Determination of the dislocation densities in GaN on c-oriented sapphire
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Internet Journal o f
Nitride S emiconductor Research
Volume 1, Article 40
Determination of the dislocation densities in GaN on c-oriented sapphire A. Pelzmann, M. Mayer, C. Kirchner, D. Sowada, T. Rotter, Markus Kamp , K. J. Ebeling Abteilung Optoelektronik, Universität Ulm S. Christiansen, M. Albrecht, H. P. Strunk Institut für Werkstoffwissenschaften, Lehrstuhl VII, Universität Erlangen-Nürnberg B. Holländer, S. Mantl Forschungszentrum Jülich, Inst. für Schicht-und Ionentechnik This article was received on July 31, 1996 and accepted on December 16, 1996.
Abstract We report on a comprehensive study of the defect structure in GaN grown on c-oriented sapphire by gas source molecular beam epitaxy and metal organic vapour phase epitaxy. Transmission electron microscopy is used to investigate the defect structures which are dominated by threading dislocations perpendicular to the sapphire surface and stacking faults. Additionally, dislocation densities are determined. For determination of dislocation densities by x-ray diffraction we employ a model that uses the linewidth of x-ray rocking curves for this purpose. Finally, Rutherford backscattering spectrometry is performed to complement the structural investigation.
1. Introduction Group III Nitrides (e.g. AlN, GaN, InN and their alloys) are predestinated for optoelectronic applications from the long visible light range down to UV wavelengths since they cover a bandgap range from 1.6 to 6.2 eV. Nowadays, one of the most important optoelectronic applications of group III Nitrides are GaN based blue and green LEDs. High-brightness GaN based blue LEDs were first fabricated by S. Nakamura using c-oriented sapphire as substrate material and two-flow metal organic vapour phase epitaxy (MOVPE) for the epitaxial growth [1]. So far, best results in growth of high quality group III nitrides are achieved by MOVPE techniques. However, the growth of group III nitrides by gas source molecular beam epitaxy (GSMBE) using On Surface Cracking (OSC) of NH3 appears also to be very promising [2] [3]. Particularly in our department we have the opportunity to compare both growth techniques directly since we are equipped with a MOVPE and a GSMBE system. The lattice mismatch between sapphire and GaN is very large (∆a/a=16 %). 6H SiC is a more suitable substrate material for the epitaxial growth of GaN (∆a/a=3.5 %). However, due to the high price and the low availability of suitable 6H SiC, most groups use the less expensive sapphire for the growth of wurtzite-type nitrides. Due to the large mismatch of the lattice constants and the thermal expansion coefficients, GaN grown on sapphire has very high defect densities. The first step in the fabrication of optoelectronic semiconductor devices is the optimization of the grown material referring to optical, electrical and crystalline quality. In our investigations we focus on transmission electron microscopy (TEM), Rutherford backscattering spectrometry (RBS) and high resolution x-ray diffraction (XRD) to examine the crystalline quality.
2. Ex
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