Theory of amorphous Si${\mathrm{O}}_2$ and $\mathrm{Si}{\mathrm{O}}_x$. III. Electronic structures of $\mathrm{Si}{\mathrm{O}}_x$

The electronic structures of $\mathrm{Si}{\mathrm{O}}_x$, with $x=1.5, 1.0, \mathrm{and} 0.5$, are studied by means of a first-principles method and are discussed in connection with the electronic structures of the Si-Si${\mathrm{O}}_2$ interface regions. States at the upper part of the valence band are found to consist of mainly Si orbitals and the localization of these states is proportional to $x$. The density-of-states (DOS) curves are resolved into partial components of Si atoms with five different types of local bonding configurations. The calculated DOS and joint DOS curves are critically compared with relevant experimental results. The electronic structures of $\mathrm{Si}{\mathrm{O}}_x$ based on a random-bond model are found to be in closer agreement with experiments than those based on the random-mixture model.