photoluminescence spectral transformation between the 2-luminescence system and the 'self-activation' band by high-temperature treatments and aging at room temperature. We propose that some residual impurity is responsible for both Z-recombination (isolated impurity) and the self-activation band at -1.45 eV (complex with a shallow donor). There is reason to believe that a key to understanding the compensation mechanism is the formation of impurity complexes consisting of the shallow impurity (donor or acceptor) and the Z centre.

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