Non-exponential decay of persistent photocarriers in an AlGaN/AlN/GaN heterostructure

We report on the kinetics of persistent photoconductivity (PPC) in AlGaN/AlN/GaN heterostructures grown on silicon substrates. Under sub-bandgap illumination (1.91, 2.10, and 2.73 eV), photocarriers from ionized, deep-level defects increase the 2DEG conductivity which persists for hours to days after the illumination has been removed. Using a novel Hall Effect characterization protocol, a logarithmic decay of the persistent carrier concentration, independent of carrier scattering, was observed. The measured logarithmic decay is consistent with a physical separation of defects from the conducting channel and is thought to result from a recombination front propagating through the heterostructure. Persistent photocarrier concentration kinetics were studied as a function of photon dose, wavelength, and temperature. At the shortest wavelength, temperature-dependent studies show the coexistence of two logarithmic decay channels and could indicate PPC in this heterostructure arises from multiple defect species or, perhaps, the defects have different spatial proximity to the conducting channel.