Synchrotron X-ray Topography Studies of Epitaxial Lateral Overgrowth of GaN on Sapphire
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STUDIES
OF
EPITAXIAL
LATERAL
PATRICK J. MCNALLY*, T. TUOMI**, R. RANTAMAKI**, K. JACOBS***, CONSIDINE****, M. O'HARE *, D. LOWNEY*, A.N. DANILEWSKY***** *Microelectronics Research Laboratory, Dublin City University, Dublin 9, Ireland. **Optoelectronics Laboratory, Helsinki University of Technology, 02015 TKK, Finland. ***Dept of Information Technology (INTEC), Univ. of Gent, B-9000, Belgium. ****Thomas Swan & Co. Ltd., Harston, Cambridge CB2 5NX, U.K. *****D-79108 Freiburg, Germany.
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ABSTRACT Synchrotron white beam x-ray topography techniques, in section and large-area transmission modes, have been applied to the evaluation of ELOG GaN on A120 3. Using the openings in 100 nm thick Si0 2 windows, a new GaN growth took place, which resulted in typical overgrowth thicknesses of 6.8 gtm. Measurements on the recorded Laue patterns indicate that the misorientation of GaN with respect to the sapphire substrate (excluding a 30' rotation between them) varies considerably along various crystalline directions, reaching a maximum of a -0.660 rotation of the (0001) plane about the [01.1] axis. This is -3% smaller than the misorientation measured in the non-ELOG reference, which reached a maximum of 0.680. This misorientation varies measurably as the stripe or window dimensions are changed. The quality of the ELOG epilayers is improved when compared to the nonELOG samples, though some local deviations from lattice coherence were observed. Long range and large-scale (order of 100 pgm long) strain structures were observed in all multi quantum well epilayers.
INTRODUCTION Epitaxial lateral overgrowth (ELOG) holds out the potential for significant reductions in threading dislocation densities for mismatched hexagonal-GaN on sapphire epitaxy [1-3]. Using openings in a relatively thick SiO 2 mask a new MOVPE GaN growth is carried out. After an initial phase of vertical growth upward through the mask window, the growth then proceeds laterally over the mask itself. It is thought that a significant reduction in threading dislocation densities can be achieved via mask blocking of vertically propagating dislocations and via a redirection of the propagation of some dislocations at the growth front [4-5]. Studies have shown that the ELOG technique can result in dislocation densities almost three orders of magnitude lower than in the nonELOG case, wherein typical densities of -1 x 1010 cm 2 are often observed [3]. GaN-based opto- and electronic devices are expected to benefit from this reduction in dislocation density and this technique has recently been applied to GaN blue laser production [6-7]. However, an understanding of the processes active during the ELOG procedure, and their impact on strain and dislocation generation, is still far from complete. In this study, synchrotron white beam x-ray topography, in section and large-area transmission [8-9] modes, have been applied to the evaluation of ELOG GaN on A120 3.
327 Mat. Res. Soc. Symp. Proc. Vol. 572 ©1999 Materials Research Society
EXPERIMENTAL The measurements were perfor
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