J. Phys. Chem. C 2008, 112, 18821–18824
18821
Catalyst-Free Growth of Well Vertically Aligned GaN Needlelike Nanowire Array with Low-Field Electron Emission Properties Chaotong Lin, Guanghui Yu,* Xinzhong Wang, Mingxia Cao, Haifeng Lu, Hang Gong, Ming Qi, and Aizhen Li State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of microsystem and information technology, Chinese Academy of Sciences, Shanghai 200050, People’s Republic of China ReceiVed: September 10, 2008; ReVised Manuscript ReceiVed: October 7, 2008
An array of high-density, vertically aligned GaN nanowires is fabricated through thermal evaporation of GaN powder with the assistance of HCl gas. All GaN nanowires with needlelike tips are well-aligned with the axis direction perpendicular to the substrate without the use of catalysts. A possible growth mechanism of the vertical GaN nanowires array is proposed. Furthermore, field emission measurement shows that the obtained GaN nanowires array has a lower turn-on field of 2.1 V/µm, and the current density is about 1 mA/cm2 at a bias field of 4.5 V/µm, which means such GaN nanowires are good candidates for large area and uniform flat display applications. Introduction GaN, an important direct band gap semiconductor material, has been widely used in UV or blue emitters and hightemperature/high-power electronic devices. GaN is also a promising material for field emitters because of its low electron affinity as well as its excellent physical and chemical stabilities. Since one-dimensional semiconductor nanowires possess unique properties, such as a high surface-to-volume ratio and quantum confinement effect, in recent years, single-crystalline GaN nanowires have already shown their ability in the realization of nanoscale devices: blue light emitting diodes,1 short-wavelength ultraviolet nanolasers,2 gas sensors,3 field effect transistors,4 Schottky diodes,5 and field emitters.6,7 To date, several methods, such as carbon-nanotube-confined reactions,8 laser ablation,9,10 and catalytic chemical vapor deposition,11,12 have been reported to grow GaN nanowires, but the obtained nanowires are always disordered. Nevertheless, for commercial device application in the future, fabrication of well-ordered nanostructures with high density is very important because they can be effectually incorporated into devices.13,14 Thus, preparation of a GaN nanowire array with high alignment has been attracting considerable interest, and various approaches have been developed. Vertically aligned and faceted GaN nanorods have been produced by Parijat et al.15 using a catalyst-free templated approach that employs a silicon dioxide mask fabricated using a porous anodic alumina template. Quasi-aligned GaN nanowire arrays have been fabricated by B. Liu et al.16 via a thermal evaporation of the starting reactants Ga2O3/GaN. G. Wang et al.17 report the growth of well-aligned and vertically oriented GaN nanowires on (1-102) r-plane sapphire by metal organic chemical vapor deposition (MOCVD). Q. Li et al.18 reported a route to ultrahigh-density and highly aligned single-crystalline GaN nanowires on sapphire by employing ultrathin Ni catalyst films with submonolayer thicknesses. However, a catalyst or pattern has been used in the above-mentioned papers. The existence of the catalysts or the patterns induces the associated concerns of contamination caused by the catalyst metal or * Corresponding author.
decomposed products from the foreign patterns, which may be prohibitive for many device applications. In this letter, we have demonstrated a novel method to fabricate high-density, well-aligned GaN needlelike nanowire arrays over large areas without any catalyst or pattern. Uniformly single-crystal GaN nanowires have been prepared through a thermal evaporation of GaN powder with the assistance of HCl gas. The formed nanowires with needlelike tips show no lateral growth but are vertically aligned. It is expected that the sharp, needlelike tips enable such GaN nanowires to exhibit better electron emission properties due to their high aspect ratio and small tip radii of curvature, which could provide a sufficiently high geometric enhancement factor. Therefore, low-field electron emission from the GaN nanowires in this work is measured. Experiments The GaN nanowires were prepared in a homemade chemical vapor deposition system, which could be used for 3-in. wafer growth. There were two vertically placed quartz boats for loading the source and the substrate inside the chamber. The vertical distance between the source and the substrate is about 1 cm. A 3-µm-thick GaN film was deposited on a sapphire substrate by MOCVD. The full width at half-maximum values of high-resolution X-ray diffraction rocking curves for this GaN film were 231 arc sec (002 reflection) and 328 arc sec (102 reflection), respectively. Then such substrate was transferred into the growth chamber. For the growth of GaN nanowires, GaN powder was utilized as the source and 1000 sccm N2 was used as the carrier gas in the whole experimental process. When the temperature reached 780 °C, 50 sccm pure HCl gas was introduced. After one hour of growth, the furnace was cooled to room temperature, and a light white layer was found on the surface of the substrate. As a result, the straight and well aligned GaN nanowires with high density were uniformly grown in GaN template, as observed in Figure 1. Field-emission scanning electronic microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX) were employed to characterize the morphological features and composition of the resulting GaN materials. The crystal structures and growth
10.1021/jp808034m CCC: $40.75 2008 American Chemical Society Published on Web 11/06/2008
18822 J. Phys. Chem. C, Vol. 112, No. 48, 2008
Lin et al.
