J. Phys. Chem. B 2005, 109, 151-154
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Fabrication and Optical Property of Silicon Oxide Layer Coated Semiconductor Gallium Nitride Nanowires Jun Zhang,* Lide Zhang, Feihong Jiang, Yongdong Yang, and Jianping Li Department of Physics, Yantai UniVersity, Yantai, 264005, Peoples Republic of China ReceiVed: July 2, 2004; In Final Form: October 20, 2004
Quasi one-dimensional GaN-SiO2 nanostructures, with a silicon oxide layer coated on semiconductor GaN nanowires, were successfully synthesized through as-synthesized SiO2 nanoparticles-assisted reaction. The experimental results indicate that the nanostructure consists of single-crystalline wurtzite GaN nanowire core, an amorphous SiO2 outer shell separated in the radial direction. These quasi one-dimensional nanowires have the diameters of a few tens of nanometers and lengths up to several hundreds of micrometers. The photoluminescence spectrum of the GaN-SiO2 nanostructures consists of one broad blue-light emission peak at 480 nm and another weak UV emission peak at 345 nm. The novel method, which may results in high yield and high reproducibility, is demonstrated to be a unique technique for producing nanostructures with controlled morphology.
1. Introduction The nanocable has the one-dimensional (1-D) features of both nanowire and nanotube in the axial direction, and it could be an ideal system for building semiconductor-insulator-semiconductor or semiconductor-insulator-metal heterojunction in the radial direction and has great potential applications for optoelectronic nanodevices and electronic transportation.1 Over the last several years, there have been considerable efforts to explore new routes to nanocable, progress has also been made in fabricating solid nanowires by filling the hollow cavity of the nanotubes with elements or compounds2,3 and nanorods through a carbon nanotube-confined reaction.4,5 Because of a wide direct band gap (3.4 eV, at room temperature) semiconductor, gallium nitride (GaN) material has promising applications for UV or blue emitters, detectors, high-speed field-effect transistors, and high-temperature micro-electronic devices.6 A number of research groups have developed various methods for synthesizing 1-D GaN nanostructures (nanowires or nanorods).7 In addition, GaN-based nitride can be both p- and n-type doped, has a direct band gap, and forms heterostructures for device applications. There has recently been a great deal of interest in the preparation and characterization of nanotube-based nanocomposites to obtain unique physical and or mechanical properties. Han et al. reported an efficient route to graphitic carbon-layer-coated gallium nitride nanorods.8 Coaxial nanocables have been demonstrated the great potential in nanodevice applications, such as coaxial-gated transistor and laser diodes. However, the challenge of synthetically controlling the shape of nanocables has been met with limited success. An important issue of current research in nanocables is how to rationally control the size, shape, and geometrical arrangement of hetero-nanostructures in order to produce specific properties toward desired function in nanodevices. Here, we report a novel nanoparticle-assisted route to the large-scale synthesis of 1-D GaN-SiO2 nanocables. This method, with a silicon oxide layer coated on 1-D GaN nanowires in the radial * To whom correspondence should be addressed. E-mail: jzhang@ ytu.edu.cn.
direction, has a very high yield and high reproducibility. To our knowledge, using the oxide nanoparticles assisted method to synthesize 1-D GaN-SiO2 nanocables and unique luminescence property has not been reported to date. 2. Experimental Section In the experiment, quasi 1-D GaN-SiO2 nanocables were synthesized using a conventional thermal chemical vapor. Ga2O vapor was reacted with ammonia gas to form GaN nanowire cores and silicon oxide was coated on the GaN nanowires. The method could be exploited to prepare nitride coaxial nanocables: nitride nanowire cores sheathed with another material nanotubes. The reaction can be expressed as
Ga2O3(s) + 4Ga (s) f 3Ga2O(g) Ga2O(g) + 2NH3 f 2GaN(nanowires) + H2O + 2H2 (1) The reaction was carried out in a conventional furnace with a horizontal quartz tube. A mixed powder of metal Ga, Ga2O3 powder, and SiO2 nanopowders was placed in an alumina crucible. The mixture of Ga and Ga2O3 powder (4:1 molar) was used as the starting material to produce Ga2O. The crucible was placed in the hot zone inside the quartz tube and was held in a flowing ammonia atmosphere (400 standard cubic centimeters per minute) at 950-970 °C for 1 h. Then the quartz tube was cooled to room temperature. After recovering the crucible from the furnace, we found a layer of light-yellow wool-like products on the surface and walls of the crucible. The morphology of the wool-like products was characterized by scanning electron microscopy (SEM) (JEOL JSM-5610LV). The X-ray powder diffraction (XRD) pattern was recorded on a MXP18AHF (MAC Science Co. Ltd.) X-ray diffractometer with Cu KR radiation (λ ) 1.54178 Å). Raman spectra were obtained by illuminating the sample with the 514.5 nm line of an Ar+ laser and 1 mW output power of a laser Raman scattering spectrometer (LABRAM-HR, JY, France). Spectra of the 1-D GaN-SiO2 nanocables are in the frequency range between 100 and 1000 cm-1, and the spectral resolution is 1.0 cm-1. The
10.1021/jp0470795 CCC: $30.25 © 2005 American Chemical Society Published on Web 12/04/2004
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Figure 2. Room-temperature Raman spectra of 1-D GaN-SiO2 nanocables (A) and 1-D GaN nanowires (B), respectively.
