Evolution of sp2 bonding with deposition temperature in tetrahedral amorphous carbon studied by Raman spectroscopy

Two transitions in the bonding are found in tetrahedral amorphous carbon $\mathrm{(ta-}\mathrm{C})$ films as a function of deposition temperature. The total ${\mathrm{sp}}^3$ fraction shows a sharp decrease at a transition temperature of order 250 °C. In contrast, visible Raman finds that the ${\mathrm{sp}}^2$ sites show a gradual ordering into the graphitic clusters through the sharp bonding transition. The optical gap and resistivity show a similar, gradual transition. This indicates that the ${\mathrm{sp}}^2$ cluster size determines the optical gap, even when the ${\mathrm{sp}}^2$ content does not change. The Raman $\mathrm{I(D)/I(G)}$ peak ratio is found to vary inversely with the square of the gap.