An X-ray and neutron scattering study of amorphous hydrogenated carbon

A series of thin film and powder samples of amorphous hydrogenated carbon (a-CH) have been deposited in a fast atom source, from one of three hydrocarbon precursor gases, at a deposition energy ranging between 0.5keV and 1.2keV. Neutron scattering studies, sensitive to both the carbon and hydrogen environments, have been carried out on three of these samples, deposited at the same impact energy, having varying degrees of isotopic enrichment. The measured scattering profiles have been used to derive the partial C-C, C-H and H-H correlation functions for the material, affording an insight into the structural configuration of this phase of a-C:H of unique clarity. Complementary X-ray diffraction measurements, dominated by contributions from the carbon matrix, have been made on these isotopically substituted samples and two further samples deposited at a lower deposition energy. A structural reconfiguration from a graphite-like, highly trigonally bonded network, to a more diamond-like atomic arrangement has been observed as the deposition energy falls, accompanied by a progressive change in the void structure. X-ray diffraction studies have also been made of two samples and of a-C:H, deposited from the same precursor has, at impact energies of 0.5keV and 0.85keV. Again a breakdown in the intra- and inter-layer graphite-like structural arrangement has been found to occur as the deposition energy decreases.

The Warren-Mavel fluorescence detection technique has been used to remove the incoherent scattering contribution to the measured X-ray scattering profiles and has been shown to be highly efficient at the removal of such contributions when used in conjunction with a synchrotron X-ray source. In fact, this technique has been so successful that is has highlighted the problem of obtaining suitable theoretical self scattering curbes for the coherent data accessed.

Preliminary studies on a novel glancing angle X-ray diffraction technique for the in situ analysis of thin film samples of a-C:H, mounted on a crystalline substrate, have been carried out. Two films have been studied, and early results have been encouraging, showing that not only can scattering profiles be collected for these low Z, amorphous systems from films 1μm thick, but that depth profiling of the thin film/substrate system is also possible.