Surface & Coatings Technology 203 (2008) 666–669
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Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / s u r f c o a t
GaN-core/SiOx-sheath nanowires Hyoun Woo Kim ⁎, Jong Woo Lee, Hyo Sung Kim, Mesfin Abayneh Kebede School of Materials Science and Engineering, Inha University, Incheon 402-751, Republic of Korea
A R T I C L E
I N F O
Available online 18 April 2008 Keywords: GaN Nanowires SiOx Sputtering
A B S T R A C T We have fabricated GaN-core/SiOx-sheath nanowires, sheathing the core MgO nanowires by sputtering with Si target. The product has wire-like morphology, regardless of SiOx-sheathing and subsequent annealing. EDX elemental mapping results have coincided with what can be expected for the SiOx-coated GaN nanowires. The core nanowires correspond to a hexagonal GaN structure, whereas the sheath layer is amorphous. Gaussian fitting analysis on the photoluminescence spectra of GaN-core/SiOx-sheath nanowires have exhibited two emission bands peaked at 2.4 eV and 2.9 eV, respectively. We observed that the relative intensity of 2.9 eV-peak to 2.4 eV-peak was increased by the thermal annealing. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Since one-dimensional (1D) nanometer-scale structures have contributed to the understanding of basic concepts and potential technological applications [1–4], they have attracted considerable interest. In particular, since useful functions could be tailored and further enhanced by fabricating core and sheath from different materials, the coaxial 1D nanostructures as another type of 1D nanostructures have also begun to be intensively studied [5]. In this case, different types of nanowires (as cores) and nanotubes (as sheaths) with different chemical compositions are assembled in the radial direction [6]. Since hexagonal gallium nitride (GaN) with a large bandgap has high melting point, saturation drift velocity, breakdown field, and chemical inertness, it is an ideal material for the fabrication of blue and ultraviolet light-emitting diodes and laser diodes, high temperature and high power optoelectronic devices [7,8]. In particular, the synthesis of GaN nanowires is a promising approach for future nanodevices [9–11]. In order to prevent the strong noise in nanocircuits and to improve the environmental stability of the devices, it is essential to form insulating sheaths on the exterior of nanowires [12,13]. They protect the surface from adsorption of unwanted species and surface oxidation or contamination, stop unnecessary charge injection, and partially screen the external fields [13]. With high voltage breakdown potential and dielectric constant, considerable mechanical strength, perfect insulating performance, and exceptional resilience to environmental factors, SiOx is one of the most suitable sheath materials [13–15]. Furthermore, silicon oxide processes are compatible with modern Si
⁎ Corresponding author. E-mail address:
[email protected] (H.W. Kim). 0257-8972/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2008.04.062
integrated circuit (IC) technology, and will also be acceptable to the future generation of devices [16]. In the present fabrication scheme comprising two separate processes, we have prepared GaN nanowires by heating GaN powders and subsequently sheathing the core nanowires by the sputtering technique. Since the core-sheath nanowires may undergo a high-temperature process during the subsequent fabrication, we have investigated the effects of thermal annealing on the structural and photoluminescence (PL) characteristics. 2. Experimental GaN nanowires were fabricated by heating pure GaN powders and collecting the product using gold (Au: about 3 nm)-coated Si substrates, inside a quartz tube in a horizontal tube furnace. With the constant flow of NH3 (flow rate: 20 standard cm3/min (sccm)) and Ar (flow rate: 100 sccm), the substrate temperature was set to 1050 °C for 1 h. Being similar to our previous work [17], coating experiments on the as-prepared GaN nanowires were carried out at room temperature using a DC turbo sputter coater (Emitech K575X, Emitech Ltd., Ashford, Kent, UK). A piece of p-type (100) Si wafer has been used as the sputter target. After the vacuum chamber was evacuated to a base pressure of 2 × 10− 4 Pa, the sputtering was performed at a pressure of 8 × 10− 4 Pa [17]. During the sputter process with DC power and current of 33 W and 130 mA, respectively, Ar gas was flowed for 1 min. Subsequently, thermal annealing was carried out in a vertical quartz tube furnace at a temperature of 800 °C for 1 h. Mostly, thermal process in IC fabrication is conventionally carried out at or below 800 °C. The N2 ambient gas was flowed with a flow rate of 500 sccm. The product was examined by X-ray diffraction (XRD) (Philips X'pert MRD diffractometer with CuKα1 radiation), field emission
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3. Results and discussion Fig. 1a shows a SEM image of uncoated GaN nanowires, whereas Fig. 1b and c show SEM images of as-fabricated and 800 °C-annealed GaN-core/SiOx-sheath nanowires, respectively. Being similar to the uncoated products, the coated products consist of 1D structures, whether the thermal annealing has been carried out or not. Based on enlarged SEM images shown in insets, we surmise that the surface roughness of SiOx-sheathed nanowires is mainly attributed to that of core GaN nanowires. Fig. 2 shows XRD spectra of uncoated GaN nanowires, as-fabricated and 800 °C-annealed GaN-core/SiOx-sheath nanowires. The diffraction peaks correspond to (100), (002), (101), (102), (110), (103), (112), and (201) reflections of a hexagonal wurtzite GaN structure with lattice parameters of a = 3.186 Å and c = 5.178 Å (JCPDS Card No. 02-1078). It is noteworthy that all XRD spectra are nearly identical, indicating that no significant phase change has been occurred in GaN core, regardless of SiOx-coating and thermal annealing. We have performed TEM investigations for further analysis. Fig. 3 shows the TEM-EDX concentration profiles of Ga, N, Si, and O, along the line drawing across the diameter in a typical GaN-core/SiOxsheath nanowire. Ga and N elements mainly concentrate in the core region of the coaxial nanowires. On the other hand, since the highest peaks are in the sheath region (indicated by arrowheads), Si and O elements mainly reside in the sheath region. Accordingly, the
Fig.1. SEM images of (a) GaN nanowires, (b) as-fabricated GaN-core/SiOx-sheath nanowires, and (c) annealed GaN-core/SiOx-sheath nanowires (inset: enlarged SEM images).
