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Hydrogenated
amorphous carbon (a-C:H) films 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 fit the reflectance in the range from 700 to 1800 nm and the
baselines of the infrared transmittance in the range from 1000 to
5000 cm-1, 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 film densification and increase of sp2-bonded
carbons. Detailed information on the self-bias-dependent evolution
of microscopic bonding structure in a-C:H films was revealed via
the quantitative vibration-absorption spectra in the mid-infrared
region, which was obtained after taking Urbach-tail-like electronic
absorption into consideration. More specifically, it was found that
as self-bias was increased hydrogen content decreased, olefinic
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
300 and 400 V of self-bias. |