Photoinduced discharge of SiO2 electret film. II. Theory

Photodepopulation spectroscopy is a traditional tool to study electronic localized states in SiO₂ and Si based metal-oxide-semiconductor devices especially at the Si-SiO₂ interface. Normally, this study reveals the occurrence of electron acceptor-like traps with the energy of around 3 eV below the SiO₂ conduction band. Higher photocurrent and faster photoinduced discharge with decreasing wavelength of inducing light is attributed to increasing photoionization cross section with increasing photon energy. However, both the photocurrent and photoinduced discharge do not reveal a sharp edge effect at the photon energy hv equal to the photoionization energy although the photocurrent is much weaker and the discharge is much slower at hv<3 eV. It is also worth noting that the experimentally observed kinetics of photoinduced discharge becomes strongly non-exponential at such low photon energies implying deviation from a simple single-photon excitation process. One should also bear in mind that, in addition to deep traps, a high density of shallower band-tail states must occur near a semiconductor disordered dielectric interface. In the present work we formulate an alternative model describing photoinduced discharge of SiO₂ electret films. The model considers carrier photoionization as a two-step process. On the first step a carrier makes a photon-assisted tunneling jump from a deep trap into a bandtail state, Being localized in this state for a sufficiently long time the carrier may pick up an energy from a second photon that could allow it to be released into extended states in the bulk of the SiO₂ film