Structure and physical properties of plasma-grown amorphous hydrogenated carbon films

Amorphous hydrogenated carbon (a-C:H) films were deposited in CH4 or C6H6He plasma at a frequency of 50 kHz or 13.56 MHz. Details of the atomic arrangement and bonding structure of the as-deposited and annealed films were derived from Fourier transform IR and Raman spectroscopy. The optical band gap Eg was obtained as a function of the hydrogen content of the films. An effective medium approximation model was applied to the optical constants and it was shown that, whatever the ion bombardment, all the films contained an important polymeric component. However, an increase in the ion bombardment rate and energy increased the diamond-like (sp3 carbon) component and decreased the hydrogen content. Determination of the volume stress showed that it is compressive and increased as hydrogen was lost and the density increased. An interpretation of this behaviour based on the increase in bond lengths and changes in bond angles is given which shows that a relatively large amount of hydrogen is necessary to ensure the stability of the film.

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