Nucleation of GaN on (0001) sapphire during MOCVD growth: an atomic force and high resolution electron microscopy study
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Nucleation of GaN on (0001) sapphire during MOCVD growth : an atomic force and high resolution electron microscopy study F. Degave, P. Ruterana1, G. Nouet, J.H. Je* and C.C. Kim* ESCTM-CRISMAT, Institut des Sciences de la Matière et du Rayonnement, 6 Bd Maréchal Juin, 14050 Caen Cedex, France *
Department of Materials Science and Engineering, Pohang University of Science and Engineering, Pohang,
Korea
ABSTRACT The morphology and microstructure of GaN nucleation layers have been studied using atomic force microscopy (AFM) and transmission electron microscopy (TEM). The nucleation layers were grown at 500°C by metalorganic chemical vapour deposition (MOCVD) on (0001) sapphire. Different deposition times were used in order to investigate the evolution of the nucleation layer and try to understand the growth mechanisms. Systematically, it was found that the thinnest layers are mainly defect free and have a cubic structure. The (111) GaN planes are parallel to (0001) sapphire. It is shown that the nucleation follows the VolmerWeber mechanism and as the islands height increases, the transformation from cubic to wurtzite starts from the substrate surface.
INTRODUCTION In recent years, extensive work has been done on GaN and related nitrides because of their potential applications in optoelectronic and electronic devices. One of the major difficulties encountered in obtaining high quality GaN films is the lack of a suitable lattice-matched substrate. In spite of its large mismatch in lattice constants and thermal expansion coefficients, sapphire is the most widely used substrate. To optimize the heteroepitaxial growth of GaN on sapphire, a two-step metalorganic chemical vapour deposition (MOCVD) growth process has been developed [1, 2]. It consists in the introduction of a GaN or AlN buffer layer at low temperature, followed by the growth of the GaN epitaxial layer at high temperature. Several studies have shown the significant effect of nucleation layer growth conditions on the properties of GaN layers [3-7]. Since then, the interest for the nucleation layer and high-temperature GaN epilayer growth mechanisms has considerably increased [817]. In this paper, we present the morphological and structural evolution of the GaN nucleation layer versus the growth time.
EXPERIMENTAL DETAILS The GaN nucleation layers were grown on (0001) sapphire at 500°C in a MOCVD (Emcore Co.) chamber equipped with a high-speed rotating disk reactor, using trimethylgallium (TMGa) and ammonia (NH3). The growth was interrupted after 20 sec, 1 min, 2 min and 3
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Corresponding author, tel : 33 2 45 26 53, fax : 33 2 45 26 60, e-mail : [email protected]
G3.41.1
min, respectively. The nucleation layers were characterized using atomic force microscopy (AFM) and cross-sectional transmission electron microscopy (TEM). Samples for TEM were thinned down to 100 µm by mechanical grinding and dimpled down to 10 µm. Electron transparency was achieved by ion milling with the sample set on a liquid-N2 cooled stage
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