Model for the amorphous roughening transition in amorphous semiconductor deposition

The evolution of surface roughness on hydrogenated amorphous silicon (a-Si:H) films prepared by plasma-enhanced chemical vapor deposition exhibits initial smoothening due to nuclei coalescence, a region of surface stability, and finally a prominent roughening transition (designated a → a) at a critical thickness. The thickness at which the a → a transition occurs, as measured by real time spectroscopic ellipsometry (SE), is found to correlate closely with the electronic properties of the film. Thus, the transition has been incorporated into deposition phase diagrams that have been applied successfully to optimize solar cell performance and stability. A simple continuum model for the evolution of the 1D surface profile using an initial condition designed to be consistent with the nucleation characteristics measured by real time SE is sufficient for insights into the correlation between the roughness evolution and film properties. Good agreement between the experimental results and model calculations support the concept that optimum electronic properties of the films are associated with weakly reactive surfaces and long lifetime radicals that lead to adsorbed precursors with large surface diffusion lengths.     

Existence and Stability for Spherical Crystals Growing in a Supersaturated Solution

We consider the growth of a spherical crystal in a supersaturatedsolution. In the first part, existence and uniqueness resultsfor radially symmetric growth are obtained, provided that thesupersaturation is not too large; conversely, when the far-fieldsupersaturation exceeds a critical value, it is shown that theradially symmetric solution ceases to exist in finite time.In the second part, we examine the linear stability of a radiallysymmetric similarity solution (in which the radius grows ast?) to shape perturbations. The results are compared with previousquasi-static analyses, and, in particular, the critical radiusat which the crystal becomes unstable is found to be largerfor small supersaturations, but smaller for large supersaturations,than those predicted by the quasi-static analysis     

High-frequency excitations in glassy crystals

We present broadband dielectric loss spectra of the orientationally disordered (‘plastic’) crystal ortho-carborane, extending well into the infrared region and compare the results to the low-molecular weight glass former glycerol. Focusing on the high-frequency dynamics beyond 1 GHz, we find a loss minimum showing evidence for excess intensity, quite similar to the findings in canonical glass formers. In the THz region a strongly asymmetric peak shows up, which exhibits marked differences to the corresponding peaks found in glycerol and other glass-forming liquids. We discuss the relation of this feature to the boson peak detected, e.g., in light and neutron scattering experiments.     

A stable three-level finite difference scheme for solving the parabolic two-step model in a 3D micro-sphere heated by ultrashort-pulsed lasers

Ultrashort-pulsed lasers with pulse durations of the order of sub-picosecond to femtosecond domain possess exclusive capabilities in limiting the undesirable spread of the thermal process zone in the heated sample. Parabolic two-step micro heat transport equations have been widely applied for thermal analysis of thin metal films exposed to picosecond thermal pulses. In this study, we develop a three level finite difference scheme for solving the heat transport equations in a three-dimensional micro-sphere heated by ultrashort-pulsed lasers. It is shown that the scheme is unconditionally stable. The method is illustrated by numerical examples.     

Positive solutions for a nonlinear functional dynamic equation on a time scale

In this paper we obtain sufficient conditions for the existence of positive solutions to a nonlocal eigenvalue problem for a class of nonlinear functional dynamic equations on a time scale. We employ a cone theoretic fixed-point theorem to establish our results.     

Optical spectroscopy of a-As2Se3 under in situ laser irradiation

We report the first in situ dynamic measurements of the optical spectrum in the complete visible region of the electromagnetic spectrum to assess the kinetics of photodarkening in chalcogenide glass films. Only through such measurements, we find limitations of the commonly used Swanepoel method for determining the optical characteristics of such films. A negative change in the refractive index of a-As₂Se₃ films is observed at wavelengths longer than the wavelength of the inducing light beam. The observation of transmission spectrum as a function of time indicates that the photodarkening and photoexpansion have different origins.