Hydrogen stability in hydrogenated amorphous carbon films with polymer-like and diamond-like structure

Abstract

Hydrogen (H) stability in hydrogenated amorphous carbon (a-C:H) films with different structures grown by (biased) electron-cyclotron-resonance chemical vapor deposition has been studied against thermal annealing and swift-ion impact (2 MeV He+). For this purpose, a-C:H films with either polymer-like (PLCH) or diamond-like (DLCH) character grown on grounded or biased (−200 V) substrates, respectively, were annealed up to 450 °C. The local-order structural evolution around C sites was analyzed by x-ray absorption near-edge spectroscopy (XANES) and the H content and radiation-induced release were determined by successive elastic recoil detection analysis (ERDA) acquisitions. A relatively high H content is measured for both as-grown PLCH (∼45 at. %) and DLCH films (∼33 at. %). Upon annealing, PLCH films suffer thermal-induced surface decomposition resulting in a thickness reduction and only above 350 °C the H content in the film matrix decreases. PLCH films also display a pronounced H loss rate during ERDA measurements, whereas H is stable in DLCH. These results indicate that H bonding differs in both structures (i.e., weaker C-H bonds in PLCH). XANES shows that upon annealing both structures suffer H loss at the near surface region together with a graphitization process, although the impact is more pronounced in PLCH. XANES fine-structure reveals that aromatic clusters are formed upon annealing in PLCH due to H loss, whereas this process is partially inhibited in DLCH due to the thermal stability of the C-H bonds.