Comprehensive study of the valence band of standard alkali-metal and indium phosphorus oxide salts through x-ray photoemission spectroscopy and extended Hückel theory tight-binding analysis: A contribution towards the elucidation of local order in oxides on InP surfaces

The evolution of the valence-band shape of the homogeneous series of phosphate compounds from the orthophosphates ${\mathrm{Na}}_3$ ${\mathrm{PO}}_4$ and ${\mathrm{InPO}}_4$ to condensed phosphates such as ${\mathrm{Na}}_4$ ${\mathrm{P}}_2$ ${\mathrm{O}}_7$, of linear polyphosphates ${\mathrm{NaPO}}_3$ and In(${\mathrm{PO}}_3$ ${)}_3$, of three-dimensional phosphorous oxides ${\mathrm{P}}_2$ ${\mathrm{O}}_5$, and of phosphorus oxynitride PON have been investigated by x-ray photoelectron spectroscopy and extended Hückel theory tight-binding band-structure calculations. This study is based partly on an accurate comparison of the experimental and calculated spectra, the latter being corrected by cross-section factors, and partly (by means of calculated partial densities of states and crystal orbital overlap population curves) on the elucidation of the main orbital interactions that generate the specific features of each valence band. Emphasis has been placed not only on the convergence behavior of the shape of the valence bands when ${\mathrm{PO}}_4$ tetrahedra are condensed, eventually yielding infinite covalent networks, but also on the contribution of the cation, especially ${\mathrm{In}}^{3+}$.