Compensation mechanism in undoped, semi-insulating GaAs

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Abstract
Concentrations of neutral and ionized deep donor EL2 in undoped, semi‐insulating GaAs crystals were obtained using the near‐infrared photoabsorption method. By varying the carbon content in the crystal in a range of 1015–1016 cm−3, it was verified that only 30% of the total carbon acceptors take part in compensation of EL2, i.e., [EL2+]=0.3[C]. This contradicts with either the conventional two‐level model which demands [EL2+]=[C] or the recent findings of electron paramagnetic resonance measurements indicating [EL2+]≫[C]. A model is presented to moderate these contradictions, which includes presence of indirectly carbon‐related donor and electrically active and inactive two types of EL2. An evidence to support the model is given by a recent nuclear‐magnetic resonance measurement by the same author [Appl. Phys. Lett. 57, 398 (1990)].