Electronic States for Threefold- and Fivefold-Coordinated Si Atoms in Amorphous Si

The defect-state wave functions for threefold-coordinated and fivefold-coordinated Si atoms were calculated for small clusters by using the transferable nonorthogonal tight-binding method. The results show that the largest Si 3s orbital density of the unpaired electron for one atom is larger for the cluster with a fivefold-coordinated Si atom than for the cluster with a threefold-coordinated Si atom. A comparison with the observed results suggests that the ESR signal with g=2.0055 observed in a-Si:H does not originate from fivefold-coordinated Si atoms but from threefold-coordinated ones.