scanning electron microscopy (FE-SEM) (Hitachi, S-4200), and transmission electron microscopy (TEM) (Philips, CM-200) equipped with an energy dispersive X-ray spectroscopy (EDX). The grazing incidence (0.5°) technique was used in the XRD measurements, minimizing the contribution from the substrate [18]. PL was measured at room temperature on Acton Research spectrometer with a He–Cd laser (325 nm, 55 mW).
Fig. 2. XRD spectra of GaN nanowires, as-fabricated GaN-core/SiOx-sheath nanowires, and annealed GaN-core/SiOx-sheath nanowires.
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band in GaN has been attributed to crystal defects such as VGa-related complexes [20–22]. In addition, it is known that the Si doping in GaN contributes to the blue emission [20,22]. In the present study, hightemperature annealing process can generate various vacancy or vacancy-related complexes, contributing to the enhancement of both
Fig. 3. TEM-EDX concentration profiles of Ga, N, Si, and O, along the line drawing across the diameter in a typical as-fabricated GaN-core/SiOx-sheath nanowire.
concentration profiles (Ga, N, Si, and O) are in good agreement with what can be expected for the GaN-core/SiOx-sheath nanowires. During the sputter deposition, we suppose that Si as well as O species arrive and diffuse on the GaN surfaces, generating sheath layers. The O atoms or ions may be originated from gas flow, chamber, and Si target [17]. Fig. 4a shows a low magnification TEM image of an annealed GaNcore/SiOx-sheath nanowire, indicating that there is a layer of mist around the core nanowire. Fig. 4b shows an enlarged TEM image, exhibiting an interface between GaN-core and SiOx-sheath. The inset gives the associated selected area electron diffraction (SAED) pattern, in which the diffraction spots can be indexed as (010), (110), and (100) reflections for the [001] zone axis, according to the hexagonal structure of GaN. In addition, the pattern shows a halo possibly being associated with the amorphous SiOx sheath layer. Fig. 4c shows a lattice-resolved TEM image, enlarging an area enclosed by a dotted box shown in Fig. 4b. In spite of sheath coating, lattice fringes are clearly visible from the GaN core region, revealing its crystalline nature. The interplanar spacing between two neighboring fringes is about 0.28 nm, agreeing with the interplanar distance of the (100) plane of the hexagonal GaN lattice. The sheath layer should be amorphous due to the absence of lattice fringes. Our preliminary TEM investigations revealed that the crystalline properties of as-fabricated GaN-core/SiOx-sheath nanowire were not significantly changed by the thermal annealing at 800 °C. We have investigated the PL properties of GaN-core/SiOx-sheath nanowires. All PL spectra were measured at room temperature. Fig. 5a and b show the PL spectra of as-fabricated and 800 °C-annealed GaNcore/SiOx-sheath nanowires, respectively. As shown in Fig. 5c–e, Gaussian fitting analysis revealed that the broad emissions was divided into two Gaussian functions, being centered at 2.9 eV in the blue region and 2.4 eV in the green region, respectively. We found that peak positions of PL spectrum were not noticeably changed by the SiOx-coating, indicating that PL emission from GaN-core/SiOx-sheath nanowires is mainly ascribed to GaN core. Fig. 5a and b reveal that not only blue emission but also green emission becomes intensified by thermal annealing. It has been suggested that the green emissions are attributed to structural defects, including VGa, VGa–ON complexes, etc [19,20]. Similarly, the blue light
Fig. 4. (a) Low-magnification TEM image of an annealed GaN-core/SiOx-sheath nanowire. (b) Enlarged TEM image of an annealed GaN-core/SiOx-sheath nanowire, exhibiting the core/sheath boundary (inset: corresponding SAED pattern). (c) Latticeresolved TEM image, enlarging an area enclosed by a dotted box shown in (b).
