Nucleation control for the growth of vertically aligned GaN nanowires
© Hou et al.; licensee Springer. 2012
Received: 12 January 2012
Accepted: 26 June 2012
Published: 7 July 2012
Aligned GaN nanowire arrays have high potentials for applications in future electronic and optoelectronic devices. In this study, the growth of GaN nanowire arrays with high degree of vertical alignment was attempted by plasma-enhanced CVD on the c-plane GaN substrate. We found that the lattice matching between the substrate and the nanowire is essential for the growth of vertically aligned GaN nanowires. In addition, the initial nucleation process is also found to play a key role in creating the high-quality homoepitaxy at the nanowire-substrate interface. By controlling the nucleation stage, the growth of highly aligned vertical GaN nanowire arrays can be achieved. The reasons for the observed effects are discussed.
Vertically aligned nanowires are potentially useful for the fabrication of nanowire electronic and optoelectronic devices. Both the growth direction and the crystallographic orientation of nanowires have significant effects on the efficiency, performance, and manufacturability of nanowire devices . The nanowires grown via vapor–liquid–solid (VLS) or vapor-solid-solid mechanism are usually not well aligned on the substrates . Compared to randomly oriented nanowires, vertically aligned nanowires have several advantages including the ability to control the crystallographic orientations of nanowires and the ability to manufacture electronic and optoelectronic devices . For the vertically aligned nanowires, the uniformity of nanowire height and diameter can be more likely achieved due to the uniform mass and heat transport to each nanowire and the absence of collision and coalescence between two nanowires during the growth . The devices can be also easily fabricated on the nanowire arrays with vertical alignment using the vertical electrical integration scheme, in which the efficiencies of the devices are strongly dependent on the orientation of nanowire crystals in the device . Therefore, the epitaxial growth of vertically aligned nanowires has attracted a great deal of attention particularly on the growth techniques and the growth mechanism .
The growths of vertically aligned GaN nanowires have been demonstrated using the lattice matching or minor-mismatching substrates in several material systems, such as GaN, GaAs, InP, Si, ZnO, etc. [6–10]. Vertically aligned faceted GaN nanorods were produced by Deb et al. using a catalyst-free template approach employing a silicon dioxide mask fabricated from the porous anodic alumina . George et al. reported the growth of vertically aligned GaN nanowires on the r-plane of sapphire substrate by metal-organic chemical vapor deposition (MOCVD) . Li and Wang reported another route to grow ultrahigh-density and highly aligned single-crystalline GaN nanowires on sapphire by employing a submonolayer of Ni catalyst . Besides, Lin et al. reported the fabrication of high-density vertically aligned GaN nanowire arrays on GaN substrate through thermal evaporation of GaN powder with the assistance of HCl gas . Furthermore, Kuykendall et al. grew the vertically aligned GaN nanowires with growth orientations along and  on γ-LiAlO2 (100) and MgO (111) substrates, respectively . On the other hand, the importance of slow nucleation rate on the vertical alignment of GaN nanowires has not been reported yet.
In this study, we employed the plasma-enhanced chemical vapor deposition system using Ga source and N2 gas reactants to synthesize GaN nanowires on the c-plane GaN film grown on sapphire by MOCVD. The homoepitaxial growth of GaN nanowires on the GaN substrate using Au catalyst allowed us to grow vertically aligned GaN nanowires. However, even for homoepitaxial growth, the growth rate at the early nucleation stage still needs to be kept low in order to grow vertically aligned GaN nanowires. The high quality of GaN nanowire crystallites was confirmed by cathodoluminescence at room temperature.
The morphology of the nanowires was analyzed using scanning electron microscopy (SEM) (Philip XL-40FEG, FEI Co., Hillsboro, OR, USA). The crystal structure of nanowires was characterized by HRTEM (JEOL, JEM-2010, 200KV, JEOL Ltd., Akishima, Tokyo, Japan). Room-temperature cathodoluminescence (CL) spectra were obtained using a Gatan/Mono CL3 system (Pleasanton, CA) attached in a field-emission scanning electron microscope (JSM-7000, JEOL) with an electron beam voltage of 10 kV and a beam current of 10 nA.
Results and discussion
The growth rate of nanowires for Ga source at 25 cm was reduced to only one half of that for Ga source at 20 cm, as shown in Figures 3c,e, owing to the fact that the vapor pressure of Ga at 25 cm was only one fourth of that at 20 cm. However, the average diameter of the nanowires for Ga at 25 cm was about 150 nm, which was around twice as large as those for Ga at 15 and 20 cm. In the VLS growth of nanowires, the extremely thin Au film (3-nm thick) would sinter into Au nanoparticles before the nucleation of GaN nanowires due to the Ostwald ripening mechanism, and the diameter of nanowires was dependent on the size of the catalyst nanoparticles. During heating of the furnace from room temperature, the Ga source and the substrate were both gradually heated up. When the Ga source was heated to a temperature providing enough concentration of Ga vapor, the nucleation of GaN nanowires would start to occur through the reaction of Ga vapor with nitrogen plasma. The substrate for the Ga source at 25 cm needed to be heated up to a temperature much higher and longer than those at 15 and 20 cm to allow the Ga source to reach the same temperature as those at 15 and 20 cm for providing the same Ga vapor pressure to initiate the GaN nanowire nucleation. For the Ga source at 25 cm, the higher substrate temperature and longer heating period to initiate nucleation resulted in a higher degree of sintering forming Au catalyst nanoparticles of larger sizes, which resulted in the growth of GaN nanowires with a larger diameter.
A route to highly vertically aligned GaN nanowires on the c-plane GaN substrate is reported in this study. We have shown that the degree of vertical alignment can be improved via controlling the gallium partial pressure during the nucleation of nanowires on the substrate. First, the lattice-matched substrate is an essential requirement for the growth of vertically aligned GaN nanowires. In addition, the nucleation stage plays a key role in the vertical alignment of GaN nanowires by creating homoepitaxial interfaces between the nanowires and the substrate. As a result, a slow growth rate at the nucleation stage is required for the homoepitaxial growth of nanowires. The CL analysis has further shown that the DBD-type nitrogen plasma employed in this study can supply sufficient active nitrogen species to react with Ga vapor forming high quality GaN nanowire crystallites.
We gratefully acknowledge the support for this work from the National Science Council of Taiwan under grant numbers NSC-99-2221-E-006-197-MY3 and NSC-100-2221-E-006-147, the Ministry of Economic Affairs, Taiwan, through Projects 101-D0204-2, the High-Tech Equipment Future Technology Development Plan under grant number 302202501, and the Aim for the Top University Project from NCKU.
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