Large-scale Synthesis and Optical Properties of Hexagonal GaN Micropyramid/Nanowire HomostructuresXu Xiang, Chuanbao Cao∗, Yajie Xu, Hesun ZhuResearch Center of Materials Science, Beijing Institute of Technology,Beijing 100081, P.R. ChinaThe free-standing, hexagonal GaN micropyramid/nanowire homostructures have been successfully synthesized in a large scale by a facile carbon-assisted thermal reduction route. The product was investigated using SEM, XRD, TEM, EDS, SAED, Raman and PL. The pyramids have lengths of 5~8 µm, and the maximal lateral size of 2~3 µm, exhibiting the growth characteristic of hexagonal crystal along c-axis. A single nanowire is grown out from the end tip of the pyramid along <0001> growth direction, indicating its epitaxial relationship with the pyramid. Raman studies reveal the characteristics of the hexagonal structure and high-crystallinity of the product. The room temperature PL spectrum displays strong band-edge emission centered at 372nm. The slight redshift originates from N vacancies caused by Ga-rich growth conditions. The formation mechanism was also discussed. The unique micro/nano structures are good entities for the investigation of fundamental physical phenomenon and also as building blocks for functional nanodevices.Key Words: GaN, Micropyramid, Nanowire∗Corresponding author. Tel: +86-10-68913792; Fax: +86-10-68912001.E-mail address: cbcao@1. IntroductionDuring the past few years, increasingly more efforts have been devoted to the fascinating and exciting field of nanomaterials, especially quasi-one-dimensional nanostructures such as nanowires (or nanorods) and nanotubes, because of not only their importance in understanding the dependence of properties on size and dimensionality of materials, but also the promising applications both as interconnects and as functional building blocks for novel electrical, optical and magnetic nanodevices [1-4].Gallium nitride (GaN), an important Ⅲ-Ⅴsemiconductor with a direct band gap of 3.4eV, has stimulated the growing interest owing to its superiority in thermal conductivity, electronic transport, and light-emitting. GaN-based materials are expected to have great prospects in high-power/high-temperature electronic devices, high-speed field-effect transistors, light-emitting diodes and laser diodes [5-7].Much attention has been paid to synthesize one-dimensional GaN nanowires since the first synthesis accomplished by Fan et al. via a carbon-nanotube-confined reaction. Those synthetic techniques include template-confined method [8,9], laser ablation [10,11], arc discharge [12], catalytic chemical vapor deposition (CVD) [13-18], and oxide-assisted growth [19]. A few efforts were also made for several new low-dimensional nanostructures such as GaN nanobelts [20,21] and nanotubes [22-24]. In contrast, the studies on other nanostructures such as prisms and pyramids etc. are quite rare. Although hexagonal GaN pyramids have been epitaxially grown on the patterned substrates by MOVPE technique [25,26], the selective lateral overgrowthprocess was rather technologically complicated.It remains great challenge to grow pyramids or similar structures in large scale by a simple and low-cost method. In this paper, we successfully synthesized a novel GaN micropyramid/nanowire homostructure in large scale via a facile carbon-assisted thermal reduction route for the first time. The optical investigations reveal excellent band-gap photoemission performance, suggesting the potential application in luminescent devices. Also, the GaN homostructures have predictable applications as either probes or electron-impact devices.2. ExperimentalFirstly, a cleaned silicon (100) wafer was coated with a layer Au thin film of 3nm by sputtering for the substrate. About 0.35g gallium oxide powders were grounded and then loaded into a graphite tube reactor. The Au-coated Si substrate was positioned inside the tube with a separation of 5~8mm from source material. The graphite tube was transferred into a ceramics tube in a horizontal tube furnace. The ceramics tube was sealed and pumped to remove residue air, and then was filled with NH3 gas (purity>99.9%). Subsequently, the furnace was heated from room temperature to setting temperature (1120℃) with an increasing rate of 10℃/min under a constant flow of NH3 of 100 ml/min. The typical reaction period was held for 120min. After reaction, the furnace was naturally cooled down to room temperature. A light yellow deposit appeared on the substrate surface.The structure and purity of the product was determined by X-ray powder diffraction (XRD, Philips X’Pert Pro MPD) with Cu Kα radiation (λ＝0.15418 nm), The morphology and composition were analyzed by scanning electron microscopy (FESEM, Hitachi S-4500) and energy dispersive X-ray spectroscopy (EDS), The microstructure was characterized with transmission electron microscopy (TEM, Hitachi H-8100, accelerating voltage of 200kV). The Raman scattering measurements were performed by Raman spectroscopy (Renishaw, RM2000) with a confocal microscopy at room temperature. A 100x microscope objective lens was used for the focus of the laser beam and the collection of the scattered light. The 514.5nm line ofan Ar+ laser was used as an excitation source and the spot diameter of the focused laser beam on the sample is about 1 µm. The room temperature PL spectra were excited using a 325nm line of He-Cd laser.3. Results and discussionFig. 