坂本 昇一

With a view to elucidating the microscopic mechanisms of the Staebler-Wronski effect in a-Si:H, we study the fundamental electronic and atomic processes by means of ab-initio molecular dynamics simulations originally proposed by Car and Parrinello. Our conclusions include: (1) Weak Si-Si bonds tend to be weakened further by illumination of light or by hole injection. (2) When a Si-Si bond is weak enough, there exists a metastable site for a hydrogen atom between these two Si atoms, at which the total energy becomes local minimum. The metastable site is either "on" or "off" Si-Si bond depending on the length of the Si-Si bond. (3) It requires much less energy to move H from a typical Si-H bond into an "off-bond" metastable site in the vicinity than into some interstitial site.

Computer simulations of melting, crystallization, glass transition, and annealing for a model system of 864 Lennard-Jones (LJ) atoms under a periodic boundary condition are carried out using constant-pressure molec ular dynamics techniques with temperature control. When an fcc crystal of LJ atoms is heated, melting occurs however, an LJ liquid, when quenched slowly, crystallizes into layers with stacking faults. Each layer forms a two-dimensional, close-packed structure with occasional point defects but without dislocations. When the quench rate is high enough, an LJ liquid transforms into a disordered structure without a discontinuous change in volume. The dependence of the glass transi tion on the quench rate is determined by examining macroscopically observable physical features. Several microscopic structure parameters are introduced to ana lyze, at the atomic level, the structures of glasses pro duced by different quench rates. When annealed, a glass made with a low enough quench rate is stable against crystallization. Identifying the number of atoms in the system having local icosahedral symmetry is a prom ising method of characterizing the glass's stability. The microscopic structural changes in the annealing pro cesses are presented in the companion video to this paper. © 1990, Sage Publications. All rights reserved.

By means of isothermal-isobaric molecular dynamics simulations, we quench a 12-6 Lennard-Jones liquid composed of 864 atoms. When the quench rate is slow enough, the crystallization takes place while, with the high quench rate, the system transforms into a disordered structure via a glass transition. We monitor the occurrence of the glass transition by examining the macroscopically-observable physical properties such as thermodynamic, structural and dynamical properties. When the quench rate is high enough to form a glass, the larger the quench rate, the higher the glass-transition temperature. We also introduce several microscopic structure parameters in order to analyze, at the atomic level, the structures of the glasses produced through different quench rates. When annealed, a glass carefully made with a low enough quench rate is stable against crystallization. We find that one promising method of characterizing the stability of a glass is to study how many atoms in the system have the environment of icosahedral symmetry. We present the microscopic structural changes in the annealing processes by using a movie film. © by R. Oldenbourg Verlag München 1988

In order to realize the glass transition by means of the isothermalisobaric molecular dynamics method for a system which takes the bcc structure when crystallized, we introduce a pair potential such that the total energy of atoms with this potential is the least for the bcc structure at low temperatures. When the system is quenched from the liquid state, the glass transition is monitored by the temperature dependence of thermodynamic properties such as volume and enthalpy, the dependence being similar to that for a Lennard-Jones (LJ) system. On the other hand, the behaviours of microscopic-structure parameters accompanying the change in temperature do not suggest the saturation of the free volume at the temperatures lower than the glass-transition temperature, which is apparently different from the case of a LJ system. © by R. Oldenbourg Verlag München 1988