EL2-defect-related changes in the magnetophotoconductivity of shallow donors in bulk semi-insulating GaAs

The far-infrared photoconductivity of semi-insulating GaAs was measured as a function of magnetic field, and transitions to excited states of shallow donors observed. The measurements were performed for two states of the material: (a) with the EL2 defects in the stable EL$2^0$ state and (b) with the EL2 defects in the metastable EL$2^{\mathrm{*}}$ state. The linewidth of the donor transitions was found to be larger in the second case, in spite of the decrease in the number of charges in the sample. A model of spatial correlation between EL$2^{+}$ defects and ionized acceptors is proposed to explain this fact.