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Synthesis of GaN nanochestnuts by hydride vapour phase epitaxy
Materials Letters 64 (2010) 1238–1241
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Synthesis of GaN nanochestnuts by hydride vapour phase epitaxy M.J. Shin a, J.Y. Moon a, H.Y. Kwon a, Y.J. Choi a, H.S. Ahn a, S.N. Yi a,⁎, D.H. Ha b, Y. Huh c a b c
Department of Applied Science, Korea Maritime University, Busan 606-791, Republic of Korea Division of Convergence Technology, Korea Research Institute of Standards and Science, Daejeon 305-340, Republic of Korea Department of Analysis and Assessment, Research Institute of Industrial Science and Technology, Pohang 790-330, Republic of Korea
a r t i c l e
i n f o
Article history: Received 3 February 2010 Accepted 28 February 2010 Available online 8 March 2010 Keywords: Chemical vapour deposition Crystal structure Nanomaterials
a b s t r a c t GaN nanochestnuts with numerous nanorods and nanoneedles were synthesized on AlN/Si(111) substrate using hydride vapour phase epitaxy (HVPE) method under constant N2 carrier gas flow rate. The formation process of nanochestnuts was systematically investigated and discussed on the basis of the experimental results. The nanochestnuts were analyzed by field emission scanning electron microscopy (FE-SEM), highresolution transmission electron microscopy (HR-TEM), and cathodoluminescence (CL). GaN nanochestnuts were revealed as the composition of core, circular stacking layers, and surrounded with nanorods or nanoneedles on all sides. The resultant nanochestnuts may be a promising structure for omnidirectional nano device applications in the future. © 2010 Elsevier B.V. All rights reserved.
1. Introduction
2. Experiment
Nanostructured semiconductor materials have received considerable attention due to their technological applications, intriguing properties, and quantum size effects [1–3]. The properties of nanostructures are strongly dependent on their size, morphology, and shape [4,5]. Gallium nitride (GaN), a wide-band-gap semiconductor (Eg = 3.4 eV at room temperature), has attracted special interest due to its great potential for use in short-wavelength optical devices and high-power electronic devices [6]. Various architectural morphologies have been fabricated using GaN and other gallium compounds. Hyper-branched GaN nanowire structures have been grown using a multi-step nanocluster catalyzed vapour–liquid–solid process [7,8]. Novel geometrical structures of crystalline gallium oxide such as nanotubes and nanopaintbrushes have been directly synthesized from molten gallium pools using a microwave oxygen plasma [9]. A flower-like GaP nanomaterial has been synthesized by heating Ga2O3 and InP powder [10]. Furthermore, various GaN nanostructures such as nanoparticles, nanobelts, nanorings, nanoneedles, and nanorods have been synthesized by different synthesis processes, including arc discharge [11], metal organic chemical vapour deposition [12], and molecular beam epitaxy [13]. Another structure, a nanoflower, has been fabricated from materials such as MoS2 and GaP [14].
This study builds on our previous studies in which we investigated the growth mechanism and properties of GaN nanorods and nanoneedles [15,16]. We synthesized a special GaN structure with a chestnut-like shape. The samples used in this study were grown on Si (111) substrates with an AlN buffer layer using an atmospheric horizontal hydride vapour phase epitaxy (HVPE) system. The AlN buffer layer was deposited by RF sputtering for 25 min (∼50 nm). Subsequently, the NH3 and HCl flow rates were maintained at 725 and 29 sccm, respectively. Nitrogen was used as the carrier gas, and the experiment was performed at atmospheric pressure. GaCl vapour species were formed by reacting HCl with liquid gallium in the source region at 850 °C. GaN species were formed by reacting GaCl with NH3 in the reaction region at 1050 °C. Samples were grown over 2 h. The growth mechanism and optical properties of GaN chestnuts were investigated by field emission scanning electron microscopy (FE-SEM), high-resolution transmission electron microscopy (HR-TEM:JEOL JEM 2100F), and cathodoluminescence (CL) spectroscopy. The samples were spherical in shape and had many burs, thereby resembling chestnuts.
⁎ Corresponding author. Tel.: + 82 51 410 4448; fax: + 82 51 404 3986. E-mail address: [email protected] (S.N. Yi). 0167-577X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2010.02.065
3. Results and discussion The intriguing nanostructures have stimulated us to investigate the synthesis of GaN flower-like structures and to analyze their morphologies. In this paper, we describe the fabrication of a GaN nanostructure with a chestnut-like shape and systematically investigate its structure. The GaN nanochestnuts were synthesized at a HCl: NH3 flow ratio of 1:25 under constant N2 carrier gas flow rate of 1140 sccm.
