Catalyst-Free Growth of GaN Nanowires K.A. BERTNESS, 1,3 N.A. SANFORD, 1 J.M. BARKER, 1 J.B. SCHLAGER, 1 A. ROSHKO, 1 A.V. DAVYDOV, 2 and I. LEVIN 2 1.—National Institute of Standards and Technology, Boulder, CO 80305. 2.—National Institute of Standards and Technology, Gaithersburg, MD 20899-8555. 3.—E-mail: bertness@boulder.nist.gov We have grown GaN and AlGaN nanowires on Si (111) substrates with gas- source molecular beam epitaxy (MBE). No metal catalysts were used. The nanowires displayed a number of interesting materials properties, including room-temperature luminescence intensity greater than that of free-standing HVPE-grown GaN, relaxed lattice parameters, and the tendency of nanowires dispersed in solvents to align in response to electric fields. The wires were well separated, 50–250 nm in diameter, and grew to lengths ranging from 2 mm to 7 mm. Transmission electron microscopy indicated that the wires were free of defects, unlike the surrounding matrix layer. Key words: Gallium nitride, molecular beam epitaxy (MBE), nanostructures, nanotechnology INTRODUCTION The growth of semiconductors in nanowire struc- tures has increased dramatically upon the identifi- cation of catalytic methods for preferential growth along a single axis. The catalysts are typically drop- lets of gold, nickel, or other metals, and growth under the droplet proceeds by means of the vapor- liquid-solid (VLS) mechanism. The details of the mechanism are still under study; 1 however, the pro- gress in terms of device demonstration has been remarkable. The nanowire growth regime is partic- ularly attractive for the AlGaInN alloy system, because lattice-matched substrates for this system are not readily available. The nanowire morphology also allows greater dislocation-free strain relaxation in response to heterojunction formation and cooling after crystal growth. GaN nanowire lasers, 2 light- emitting diodes, 3,4 and transistors 5 are among early technological demonstrations emerging from these structures. Like any growth method, however, cata- lytic growth does have some limitations. Growth conditions are limited to temperature ranges com- patible with the catalyst, and preparation of the catalyst dots requires additional steps. The question of the effect on luminescent efficiency of impurity- level incorporation of the metals into the semicon- ductor has not yet been addressed quantitatively. For this reason, some applications may require the use of catalyst-free growth methods. Noncatalytic growth of nanowires has been reported by only a handful of groups, 6–10 including those groups using GaN seed crystal methods to nucleate nanowires. In this paper, we discuss growth conditions for catalyst-free growth of GaN and AlGaN nanowires on Si substrates using molecular beam epitaxy (MBE) and present data illustrating some of the interesting nanowire properties. We observed indi- cations of very high material quality, including high intensity of photoluminescence (PL) at room tem- perature, relaxed lattice parameters, and absence of structural defects as observed by transmission electron microscopy (TEM). The wires also exhibited a tendency to align in response to external electric fields, and this property has been used to assist in the placement of wires dispersed in solvents onto metal contact pads. EXPERIMENTAL PROCEDURES The nanowire growths took place in a conven- tional gas-source MBE system with evaporator cells for Ga, Al, Si, and Be and an radio-frequency plasma N source. The operating conditions during growth of GaN nanowires were a nitrogen flow of 2.1 mmol/s (3 sccm), with 450 W of RF power for the nitrogen source, and a Ga beam equivalent pressure of 1.3 3 10 ÿ5 Pa. Substrate temperatures were measured (Received August 1, 2005: accepted November 22, 2005) Journal of ELECTRONIC MATERIALS, Vol. 35, No. 4, 2006 Special Issue Paper 576