Matrix Formation Leading to Catalyst Free Growth of GaN Nanowires
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Matrix Formation Leading to Catalyst Free Growth of GaN Nanowires J. B. Halpern1; G. L. Harris2; M. He2; P. Zhou2; C. Cheek3 1 Chemistry, Howard University, Washington, DC 20059, U.S.A. 2 Howard Nanofabrication Facility, Howard University, Washington, DC 20059, U.S.A. 3 Electrical Engineering, Howard University, Washington, DC 20059, USA. ABSTRACT Catalyst-free vapor-solid GaN nanowire growth occurs when ammonia flows over Ga first forming a GaN matrix, the top layer of which is composed of hexagonal platelets. Multiphase nanowire growth occurs at nanoscale nucleation sites on the GaN platelets. Lower layers of the matrix are Ga rich, upper ones are stoichiometrically GaN. Gallium for later stages of growth is sourced from the decomposition of GaN particles and Ga rich GaN. Growth temperature exerts a strong influence on nucleation site formation. Scanning electron microscopy (SEM) and Energy Dispersive Spectroscopy (EDS) was used to characterize the matrix. INTRODUCTION An increasing number of studies of semiconductor nanowires have appeared recently [1, 2, 3] and many methods have been used to grow them. Characteristically, nanowire growth by chemical vapor deposition (CVD) occurs at lower temperatures than thin film growth and the nanowire growth occurs along preferred axes. This requires that the limiting activation energy for decomposition of the CVD source(s) be exceptionally low at the surface along which the nanowire grows. In other words, nanowire growth selects an orientation along which growth is kinetically favored. The decomposition can be either catalytic or not. Information on the most favored orientations for crystal growth could be useful for thin film growth. Catalytic GaN nanowire growth involves catalytic vapor-liquid-solid formation. [4] In that case, liquid metal droplets, usually Ni, decompose ammonia and solvate Ga metal vapor. Growth of wurtzite GaN wires oriented along the c axis then occurs at the bottom of the droplet. This paper describes a vapor-solid mechanism for catalyst free growth of GaN nanowires by reaction of Ga metal vapor with NH3 in a tube furnace. [5, 6, 7, 8] The wires can either be wurtzite, or biphasic combinations of wurtzite and zinc-blende. [9, 10, 11] The structure of the wire remains constant along its entire length. Growth temperature determines the form. Initially, a Ga rich matrix of GaN appears. In a short time the top of the matrix becomes crystalline and stoichiometrically pure GaN. Small hexagonal platelets appear on the surface. Wires grow from the edge and the center of the platelets. Their thickness (20 nm to 10 ยต) can be controlled by varying the ammonia flow rate (20-150 sccm) and temperature (800-1100 oC). Wires that grow from the edge of the platelets are as thick as the platelets and are biphasic. Wires that grow from the centers of the platelets (along the c-axis in the [0001] direction) are pure wurtzite and generally thicker. Length is determined by the duration of the growth period. EXPERIMENT Roughly 3 g of Ga metal in a small boron
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