Figure 2. EDX analysis (a) and XRD pattern (b) recorded from GaN nanowires.
Figure 1. (a) Top view, (b) 45° side view and (c) cross-sectional view field-emission SEM images of the obtained GaN nanowire array on the thin GaN film template.
orientation of the GaN nanowires were investigated by X-ray diffraction (XRD). The further structural characterizations of GaN nanowires were performed using high-resolution transmission electron microscopy (TEM) and fast Fourier transform (FFT) pattern analysis. Field emission measurements for the GaN nanowires were conducted in a vacuum chamber at a pressure of about 8 × 10-8 Torr at room temperature. Results and Discussion Figure 1 exhibits the (a) top view, (b) 45° side view, and (c) cross-sectional view SEM images of the GaN nanowire array on a thin GaN film template. Figure 1a shows the GaN nanowires are in high density and uniform. As can be seen from Figure 1b and c, the GaN nanowires with needlelike tips show good alignment with the axis direction perpendicular to the substrate. Moreover, the GaN nanowires possess quite straight morphology and have a clean surface without any particles. The diameters of the nanowires are about 80-100 nm, and their lengths are mainly approximately 1.5 µm. Figure 2a shows the EDX spectrum of the specimen. It can be seen that the product consists of only Ga and N elements and that the atomic composition ratio of Ga/N is about 1:1, which means pure GaN material was obtained. Figure 2b shows the XRD pattern of as-synthesized GaN nanowire arrays. Two diffraction peaks from the (0002) and (0004) planes of the
Figure 3. (a) Low magnification TEM image of the needlelike GaN nanowire, and (b) high-resolution TEM image of a single-crystalline wurtzite structure GaN nanowire and its corresponding FFT pattern (inset).
wurtzite-type hexagonal GaN are observed, which indicates that the formed GaN nanowires are preferentially oriented in the c-axis direction. Figure 3 displays the TEM images of the GaN nanowires scratched from the substrate. The low-magnification TEM image for a single GaN nanowire is shown in Figure 3a. It can be
GaN Needlelike Nanowire Array
Figure 4. Side view (45°) field-emission SEM images of the GaN nanowires grown at different growth times (under otherwise identical growth conditions) of (a) 3, (b) 20, and (c) 60 min.
observed that the GaN nanowires with sharp needlelike tips have a smooth surface. The high-resolution TEM image near the tip of the nanowire confirms the single-crystal structure and the c-axis growth direction of the GaN nanowires, as shown in Figure 3b. The [0001] direction is parallel to the long axis of the nanowires, indicating that the [0001] direction is the growth direction of the GaN nanowires. The inset in Figure 3 b displays the corresponding FFT pattern of the high-resolution TEM image that indicates the GaN nanowires are preferentially oriented in the c-axis direction, as well, and are indexed to the reflections of the wurtzite structure, which is consistent with the XRD result. The diffuse circular rings in the FFT pattern are due to the presence of the underlying carbon membrane used to load the sample during the TEM measurement. To observe the evolution of the GaN nanowires growth, further experiments at different growth times but under other identical growth conditions were analyzed by SEM images. Figure 4a shows that GaN materials were first nucleated as nanodroplets with abrupt tips on the GaN template at the growth time of 3 min. Then the nanodroplets turn into nanocones when at a growth time of 20 min, as shown in Figure 4b. The vertical direction of the GaN nanocones, which is the preferred growth direction, develops faster than the lateral direction on the
J. Phys. Chem. C, Vol. 112, No. 48, 2008 18823 nanocone sides at the low growth temperature.19,20 Meanwhile, only GaN powder and no additional ammonia gas introduced into the experimental process makes for the growth conditions under a relatively low V/III ratio, which has been reported to result in the vertical growth of GaN.15,21,22 Therefore, the aligned vertical GaN nanowires are found to be formed after 60 min of growth, as shown in Figure 4C with the same specimen as that in Figure 1. It should be mentioned that the elimination of HCl gas or the GaN template in the experiments leads to the deposition of only a few GaN materials. Without the assistance of the HCl gas, the amorphous GaN powder source thermally evaporated at 