The light induced effect and the change of recombination processes with Fermi level shift are investigated by dark- and photo-conductivity measurements respectively.
The fast increase of the photoconductivity $σ_{ph}$ by two orders of magnitude is observed when the dark Fermi level has moved to a position between 0.45 and 0.50eV above valence band; at this stage bimolecular recombination also takes over.
From the relation between illumination time and Fermi level shift, the relative distribution of the density of gap states is obtained. Here we note that the transition between two recombination processes should be taken into consideration.
A valley in the distribution of densitiy of states is found near 0.4 eV above valence band.
The surface effects in B-doped samples are considered in each cases.
