Fluorinated hydrogenated amorphous silicon alloys. Electronic structure due toSiFnunits and their chains

A theory for the electronic structure of amorphous fluorinated silicon alloys ($a-\mathrm{S}\mathrm{i}:\mathrm{F}$) has been discussed by using a cluster-Bethe-lattice method (CBLM). The various silicon-fluorine conformations, e.g., SiF, Si${\mathrm{F}}_2$, Si${\mathrm{F}}_3$, ${\left(\mathrm{Si}\mathrm{F}\right)}_n$, and ${\left(\mathrm{Si}{\mathrm{F}}_2\right)}_n$, in an otherwise perfect tetrahedral network of Bethe lattice discussed earlier by one of the authors in the study of their vibrational excitations have been considered. A realistic tight-binding Hamiltonian considering the interactions up to the second neighbors within the complexes has been employed for calculating the electron energy values of the complexes embedded into the bulk. The local density of states (LDOS) at the F atom and the Si atom to which they bond have been computed both for the $s$ and $p$ orbitals. The F $2p$ components dominate the local density in the valence-band region in all the configurations. In the case of simple Si${\mathrm{F}}_2$ and Si${\mathrm{F}}_3$ complexes and in fluorosilane ${\left(\mathrm{Si}\mathrm{F}\right)}_n$ and ${\left(\mathrm{Si}{\mathrm{F}}_2\right)}_n$ chains we observe some extra structure in the density of states which has not been detected in the photoemission data, probably due to the occurrence of topological disorder in the amorphous alloy. The calculated peak positions are in excellent agreement with the available photoemission data. The present study identifies the occurrence of the SiF, Si${\mathrm{F}}_2$, and Si${\mathrm{F}}_3$ conformations in these alloys. Further, in the study of the electronic structure of the fluorosilane-like configurations ${\left(\mathrm{Si}\mathrm{F}\right)}_n$ and ${\left(\mathrm{Si}{\mathrm{F}}_2\right)}_n$ embedded into the silicon matrix, one observes spiky structures in the electronic density. The smoothened peaks of these spiky structures are still in good agreement with the photoemission intensity assigned to the isolated Si${\mathrm{F}}_2$ and Si${\mathrm{F}}_3$ units. These results suggest the possible occurrence of the coupled SiF and Si${\mathrm{F}}_2$ units in $a-\mathrm{S}\mathrm{i}:\mathrm{F}$ alloys in agreement with the results of a similar analysis of the infrared data by our group.