Diamond-like carbon films prepared by reactive pulsed laser deposition in hydrogen and methane ambient

The properties of amorphous hydrogenated carbon (a-C:H) films, like their hardness and optical band gap drastically depends on their hydrogen content. By reactive pulsed laser deposition in hydrogen containing gas, the hydrogenation of the films can be achieved. The application of reactive gases of different chemical composition can influence the film properties in a different manner. Therefore, we investigated deposition of a-C:H films in H2 and CH4 atmosphere (0.001-50 Pa). The deposited films were characterized by Raman spectroscopy, spectroscopic ellipsometry and infrared spectroscopy. The optical and structural properties of the films prepared in the two different ambient gas show similarities: at low pressure below 1 Pa, the films are diamond-like, at ∼1-10 Pa, the films become graphite-like and at higher pressures, the films show polymer-like properties. Differences in the films prepared in H2 and CH4 are found in their thickness and infrared absorption. When increasing the pressure up to medium pressures the thicknesses of both series of films increase, and the films become thinner in the highest pressure domain. However, the films prepared in CH4 are ∼1.5 times thicker than the films prepared in H2. Since the mass differences of C atoms and CH4 is small, the CH x radicals are efficiently accelerated by the carbon plasma and are incorporated into the films, too