Fluorinated hydrogenated amorphous silicon alloys. II. Electronic structure of pure and fluorinated silane chains

A theoretical study of the electronic structure of hydrogenated amorphous silicon alloys (a-Si:H) has been made after employing a cluster Bethe-lattice formalism. A number of silicon-fluorine-hydrogen configurations, e.g., simple units such as the ${\mathrm{SiH}}_{\mathrm{n}}$, ${\mathrm{SiF}}_{\mathrm{n}}$ ${\mathrm{H}}_{\mathrm{m}}$ (m,n=1,2, m+n≤3) and the chainlike configurations (SiH${)}_n$, (${\mathrm{SiH}}_2$ ${)}_{\mathrm{n}}$, and (SiFH${)}_2$ embedded in amorphous silicon, have been investigated using a realistic tight-binding Hamiltonian considering nearest-neighbor and next-nearest-neighbor interactions within the cluster. The local densities of states at H and Si atoms in alloys incorporating high H concentrations reveal structures in excellent agreement with the photoemission data. For low H concentration, a low-energy peak in the photoemission data can be understood in terms of the two adjacent interacting monohydrides. Thus, the occurrence of simple units and chainlike (SiH${)}_n$ and (${\mathrm{SiH}}_2$ ${)}_{\mathrm{n}}$ configurations can explain most of the photoemission data in pure hydrogenated silicon samples very well. The same conclusion has recently been drawn after an analysis of infrared and Raman data by ourselves elsewhere. Furthermore, we predict the electronic spectra for the a-Si:F:H samples arising from the simple ${\mathrm{SiF}}_{\mathrm{n}}$ ${\mathrm{H}}_{\mathrm{m}}$ units and the (SiFH${)}_n$ chainlike configurations. We observe that the locations of the peaks induced by F atoms remain undisturbed by the incorporation of H atoms. However, the positions of H-induced peaks are altered by the presence of the F atoms. Photoemission measurements on a-Si:F:H alloys need to be performed to detect the predicted electronic structure for a-Si:F:H alloys.