shear transformations develops more quickly than in a quiescent glass, and many of these transformations are irreversible. When the macroscopic strain is reversed, we observe partial elastic recovery, followed by plastic deformation of the opposite sign, required to compensate for the irreversibly transformed regions. The average diameter of the shear transformation zones at maximum strain is 2.3 particle diameters.

Structural relaxation in amorphous (Pd_{82}Si_{18}) is studied using high-precision x-ray diffraction. The x-ray structure factor S(k) and the density ρ (determined from the x-ray absorption), are measured simultaneously as a function of the annealing conditions. The measured changes in S(k) are compared with those expected from simple densification using a Percus-Yevick model with two hard-sphere diameters. The variation in the electrical resistivity with annealing is also measured and is compared with the resistivity change estimated from the x-ray measurements using the Ziman theory. To allow a direct comparison of the x-ray and electrical measurements, we derive an approximate relationship between the x-ray atomic scattering factor and the pseudopotential as a substitute for a first-principles calculation. The combination of the low scattering rate from the amorphous samples and the high precision (<0.1%) necessary to allow direct comparison requires special techniques to maintain adequate system stability.

In molecular-beam epitaxy a monolayer of Pb on the Si(111) surface induces single-crystal growth at temperatures well below those required for similar growth on a bare surface. We demonstrate that the suppression of dopant segregation at the lower temperatures attainable by Pb-mediated growth allows the incorporation of As donors at concentrations reaching a few atomic percent. When Pb and Si are deposited on an As-terminated Si(111) substrate at 350 °C, the Pb segregates to the surface without doping the Si film while the As is buried within nanometers of the substrate–film interface. The resulting concentration of electrically active As, 1.8×10²¹cm^–3, represents the highest concentration of As donors achieved by any delta-doping or thin-film deposition method.