Electronic structure of amorphous III-V and II-VI compound semiconductors and their defects.

Abstract

The local electronic structure of bulk and defect sites has been calculated for nine amorphous III-V compound semiconductors and for two amorphous II-VI compound semiconductors using the tight-binding recursion method. We find that structural disorder in a chemically ordered, tetrahedrally coordinated network causes a smoothing of the valence-band density of states, but little movement of the band edges, so that theoretically little change in the band gap is expected. Experimentally, the optical gap is found to close up and we attribute this to the presence of significant numbers of defect states at the band edges. The principal defects studied are undercoordinated atoms (``dangling bonds'') and like-atom bonds (``wrong bonds''). In all III-V compounds we find that anion dangling bonds give rise to occupied acceptorlike states at or below the valence-band edge (${E}_{v}$) and that cation dangling bonds produce empty donorlike states at or above the conduction-band edge (${E}_{c}$). Isolated wrong bonds are found to introduce gap states in some of the compounds; usually anion wrong bonds introduce donor states near ${E}_{c}$ while cation wrong bonds introduce acceptor states near ${E}_{v}$. Overall, a much lower density of states at the Fermi level ${E}_{F}$ is expected for these compounds compared to a-Si, and this is indeed found experimentally. In the wider-gap compounds such as a-GaAs, we propose that clusters of wrong bonds are the most probable cause of mid-gap states, while in some cases like a-InP isolated cation wrong bonds may also be responsible. We argue that the dangling-bond concentration in these materials is intrinsically high, of order 1--5? and that they are the predominant defect in annealed material. We also show that stoichiometry changes produce a combination of wrong bonds and trivalent sites of the excess species. This frequently leads to an increased density of mid-gap states, but ${E}_{F}$ does not shift from mid-gap. We have also calculated the electronic structure of various hydrogen configurations and compared them to the photoemission spectra.