Effect of deposition conditions on optical properties of a-C:H:SiO_x films prepared by plasma-assisted chemical vapor deposition method

The a-C:H:SiO_x films were deposited on glass substrates by plasma-assisted chemical vapor deposition from mixtures of argon and polyphenylmethylsiloxane vapor. Optical properties of a-C:H:SiO_x films were investigated by varying deposition parameters, such as Ar pressure and amplitude of negative pulse of bipolar substrate bias. Optical properties were determined using the transmission spectra T(λ), measured in the spectral range 300–800 nm. It was shown that the integrated transmittance of a-C:H:SiO_x films in the visible wavelength range 380–780 nm is 69–89%, depending on the deposition conditions. On the basis of the transmission spectra Tauc optical band gap, wavelength of absorption edge and Urbach energy were calculated. The resulting films exhibit a wide range of properties including hydrogen content from 35% to 39%, optical band gap from 2.3 to 2.6 eV, wavelength of absorption edge from 477 to 537 nm and Urbach energy from 547 to 718 meV. The change in optical properties of a-C:H:SiO_x films is associated with a decrease in the size and concentration of graphite-like clusters in the films as a result of the enhancement of ion bombardment of a growing film with increasing of argon pressure and amplitude of negative pulse of bipolar substrate bias. The study of the optical properties of a-C:H:SiO_x films showed that they can be used as protective and antireflection films on optical instruments, in particular, on solar cells.