Changes of the occupied density of defect states of a-Si:H upon illumination

We study the light-induced transient changes of the near surface density of occupied states g(E) of undoped and boron-doped a-Si:H with photomodulated total photoelectron yield spectroscopy. The data show an increase of g(E) upon illumination between 0.35 eV above ${\mathit{E}}_{\mathit{F}}$ and 0.7 eV below ${\mathit{E}}_{\mathit{F}}$ (towards the valence band) and a decrease in the region of deep valence-band-tail states. The difference signal depends sublinearly on the laser intensity and reaches a maximum of Δg≊${10}^{17}$ ${\mathrm{cm}}^{\mathrm{-}3}$ ${\mathrm{eV}}^{\mathrm{-}1}$ at a laser intensity of 30 mW ${\mathrm{cm}}^{\mathrm{-}2}$ (λ=532 nm). Time-resolved measurements reveal rise and decay times of the order of milliseconds. The experimental results are explained quantitatively by a recombination model. In the framework of this model, a range of deep defects around mid-gap energy are singly occupied and neutral at probe-light intensities. Additional illumination with a laser leads to double occupation of these defects and a decrease of the valence-band-tail occupation. © 1996 The American Physical Society.