Ab initio calculations on metastable defects in a-Si:H: The Staebler-Wronski effect

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.