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.