Figure 1. (a) Typical SEM image of the as-prepared products. (b) XRD pattern of the as-prepared products.
structural and elemental analyses of an individual GaN-SiO2 nanocable were performed using HRTEM (JEOL JEM-2010, 200 kV) and energy-dispersive X-ray fluorescence spectroscopy (EDS) (EDAX, DX-4) attached to the HRTEM. Photoluminescence (PL) spectrum of the 1-D GaN-SiO2 nanocables was measured in a Hitachi 850 fluorescence spectrophotometer with a Xe lamp at room temperature. The excitation wavelength was 254 nm, and the filter wavelength was 310 nm. 3. Results and Discusses A typical SEM image shown in Figure 1a indicates that the material resulting from the reaction of the mixture Ga2O vapor and SiO2 nanopowders with ammonia produced a high yield of nanometer wire-like structures. The nanowires are straight and smooth. The diameters of the nanowires are 10-30 nm, and the lengths are up to several hundreds micrometers. In the XRD pattern in Figure 1b, GaN has crystallized in the hexagonal structure and the lattice parameters have been calculated as a0 ) 3.186 Å and c0 ) 5.174 Å, which are in very good agreement with the reported values.7 The strong intensities of the GaN diffraction peaks indicated that the resulting product had high purity of the GaN hexagonal wurtzite phase. In addition, the result from the lower angle of the XRD spectrum indicates that there is SiO2 remainder (SiO2 nanopowders or SiO2 outer layer) in the product. Figure 2A shows the Raman spectrum of the 1-D GaN-SiO2 nanocables. Three features of GaN are clearly evident as shown in the figure. The observed Raman-active phonon band 125 cm-1 has E2 (low) symmetry. The position of the Raman mode shifts by a large amount to lower frequency (by a few tens cm-1) as compared with the previous data of the single crystals.9 The strong E2 (high) phonon line in the Raman spectra which reflects the characteristics of wurtzite phase of GaN nanowires has been screened in the GaN-SiO2 nanocable structures. Two additional
Raman modes are also observed at 229 and 416 cm-1, all of which are not allowed by the C6V4 space group in the first-order Raman-scattering at the zone center. The position of the Raman mode (229 cm-1, Zone-boundary phonon) shifts by a large amount to lower frequency (a few tens cm-1) and the mode 416 cm-1 shifts also very slightly to lower frequency (a few cm-1) as compared with those previous data of GaN nanowires (shown in Figure 2B) and the single crystals.9 These results indicate that the structural properties of the GaN-SiO2 nanocable structures have difference from the GaN nanowires. Yu et al.10 reported that the Raman spectrum of amorphous silica nanowires was the same as that of bulk noncrystalline SiO2 solids, whereas some other approaches showed that there was no scattering peak in the stoichiometric SiO2 nanowires.11 Moreover, it has been reported that there is a peak at 480 cm for bulk amorphous silicon12 and a peak at 520 cm for crystalline silicon.13 Wang et al. also14 reported that the Raman scattering spectrum on a bulk quantity of SiO2 nanowires had a sharp peak around 505 cm. However, in our experiment, there is no any scattering peak of SiO2 coating layers or remainder. Our investigations verified that there was no trace of a crystalline phase in SiO2 coating layers or remainder. A typical TEM image shown in Figure 3a indicates that an individual GaN nancable has uniform diameter. The nanocable has high aspect ratios with length of up to several hundreds of micrometers and diameter of several tens of nanometers. A magnified TEM image in Figure 3b shows that there is a bundled mass of nanocables, and the GaN cores were sheathed with a layer SiO2. Two typical TEM images in Figure 3, parts c and d, show that the nanocables were smooth and straight, and the average diameter of the nanowires were 20 nm. The images show that the wire has a crystalline core and a surrounding amorphous layer. Selected area electron diffraction patterns [inset in Figure 3, parts c and d], taken from a single nanowire, are consistent with the single crystalline nature of the sample. The patterns can be indexed to the reflection of wurtzite GaN [100] [in Figure 3c] and GaN [001] [in Figure 3d]. In a highresolution image shown in Figure 3e, we can see an additional silicon oxide sheath outside the amorphous layer. The magnified lattice fringes from the sheath [bottom left inset of Figure 3e] indicate that the interlayer distance is ∼0.352 nm. From the crystalline GaN core [top right inset of Figure 3e], the [001] direction is indicated by an arrow. The space of about 0.261 nm between arrowheads corresponds to the distance between two (002) planes. We can see the typical projection of a wurtzite structure with (002) plane spacing of 0.261 nm. An elemental analysis shown in Figure 3f of the individual GaN-SiO2
Gallium Nitride Nanowires
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Figure 3. (a) Typical TEM image of an individual GaN-SiO2 nanowire with uniform diameter. (b) A bundled mass of GaN-SiO2 nanowires. (c-d). The magnified TEM images show a crystalline core and an amorphous layer. The selected area electron diffraction (SAED) pattern (inset image in 3c) indicates that the crystalline core is wurtzite phase GaN with the [100] diffraction pattern. The inset image in 3d shows that the crystalline core is the [001] diffraction pattern. (e) A HRTEM image of the 1-D GaN-SiO2 nanocable. The magnified lattice fringes of the outer SiO2 layers (left-bottom inset). A magnified lattice image of the wurtzite phase single-crystalline GaN nanowire core (top-right inset). 3(f) EDS spectrum of the individual GaN-SiO2 nanocable.
