Electronic and optical investigation of hydrogenated amorphous carbon (a-C:H) by X-ray photoemission spectroscopy and spectroscopic ellipsometry

Hydrogenated amorphous carbon (a-C:H) films were prepared by low-pressure PECVD of CH4 in a dual electron cyclotron resonance (ECR)–radio frequency (RF) discharge by applying an independently controlled RF substrate bias voltage from 0 to −200 V. The films were analyzed with core-level and valence band (VB) X-ray photoemission spectroscopy (XPS) as well as spectroscopic ellipsometry. A progressive change from soft transparent polymer-like to hard absorbing diamond-like was observed with increasing bias voltage in this range. The C 1s core-level spectra were fitted by using a Doniach–Sunjic photoemission line-shape. The C 1s spectra showed the enhanced asymmetric behavior with increasing bias voltage. In addition, the VB spectra exhibited distinct π density of states according to the bias. Such XPS features were analyzed and interpreted in terms of the π→π* electron transition model on amorphous carbon through spectroscopic ellipsometry.