Microstructure of epitaxial AlN layers on sapphire substrates deposited by physical vapor deposition
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Microstructure of epitaxial AlN layers on sapphire substrates deposited by physical vapor deposition Sandeep Kohli, Boris Druz, Adrian Devasahayam, Arindom Datta, Frank Cerio Veeco Instruments Inc., 1 Terminal Drive, Plainview NY 11803, USA ABSTRACT Asymmetric (10L) XRD peaks have been employed as a measure of epitaxial quality for aluminum nitride (AlN) nucleation layers (NL) deposited on sapphire substrate. Epitaxial AlN films have been deposited on 2-6” sapphire substrate by reactive sputtering. FWHM of AlN (103) and (105) were found to be an excellent indicator of quality of AlN films for GaN growth. AlN films produced nucleation layers with highly reproducible microstructure and GaN film growth. NLs had in-plane and out-of-plane texture as evident by the pole-figure results and selected area diffraction pattern. Based on electron microscopy results, AlN film thickness for complete atomic ordering was estimated to be 6-7 nm and most of the edge dislocations were seen in the first 20 nm of the film. Excellent thickness and texture uniformity were seen on planar and patterned sapphire substrates. A compressive stress of 2.9±0.2 GPa was seen in our BKM films. The maximum screw and edge dislocation densities of films were found to be ~3 x 108 cm-2 and ~9 x 109 cm-2 respectively. The root mean square roughnesses of A-polar films were found to be < 0.3 nm. INTRODUCTION Since the earliest work by Nakamura etal1 Gallium Nitride (GaN) based solid state lighting (SSL) has matured enough to enable commercialization of this technology for large scale application in consumer electronics as well as recent entry in the area of house-hold lighting2. While GaN growth on sapphire and silicon carbide 3has been the mainstay of the technology, GaN on silicon (Si) has made some recent news for SSL 4. However, GaN on Si is yet to mature enough for large scale consumer applications. Efforts are being continuously made to improve status of current GaN based SSL on sapphire. Amongst the methods being explored, presence of aluminum nitride (AlN) nucleation layer5, -8 and use of patterned sapphire substrates9 are touted as some major improvements in the technology to improve, the external as well as internal quantum efficiencies of GaN based SSL technology on sapphire leading to lower costs and more affordability for mainstream house-hold lighting. Some recent results have reported significant results in this direction 10. Also, presence of a thin seed AlN layer also eliminates 1st step of the growth of low temperature GaN layer (~2.5 um) that is frequently encountered in standard 2-step nucleation process (MOCVD process for LEDs grown on sapphire). Although, AlN may be deposited easily using metalorganic chemical vapor deposition (MOCVD) methods, the slow rate of deposition using MOCVD is a limiting factor. Hence, physical vapor deposition (PVD) techniques like reactive sputtering that enable comparatively faster growth rates and low cost of ownership may be preferred. However,
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the literature in this direction has been limited in s
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