We have shown that the SPV spectral behaviour in bulk SI GaAs:Cr crystals with real (100) surfaces is entirely determined by the photo-ionisation of the main deep centers (Cr~(2+) acceptors, EL2 donors) and shallow levels in the sample bulk. This model is consistent with our earlier investigations, which have shown that in SI GaAs:Cr the surface states are optically inactive in the extrinsic spectral range up to the band-gap energy. This work gives an alternative approach for deep level characterisation in semiconductors, based on extrinsic SPV spectroscopy. Financial support from the Bulgarian National Science Fund and the Sofia University Research Fund is gratefully acknowledged.
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