Stages in the interaction of deuterium atoms with amorphous hydrogenated carbon films: Isotope exchange, soft-layer formation, and steady-state erosion

Abstract

We report studies of the interactions of quantified deuterium (hydrogen) atom beams with hard amorphous hydrogenated carbon films at a substrate temperature of ∼330 K in an ultrahigh-vacuum chamber. The modification/erosion of a-C:H (a-C:D) films was monitored in situ by ellipsometry in real time. By interpreting the ellipsometric information and combining it with measurements of the absolute D areal density changes in the a-C:H (a-C:D) films by ion beam analysis as a function of D (H) atom fluence, we are able to distinguish three sequential stages of D interaction with hard a-C:H films. The first stage is replacement of bonded hydrogen by deuterium up to an areal density of ∼5×1015 D cm−2 to a depth of ∼1.4 nm from the surface. This phase is complete after a deuterium fluence of ≈2×1018 cm−2. The effective cross section for isotopic exchange of H with D atoms for the a-C:H layer is found to be σ=2.0×10−18 cm2, and is close to the cross section for H abstraction from a carbon surface. This may indicate that H abstraction by D from the a-C:H surface is the rate limiting step for isotope exchange in this situation. Hydrogen replacement is followed by creation of additional C–D bonds in the near-surface region and increases the D areal density by about 2.5×1015 D cm−2. By ellipsometry this process can be observed as the formation of a soft a-C:D layer on top of the hard a-C:H bulk film, with the soft layer extending about 1.4 nm from the surface. This stage is complete after a deuterium fluence of about 2×1019 cm−2. Subsequently, steady-state erosion of the a-C:H film takes place. Here, a soft a-C:D layer with roughly constant thickness (∼1.4 nm) remains on the hard a-C:H substrate and is dynamically reformed as the underlying hard a-C:H film becomes thinner. A similar sequence of processes takes place at a substrate temperature of 650 K, albeit at a much faster rate.