The temperature dependence of photo-quenching of the EL2 level in semi-insulating material was studied by using photo-current and thermally stimulated current methods. These observations, plus published data, revealed that the metastable transformation of EL2 could not be fully attained when the sample temperature was above 85K. These results were compared with the thermal recovery of the EL2 ground state from its metastable configuration, EL2*, which was known to occur between 120 and 130K. This study thus revealed the existence of a marked thermal hysteresis between the transitions, EL2  EL2* and EL2*  EL2. This was analyzed in terms of the presence of a level that played the role of an actuator in the metastable transformation of EL2. The charge state of this level could be altered by optical excitation or heat. At temperatures above 85K, it would be thermally emptied and be in a charge state configuration which could not activate the metastable transformation of EL2.

J.Jiménez, A.Alvarez, M.Chafai, J.Bonnafe: Physical Review B, 1994, 50[19], 14112-8