An EXAFS study of the local structure of Ce---Si---O amorphous thin films

An EXAFS study has been made on the structure of three composition ranges of Ce---Si---O amorphous thin films prepared by RF sputtering. The measurements, carried out on the K edge of silicon and the L₃ edge of cerium, reveal that in the stoichiometric oxygen films of the general formula (Ce, Si) O₂, both cerium and silicon are four-coordinated by oxygen regardless of the O : Si ratio. In the oxygen-deficient films cerium remains four-coordinated by oxygen, but, around silicon, the oxygen atoms are progressively replaced by silicon as the oxygen content of the films is reduced. In silicon-rich films which are very deficient in oxygen, the oxygen atoms prefer to ramain coordinated with cerium, rather than silicon.

A definite decrease in the Si---O distance with increase in Si---O coordination has been found. The effect is attributed to an increase in the charge of silicon with oxygen coordination, and supports a randomly bonded model for the structure.

The total oxygen coordination, derived from a consideration of bond conservation, indicates that the film structures are probably SiO₂-type continuous random networks.