Gallium Nitride Epitaxy on Silicon: Importance of Substrate Preparation
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G. A. MARTIN *, B. N. SVERDLOV *, A. BOTCHKAREV *, H. MORKO *, D. J. SMITH **, S.-C. Y. TSEN **, W. H. THOMPSON ***, M. H. NAYFEH *** * Coordinated Science Laboratory and Materials Research Laboratory, University of Illinois at Urbana-Champaign, Urbana, IL 61801 ** Department of Physics and Astronomy and Center for Solid State Science, Arizona State University, Tempe, AZ 85287 *** Department of Physics, University of Illinois at Urbana-Champaign, Urbana, IL 61801
ABSTRACT Hexagonal GaN films grown on non-isomorphic substrates are usually characterized by numerous threading defects which are essentially boundaries between wurtzite GaN domains where the stacking sequences do not align. One origin of these defects is irregularities on the substrate surface such as surface steps. Using Si substrates and a substrate preparation procedure that makes wide atomically flat terraces, we demonstrate that reduction of these irregularities greatly improves the crystalline and luminescent quality of GaN films grown by plasma-enhanced molecular beam epitaxy. X-ray rocking curve width decreases from over 1 degree to less than 20 minutes, while PL halfwidth decreases from over 15 meV to less than 10 meV.
INTRODUCTION The Group III nitrides have great potential for optoelectronic devices operating across the entire visible spectrum and into the ultra-violet [I]. However, growth of nitride materials has been plagued by dense networks of threading defects in almost all nitride epitaxial layers grown to date by molecular beam epitaxy (MBE) [2] and metalo-organic chemical vapor deposition (MOCVD) [3]. The threading defects are generally identified as stacking mismatch boundaries (SMBs), originating from the intersection of two domains where the stacking sequences are not aligned. Recent work demonstrated that in wurtzite epitaxy on non-isomorphic substrates each substrate surface step has a high probability of leading to an SMB [4]. A natural avenue for exploration is to reduce the number of surface steps and observe whether the density of threading defects decreases. Unfortunately, most substrates, whether mechanically or chemically polished, are far from atomically smooth and contain a very high density of surface steps. Previous attempts to grow wurtzite GaN on Si have generally yielded unsatisfactory results [5-7]. However, the Si surface, when treated with certain etches, is reported to exhibit atomically flat terraces extending for thousands of angstroms [8-10], greatly increasing the distance between surface steps and thus decreasing their density. In this work we have systematically investigated substrate preparation procedures for improving the growth of GaN on Si by decreasing the density of substrate steps.
67 Mat. Res. Soc. Symp. Proc. Vol. 395 ©1996 Materials Research Society
EXPERIMENT Three-inch n-type 50 ohm-cm ± 0.10' silicon substrates from Virginia Semiconductor were used in this work. All substrate preparations began with a standard RCA cleaning procedure [11] (without thermal oxide growth). Later substrate prep
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