Annealing Effect on Passivated Deep Levels in GaN Epilayers

Transient extrinsic photoconductivity (PC) measurements on as-grown, hydrogenated, and hydrogenated and annealed GaN epitaxial layers grown on sapphire substrates using plasma-assisted molecular-beam epitaxy were performed to investigate the annealing effect on passivated deep levels in a GaN epilayer. A PC spectrum at room temperature showed that the deep-level state in an as-grown GaN epilayer was located 2.9 eV below the conduction-band minimum of the GaN epilayer. The photoionization cross section and the concentration of the deep levels for the as-grown GaN epilayers, determined from the dependence of the rise and decay times of the extrinsic PC response on the illumination intensity of the blue light-emitting diode with 2.9 eV, were 9.7×10-15 cm2 and 4.5×1014 cm-3, respectively. The concentration of the as-grown deep levels in the GaN epilayer was significantly reduced by the hydrogenation treatment, and this passivation effect was still effective after thermal annealing at a temperature up to 700°C. The activation energy of the deep levels and the dissociation energy of the complexes between the deep levels and hydrogen atoms were 0.55 eV and 3.0 eV, respectively. These results provide important information for improving the quality of devices utilizing GaN epilayers.