Superior Structural Quality of Newly Developed GaN Pendeo-Epitaxial Layers.
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Superior Structural Quality of Newly Developed GaN Pendeo-Epitaxial Layers. Z. Liliental-Weber,a J. Jasinski,a D. Cherns,b M. Baines,b and R. Davisc a
Lawrence Berkeley National Laboratory, Berkeley, CA 94720 m/s 62/203 H.H. Wills Physics Laboratory, University of Bristol, Bristol, UNITED KINGDOM c Department of Materials Science and Engineering, University of North Carolina, Raleigh, NC. b
ABSTRACT: Transmission electron microscopy of plan-view and cross-section samples of pendeo-epitaxial layers is described. Samples grown with and without silicon nitride masks are compared. A large misorientation of the GaN grown above the mask was observed, with 2-3° tilt between wing and seed areas, caused by additional nucleation on the mask layer. Some misorientation was also observed between wing/wing areas of the sample. Samples grown without silicon nitride masks show much smaller misorientations and contain different types of defects. INTRODUCTION The lack of large, free standing GaN substrates for the growth of homoepitaxial layers is forcing investigators in the III- nitride community to develop special growth methods in order to decrease dislocation density in heteroepitaxially grown material, used for different device applications. Two related methods: laterally epitaxial overgrowth (LEO) [1-4] and pendeoepitaxy (PE) have been developed [5-8]. Application of both these methods leads to a much lower density of dislocations, especially those that propagate in the growth direction. In the pendeo-epitaxy method, growth does not initiate through the open windows on the (0001) as it does in LEO structures, but instead it is forced to begin on the sidewalls etched into the seed crystal to form overhanging wings. All layers studied in this paper were grown on SiC with an AlN buffer layer, which had then been selectively etched; etching continued into the SiC. The details of the growth procedure are described in several previous papers [5,7]. Here the microstructure of two (masked and unmasked) PE structures will be described in detail. EXPERIMENTAL Masked PE structures consisted of a 100 nm silicon nitride layer deposited onto the etched GaN/AlN seeds (about 1.2 µm in height). The seeds (2 µm wide strips) were about 3 µm apart. The PE layers coalesced and the total thickness of the layers measured between the seeds was about 2 µm, with about 1.2 µm above the Si3N4 mask. In the second sample, which was prepared without a silicon nitride cap, the seed height was only about 0.15 µm, and the seeds were separated by about 4 µm. The total PE layer thickness between the seeds was measured to be about 3.5 µm. Schematic drawings of these samples are shown in Fig. 1. Thin foils, transparent to electrons, were prepared using mechanical polishing (dimpling) and Argon ion-milling, in the cross-section perpendicular to the [1100] seed-stripe direction, and also in plan-view orientation, where the thinning was performed from the substrate in order to obtain information from the area I5.7.1
close to the sample surface. A Topcon 002B elec
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