Deep Centers and Their Capture Barriers in MOCVD-Grown GaN
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Deep Centers and Their Capture Barriers in MOCVD-Grown GaN Daniel K. Johnstonea, Mohamed Ahoujjab, Yung Kee Yeoc, Robert L. Hengeholdc, Louis Guidod a Air Force Office of Scientific Research, Arlington, VA 22203, USA b University of Dayton, Dayton, OH 45469, USA c Air Force Institute of Technology, Wright-Patterson AFB, OH 45433, USA d Virginia Polytechnic Institute and State University, Blacksburg, VA 24061, USA ABSTRACT GaN and its related alloys are being widely developed for blue-ultraviolet emitting and detection devices as well as high temperature, high power, and high frequency electronics. Despite the fast improvement in the growth of good quality GaN, a high concentration of deep level defects of yet unconfirmed origins are still found in GaN. For both optical and electronic devices, these deep carrier traps and/or recombination centers are very important and must therefore be understood. In the present work, deep level defects in GaN grown on sapphire substrates by metal organic chemical vapor deposition (MOCVD) have been investigated using Isothermal Capacitance Transient Spectroscopy (ICTS) and Current Voltage Temperature (IVT) measurements. Several deep level electron traps were characterized, obtaining the emission energy, concentration, and capture cross section from a fit of exponentials to the capacitance transients. ICTS was also used to reveal information about the capture kinetics involved in the traps found in GaN by measuring the amplitude of the capacitance transient at each temperature. At a reduced filling pulse where the traps were not saturated, several of them showed marked reduction in capacitance transient amplitude when compared to the transient amplitude measured under conditions where the filling pulse saturates the traps. This reduction in transient amplitude indicates that there is a barrier to carrier capture, in addition to the emission barrier. It has been found that several traps had capture barriers that were significant fractions of the emission energies up to 0.32 eV. These capture barriers may lead to persistent photoconductivity and reduced trapping. In this paper, deep level emission energies as well as capture barrier energies found in MOCVD-grown GAN will be discussed. INTRODUCTION In spite of considerable work to develop gallium nitride based materials, there is still much to be done to understand the nature of the defects. GaN is desirable for applications toward higher density memory,1 high power microwave devices and high temperature devices,2-4 and UV photodetection.5-7 Current studies of the material include investigations of dopants to increase the carrier activation,8 metallizations to reduce contact resistance or improve rectifying ideality,9-11 extensive efforts to improve the quality of the epitaxial material, and myriad characterization methods that have been employed to understand the origin and role of defects.12-14 Lack of high quality lattice-matched substrates for epitaxy is receiving most of the development focus. Epitaxial growth on sapphire and silic
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