Control of growth orientation of GaN nanowires

Bulk-quantity GaN nanowires of wurtzite hexagonal structure were synthesized by using hot-filament chemical vapor deposition. GaN nanowires showed two distinctive temperature-dependent growth directions. At a substrate temperature of 900–950 C, the growth direction of GaN nanowires was perpendicular to the f10 11g plane, while at 800–900 C, the growth direction was perpendicular to {0 0 0 2}plane. The two directions are different from the h10 10i direction, which is common for GaN nanowires grown to date. The difference in growth direction may be due to different growth mechanisms. In this study, the nanowires grew via a vapor–solid mechanism instead of the VLS growth mechanism. 2002 Elsevier Science B.V. All rights reserved. GaN-based III–V nitrides are attractive materials for the fabrication of optoelectronic components in the short wavelengths from blue to ultraviolet. In recent years, it has become possible to grow single crystal films of GaN on various substrates by metal–organic chemical vapor deposition (MOCVD) [1] and molecular beam epitaxy (MBE) technique [2]. Although nanostructured GaN materials such as zero-dimensional (0D) quantum dots [3,4] and two-dimensional (2D) quantum well structures [5,6] have been studied for several years, investigations of one-dimensional (1D) GaN nanowires are limited due to difficulties associated with their synthesis. GaN nanowires were first achieved through a carbon nanotubeconfined reaction [7]. Since then, several other methods have been reported to synthesize GaN nanowires [8–10]. All these methods required nanometer-sized metal catalysts such as Fe [8] or In [10] or nano-confined substrates such as carbon nanotube [7] or anodic alumina membrane [9] to control the diameter of the nanowires. All GaN nanowires reported to date has growth direction along h10 10i of hexagonal wurtzite structure. Recently, we reported that, without any of these nanometer-sized confinement effects, high-quality single crystalline GaN nanowires can be synthesized by using a hot filament chemical vapor deposition (CVD) method [11]. In the present Letter, we show that the substrate temperature can be used to control the growth orientation of GaN nanowires. It is well known that the lattice orientation in a wire can provide a tuning parameter, unavailable in quantum dots, to adjust materials 20 June 2002 Chemical Physics Letters 359 (2002) 241–245 www.elsevier.com/locate/cplett * Corresponding author. Fax: +852-2784-4696. E-mail address: [email protected] (S.T. Lee). 1 Present address: Physics Department, Hong Kong University of Science and Technology, Hong Kong, China. 0009-2614/02/$ see front matter 2002 Elsevier Science B.V. All rights reserved. PII: S0009-2614 (02 )00644-9 properties to suit particular applications as well as to provide an opportunity to test fundamental quantum mechanical concepts [12]. To the best of our knowledge, this Letter gives the first report of achieving tunable crystallographic orientation of GaN nanowires. The hot filament CVD set-up was described before [11]. Three straight tungsten wires of 0.75 mm diameter were used as the hot filaments in the CVD chamber. The solid source mounted above the hot filaments was a mixture of Ga2O3 and C powders (molecular ratio 1:1) and was made under a hydraulic press (3:2 10 Pa) at room temperature for 48 h. The substrate placed below the filament was a graphite plate, which had been ultrasonically cleaned in acetone, ethanol and deionized water for 10 min each before loading into the chamber. Through adjusting the distance between the substrate and the filament, we can control the substrate temperature during reaction (measured by a thermocouple in contact with the backside of the substrate). Ultra-high-purity NH3 (flow rate: 100 sccm) was introduced into the CVD chamber at a total pressure of 200 Torr. The reaction time was about 1 h. The wool-like product on the graphite substrate was examined by a scanning electron microscope (SEM, Philips XL 30 FEG). Some samples were directly mounted on Cu folding grids and studied with a Philips CM20 TEM at 200 kV. The HRTEM study was carried out on a Philips CM200 FEG TEM at 200 kV. After reaction, we found that the wool-like products on the substrate appeared in two distinguished colors. As schematically shown in Fig. 1a, the central round part of the substrate (about 900– 950 C) was dark gray, while the surrounding edge part (about 800–900 C) was light yellow. The boundary between these two regions was quite sharp. Typical SEM images of the products in the central part (referred to as sample A thereafter) and the edge part (referred to as sample B thereafter) are shown in Figs. 1b,c, respectively. Clearly, both samples A and B contained bulk quantity of nanowires, but their morphologies appeared quite different. Nanowires in sample A were very straight and smooth with diameters less than 15 nm, while nanowires in sample B were rough and exhibited a periodic zigzag structure with an average diameter of 30 nm. The nanowires in both samples had lengths greater than several micrometers. The energy dispersive X-ray spectroscopy (EDX) analyses indicated that both types of nanowires mainly consisted of Ga, N and O. The molecular ratio of Ga/N calculated from EDX data was close to 1:1, and the O content was very low. Fig. 1. Schematic diagram showing the distribution of GaN nanowires on the graphite substrate: sample A was in the central part and sample B in the edge part. (b, c) SEM images showing the typical morphologies of samples A and B, respectively. 242 H.Y. Peng et al. / Chemical Physics Letters 359 (2002) 241–245