Wide wavelength-range optical studies of hydrogenated amorphous carbon films: from 700 nm to 10 mu m

Hydrogenated amorphous carbon (a-C:H) films were deposited on-double-side p olished germanium substrates by RF plasma-assisted chemical vapour depositi on method using benzene as a precursor, and their optical properties were i nvestigated in the wavelength range from 700 nm to 10 mum. In particular, w e used a dispersion-function-based non-linear regression to fit the reflect ance in the range from 700 to 1800 nm and the baselines of the infrared tra nsmittance in the range from 1000 to 5000 cm(-1), respectively. Optical con stants determined in both ranges, together with their respective thickness values, showed overall consistency. As a function of self-bias, we found th at the refractive index increased while the band gap decreased, which was i ndicative of film densification and increase of sp(2)-bonded carbons. Detai led information on the self-bias-dependent evolution of microscopic bonding structure in a-C:H films was revealed via the quantitative vibration-absor ption spectra in the mid-infrared region, which was obtained after taking U rbach-tail-like electronic absorption into consideration. More specifically , it was found that as self-bias was increased hydrogen content decreased, olefinic sp(2)-bonding decreased while aromatic sp(2)-bonding increased, an d more carbon bonding was distorted due to increased cross-linking. Maximum sp(2)-bonding was observed between -300 and -400 V of self-bias. (C) 2001 Elsevier Science B.V. All rights reserved.