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Fig. 5. Room temperature PL spectra of GaN-core/SiOx-sheath nanowires, which was (a) as-fabricated and (b) annealed at 800 °C. (c) Gaussian fitting analysis of uncoated GaN nanowires. Gaussian fitting analyses of (d) as-fabricated and (e) annealed SiOx-coated GaN nanowires indicate that both emission bands are a superimposition of two major peaks.
green and blue emissions. Also, by comparing Fig. 5e with Fig. 5d, we reveal that the relative intensity of blue peak to green peak is increased by the thermal annealing. We suppose that the thermal diffusion of Si atoms from sheath layer into the GaN lattice further enhances the blue emission, increasing the relative intensity of blue to green emission. Further detailed investigation is underway. 4. Conclusions In summary, we have prepared GaN-core/SiOx-sheath nanowires and investigated the effects of subsequent thermal annealing. XRD spectrum of GaN-core/SiOx-sheath nanowires is nearly identical to that of core GaN nanowires, corresponding to the hexagonal GaN structure. TEM investigations reveal that sheath layers wrap the core nanowires, whereas GaN core and SiOx sheath are crystalline and amorphous, respectively. EDX elemental mapping analysis reveals that the concentration profiles of Si and O elements are valley-like, suggesting that the sheath layer comprises Si and O elements. The overall PL intensity of GaN-core/SiOx-sheath nanowires increases by the thermal annealing. The PL spectra could be divided into blue and green emission bands by the Gaussian fitting analysis, being attributed to GaN core. The relative intensity of blue to green emission becomes enhanced by the thermal annealing, presumably due to the Si doping effects. Not only this is a first report on the fabrication of GaN-core/ SiOx-sheath nanowires, but also this result will significantly contribute to the potential applications of SiOx-sheathed coaxial 1D nanostructures to a variety of nanodevices. Acknowledgement This work was supported by a Korea Research Foundation Grant funded by the Korean Government (MOEHRD) (KRF-2007-521-D00216).
The main calculations were performed by the supercomputing resources of the Korea Institute of Science and Technology Information (KISTI). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22]
A.P. Alivisatos, Science 271 (1996) 933. X. Duan, Y. Huang, Y. Cui, J. Wang, C.M. Lieber, Nature 409 (2001) 66. W.A. de Heer, A. Chatelain, D. Ugarte, Science 270 (1995) 1179. A.M. Morales, C.M. Lieber, Science 279 (1998) 208. Y. Yin, Y. Lu, Y. Sun, Y. Xia, Nano Lett. 2 (2002) 427. W.-S. Shi, H.-Y. Peng, L. Xu, N. Wang, Y.-H.H. Tang, S.-T. Lee, Adv. Mater. 12 (2000) 1927. J. Zolper, R. Shul, A. Baca, R. Wilson, S. Pearton, R. Stall, Appl. Phys. Lett. 68 (1996) 2273. B. Liu, Y. Bando, C. Tang, F. Xu, J. Hu, D. Golberg, J. Phys. Chem. B 109 (2005) 17082. X.M. Cai, A.B. Djurišić, M.H. Xie, Thin Solid Films 515 (2006) 984. H.M. Kim, Y.H. Choo, H. Lee, S.I. Kim, S.R. Ryu, D.Y. Kim, T.W. Kang, K.S. Chung, Nano Lett. 4 (2004) 1059. Y. Huang, X. Duan, Y. Cui, C.M. Lieber, Nano Lett. 2 (2002) 101. Y.B. Li, Y. Bando, D. Golberg, Y. Uemura, Appl. Phys. Lett. 83 (2003) 3999. X. Liang, S. Tan, Z. Tang, N.A. Kotov, Langmuir 20 (2004) 1016. X.-M. Meng, J.-Q. Hu, Y. Jiang, C.-S. Lee, S.-T. Lee, Appl. Phys. Lett. 83 (2003) 2241. N.I. Kovtyukhova, T.E. Mallouk, T.S. Mayer, Adv. Mater. 15 (2003) 780. C.-Y. Wang, L.-H. Chan, D.-Q. Xiao, T.-C. Lin, H.C. Shih, J. Vac. Sci. Technol. B 24 (2006) 613. H.W. Kim, S.H. Shim, J.W. Lee, Carbon 45 (2007) 2692. M. Salluzzo, A. Fragneto, G.M. de Luca, U. Scotti di Uccio, X. Torrelles, Thin Solid Films 486 (2005) 178. M.A. Reshchikov, H. Morkoc, S.S. Park, K.Y. Lee, Appl. Phys Lett. 78 (2001) 3041. M.A. Reshchikov, H. Morkoc, J. Appl. Phys. 97 (2005) 061301. M.A. Reshchikov, R.Y. Korotkov, Phys. Rev. B 64 (2001) 115205. H.C. Yang, T.Y. Lin, Y.F. Chen, Phys. Rev. B 62 (2000) 12593.
