Real Time Observation and Characterization of Dislocation Motion, Nitrogen Desorption and Nanopipe Formation in GaN
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REAL TIME OBSERVATION AND CHARACTERIZATION OF DISLOCATION MOTION, NITROGEN DESORPTION AND NANOPIPE FORMATION IN GaN ERIC A. STACH,* C.F. KISIELOWSKI,* W.S. WONG,†,# T. SANDS† AND N.W. CHEUNG§ * National Center for Electron Microscopy, Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720: email: [email protected] ; http://ncem.lbl.gov † Department of Materials Science and Engineering, University of California, Berkeley, CA 94720; § Department of Electrical Engineering and Computer Science, University of California, Berkeley 94720. #
Present address: Xerox Palo Alto Research Center, Palo Alto, CA 94304.
ABSTRACT Despite the considerable attention focused on GaN and related alloys during the past decade, many outstanding questions remain regarding the mechanisms of defect formation in these materials. In this work, we take advantage of a recently developed processing technique known as laser lift-off to examine the behavior of thin, free-standing, nearly stress-free single crystals of GaN subjected to thermal stimulus. GaN layers of 7 µm thickness were removed from their sapphire growth substrate using the laser lift-off method, then ion-milled to electron transparency. The samples were then annealed at temperatures between 850 and 1025 °C within the objective lens of a 200 kV transmission electron microscope. This allowed real time observation of defect formation via diffraction contrast imaging. Above 925 °C nitrogen desorption first becomes visible at the thinnest edges of the TEM sample. Concomitant with this is the preferential desorption of nitrogen along the cores of those dislocations which are pure screw in character, resulting in the formation of nanopipes. In regions with small residual stresses, those dislocations with mixed edge and screw components propagate parallel to the basal plane in the three close-packed {1 1 00} directions, leaving a hollow tube in their wake. At these temperatures, no motion of dislocations with pure edge character is observed. These results indicate that desorption and plasticity may occur simultaneously in these materials. Additionally, it appears that nanopipe formation may be unavoidable during heteroepitaxial growth by chemical vapor deposition and hydride vapor phase epitaxy, as these deposition methods require substrate temperatures of this magnitude. INTRODUCTION GaN and related III-nitride alloys are fully miscible, with direct band gaps that span the range from 1.9 eV (amber) to 6.2 eV (ultraviolet). This has permitted the creation of opto-electronic devices such as blue and green lasers and the full color spectrum of light emitting diodes from these materials. 1,2 The majority of III-nitride materials are grown heteroepitaxially onto (0001) sapphire substrates via deposition methods such as molecular beam epitaxy (MBE), metallorganic chemical vapor deposition (MOCVD) and hydride vapor phase epitaxy (HVPE). However, because of the large lattice mismatch between these two materials (on the order of 14% for GaN / Al2O3) the heteroepitaxial
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