Stress Evolution during the Early Stages of AlN Vapor Growth
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Stress Evolution during the Early Stages of AlN Vapor Growth B. Wua, J. Baib, V.L. Tassevc, M. Lal Nakarmid, W. Sune, X. Huangb, M. Dudleyb, H. Zhanga, D. F. Blissc, J. Lind, H. Jiangd, J. Yange, M. Asif Khane a Department of Mechanical Engineering, State University of New York at Stony Brook, Stony Brook, NY 11794-2300 b Department of Materials Science and Engineering, State University of New York at Stony Brook, Stony Brook, NY 11794-2275 c Air Force Research Laboratory, Sensors Directorate, 80 Scott Road, Hanscom AFB, MA 01731 d Department of Physics, Kansas State University, Manhattan, KS 66506-2601 e Departement of Electrical Engineering, Swearingen Engineering Center, Univeristy of South Carolina, SC 29208 ABSTRACT The evolution of stress during the MOCVD growth of AlN thin films on sapphire substrates under both low and high temperature conditions has been evaluated. The final stress state of the films is assumed to consist of the summation of stresses from three different sources: (1) the stress which arises from residual lattice mismatch between film and substrate i.e. that which persists after partial relaxation by misfit dislocation formation. The extent of relaxation is determined from High Resolution TEM analysis of the substrate/film interface; (2) the stress arising from the coalescence of the 3D islands nucleated in this high mismatch epitaxy process. This requires knowledge of the island sizes just prior to coalescence and this was provided by AFM studies of samples grown under the conditions of interest; and (3) the stress generated during post-growth cooling which arises from the differences in thermal expansion coefficient between AlN and sapphire. The final resultant stress, comprising the summation of stresses arising from these three sources, is found to be tensile in the sample grown at lower temperature and compressive in the sample grown at higher temperature. These results are in general qualitative agreement with results of TEM and High resolution X-ray diffraction (HRXRD) studies, which show evidence for tensile and compressive stresses in the low temperature and high temperature cases, respectively. INTRODUCTION The group III nitride semiconductors, AlN, GaN, and InN, with their wide and direct band gaps which range from 0.7 – 6.2 eV (spanning the whole of the visible spectrum and extending into the ultraviolet region) [1] have been successfully fabricated into various optoelectronic devices. Among the nitrides, AlN has one of the larger band gaps, a relatively high thermal conductivity, high hardness, and a resistance to high temperature and corrosive environments. It also is closely lattice matched to GaN, as well as zinc oxide and silicon carbide, and thus has seen extensive use as a buffer layer. Growth of bulk crystals of AlN is also being actively pursued for use as native substrates for III-nitride ultraviolet emitters and detectors. Currently, the most widely used substrates for AlN and related alloy epi-growth are SiC and sapphire. Stresses in the film are expec
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