The observation of sp 2 fraction disorder using dual wavelength Raman spectroscopy in a-C:H films fabricated using an open inductively coupled plasma reactor

Abstract

Amorphous hydrogenated carbon (a-C:H) films were synthesised using an inductively coupled plasma reactor under varying DC substrate bias voltage. Films were characterised using nanoindentation measurements, UV (325 nm) and visible (633 nm) Raman spectroscopy. Film microhardness was found to vary from 15.2 to 18.6 GPa with harder films deposited at higher bias. While visible Raman spectroscopy is a routinely accepted technique used to study the sp 2 fraction in a-Cs, we found that it was ineffective in our instance to identify differences between the samples fabricated, as indicated by largely unchanged G peak position and D peak intensity for the selected films. UV Raman analysis revealed that the sp 3 fraction is essentially the same for all the films studied, while the sp 2 fraction was found to have higher degree of bond disorder in both aromatic rings and olefinic chains in films deposited at higher substrate bias. Furthermore, an additional peak at 1200 cm − 1 appeared in both visible and UV spectra of all films examined. We believe that the peak originates from the small, hydrogenated sp 2 sites with a narrow band gap and arranged in minute clusters. We hypothesize that the observed variation of the mechanical properties of a-C:H films are not controlled by the amount of the sp 3 fraction but are also determined by the nature of the disorder within the sp 2-bonded fraction. a-C:H films displaying higher hardness and Young's modulus have sp 2-bonded aromatic sites arranged in smaller domains with a higher degree of ring distortion and have sp 2-bonded olefinic sites with higher degree of clustering. This phenomenon was observed as a result of a low temperature plasma deposition.