The metastable transformation of EL2 is modeled in terms of an additional acceptor level, whose role is the activation of the metastable transformation of EL2. This level, called the actuator of the metastability, enables the metastable transformation after capturing a hole. Thermal emission of this hole at 80–85 K renders the metastable transformation of EL2 quite ineffective above this temperature. The double acceptor 78/203 meV is proposed as a possible actuator.
Semi-insulating Fe-doped InP was annealed under different conditions and investigated by Hall effect, extrinsic photocurrent mapping, chemical etching, and optical microscopy. The resistivity is increased for any treatment, particularly in wafer-annealed InP. This result is probably due to strong losses of shallow donors. Remarkable differences exist between the structural properties of the wafer and ingot annealed material; wafer annealing produces a quick elimination of growth striations and decoration microdefects while ingot-annealed InP still retain these microdefects, both along dislocations and in the matrix. The photocurrent maps indicate that the thermal treatments normally improve the doping uniformity (especially the short-range fluctuations). The best uniformity is achieved for the long annealing time (⩾50 h), while a slow cooling rate improves the mobility.
The temperature dependence of the photoquenching of the EL2 level in semi-insulating GaAs is studied by photocurrent and thermally stimulated currents. The observations made in these experimental procedures together with other results reported in the literature reveal that the metastable transformation of EL2 cannot be fully achieved when the sample temperature is above 85 K. This observation is compared with the thermal recovery of the EL2 ground state from its metastable configuration EL2*, which is known to take place between 120 and 130 K; showing thus the existence of a conspicuous thermal hysteresis between both transitions, EL2~EL2 and EL2~EL2. This is analyzed in terms of the existence of a level that would play the role of an actuator of the metastable transformation of EL2. The charge state of this level can be altered by both optical excitation and temperature. Above 85 K it would be thermally emptied, being in such a charge state configuration unable to activate the metastable transformation of EL2.
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