Wide wavelength-range optical studies of hydrogenated amorphous carbon ®lms: from 700 nm to 10 mm

Hydrogenated amorphous carbon (a-C:H) ®lms were deposited on double-side polished germanium substrates by RF plasma-assisted chemical vapour deposition method using benzene as a precursor, and their optical properties were investigated in the wavelength range from 700 nm to 10 mm. In particular, we used a dispersion-function-based non-linear regression to ®t the re¯ectance in the range from 700 to 1800 nm and the baselines of the infrared transmittance in the range from 1000 to 5000 cmÿ1, respectively. Optical constants determined in both ranges, together with their respective thickness values, showed overall consistency. As a function of self-bias, we found that the refractive index increased while the band gap decreased, which was indicative of ®lm densi®cation and increase of sp2-bonded carbons. Detailed information on the selfbias- dependent evolution of microscopic bonding structure in a-C:H ®lms was revealed via the quantitative vibrationabsorption spectra in the mid-infrared region, which was obtained after taking Urbach-tail-like electronic absorption into consideration. More speci®cally, it was found that as self-bias was increased hydrogen content decreased, ole®nic sp2-bonding decreased while aromatic sp2-bonding increased, and more carbon bonding was distorted due to increased cross-linking. Maximum sp2-bonding was observed between ÿ300 and ÿ400 V of self-bias. # 2001 Elsevier Science B.V. All rights reserved.