Characterization of low pressure deposited diamond films by X-ray photoelectron spectroscopy

Abstract

X-ray photoelectron spectroscopy (XPS) has been utilized to study the electronic structure of carbon films grown by a low pressure hot filament assisted chemical vapor deposition technique with a view to optimizing the growth parameters for the diamond films. A constant flow of a mixture of high purity methane and hydrogen was maintained at a pressure of 25 Torr in the reaction chamber. The films were prepared at three different concentrations of methane: 0.25%, 0.5%, and 1.0% by density. The XPS C 1s core level and valence band spectra of these films are compared with those of graphite. Diamond has covalent sp 3 bonding while graphite has sp 2 bonding. XPS spectra exhibit features related to the difference in bonding. The plasmon loss shoulder (characteristic of graphite) associated with the main C 1s peak is found to be absent in the spectrum of films grown when the gas composition contains 0.25% methane, while it is found to build up with increasing concentration of methane. The binding energy position of the C 1s peak shows that appreciable charging occurs for the 0.25% methane concentration film. These results show that the films grown with 0.25% methane concentration closely resembled diamond. The valence band region for films grown using a gas composition consisting of 0.25% methane concentration shows considerable s-p mixing as compared with graphite. The core level and valence band results show that the film grown with a gas composition consisting of 0.5% methane concentration represents a composite of diamond and graphite. Scanning electron micrographs and Raman spectra support the conclusions drawn on the basis of XPS investigation.