780° might produce deficient precursors for the GaN nanowires’ growth. Furthermore, it is known that there is a large mismatch of lattice parameters existing between the GaN and sapphire substrate, and the surface of the bare sapphire substrate without any treatment is not conducive to GaN material nucleation. The GaN template grown by MOCVD at the early stage could serve as nucleation sites for subsequent homoepitaxial growth of GaN nanowires with good vertical alignment. Therefore, the HCl gas and GaN template are key roles for the growth of GaN nanowires in these experiments. On the basis of the above results, we propose a possible growth mechanism. During the process of thermal evaporation of the GaN powders under the atmosphere of the HCl gas, GaCl and NH3 are produced in a direct reaction between the amorphous GaN powders and the HCl gas. The GaCl reacts with NH3 to grow GaN nanowires on the GaN template at the low growth temperature. The field emission characteristics from the synthesized GaN needlelike nanowire array were studied. The field emission measurement for the GaN nanowires was performed in a vacuum chamber with a pressure of ∼8 × 10-8 Torr. The measurements were made by adjusting the voltage from 1 to 1300 V at a fixed distance of 270 µm between the phosphor screen (anode) and the sample (cathode). The dependence of field emission current density on the applied electric field is shown in Figure 5a. The turn-on field, which is defined as the field required to detect an emission current density of 1 µA/cm2, is evaluated to be about 2.1 V/µm. The emission current density of the GaN nanowires array reaches 1 mA/cm2 at a bias electric field of 4.5 V/µm. The turn-on field and threshold field are much lower than the previously reported values of GaN nanowires without needlelike tips.6,7,12 It is believed that the excellent electron emission property is mainly attributed to the geometrical configuration of the sharp needlelike tip, which can provide sufficiently high geometrical enhancement factors. The corresponding FowlerNordheim (F-N) plots are shown in Figure 5b. The variation of ln(I/E2), with 1/E being a nearly straight line, indicates that the field emission process from the GaN nanowires is a barrier tunneling, quantum mechanical process (F-N tunneling).23,24 According to the F-N electron emission theory, emission current can be expressed as
J ) A(βE)2 ⁄ (Φ) exp(-BΦ3⁄2 ⁄ βE) where A ) 4.43 × 10-22 (AV-2 eV), B ) 6.83 × 109 (VeV-3/2 V m-1), β is a field enhancement factor dependent on emitter geometry, and Φ is the work function (GaN ) 4.1 eV). From the slope of the F-N plots, β was estimated to be ∼2835. Compared with some other GaN nanostrures,25-27 the vertically aligned GaN needlelike nanowires show a relatively larger β value. Conclusions We have fabricated a high-density, vertically aligned GaN nanowire array through thermal evaporation of GaN powder
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Lin et al. Acknowledgment. The authors thank Tao Feng and Lifeng Lin from East China Normal University for assistance in field emission measurement. This work was supported by the Natural Science Foundation Project (05ZR14139) and International Cooperation Project (055207043) of the Shanghai Government and National Science Foundation Project (60676060). References and Notes
Figure 5. (a) Electron field emission current density versus applied electric field curve for the GaN needlelike nanowires and (b) the corresponding Fowler-Nordheim plot.
with the assistance of HCl gas. It is noted that this method is free of foreign catalysts and pattern. Field-emission SEM images show that the GaN nanowires with needlelike tips are wellaligned with the axis direction perpendicular to the substrate. The XRD pattern and high-resolution TEM image indicate that the obtained GaN nanowires have high single-crystal quality with preferential growth along the [0001] direction. The possible growth mechanism of the vertical GaN nanowires array is discussed. Field emission measurement shows that the turn-on field of GaN nanowires is 2.1 V/µm and the current density is about 1 mA/cm2 at a bias field of 4.5 V/µm, which indicates such GaN nanowires are good candidates for large area and uniform flat display applications. It is also believed that the good alignment allows the GaN nanowire arrays to be extensively applied in future nanodevice design and integration.
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