Identification of the Dangling-Bond State within the Mobility Gap ofa-Si: H by Depletion-Width-Modulated ESR Spectroscopy

The first observation is reported of an absorption (dark) ESR signal specifically from unoccupied gap states within the space-charge region of $n$-type $a$-Si: H diode junctions. The detected resonance with $g=2.0053\mathrm{}\pm 0.0003$ is associated with the singly occupied dangling-bond defect. The applied-bias and temperature dependence identifies this signal with the well defined midgap defect band observed in deep-level transient spectroscopy. The results also imply a positive correlation energy, $U\gtrsim 0.2$ eV, between the singly and doubly occupied centers.