1 displays typical SEM images of the product. The overall morphology is shown in Fig. 1(a) and (b), exhibiting large quantity of pyramid-shape and wire-like structures grown on the substrate surface. The axial length of the pyramids is 5~8 µm, and the maximal lateral size is 2~3 µm. The pyramids show symmetric shape, clear side facets and a pyramidal end, revealing the characteristics of hexagonal crystal growing along c-axis. The nanowires have the diameters of several tens of nanometers, and lengths of several micrometers. Fig. 1(c) exhibits the growing status of two pyramids, where one grows across the middle of another. The end of an individual pyramid is shown in Fig. 1(d), which clearly displays an interesting phenomenon, i.e.a single nanowire growing out from the end tip of pyramid. Similar structures were also observed from other pyramids, indicating that the product was a unique GaN micropyramid/nanowire homostructure. The EDS measurements indicate that the product consists of the elements nitrogen and gallium, and the atomic ratio of N and Ga is 52.66: 47.34 (i.e. 1.1), near to the chemical stoichiometry of GaN.Fig. 2 shows a typical XRD pattern of the light yellow product grown on the substrate. The reflection peak at 2θ = 61.67°corresponds to the Si(400)K β reflection from single-crystalline silicon substrate. And other diffraction peaks in the pattern can easily be indexed to hexagonal GaN with wurtzite structure. Miller indices of hexagonal GaN were marked in this pattern. The calculated lattice parameters are a=3.189 and c=5.184, which agree well with those of bulk GaN (JCPDS Card No.76-0703). The sharp and narrow peaks reveal the product is highly crystalline. The XRD results suggest that the product is pure, single-phase hexagonal GaN.o A o A The TEM micrographs of a single GaN pyramid are shown in Fig. 3(a) and (b), revealing a typical symmetric pyramid-shape appearance. The pyramid appears no transparent morphology because of the thickness at micrometer scale. Noticeably, the lateral maximal size is not at the bottom of pyramid, but at the position from the bottom about 1/3 of the total length. Fig. 3(c) shows a GaN micro/nano homostructure, where a single GaN nanowire with diameter of 40~50nm grows out from the end of the pyramid. The inset is the corresponding SAED pattern, clearly displaying regular reflection spots. The electron beam is parallel to [100] zone axis, revealing the nanowire grows along <001> direction. The electron diffraction taken from differentplace along the axis of nanowire exhibits the identical pattern, indicating the single-crystalline nature of nanowire. According to SAED analysis, the nanowire is epitaxially grown along <0001>, which is identical with the axial direction of pyramid.Raman scattering of GaN has been intensively investigated by many a group. It is well known that hexagonal GaN bulk crystal (or GaN epilayer) has six first-order Raman active phonons, at 533, 735, 559, 743, 569 and 145cm-1, corresponding to A1(TO), A1(LO), E1(TO), E1(LO), E2(high), E2(low), respectively [27]. Here the Raman scattering spectrum of GaN pyramid/nanowire homostructures is shown in Fig.4. The shifts at 567, 531, 420, 143, and 669cm-1, can be identified. The peaks at 567, 531 and 143cm-1 can be assigned to E2(high), A1(TO), and E2(low) mode, respectively. The symmetric and strong E2(high) phonon line at 567 cm-1 reflects the characteristics of the hexagonal phase and high-crystallinity of GaN. Two distinguished features are also observed in the spectrum. Firstly, a weak phonon peak appears at 420cm-1, which is not allowed by P63mc space group in first-order Raman scattering at the zone center. Recent studies [28] by Liu et al. suggested this peak could be assigned to zone-boundary phonon activated by finite-size effects and surface disorder, or acoustic overtone of GaN. Another band at 669cm-1 displays a broad and asymmetric feature. Chen et al. regarded it as defect-induced phonon in their previous work [13], however, we consider that it is likely associated with the surface state levels due to the unique structure of GaN.The photoluminescence (PL) spectrum obtained by a He-Cd laser excitation atroom temperature is shown in Fig. 5. An intensive and symmetric UV emission at 372nm (3.33eV) is observed, and the yellow luminescence band [29] centered at 2.2~2.3eV is negligible in this spectrum. The UV emission band can be attributed to band-edge transition of hexagonal GaN [30]. The strong band-edge emission reveals highly crystallographic quality of the product. The redshift of band-edge emission is about 60meV, compared with the bulk GaN band-gap of 3.39eV [31]. A plausible explanation for the redshift can be C impurity considering the carbon-thermal growth conditions. However, the ionization energy of carbon was estimated to be 200meV regardless carbon atom occupies a Ga site or a N site [32], which is much higher than 60meV. Therefore, C impurity is not a primary reason for the redshift. It has been predicted