PHONONS AND PHONON LOCALIZATION IN A-SI - COMPUTATIONAL APPROACHES AND RESULTS FOR CONTINUOUS-RANDOM-NETWORK-DERIVED STRUCTURES

Three different models for a-Si are studied with respect to the vibrat ional density of states (VDOS) and phonon localization. The degree of disorder is varied for each model in a large range. For all models str uctural properties are investigated in connection with the VDOS. Phono n localization is examined via scaling approaches and mobility edges a re quantified. Two of the models are continuous random networks (CRN's ): the vacancy model and the Wooten-Winer-Weaire (WWW) model both rela xed with the Keating potential. The vacancy model causes the appearanc e of an artificial high-energy shoulder of the TO peak, which leads to wrong predictions on localization too. This shortcoming of the vacanc y model is caused by a second maximum of the bond angle distribution a t large angles. The WWW model is here the superior CRN model for a-Si. It allows a good reproduction of the experimental VDOS and possesses only about 1% localized states at the upper edge of the VDOS. In the t hird model, the WWW model relaxed with the Stillinger-Weber potential, dangling bonds and floating bonds are introduced. Its only shortcomin g is an artificial maximum in the radial distribution function below t he second diffraction peak. Due to defects extra modes at low energies are found that are highly dependent on the quality of the relaxation. The VDOS is well reproduced. About 2% of the modes at high energies a re localized. The modes at lowest energies look localized, when system s below 2000 atoms are studied. It turns out that large systems up to 8000 atoms and many independent realizations are required to interpret the phonon properties correctly. The amount of localization is found to be independent of the degree of disorder present in the model, but an increase in the number of localized states with decreasing density is observed. The present investigation permits statements about the su itability of models for amorphous solids, relaxation procedures, stand ard potentials, and procedures to determine the localization character of states.