Transient Hall Effect characterization of persistent photo-generated carriers in GaN/AlGaN heterostructures

We report on the transient conductivity and scattering of charge carriers generated by photo-ionization of deep-level defects in GaN/AlGaN heterostructures grown on silicon and sapphire substrates. Under sub-bandgap (455 nm) illumination, photo-generated carriers increase the conductivity of on-silicon devices while, due to enhanced carrier-carrier scattering, on-sapphire devices show a conductivity decrease. After the illumination has been removed, the photo-induced carrier concentration persists for hours to days in these materials. Queisser-like logarithmic decay kinetics were observed for the on-sapphire devices at room temperature while the on-silicon devices display a more complex decay transient indicative of serial or multichannel decay mechanisms. In this work, a novel Hall effect acquisition methodology removes offset voltages from the Hall signal without the need for magnetic field reversal enabling acquisition of the Hall voltage at millisecond timescales. Carrier scattering rates were extracted by subsequent measurement of the transient conductivity.