Correlation between deep defects and persistent photoconductivity in undoped GaN and AlGaN layers

Undoped GaN and AlGaN-layers with Al- contents between 7.5 % and 21% grown by metal organic vapor phase epitaxy on (0001) sapphire substrates were investigated with thermally stimulated currents, temperature dependent currents, and optical admittance spectroscopy for deep defect analysis and with temperature dependent and persistent photocurrent. Mainly, two defect groups were distinguished at low temperatures with activation energies between 70 and 110 meV (trap T1) and between 250 and 400 K with spreading energies between 320 and 480 meV (trap T3). The deep trap T3 causes the mai*n transition in the blue band region in optical admittance spectra. The peak height of this defect increases significantly with increasing Al content compared with the low temperature peaks. Furthermore, the thermal emission of the trap T3 causes a strong decrease of the photocurrent and of the time constant of the persistent photocurrent effect which is thermally activated with an energy of (508 +/- 30) meV.