A study was made of light-induced transient changes, in the near-surface density of occupied states of undoped and B-doped amorphous hydrogenated material, by using photo-modulated total photo-electron yield spectroscopy. Upon illumination, the data revealed an increase in the near-surface density of occupied states between 0.35eV above EF and 0.7eV below EF, and a decrease in the region of deep valence-band tail states. The difference depended sub-linearly upon the laser intensity, and attained a maximum value of about 1017/cm3eV at a laser intensity of 0.030W/cm2 (532nm). Time-resolved measurements revealed rise and decay times which were of the order of ms. The experimental results were quantitatively explained in terms of a recombination model. According to the predictions of this model, a range of deep defects around the mid-gap energy were singly occupied and neutral at probe-light intensities. Additional illumination, using a laser, led to double occupation of these defects, and to a decrease in the valence-band tail occupation.

W.Graf, K.Leihkamm, M.Wolf, J.Ristein, L.Ley: Physical Review B, 1996, 53[8], 4522-33