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

Abstract

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 50mTorr 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 550nm, while the imaginary part also decreases from 0.38 to 0.02 at 550nm, 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 0.80eV to 2.06eV. RMS surface roughnesses of the films increased from 0.95nm to 4.69nm 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.