GaN Epitaxial Growth by Molecular Beam Epitaxy utilizing AlGaN Buffer Layer with Nanopipes
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GaN Epitaxial Growth by Molecular Beam Epitaxy utilizing AlGaN Buffer Layer with Nanopipes F. Yun1, L. He1, M. A. Reshchikov1, H. Morkoç1, J. Jasinski2 and Z. Liliental-Weber2 1
Department of Electrical Engineering and Physics Department, Virginia Commonwealth University, Richmond, VA 23284 2
Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720 ABSTRACT GaN layers were grown on AlGaN with nanopipes by molecular beam epitaxy (MBE) and analyzed. AlGaN films were grown by MBE using rf-plasma nitrogen source under metal-rich condition. Within the Al composition range of 0.5-0.6, open-end nanopipes were formed at the surface of AlGaN films with a density of ~6x109 cm-2 and a size ranging from 10 to 20 nm. These nanopipes, observed within ~300 nm of the surface, served as a nanoporous AlGaN template for re-growth of GaN epilayers. GaN epilayers grown to different thickness by MBE were studied for their microstructural and optical properties. For an AlGaN buffer layer with dislocation density of 3x1010 cm-2 near its surface, the overlaying GaN layers with thickness ranging from 0.1 µm to ~2µm were grown and analyzed by transmission electron microscopy for dislocation density. The GaN layer started with hexagonal islands on the nanopiped AlGaN and began to coalesce at about 0.1µm thickness. At a thickness of 2.0 µm, the dislocation density reduced to ~1x109 cm-2. Low temperature photoluminescence data demonstrate the improved optical quality of GaN epilayer grown on the porous AlGaN buffer layer. INTRODUCTION For application in electronic devices such as field effect transistors, high-quality GaN epilayers grown by molecular beam epitaxy (MBE) is essential. The key to improving the quality is the reduction of electrically active dislocation density during the epitaxial growth. While selective and lateral overgrowth has been successfully incorporated for GaN metal organic chemical vapor deposition - growth to reduce the extended defect density,1,2 the case in MBE is not well investigated. Recently, there has been some effort in growing GaN on nanopatterned substrate for defect reduction.3,4,5,6 There have also been reports on growth of GaN on ternary alloy of AlGaN instead of AlN.7 It is believed that the use of ternary alloy as a buffer layer would be beneficial for reducing lattice and thermal mismatch, while the porous structure of the AlGaN surface would presumably enhance the strain relaxation and defect looping as well as lateral overgrowth of GaN in order to get rid of the threading dislocations along the growth direction. The combination of these two approaches for the reduction of dislocation density during GaN epitaxial growth is therefore of great interest. Up to now, there has been no report of GaN growth on the nanoporous ternary AlGaN buffer layer. In this work, we report preliminary growth and characterization results on GaN thin films deposited on top of porous AlGaN templates by MBE.
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