Evolution of sp(2) bonding with deposition temperature in tetrahedral amorphous carbon studied by Raman spectroscopy

Two transitions in the bonding are found in tetrahedral amorphous carbon (t a-C) films as a function of deposition temperature. The total sp(3) fractio n shows a sharp decrease at a transition temperature of order 250 degrees C . In contrast, visible Raman finds that the sp(2) sites show a gradual orde ring into the graphitic clusters through the sharp bonding transition. The optical gap and resistivity show a similar, gradual transition. This indica tes that the sp(2) cluster size determines the optical gap, even when the s p(2) content does not change. The Raman I(D)/I(G) peak ratio is found to va ry inversely with the square of the gap. (C) 2000 American Institute of Phy sics. [S0003-6951(00)02311-1].