Optical, structural and bonding properties of diamond-like amorphous carbon films deposited by DC magnetron sputtering

• DLC thin films were deposited on glass substrates at different working pressures.
• Optical transmission increased from 10% to 80% with increasing working pressure.
• The films with refractive indices varying between 1.6 and 2.4 at 550 nm were deposited.
• Refractive index decreased as Tauc gaps increased with increasing sp3 fraction.
• The film with diamond-like optical property has a lower film density and a higher roughness.

In the present study effects of working pressure on the optical, structural and bonding properties of diamond-like amorphous carbon (DLC) thin films prepared by direct current (DC) magnetron sputtering technique were investigated. XPS analysis is used to determine the bonding properties and optical spectroscopy to determine optical constants. Increasing the working pressure from 2 to 50 mTorr is shown to strongly influence the optical and structural properties due to a large increase in the sp3/sp2 ratio from 0.51 to 2.81. With increasing pressure, the real part of the refractive index decreases from 2.4 to 1.6 at 550 nm, while the imaginary part also decreases from 0.38 to 0.02 at 550 nm, leading to a large increase in optical transmission in the visible region from 10% to 80% for films of approximately equal thickness, while the Tauc gap of the films increases from 0.80 eV to 2.06 eV. RMS surface roughnesses of the films increased from 0.95 nm to 4.69 nm with increasing working pressure. These results indicate that the sp3 sites mainly consist of C–H bonds at higher working pressures and the optical and structural properties are consistent with a change from graphitic to a more polymer-like DLC. The resulting strong increase of transparency in the visible range extends the optically useful range for DLC films into the visible part of the spectrum, for applications such as low-e coatings for windows.