Self-oriented Growth of GaN Films on Molten Gallium
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E11.34.1
Self-oriented Growth of GaN Films on Molten Gallium Hongwei Li1, Hari Chandrasekaran1, Mahendra K Sunkara1, Ramon Collazo2, Zlatko Sitar2, Michael Stukowski3, Krishna Rajan3 1 Department of Chemical Engineering, University of Louisville, Louisville, KY 40292 2 Department of Materials Science and Engineering, North Carolina State University, 1001 Capability Drive, Research 1, Box 7919, Raleigh, NC 27695-7919 3 Department of Materials Science & Engineering, Rensselaer Polytechnic Institute, 110 Materials Research Center, 110 8th Street, Troy, NY 12180 ABSTRACT This paper presents a concept of growing near single crystal quality GaN films over large areas through self-oriented growth of GaN platelets on molten gallium. The experiments were performed by nitridation of Ga films on amorphous quartz substrates using nitrogen plasma at low pressures of few mTorr. XRD texture analysis of the free standing GaN flakes with areas over 25 mm2 exhibited an overall c-axis tilt of 2.2o, while showing primary reflections from (0002) and (0004) planes. Further more, the cross-sectional TEM micrographs showed that the resulting GaN films are free from dislocation crops inside the grains but showed diffraction contrast due to small misorientation between the grains. The twist and tilt angles between adjacent columnar grains were determined using convergent beam electron diffraction technique to be less than 8o and 1o, respectively. HRTEM micrographs of the grain boundaries showed sharp interfaces with both twisted and perfect attachments. INTRODUCTION Group III-nitrides (InN, GaN, AlN) are attractive materials for applications in optoelectronics as well as high temperature, high power electronics [1-3]. Their ability of forming continuous alloy makes a wide range of band gap from 0.7eV (InN) to 6.2eV (AlN) possible, which is essential for producing any specific wavelength emitters. Due to the unavailability of native GaN substrates, heteroepitaxy growth on substrates is employed commercially with sacrifices on the quality of the material, e.g., high density of dislocations on the order of 109-1010/cm2. Engineered techniques such as lateral epitaxial overgrowth (LEO) [4] or pendeo-epitaxy [5] have achieved significant reduction in the dislocation density to 106 to 107/cm2. However, the only way to reduce the dislocation density further to a level of 104/cm2 or lower is by using a bulk synthesis method, i.e., nucleation and growth of GaN directly from molten Ga. Bulk GaN growth under high temperature, high-pressure (>1500 oC, >15kbar) has produced maximum sized single crystals of 17mm with dislocation density lower than 103/cm2 [6], but further scale-up is complicated due to the extreme conditions involved. Other techniques such physical vapor transport (PVT) [7], ammonothermal [8] and sodium flux solution growth [9] also show promise, but the crystal size still limited to mm scale. Reaction between Ga and atomic nitrogen for bulk synthesis of GaN at sub-atmospheric pressures had also been demonstrated. But the resulting GaN wa
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