Solid Phase Epitaxial Recrystallization of AlN Films on Sapphire (0001): A Novel Substrate Approach for GaN Epitaxy
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Solid phase epitaxial recrystallization of AlN films on sapphire (0001): A novel substrate approach for GaN epitaxy R. D. Vispute1, A. Patel1, R. P. Sharma1, T. Venkatesan1, T. Zheleva2 and K. A. Jones2 1 CSR, Department of Physics, University of Maryland, College Park, MD 20742 2 U.S. Army Research Laboratory, Adelphi, MD 20783. ABSTRACT High quality and lattice matched buffer layers are needed for the growth of device quality GaN thin films on sapphire for optoelectronic applications. In this context, we report the fabrication of AlN thin films having low defect densities through a novel process called solid phase epitaxial recrystallization (SPER). In this process, as-grown crystalline AlN thin films, having a large defect concentration (such as threading dislocations due to a large lattice mismatch between AlN and sapphire and low angle grain boundaries), were thermally annealed in an inert atmosphere at various temperatures ranging from 1200-1600°C for 30 min. The as-grown and annealed samples were characterized using x-ray diffraction, transmission electron microscopy (TEM), Rutherford backscattering spectroscopy (RBS), atomic force microscopy (AFM) and UV-visible spectroscopy. The ion channeling/RBS and TEM results clearly indicate a substantial reduction in the defect density for the recrystallized AlN films. The surface morphology of the SPER AlN films was smooth with a surface roughness close to the unit cell height. The optical bandgap was sharp as compared to as-grown films, with a bandgap of 6.2 eV. The recrystallized films having smooth surface morphology and low defect densities may be useful for the growth of device quality GaN films on sapphire. INTRODUCTION III-V nitrides (GaN, AlN, InN, and Ga-Al-In-N alloys) have now been recognized as potential materials for the generation of optoelectronic devices and hightemperature, high-power devices [1]. However, the success of utilizing the full potential of these materials, has not been fully realized due to the lack of high-quality and latticematched substrates. Thus, current research has been directed towards the fabrication of lattice-matched substrates and the subsequent growth of high quality thin films with low dislocation densities (104-105 cm-2). Despite its poor structural and thermal match to GaN, sapphire has been the substrate material of choice due to its low cost, availability in large-area wafers, and its high optical transparancy in the UV-visible region. However, as-grown nitride films on sapphire are known to contain a high density of defects (mainly threading dislocations) [2], which affect both the electrical and optical properties and the life time of the devices. Due to the large lattice mismatch (16%) and the large interface energy between GaN and sapphire, GaN tends to grow 3-dimensionally (3D) rather than layer by layer or 2dimensionally (2D). It has been shown in the past that 3D growth can be effectively suppressed, and that the crystallinity, as well as surface quality, can be improved with a two step growth process that utili
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