nanocable was obtained by EDS. The result indicates that the nanocable consisted of Ga, N, Si, and O, the atomic ratio of Ga and N is near unity, which suggests that Ga and N form the GaN. The Si:O ratio is very near 1:2 at the amorphous region, which indicates that the sheathed layer is amorphous SiO2. Photoluminescence (PL) spectrum of the 1-D GaN-SiO2 nanostructures was measured in a Hitachi 850-fluorescence spectrophotometer with a Xe lamp at room temperature. The excitation wavelength was 254 nm, and the filter wavelength was 310 nm. The PL spectrum of the 1-D GaN-SiO2 nanostructure was shown in Figure 4a. The PL spectrum consists of one broad blue-light emission peak at 480 nm and another weak UV emission peak at 345 nm, which is similar to the PL spectra of GaN nanowires reported previously.15 In Chen’s report,15 they observed the existence of a strong and broad band centered at 342 nm and another strong and broad peak centered a 470 nm. The full-width at half-maximum (fwhm) of the blue-light emission band is about 160 nm. The blue shift of the band gap emission compared with the 365 nm band of bulk GaN16 might
be ascribed to the quantum confinement effect since a considerable fraction of the nanowires have diameters less than the Bohr exciton radius for GaN 11 nm.17 Another broad peak, centered at 480 nm, might be ascribed to the existence of defects or surface states. Previously, Wang et al.11 reported that the fully oxidized SiO2 nanowires have a weak PL peak at about 600 nm. Yu et al.10 showed two broad PL peaks of SiO2 at 470 and 420 nm. Wang et al.14 also reported that the SiO2 nanowires have a PL peak at about 446 nm. In addition, Zhu et al.18 reported that the two broad PL peaks of SiOx were at around 430 and 570 nm. To further make clear that the peak is not contributed from SiO2 nanopowders or SiO2 coating layers, a comparison experiment of PL spectrum from the as-prepared SiO2 nanopowders is shown in Figure 4b. In this image, there is no any PL peak in SiO2 nanopowders. Therefore, this peak position (480 nm) is not contributed from the remainder of SiO2 nanopowders. The importance of the 1-D GaN-SiO2 nanostructures is not only the superior structure itself but also the control of the
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Zhang et al. The thermomechanical, optical, and electronic properties of the nanostructures can be revealed from combining different nanowires and coating layers. Therefore, the successful preparation of these nitride nano-structures indicates that the method described in this report has high yield and high reproducibility and could be used to fabricate other 1-D nanostructures, which offer opportunities for nanotechnological applications. Acknowledgment. This work was supported by the National Natural Science Foundation of China (Grant No. 60277023). References and Notes
Figure 4. Photoluminescence spectra of 1-D GaN-SiO2 nanostructures (a) and the as-prepared SiO2 nanopowders (b).
structure formation. In previous studies, without catalysts and carbon nanotubes, the products were GaN powders from the reaction of Ga2O and ammonia.19 Using catalysts or carbon nanotubes, the products from the reaction of Ga2O and ammonia were GaN nanowires or nanorods.7 In our experiment, Ga2O vapor was generated through a reaction Ga2O3 (s) + 4Ga (l) ) 3Ga2O (v) and N radicals decomposes from the NH3 at high temperature. The crystalline GaN nanowires grow through the reaction between Ga2O and N ions. Because SiO2 nanoparticles (2∼5 nm) are the effect of the oxygen deficiencies and are very active at high temperature, GaN nanowires are sheathed with a layer of silicon oxide. The experimental results provide further support for the speculation that the GaN-based material nanocables formed through reaction 1. GaN is a direct band gap semiconductor with a wide band gap of 3.4 eV (364.7 nm) at room temperature, and amorphous silicon oxide films are widely used as passivation or insulation layers in integrated circuits. The GaN-SiO2 nanocable, where a shell of an insulator is overgrown along the semiconductor nanowire, might provide further control over the thermomechanical and electronic properties of nanowire. The silicon oxide layers can act as chemically inert protecting layers for GaN nanowires. We also expect other beneficial effects of silica layers, such as enhanced stability, for example, of such nanowires against photobleaching, when used as solid-state injection nanolasers. 4. Conclusions SiO2 coatings on GaN nanowires have been obtained by heating SiO2 nanopowders and Ga2O vapor under NH3 atmosphere at high temperature. Using as-prepared nanoparticles, self-organization of nanocables with different components is also possible by changing chemicals in the starting materials.
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