Structural, electronic, and dynamical properties of amorphous gallium arsenide: a comparison between two topological models

We present a detailed study of the effect of local chemical ordering on the structural, electronic, and dynamical properties of amorphous gallium arsenide. Using the recently-proposed ``activation-relaxation technique'' and empirical potentials, we have constructed two 216-atom tetrahedral continuous random networks with different topological properties, which were further relaxed using tight-binding molecular dynamics. The first network corresponds to the traditional, amorphous, Polk-type, network, randomly decorated with Ga and As atoms. The second is an amorphous structure with a minimum of wrong (homopolar) bonds, and therefore a minimum of odd-membered atomic rings, and thus corresponds to the Connell-Temkin model. By comparing the structural, electronic, and dynamical properties of these two models, we show that the Connell-Temkin network is energetically favored over Polk, but that most properties are little affected by the differences in topology. We conclude that most indirect experimental evidence for the presence (or absence) of wrong bonds is much weaker than previously believed and that only direct structural measurements, i.e., of such quantities as partial radial distribution functions, can provide quantitative information on these defects in a-GaAs.Comment: 10 pages, 7 ps figures with eps

Interstitial-fluoride and substitutional-oxygen charge compensations of Er

A detailed crystal-field analysis, based on the Racah' theory, was carried out for the so-called A, B and G1 isolated charge-compensation centers of Er3+ ion doped in CaF2 crystal. Three sets of crystal-field parameters were obtained by a least-squares fitting of the optical data of Er3+ ion diluted in epitaxial Ca1−xErxF2+x thin film. This theoretical analysis confirms the expected $C_{4\nu}$ site symmetry for the A center and the $C_{3\nu}$ site symmetry for the G1 center. For the B center, however, the site symmetry is not exactly $C_{3\nu}$ in contrast to what is believed