Substrate bias effects on the structural and electronic properties of tetrahedral amorphous carbon

Tetrahedral amorphous carbon deposited by a filtered cathodic are has a very high unpaired electron spin density of around 10(20)-10(21) spin/g (c.f. 10(18)-10(20) spin/g for a-Si and a-Ge). Trap states associated with such unpaired spins have a detrimental effect on the electronic properties of both amorphous silicon and germanium and it is therefore desirable to reduce them in tetrahedral amorphous carbon (ta-C). In this paper, we report on the effect of a negative substrate bias, applied during deposition, on the electron spin resonance (ESR) of ra-C films and on their structure, bonding, conductivity, and Raman spectra. The results show a slow decrease in the unpaired electron spin density with increasing bias up to -1350 V followed by an abrupt decrease at -1750 V. Electron energy loss and electrical conductivity measurements indicate a steady increase in the sp(2) hydridized bonding in these films with bias, and it is understood that it is the coupling between electrons at these sp(2) sites that leads to a reduction in the ESR signal. Investigations using electron diffraction and Raman spectroscopy show the formation of sp(2)-bonded graphitelike planes in the sample deposited at -1750 V, which provides a mechanism for the absence of unpaired electron spins at this deposition bias.