DIET in the bulk: Evidence for hot electron cleavage of Si-H bonds in SiO₂ films

The observed increase in leakage current through SiO₂ films after hot electron exposure is ascribed to dissociation induced by electronic transitions ("DIET") of bulk Si-H bonds, producing mobile hydrogen. We use ab initio supercell bandstructure calculations at the local density functional level to locate features produced by hydrogen-containing defects in α-SiO₂. The edge of the Si-H σ* resonance is found to be about 2.7 eV above the conduction band rise, in good agreement with the observed threshold for hot electron induced damage in amorphous SiO₂ films grown on Si substrates. The O-H σ* resonance is almost 4 eV higher. Removing H from O-H in the supercell does not affect the gap region (-O^- forms); however, removing H from Si-H produces a mid-gap state, suggesting leakage current by hopping conductivity between Si dangling bonds. A Morse potential model is used to explore the dynamics of bond scission by short-lived (< 1 fs) hot electron σ* capture. Supercell calculations on interstitial atomic hydrogen indicate the energy cost to break an embedded Si-H bond is about 0.6 eV less than in the gas phase. The DIET yield is substantially increased by reducing both ground and electron-attached state binding by this amount. While uncertainty over the displaced equilibrium in the electron-attached excited state remains, the computed DIET cross-section for reasonable parameters is ≈ 10^-18 cm², and is in agreement with the semi-empirically derived value for trap creation. Comparisons are made to surface DIET processes involving Si-H bonds.