M.J. Shin et al. / Materials Letters 64 (2010) 1238–1241
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Fig. 1. Schematic diagram of a possible growth mechanism of GaN chestnut-like nanostructures.
Fig. 1 shows a schematic diagram of one possible growth mechanism of GaN chestnut-like nanostructures in a thermodynamic process. In the source zone, GaCl vapour species are formed by reacting HCl with liquid gallium at about 850 °C. The furnace temperature increases to 1050 °C in the reaction zone, where precursor ammonia is decomposed into nitrides and hydrides, leading to nucleation of GaN. Most GaN nuclei generate GaN droplets on the substrate in the growth zone, which is maintained at 600 or 650 °C, and they coalesce to form GaN nanostructures. However, some GaN nuclei float in the N2 carrier gas and behave as colloidal particles. While the nanostructures on the substrate grow vertically, the colloidal particles produce chestnut-like nanostructures, which gradually grow in size until their mass exceeds the lift-off force of
the N2 carrier gas. Individual nanostructure chestnuts then fall and grow larger together with the nanoneedle structures under them. The resulting morphologies are depicted in the inset of Fig. 1. Fig. 2(a) and (c) shows FE-SEM images of a GaN nanorod chestnut grown at 650 °C and a nanoneedle chestnut grown at 600 °C, respectively. We cut the nanochestnuts using a focused ion beam system to investigate the growth mechanism. Fig. 2(b) and (d) shows cross-sectional images of nanorod- and nanoneedle chestnuts, respectively, revealing that the spheres are surrounded by burs of nanorods and nanoneedles. The nanorod- and nanoneedle chestnuts are both about 10 μm in diameter, while a single rod and a single needle have diameters of about 300 and 100 nm, respectively. Based on observations of the boundary line between the nanochestnut and
Fig. 2. FE-SEM images of (a) a GaN nanorod chestnut, (b) a cross-sectional image of a nanorod chestnut, (c) a nanoneedle chestnut, and (d) a cross-sectional image of a nanoneedle chestnut.
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the substrate, we conjecture that the nanochestnut did not grow directly from the substrate; instead, it grew from the nanorods or nanoneedles that had grown vertically on the substrate. The growth rate of the upper part of the nanochestnut is higher than that of the lower part after the nanochestnut had fallen onto the vertical nanorods or nanoneedles. Thus, nanochestnuts do not have completely circular cross-sections. HR-TEM observations were performed for a nanorod chestnut to determine its growth mechanism. Fig. 3(a) shows that coaxial stacking layers surround the core and omnidirectionally grown nanorods are on the outside. Fig. 3(b) shows a schematic diagram of the nanorod chestnut. The diameters of the core, the stacking layers, and the whole chestnut are 0.85, 4.3, and 10 μm, respectively. Fig. 3(c) and (d) shows a HR-TEM lattice image and a selected-area electron diffraction pattern, respectively, which were obtained from the area indicated by the white solid circle in Fig. 3(a) (point 1 in Fig. 3(b)). Fig. 3(c) and (d) reveals its polycrystalline nature. Fig. 3(e) and (g) was obtained from the middle (point 2 in Fig. 3(b)) and the periphery
(point 3 in Fig. 3(b)) of a nanochestnut, respectively. Planes (0004) and (112̄0) in Fig. 3(f) confirm that the area indicated by the white circle in Fig. 3(e) consists of two single crystal planes, and plane (0004) in Fig. 3(h) confirms that the area indicated by the white circle in Fig. 3(g) is composed of a single crystal. Initially, the nanochestnut grows radially as a single crystal. Subsequently, GaN fills the air gaps in the lower regions between the nanorods in a regular sequence to form a polycrystal; as this process continues, the nanochestnut grows in size. Fig. 4 shows CL spectra of nanorod- and nanoneedle chestnuts at room temperature. Characteristic emission peaks at 380 and 390 nm are observed in the spectra of nanorod- and nanoneedle chestnuts, respectively. The peak at 380 nm is due to recombination of free electrons with holes bound at acceptors (eA transitions) [17]. The peak at 390 nm in the nanoneedle chestnut spectra is also due to eA transitions, but it is shifted by 10 nm relative to the peak in the nanorod chestnut spectra. The energy band gap increases when compressive strain is present and decreases when residual tensile
Fig. 3. HR-TEM images of a nanorod chestnut, showing the (a) core and stacking layers, (e) middle, and (g) periphery regions of a nanorod chestnut. (c) and (d) show the HR-TEM lattice image and electron diffraction pattern obtained for the area indicated by the white circle in panel (a). The inset figures (f) and (h) show HR-TEM lattice images obtained for the areas indicated by white circles in panels (e) and (g), respectively. (b) Schematic diagram of a nanorod chestnut.
