Dopant states ina-Si: H. II. Effects of H and F

Abstract

The electronic structures of a variety of Si-H, Si-F, dopant-H, and dopant-F configurations have been calculated with the use of the tight-binding method. Monohydrogenated dopant sites are found to give levels no deeper than their parent sites with four Si neighbors, in general, while fluorinated donor levels are slightly deeper. We suggest that the size of an observed field-induced doping effect cannot be explained by changes in dopant levels with hydrogen attachment, and prefer an explanation based on Fermi-level—dependent dopant coordinations. New tight-binding parameters are presented for the Si—H bond using only first-neighbor interactions but including the H $2p$ state. This gives more easily transferable parameters for the amorphous states, which also represent the polarized H-charge distribution in the Si—H bond. With these parameters we find that monohydride units, including the idealized hydrogenated vacancy, do not give gap states but that the conduction bond of polysilane ${\left(\mathrm{Si}{\mathrm{H}}_2\right)}_n$ extends down to 0.3 eV. We show that the valence band of $a$-Si:H retreats by a maximum of $V\left(pp\pi \right)$ if only monohydride units are present.