Magnetic freeze-out experiments under hydrostatic pressure have been performed in the temperature range 4.2-77 K on direct-band-gap Si-doped AlGaAs samples. Successive illuminations have been used to monitor the concentrations of the metastable shallow states arising from the DX centres. With increasing concentrations of those states, evidence is given for an insulator-metal transition confirming the r character of these shallow impurity states. Moreover a shallow-deep transition is shown to hold at x = 0.32 as a result of the appearance in the gap at x = 0.30 of a localized level also originating from t h e DX centre but with no potential barrier for carrier capture.
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