Morphology and surface reconstructions of m-plane GaN
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Morphology and surface reconstructions of m-plane GaN C. D. Lee,1 R. M. Feenstra,1 J. E. Northrup,2 L. Lymperakis,3 J. Neugebauer 3 1 Department of Physics, Carnegie Mellon University, Pittsburgh, PA 15213 2 Palo Alto Research Center, 3333 Coyote Hill Road, Palo Alto, CA 94304 3 Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, D-14195 Berlin, Germany ABSTRACT M-plane GaN(1 1 00) is grown by plasma assisted molecular beam epitaxy on ZnO(1 1 00) substrates. A low-temperature GaN buffer layer is found to be necessary to obtain good structural quality of the films. Well oriented (1 1 00) GaN films are obtained, with a slate like surface morphology. On the GaN(1 1 00) surfaces, reconstructions with symmetry of c(2x2) and approximate "4×5" are found under N- and Ga-rich conditions, respectively. We propose a model for Ga-rich conditions with the "4×5" structure consisting of ≥ 2 monolayers of Ga terminating the GaN surface. INTRODUCTION Wurtzite GaN heteroepitaxy and surface reconstructions have been extensively studied in the past 5-10 years [1,2]. It is well known that GaN films with (0001) and (000 1 ) surface orientations have pyroelectric and piezoelectric properties [3,4], leading to strong electric fields along the crystal c-axis which have useful device applications such as the formation of a twodimensional electron gas near a heterointerface. However these strong electric fields also produce a shift in the wavelength of emission lines and reduced quantum efficiency because of the spatial separation of electrons and holes in quantum wells. Thus, several growth studies of (1 1 00) oriented (m-plane) GaN have been performed, since for this orientation the crystal symmetry precludes pyroelectric and piezoelectric effects (at least in the absence of shear stresses in the growth plane) [5-7]. In this work we have used plasma assisted molecular beam epitaxy (PAMBE) to grow GaN films on ZnO(1 1 00) substrates. We find that a low temperature (~500°C) buffer layer is needed to obtain smooth morphology. Several reconstructions on the GaN(1 1 00) surfaces are observed for both Ga- and N-rich conditions. The most dominant of these occurs under Ga-rich conditions and it has an approximate surface unit cell of 4×5. We propose a model for this structure in which it consists of a contracted and distorted arrangement of a Ga bilayer on the surface. EXPERIMENTAL ZnO(1 1 00) substrates were obtained from Cermet, Inc. The substrates were prepared by mechanical polishing; atomic force microscopy (AFM) revealed many scratches and polishing marks on the substrate surfaces. The MBE growth and subsequent scanning tunneling microscopy (STM) were performed using a system previously described [1,2]. The ZnO substrates were cleaned inside the growth chamber prior to growth simply by heating them to about 550°C for 20 min. Growth was initiated by exposing the ZnO surface simultaneously to
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incident Ga and N fluxes. A ≈20 nm thick GaN layer is grown at a relatively low temperature of about 500°C, and subsequent
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