Nature of paramagnetic centers ina-Si anda-Si:H

Abstract

A comparative ESR study has been undertaken of various kinds of a-Si and a-Si:H. Experiments on sputtered samples reveal that the local configuration of the usual dangling-bond (DB) centers (g≊2.0055) depends on the details of deposition. The g value may gradually be reduced to values as low as 2.0041 as a result of local-structure change. Regarding the g=2.004 signal originated from conduction-band-tail (CBT) states, it is shown that the effective correlation energy ($U_{\mathrm{eff}}$) is large and the temperature dependence of its spin density is due to the presence of donor levels—not due to a small-$U_{\mathrm{eff}}$ effect as has previously been concluded. Related to this, ample evidence is provided, indicating that the g=2.004 signal in P-doped a-Si:H and the g=2.005 DB line are in fact variants of the same kind of defect. Systematic analysis identifies the g=2.004 CBT states signal with $T_3^{\mathrm{-}}$-$T_3^{+}$+$e^{\mathrm{-}}$ defect centers thus showing that a great deal of the CBT states are defect states rather than disorder-induced localized states. Regarding the g=2.013 signal in a-Si:H, evidence is provided showing that this cannot be associated with valence-band-tail states. Rather it concerns defects positioned at ≊0.6 eV above the valence-band mobility edge, whose nature is as yet to be determined. Their presence relates directly or indirectly to H incorporation.