M.J. Shin et al. / Materials Letters 64 (2010) 1238–1241
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the nanostructure to grow until its mass exceeds the lift-off force. It then falls and grows larger together with the nanostructures under it. Cross-sectional images of nanochestnuts reveal a core, concentric circular stacking layers, and omnidirectionally grown nanoburs on all sides. Both nanorod- and nanoneedle chestnuts have eA transitions, but the peak in the CL spectra of nanoneedle chestnuts is shifted because of residual tensile strain in the nanoneedles.
References
Fig. 4. CL spectra of a nanorod- and a nanoneedle chestnut measured at room temperature.
strain exists [18]. Thus, the observed energy shift is caused by residual tensile strain in nanoneedle chestnuts, which has thinner burs than do nanorod chestnuts. The CL spectra also exhibit a broad yellow luminescence peak, which is probably attributable to intrinsic point defects, such as Ga vacancies or impurities [19]. 4. Conclusions We synthesized spherical nanorod- and nanoneedle chestnuts using HVPE. A possible growth mechanism is that GaN nuclei cause
[1] Duan X, Huang Y, Agarwal R, Lieber CM. Nature 2003;421:241–5. [2] Polenta L, Rossi M, Cavallini A, Calarco R, Marso M, Meijers R, et al. ACSNANO 2008;2:287–92. [3] Yi SN, Na JH, Lee KH, Jarjour AF, Taylor RA, Park YS, et al. Appl Phys Lett 2007;90: 101901–3. [4] Mahan GD, Gupta R, Xiong Q, Adu CK, Eklund PC. Phys Rev B 2003;68:073402–4. [5] Duan X, Lieber CM. J Am Chem Soc 2000;122:18189. [6] Nakamura S. Science 1998;281:956–61. [7] Wang D, Qian F, Yang C, Zhong ZH, Lieber CM. Nano Lett 2004;4:871–4. [8] Lan ZH, Liang CH, Hsu CW, Wu CT, Lin HM, Dhara S, et al. Adv Funct Mater 2004;14: 233–7. [9] Sharma S, Sunkara MK. J Am Chem Soc 2002;124:12288–93. [10] Jiang CY, Sun XW, Lo GQ, Kwong DL, Wang JX. Appl Phys Lett 2007;90:263501–3. [11] Han W, Redlich P, Ernst F, Ruhle M. Appl Phys Lett 2000;76:652–4. [12] Khanderi J, Wohlfart A, Parala H, Devi A, Hambrock J, Birkner A, et al. J Mater Chem 2003;13:1438–46. [13] Kim YH, Lee JY, Lee SH, Oh JE, Lee HS. Appl Phys A 2005;80:1635–9. [14] Liu BD, Bando Y, Tang CC, Golberg D, Xie RG, Sekiguchi T. Appl Phys Lett 2005;86: 083107-3. [15] Moon JY, Kwon HY, Choi YJ, Shin MJ, Yi SN, Yun YJ, et al. J Alloys Compd 2009;480: 853–6. [16] Kwon HY, Shin MJ, Choi YJ, Moon JY, Ahn HS, Yi SN, et al. J Cryst Growth 2009;311: 4146–51. [17] Turnbull DA, Li X, Gu SQ, Reuter EE, Coleman JJ, Bishop SG. J Appl Phys 1996;80: 4609–14. [18] Nasr FB, Matoussi A, Salh R, Guermazi S, Fitting H-J, Fakhfakh Z. Physica E 2009;41: 454–9. [19] Reshchikov MA, Morkoc HJ. Appl Phys 2005;97:06130101–95.
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Synthesis of GaN nanochestnuts by hydride vapour phase epitaxy M.J. Shin a, J.Y. Moon a, H.Y. Kwon a, Y.J. Choi a, H.S. Ahn a, S.N. Yi a,⁎, D.H. Ha b, Y. Huh c a b c
Department of Applied Science, Korea Maritime University, Busan 606-791, Republic of Korea Division of Convergence Technology, Korea Research Institute of Standards and Science, Daejeon 305-340, Republic of Korea Department of Analysis and Assessment, Research Institute of Industrial Science and Technology, Pohang 790-330, Republic of Korea
a r t i c l e
i n f o
Article history: Received 3 February 2010 Accepted 28 February 2010 Available online 8 March 2010 Keywords: Chemical vapour deposition Crystal structure Nanomaterials
a b s t r a c t GaN nanochestnuts with numerous nanorods and nanoneedles were synthesized on AlN/Si(111) substrate using hydride vapour phase epitaxy (HVPE) method under constant N2 carrier gas flow rate. The formation process of nanochestnuts was systematically investigated and discussed on the basis of the experimental results. The nanochestnuts were analyzed by field emission scanning electron microscopy (FE-SEM), highresolution transmission electron microscopy (HR-TEM), and cathodoluminescence (CL). GaN nanochestnuts were revealed as the composition of core, circular stacking layers, and surrounded with nanorods or nanoneedles on all sides. The resultant nanochestnuts may be a promising structure for omnidirectional nano device applications in the future. © 2010 Elsevier B.V. All rights reserved.
