A THEORETICAL-STUDY OF STOICHIOMETRIC AND AS-RICH AMORPHOUS GAAS

We have investigated the electronic and atomistic structure of stoichi ometric and As-rich GaAs by treating this material as a ternary alloy of fourfold-coordinated As4+, Ga3+ and threefold-coordinated As3+. Usi ng the configurational averaging technique of Verges we have calculate d the density of states (DOS) of this material, not only in terms of t he stoichiometric index x and proportion of As3+ atoms alpha, but also in terms of the short-range order (SRO) parameters. By correlating wi th experiment we obtain valuable information about the SRO. In particu lar, we have shown that compensation occurs at a value of alpha that i s a function of x only. Using this value of alpha and assuming that le ss than 1% of As4+-As4+ bonds occur (all other As-As bonds being allow ed) we were able to (i) reproduce the EXAFS data and the variation of the optical gap with x and (ii) account for the observed shoulder in t he lowest peak of the valence band and the subsidiary shell of As neig hbours of Ga. Finally, at other values of the SRO parameters, we find features in the Dos that could explain some experiments on other amorp hous III-V semiconductors.