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N. NEWMAN*, A. GASSMANN*,

X. LIU*,

L. SCHLOSS*, E.R. WEBER*, I. GRZEGORY**, M. BOCKOWSKI**, J. JUN**, T. SUSKI**, K. PAKULA**, J. BARANOWSKI**, S. POROWSKI**, H. AMANO*** , and I. AKASAKI*** * Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720 **High Pressure Research Center "Unipress" Polish Academy of Sciences, Warsaw, Poland ***Meijo University, Nagoya, Japan ABSTRACT The microstructure and characteristic defects of heteroepitaxial GaN films grown on sapphire using molecular beam epitaxy (MBE) and metal-organic-chemical-vapor-deposition (MOCVD) methods and of homoepitaxial GaN grown on bulk substrates are described based on transmission electron microscopy (TEM), x-ray diffraction, and cathodoluminescence (CL) studies. The difference in arrangement of dislocations along grain boundaries and the influence of buffer layers on the quality of epitaxial films is described. The structural quality of GaN epilayers is compared to that of bulk GaN crystals grown from dilute solution of atomic nitrogen in liquid gallium. The full width at half maximum (FWHM) of the x-ray rocking curves for these crystals was in the range of 20-30 arc sec, whereas for the heteroepitaxially grown GaN the FWHM was in the range of 5-20 arc min. Homoepitaxial MBE grown films had FWHMs of about 40 arc sec. The best film quality was obtained for homoepitaxial films grown using MOCVD; these samples were almost free from extended defects. For the bulk GaN crystals a substantial difference in crystal perfection was observed for the opposite sides of the plates shaped normal to the c direction. On one side the surface was almost atomically flat, and the

underlying material was free of any extended structural defects, while the other side was rough, with a high density of planar defects. This difference was related to the polarity of the crystal. A large difference in crystal stoichiometry was also observed within different sublayers of the crystals. Based on convergent beam electron diffraction and cathodoluminescence, it is proposed that GaN antisite defects are related to the yellow luminescence observed in these crystals. INTRODUCTION Gallium nitride has attracted much attention in recent years, due to its wide band gap (3.4 eV) and therefore its possible applications for short-wavelength light-emitting devices [1]. Considerable effort in crystal-growth engineering (MBE, MOCVD) has improved the quality of GaN epitaxial layers to the extent that x-ray rocking curves have improved from a FWHM of 25 arc min to values that are now below 2 arc min [2-5]. Blue-light-emitting diodes could be built based on GaN and GalnN alloys [1]. It has been reported, surprisingly, that, the high emitting efficiency occurred in spite of the high dislocation densities (of the order of 1010 cm- 2 ) [6]. Further improvement in structural perfection of GaN thin films will depend strongly on the availability of substrate materials having a good lattice match to GaN crystals and a good match in thermal expansion coefficients. Since homo