that the energy of N vacancies lies about several tens of meV below the conduction band of GaN [33], which agrees well with the present redshift value. In addition, N vacancies are easily produced because N atoms are not enough to bond with abundant Ga atoms under the Ga-rich conditions. It is most likely that the redshift originates from N vacancies caused by Ga-abundant growth environment.In our present work, GaN pyramid/nanowire homostructures are grown on Au-coated silicon substrate in a space-confined reactor under carbon-thermal conditions, where graphite acts as carbon source. The following reactions could occur.Ga2O3(s) + 2C(graphite) Ga2O(g) + 2CO(g) (1) Ga2O(g) + 2NH3(g) 2GaN(s) + H2O(g) + 2H2(g) (2) On one hand, at high temperature, Ga2O3 source is reduced by graphite to produce gaseous Ga2O molecules. On the other hand, Au film is broken to form liquid Au clusters, uniformly dispersing on the substrate surface. Au clusters, as energy favorable sites, adsorb and capture Ga2O molecules in the gas phase, which act as “adsorbers”. Ga2O and NH3 molecules bond to produce GaN nuclei, and with continuous dissolve of gas species to nuclei, GaN begins to grow when the condition reaches supersaturation. As to the faceted pyramids, we believe it is strongly associated with both the intrinsic structure of material and the growing parameters. It is well-known GaN belongs to hexagonal crystal, and bulk GaN commonly presents hexagonal platelet or column shape, where the growth is along c-axis and is terminated by (0001) or (000-1) planes. In the present growing conditions, GaN grows along c-axis, enclosed by six smooth side facets, and finally forms pyramidal shape. Au clusters not only play the role of “adsorbers” but also influence the growth rate of different crystallographic planes. As for the growth of nanowires, we attribute it to the secondary nuclei effect, resulting from the imperfect of pyramid tapered end. The unique structure is favorable to the secondary nuclei. The formation of nanowires could be attributed to Vapor-Solid Epitaxy (VSE) mode with regard to the epitaxial growth relationship. It should be pointed out that we haven’t observed the existence of Au particles on pyramids or nanowires, and no trace Au was detected by XRD and EDS within the detection limit. Therefore, the further studies are necessary to understand the effect of Au in the growth process and to provide the experimental evidence for details.4. ConclusionsWe have successfully synthesized a novel GaN micropyramid/nanowirehomostructure via a carbon-assisted thermal reduction route. The results indicate that the product is highly crystalline hexagonal GaN. The pyramids have the lengths of 5~8 µm and the lateral maximal widths of 2~3 µm, which show hexagonal pyramidal shape and are enclosed by six smooth side facets. The nanowire, with the growth direction parallel to <0001>, is grown out from the top end of each pyramid. The growth process could correspond to Vapor-Solid Epitaxy mode. The Au plays an important role for the formation of the unique micro/nano structures. The PL spectrum shows intensive and narrow band-gap emission peaked at 372nm, revealing the excellent luminescent properties. The slight redshift is attributed to N vacancies caused by Ga-rich growth conditions.AcknowledgementsThis work was supported by the Research Fund for the Doctoral Program of Higher Education of China (Grant No. 20020007029). One of the authors (X. Xiang) also thanks the support from Yumiao Foundation at Beijing Institute of Technology.References[1] Holmes J D, Johnston K P, Doty R C and Korgel B A 2000 Science287 1471[2] Cui Y, Wei Q Q, Park H K and Lieber C M 2001 Science293 1289[3] Huang M H, Mao S, Feick H, Yan H, Wu Y, Kind H, Weber E, Russo R and YangP D 2001 Science292 1897[4] Kong X Y, Ding Y, Yang R and Wang Z L 2004 Science303 1348[5] Zhong Z, Qian F, Wang D and Lieber C M 2003 Nano. Lett.3 343[6] Han S, Jin W, Zhang D, Tang T, Li C, Liu X, Liu Z, Lei B and Zhou C 2004Chem. Phys. Lett. 389 176[7] Qian F, Li Y, Gradec¡¦ak S, Wang D, Barrelet C J and Lieber C M 2004 Nano.Lett. 4 1975[8] Han W, Fan S, Li Q and Hu Y 1997 Science277 1287[9] Cheng G S, Zhang L D, Zhu Y, Fei G T, Li L, Mo C M and Mao Y Q 1999 Appl.Phys. Lett.75 2455[10] Duan X and Lieber C M 2000 J. Am. Chem. Soc. 122 188[11] Shi W, Zheng Y, Wang N, Lee C and Lee S 2001 Adv. 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The typical SEM images of the product, an overall view (a) and (b), one pyramid crossing another(c), a close view of the end of a single homostructure at high-magnification (d).Figure 2. The XRD pattern of the GaN homostructures, showing sharp and narrow reflections.Figure 3. TEM images of individual GaN pyramid (a) and (b), and a typical homostructure end (c), the inset presents the SAED pattern of the nanowire grown from the pyramid end.Figure 4. Raman scattering spectrum of GaN micro/nano homostructures. The excitation wavelength is 514.5nm from an argon ion laser.Figure 5. The room temperature PL spectrum of GaN homostructures. The excitation wavelength is 325nm from a He-Cd laser.Figure 1.Figure 2.Figure 3.Figure 4.Figure 5.。
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