PHOTOCURRENT TRANSIENTS IN SEMIINSULATING GAAS, EFFECTS OF EL2 AND OTHER DEFECTS

Photocurrent transient effects in undoped semi-insulating gallium arse nide have elicited conflicting explanations. The photocurrent quenchin g, enhanced photocurrent, and persistent photocurrent effects are revi ewed and new results on these effects are reported. A comparison of th e photocurrent transients with optical absorption quenching along with other experiments shows that photocurrent quenching is due to the met astable transformation of the deep donor EL2 and that the enhanced and persistent photocurrent effects are due to a seperate effect that is most likely a metastable transformation of another defect. Thermal rec overy experiments show that the activated state responsible for the en hanced photocurrent recovers prior to the recovery of the metastable s tate of EL2. Furnace annealing experiments are presented showing that the enhanced and persistent photocurrent effects are significantly red uced after anneals at 500-degrees-C, well below the annealing temperat ure of quenchable EL2. After reviewing existing models we suggest that the absence of a persistent photocurrent immediately after quenching is due to compensation of residual impurities by the second level of E L2. A complex defect, perhaps including EL6 as an intermediary, is pro posed for the defect responsible for the charge transfer resulting in the enhanced and persistent photocurrent effects.