White Beam Synchrotron X-ray Topography and X-ray Diffraction Measurements of Epitaxial Lateral Overgrowth of GaN
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White Beam Synchrotron X-ray Topography and X-ray Diffraction Measurements of Epitaxial Lateral Overgrowth of GaN W.M. Chen1, P.J. McNally1, K. Jacobs2, T. Tuomi3, A.N. Danilewsky4, D. Lowney1, J. Kanatharana1, L. Knuuttila3, J. Riikonen3. 1 Microelectronics Group, Research Institute for Networks and Communications Engineering (RINCE), Dublin City University, Dublin 9, Ireland. 2 Department of Information Technology (INTEC), University of Gent, B-9000, Belgium. 3 Optoelectronics Laboratory, Helsinki University of Technology, FIN-02015 TKK, Finland. 4 D-79108 Freiburg, Germany. ABSTRACT The Epitaxial Lateral Overgrowth (ELO) of GaN on Al2O3 using a SiO2 mask with different fill factors (ratio of stripe opening width to stripe period) is examined with White Beam Synchrotron X-ray Topography (WBSXT) and X-ray rocking curve analysis. The sapphire substrate was identified with a dislocation density of the order of ~106cm-2. WBSXT in both transmission and back reflection mode is used to image the ELO GaN and confirms that crystal planes in the lateral overgrown part (wing) are tilted, and that the wing tilt increases as the fill factor increases. X-ray rocking curve and WBSXT measurements confirm the same wing tilt tendency as the fill factor changes. The WBSXT method provides a measure of the maximum wing tilt, while the X-ray rocking curve method gives the average wing tilt. The average wing tilt reaches about 1602 arcsec at a fill factor of 0.625, but the maximum wing tilts can reach values as large as 2372 arcsec when the fill factor is only 0.571. This study shows that WBSXT is an effective method in dislocation and wing tilt determination for the GaN on Al2O3 ELO epilayer system or indeed for similar systems. The tilted wings induce a slightly lower compressive stress in the coalesced region of the GaN epilayer.
INTRODUCTION GaN and the related materials have drawn much attention in recent years [1-3]. Usually heteroepitaxial GaN thin films are used for device purposes due to the difficulty of growing large (>1 cm) bulk GaN single crystal [1]. At present (00ยท1) sapphire [1-6] is the most commonly used substrate, but standard epitaxial deposition of GaN on Al2O3 can lead to very high threading dislocation densities in the GaN epilayer, usually in the range of 1010cm-2 [1,6]. In the past few years the epitaxial lateral overgrowth (ELO) method was introduced to improve GaN heteroepilayer quality [1-6]. On the whole the ELO technique can result in dislocation densities almost three orders of magnitude or more lower than in the non-ELO case [6-7]. GaN based devices were reported to have benefited from the reduced dislocation density [3,8-9]. However the ELO GaN is far from perfect at present. An understanding of the processes active during the ELO procedure, and their impact on defect and strain generation, will help promote ELO GaN quality improvement. In this study White Beam Synchrotron X-ray Topography (WBSXT) is applied to the evaluation of ELO GaN on sapphire and the sapphire substrate itself. The crystal misorien
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