SUBSTRATE BIAS EFFECTS ON THE STRUCTURAL AND ELECTRONIC-PROPERTIES OFTETRAHEDRAL 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) spi n/g (c.f. 10(18)-10(20) spin/g for a-Si and a-Ge). Trap states associa ted with such unpaired spins have a detrimental effect on the electron ic properties of both amorphous silicon and germanium and it is theref ore desirable to reduce them in tetrahedral amorphous carbon (ta-C). I n this paper, we report on the effect of a negative substrate bias, ap plied 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 densi ty with increasing bias up to -1350 V followed by an abrupt decrease a t -1750 V. Electron energy loss and electrical conductivity measuremen ts 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 s ignal. Investigations using electron diffraction and Raman spectroscop y show the formation of sp(2)-bonded graphitelike planes in the sample deposited at -1750 V, which provides a mechanism for the absence of u npaired electron spins at this deposition bias.