Effect of the polar surface on GaN nanostructure morphology and growth orientation
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Effect of the polar surface on GaN nanostructure morphology and growth orientation C. Y. Nam, D. Tham and J. E. Fischer* Department of Materials Science and Engineering, University of Pennsylvania, 3231 Walnut Street, Philadelphia, PA 19104-6272 E-mail address: [email protected] * Corresponding author ABSTRACT Wurtzite gallium nitride nanostructures were synthesized by catalyst-free thermal reaction of gallium oxide and ammonia. Depending on ammonia flow rate the resulting morphology varied. At 75 sccm only nanowires were obtained, while polyhedral crystals and nanobelts were observed at 175 sccm. From scanning electron microscopy and transmission electron microscopy, both thin smooth and thick corrugated nanowires were observed. The growth orientations of most of the smooth ones, as well as the nanobelts, were perpendicular to the c-axis (), while the corrugated nanowires and the large polyhedra grew parallel to . We propose a model to explain these variations of morphology and growth orientation in terms of the Ga/N ratio and the different characteristic length of {0001} polar surface in each morphology. 1. INTRODUCTION Along with recent extensive research interest in nanotechnology, gallium nitride (GaN) one-dimensional nanostructures, such as nanowire and nanobelt (or nanoribbon), are actively studied for possible future device applications. Generally, nanostructure synthesis involves one of two typical growth mechanisms. The vapor-liquid-solid (VLS) mechanism utilizes a transition metal catalyst while the vapor-solid (VS) mechanism relies on direct crystallization from the vapor. For VLS the dominant morphology is generally small-diameter nanowire because nucleation and growth are defined by a liquid catalyst particle [1]. In contrast, for VS more varied morphologies, e.g. nanobelt, are possible due to the absence of constraints by the catalyst. Accordingly, numerous groups reported the synthesis of various GaN nanostructures, e.g. nanowire [1] and nanobelt [2], using the VS type growth methods. To the best of our knowledge, there has been no attempt to understand or control the variation of morphology and crystallographic growth orientation among these structures. Such information would be critically important for building up devices out of these nanostructures, and for tuning the device characteristics given the highly anisotropic physical properties of GaN. In the present report, we studied the morphological evolution of GaN nanostructure, from nanowires to polyhedral crystals to nanobelts, by varying the ammonia flow rate in the catalystfree thermal reaction of gallium oxide (Ga2O3) and NH3. We propose that the Ga/N reactant ratio, i.e. reaction condition (N- or Ga- rich), in the vapor phase plays an important role in determining the resulting morphology and growth orientation based on the observed relationship between the morphology and the characteristic length of {0001} polar surface. 2. EXPERIMENTAL DETAILS
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Figure 1. Schematic illustration of a GaN reactor. The substrate tempera
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