The structure and optical properties of amorphous oxide films.

The structure and optical bandgap properties of amorphous oxide films are studied by reference to the specific examples of r.f.-sputtered ternary a-Cex Siy Oz and binary a-SiOx : Hy thin films. Atomic structure is investigated by IR, EXAFS and Urbach edge measurements, while the bandgaps are determined from the optical absorption edge. In addition, details of the X-ray edge structure are used for the first time to obtain structural information further to that derived from EXAFS. The EXAFS study of the ternary system indicates that most films favour a continuous-random-network structure. Comparison of the optical bandgaps with the coordination numbers shows that it is the nearest-neighbour environment of Si, not Ce, that determines the former quantities. The downward shift of the IR O-stretch wSO peak position with Ce addition in these films is probably due to Ce-Si bond formation. The IR results of the a-SiOx :Hy films show a rapid reduction in film H content as the oxygen content increases, and indicates H bonded in both the mono- and di-hydride geometries. Analysis of the Si-H stretch peak fine structure points explicitly to the presence of specific tetrahedral units comprising the film structures. A combination of IR and EXAFS techniques is suggested for obtaining a detailed picture of the distribution of such microstructural units. The sensitivity of the X-ray edge features to the Si environment is used to detect small changes in its coordination due to H incorporation, as well as re-affirming the general coordination picture obtained from EXAFS. The trends in the bandgaps of the binary films with hydrogenation is shown, after a consideration of the calculated DOS in the gap region, to be due to the passivation of Si dangling bonds and replacement of weak Si-Si bonds. Oxygen 2p lone-pair interactions are identified as the major cause of bandtailing in the non-stoichiometric films, and the variation of Eo with the O/Si ratio is explained in terms of a change in connectivity of the structures. The anomalous behaviour of the stoichiometric ternary structures is attributed to the passivation of non-bridging oxygen centres by cerium.