Deep levels in GaN epilayers grown on sapphire substrates

Steady state and transient extrinsic photoconductivity (PC) measurements on unintentionally doped GaN epilayers grown on sapphire substrates by using metalorganic chemical vapor deposition were performed to investigate deep e lectron levels. The photoionization cross section and the concentration of the deep levels were determined from the dependence of the rise and the dec ay times of the extrinsic PC response on the illumination intensity. The co ncentrations on the deep levels, located at 2.9 and 1.42 eV below the condu ction band minimum of the GaN were found to be 2 x 10(13) and 1.6 X 10(11) cm(-3), respectively. The typical relaxation time of the excess carriers co ntrolled by the deep levels was approximately 10(-3) s. The thermal activat ion energies of the deep levels were determined from the temperature depend ence of the relaxation time of the extrinsic PC response. These results can help improve understanding for potential applications in optoelectronic de vices based on GaN epilayers. (C) 1999 Published by Elsevier Science Ltd. A ll rights reserved.