Quantitative Defect Analysis of GaN Thick Films by TEM and AFM
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Quantitative Defect Analysis of GaN Thick Films by TEM and AFM Praveena Bhaskara, Changmo Sung Department of Chemical and Nuclear Engineering, University of Massachusetts Lowell Lowell, MA, USA. David Bliss, Michael Suscavage Air Force Research Laboratory, Hanscom AFB, MA, USA Abstract Ever since the discovery of the astonishing properties of GaN, many research groups have been involved in the processing of the perfect GaN crystal. Iodine Vapor Phase Growth (IVPG) technique was employed to grow GaN epilayer on a MOCVD pre-deposited buffer layer. This new epitaxial system was characterized by TEM, AFM and EFM. A complete AFM study involved the polarity measurements and the etch pit density measurements. For the first time a systematic study was performed of the dislocation density changing as a function of distance from the substrate. TEM performed on the cross-section, as well as the plan view, of the samples showed a remarkable decrease in the dislocations in the current system, compared to the samples that were solely deposited by MOCVD. Advanced analytical methods of polarity and dislocation density measurements have been established to understand the relation between microstructure and electrical properties of the thick film GaN. Electrostatic Force Microscopy has been suggested as a potential tool for obtaining polarity information. Introduction The prime difficulty encountered in the processing of GaN thick films, is the reduction of dislocations. Different processes like MOCVD, MBE, HVPE etc. are being tried out to decrease the occurrence of the dislocations. The presence of a buffer layer is believed to reduce the dislocation density in the films. A combination of techniques is being experimented with. Iodine Vapor Phase Growth (IVPG) is a new technique developed for growing GaN films and single crystals using elemental iodine instead of HCl, which promises better results. A complete analysis of the system would give a better idea of the process control. In this paper, a complete study has been performed for the first time on dislocation density of the thick films. Dislocation density variation with the distance from the interface, which is an important section of this study, would enable one to determine the right conditions for processing, and the appropriate thickness of the thick film that should be lifted off for the use in the devices. A complete analysis of polarity of the samples has been carried out with AFM. The information would be beneficial in the selection of the right processing conditions to enable the proper device designing. In a new attempt, Electrostatic Force Microscopy has been used to further complete the polarity information. Experimental: A set of nine samples, all processed by IVPG, were analyzed by TEM and AFM. The thick layer consisted of a low temperature layer deposited on the sapphire substrate. A high temperature template was deposited on this low temperature template. The low temperature and high temperature templates together formed the buffer layer and were laid down by MOC
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