Self-branching in GaN Nanowires Induced by a Novel Vapor-Liquid-Solid Mechanism
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1058-JJ04-03
Self-branching in GaN Nanowires Induced by a Novel Vapor-Liquid-Solid Mechanism Chang-Yong Nam1, Douglas Tham2, and John E. Fischer3 1 Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, NY, 119735000 2 Department of Materials Science, California Institute of Technology, Pasadena, CA, 91125 3 Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, PA, 19104 ABSTRACT Investigations of the growth morphology of AuPd-catalyzed GaN nanowires lead us to propose a vapor-liquid-solid (VLS) mechanism distinct from the well-established VLS growth of elemental wires. Here, nucleation and growth of GaN nanowires proceeds by direct spontaneous reaction between NH3 vapor and Ga dissolved in liquid AuPd alloy, rather than by solubilitylimited supersaturation. A frequently observed self-branching growth mode can be explained by the proposed VLS scheme and the migration of Ga-enriched AuPd liquid on Ga-stabilized polar surfaces of mother nanowires. (b)
Figure 1. (a) SEM image showing as-grown VLS GaN NWs on a growth substrate. (b) Bright field TEM image of individual GaN NW showing metal catalyst at the tip. INTRODUCTION GaN nanowires (NWs) are of interest for nanoscale optoelectronic device applications [1-4]. Synthesis can be achieved by several methods involving vapor-liquid-solid (VLS) growth, such as thermal chemical vapor deposition (CVD) [5-7], hydride vapor phase epitaxy (HVPE) [8], molecular beam epitaxy (MBE) [9], and metalorganic CVD (MOCVD) [10]. The low N solubility in transition metals implies that nucleation of GaN from supersaturated catalyst droplets is unlikely, in contrast to elemental structures such as Si whiskers and NWs [11]. Here we study GaN NWs grown by thermal CVD using Ga2O3, NH3 and AuPd catalyst. The observed morphologies suggest a VLS mechanism distinct from the nucleation and growth of elemental nanostructures from supersaturated liquid. We consider a hypothetical pseudo-binary phase diagram and thermodynamics of GaN formation reaction to propose that NW nucleation and growth occur via direct reaction of dissolved Ga with NH3 vapor. We also observe spontaneous ‘self-branching’ of large-diameter NWs into smaller diameter progeny. This is explained by the proposed VLS scheme and the high mobility of Ga-rich liquid catalyst on polar NW surfaces.
EXPERIMENTAL DETAILS Ga2O3 powder (Alfa Aesar, 99.999%) in an alumina boat is placed at the center of a quartz tube in a horizontal furnace. SiOx/Si substrates with sputter-coated AuPd (60:40 wt.%, thickness 50 wt.% for liquid Ga-Pt formation at our growth temperature 1000 .
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Figure 4. (a),(b) High resolution SEM images of self-branched growth from a stem of thicker NW. (a) is taken from an as-grown substrate, and (b) shows a broken piece of a NW with branches after dry-transfer to a clean Si substrate. The inset reveals spherical particles at the tip of branch A, suggesting VLS-type branch growth. (c) Schematic of the self-branching mechanism. See the text for more details. As a fi
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