Abstract
We use real-space local density functional theory to investigate the formation and stability of paramagnetic defects involving H in a-Si:H. We have considered the mechanism for dangling-bond formation by exciting an electron-hole pair at a strained Si-Si bond, using the clusters Si8H18, Si7H16 and Si26H30. We find in these clusters that a Si-Si bond strained by ∼12% can break once an electron is promoted from the bonding to the antibonding level. Calculations on the clusters Si5H12, Si12H22 and Si26H40 serve to show that metastable defects comprising Si-dangling bonds and bond-centred H atoms can form with energies of 0·0–3·0eV, depending on the initial strain in the system. These results support the argument that light induces H motion in locally strained regions and causes the Staebler-Wronski effect.