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 (ta-C) films as a function of deposition temperature. The total sp³ fraction shows a sharp decrease at a transition temperature of order 250°C. In contrast, visible Raman finds that the sp² 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 sp² cluster size determines the optical gap, even when the sp² content does not change. The Raman I(D)/I(G) peak ratio is found to vary inversely with the square of the gap.