EL2Deep Donor State in Semi-Insulating GaAs Revealed by Frequency Dependent Positron Mobility Measurements

Positron mobility measurements carried out on semi-insulating GaAs, using the Doppler shift in annihilation radiation technique, show a sharp transition from a high mobility value ∼120 ${\mathrm{cm}}^2$ ${\mathrm{V}}^{-1\mathrm{}}$ ${\mathrm{s}}^{-1\mathrm{}}$ to a lower value ∼45 ${\mathrm{V}}^{-1\mathrm{}}$ ${\mathrm{s}}^{-1\mathrm{}}$ just below room temperature. The temperature of the transition is found to be dependent on the frequency of the applied AC bias. We show that this effect is an artifact due to the thermal ionization of the $\mathrm{EL}2\mathrm{}$ deep donor state, which in its ionized state forms a positive space charge that causes the positron to experience large electric fields. This observation suggests a new positron annihilation-based deep-level transient technique applicable to semi-insulating materials.