Defect states at floating and dangling bonds in amorphous Si

Defect-state wave functions and energies are calculated for dangling bonds and recently proposed "floating bonds" in $a-\mathrm{Si}$, using a Bethe-lattice terminated nine-atom cluster and a tight-binding approach. The dangling-bond state is much more localized than the floating-bond state, consistent with a rigorous result we prove for the wave-function amplitudes. Our results suggest that the $D$-state electron-spin-resonance signal in $a-\mathrm{Si}:\mathrm{H}$ arises from dangling bonds.