Deep levels in GaN epilayers grown on sapphire substrates T.W. Kang a, * , Sh.U. Yuldashev a,1 , C.S. Park a , C.S. Chi a , S.H. Park a , Y.S. Ryu a , T.W. Kim b a Department of Physics, Dongguk University, 3-26, Pildong, Chungku, Seoul 100-715, South Korea b Department of Physics, Kwangwoon University, 447-1 Wolgye-dong, Nowon-ku, Seoul 139-701, South Korea Received 12 June 1999; accepted 17 August 1999 by C.N.R. Rao Abstract 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 electron levels. The photoionization cross section and the concentration of the deep levels were determined from the dependence of the rise and the decay times of the extrinsic PC response on the illumination intensity. The concentrations on the deep levels, located at 2.9 and 1.42 eV below the conduction band minimum of the GaN were found to be 2 × 10 13 and 1:6 × 10 11 cm -3 ; respectively. The typical relaxation time of the excess carriers controlled by the deep levels was approximately 10 -3 s. The thermal activation energies of the deep levels were determined from the temperature dependence of the relaxation time of the extrinsic PC response. These results can help improve understanding for potential applications in optoelectronic devices based on GaN epilayers.  1999 Published by Elsevier Science Ltd. All rights reserved. Keywords: A. Heterojunctions; C. Impurities in semiconductors; D. Photoconductivity and photovoltaics 1. Introduction GaN and its related materials have been particularly attractive because of interest in both investigations of the fundamental physical properties and potential applications in optoelectronic devices ranging from the visible to the ultraviolet region of the spectrum [1–5]. Since GaN/ sapphire heterostructures have inherent problems with obtaining high-quality epitaxial growth, device fabrications utilizing GaN/sapphire have several problems, such as leak- age currents and lifetime limitations. These problems origi- nate from the deep levels in GaN, and they affect the optical properties of the devices [6]. Therefore, detailed studies of the deep levels in GaN/sapphire heterostructures are very important for achieving high-performance devices [7]. The deep levels in GaN epitaxial films have been extensively investigated by using photoluminescence [8,9], deep-level transient spectroscopy [10], and other measurements [11,12]. Recently, a broad distribution of the impurity states existing in the GaN energy gap was investigated by photo- conductivity (PC) spectroscopy measurements [13], and two exponential tails, one abrupt tail located near the band edge and another gradual tail located in the energy gap, were observed [13]. While the abrupt tail was related to the Urbach edge, the gradual tail was attributed to a deep defect state. Extrinsic and intrinsic PC measurements are useful tools for optically investigating the impurities and the localized centers existing in semiconductors. The energy level, the capture cross section, the trap density, and the relaxation time of the excess carriers can be determined by extrinsic PC measurements. Since the penetration depth of the light for extrinsic PC measurements is relatively large due to the small absorption coefficient, the surface recombination effect for the measurements can be neglected. More recently, preliminary results of the deep levels located at 1.44 and 1.56 eV from the conduction band minimum of the GaN were obtained by using extrinsic PC measurements [14]; however, to the best of our knowledge, detailed studies of the deep localized states in GaN have not yet been done. Solid State Communications 112 (1999) 637–642 0038-1098/99/$ - see front matter  1999 Published by Elsevier Science Ltd. All rights reserved. PII: S0038-1098(99)00406-8 PERGAMON www.elsevier.com/locate/ssc * Corresponding author. Tel.: +82-2-2260-3205; fax: +82-2- 2278-4519. E-mail address: twkang@cakra.dongguk.ac.kr (T.W. Kang) 1 Permanent address: Heat Physics Department, Academy of Sciences, Katartal str. 28, Tashkent, 700135 Uzbekistan.