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A theoretical study of stoichiometric and As-rich amorphous GaAs

We have investigated the electronic and atomistic structure of stoichiometric and As-rich GaAs by treating this material as a ternary alloy of fourfold-coordinated As4+, Ga3+ and threefold-coordinated As3+. Using the configurational averaging technique of Verges we have calculated the density of states (DOS) of this material, not only in terms of the stoichiometric index x and proportion of As3+ atoms alpha , but also in terms of the short-range order (SRO) parameters. By correlating with experiment we obtain valuable information about the SRO. In particular, we have shown that compensation occurs at a value of alpha that is a function of x only. Using this value of alpha and assuming that less than 1% of As4+-As4+ bonds occur (all other As-As bonds being allowed) 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 the lowest peak of the valence band and the subsidiary shell of As neighbours of Ga. Finally, at other values of the SRO parameters, we find features in the DOS that could explain some experiments on other amorphous III-V semiconductors.