1. Introduction
2. Experiment
Nanostructured semiconductor materials have received considerable attention due to their technological applications, intriguing properties, and quantum size effects [1–3]. The properties of nanostructures are strongly dependent on their size, morphology, and shape [4,5]. Gallium nitride (GaN), a wide-band-gap semiconductor (Eg = 3.4 eV at room temperature), has attracted special interest due to its great potential for use in short-wavelength optical devices and high-power electronic devices [6]. Various architectural morphologies have been fabricated using GaN and other gallium compounds. Hyper-branched GaN nanowire structures have been grown using a multi-step nanocluster catalyzed vapour–liquid–solid process [7,8]. Novel geometrical structures of crystalline gallium oxide such as nanotubes and nanopaintbrushes have been directly synthesized from molten gallium pools using a microwave oxygen plasma [9]. A flower-like GaP nanomaterial has been synthesized by heating Ga2O3 and InP powder [10]. Furthermore, various GaN nanostructures such as nanoparticles, nanobelts, nanorings, nanoneedles, and nanorods have been synthesized by different synthesis processes, including arc discharge [11], metal organic chemical vapour deposition [12], and molecular beam epitaxy [13]. Another structure, a nanoflower, has been fabricated from materials such as MoS2 and GaP [14].
This study builds on our previous studies in which we investigated the growth mechanism and properties of GaN nanorods and nanoneedles [15,16]. We synthesized a special GaN structure with a chestnut-like shape. The samples used in this study were grown on Si (111) substrates with an AlN buffer layer using an atmospheric horizontal hydride vapour phase epitaxy (HVPE) system. The AlN buffer layer was deposited by RF sputtering for 25 min (∼50 nm). Subsequently, the NH3 and HCl flow rates were maintained at 725 and 29 sccm, respectively. Nitrogen was used as the carrier gas, and the experiment was performed at atmospheric pressure. GaCl vapour species were formed by reacting HCl with liquid gallium in the source region at 850 °C. GaN species were formed by reacting GaCl with NH3 in the reaction region at 1050 °C. Samples were grown over 2 h. The growth mechanism and optical properties of GaN chestnuts were investigated by field emission scanning electron microscopy (FE-SEM), high-resolution transmission electron microscopy (HR-TEM:JEOL JEM 2100F), and cathodoluminescence (CL) spectroscopy. The samples were spherical in shape and had many burs, thereby resembling chestnuts.
⁎ Corresponding author. Tel.: + 82 51 410 4448; fax: + 82 51 404 3986. E-mail address: [email protected] (S.N. Yi). 0167-577X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2010.02.065
3. Results and discussion The intriguing nanostructures have stimulated us to investigate the synthesis of GaN flower-like structures and to analyze their morphologies. In this paper, we describe the fabrication of a GaN nanostructure with a chestnut-like shape and systematically investigate its structure. The GaN nanochestnuts were synthesized at a HCl: NH3 flow ratio of 1:25 under constant N2 carrier gas flow rate of 1140 sccm.
M.J. Shin et al. / Materials Letters 64 (2010) 1238–1241
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Fig. 1. Schematic diagram of a possible growth mechanism of GaN chestnut-like nanostructures.
Fig. 1 shows a schematic diagram of one possible growth mechanism of GaN chestnut-like nanostructures in a thermodynamic process. In the source zone, GaCl vapour species are formed by reacting HCl with liquid gallium at about 850 °C. The furnace temperature increases to 1050 °C in the reaction zone, where precursor ammonia is decomposed into nitrides and hydrides, leading to nucleation of GaN. Most GaN nuclei generate GaN droplets on the substrate in the growth zone, which is maintained at 600 or 650 °C, and they coalesce to form GaN nanostructures. However, some GaN nuclei float in the N2 carrier gas and behave as colloidal particles. While the nanostructures on the substrate grow vertically, the colloidal particles produce chestnut-like nanostructures, which gradually grow in size until their mass exceeds the lift-off force of
the N2 carrier gas. Individual nanostructure chestnuts then fall and grow larger together with the nanoneedle structures under them. The resulting morphologies are depicted in the inset of Fig. 1. Fig. 2(a) and (c) shows FE-SEM images of a GaN nanorod chestnut grown at 650 °C and a nanoneedle chestnut grown at 600 °C, respectively. We cut the nanochestnuts using a focused ion beam system to investigate the growth mechanism. Fig. 2(b) and (d) shows cross-sectional images of nanorod- and nanoneedle chestnuts, respectively, revealing that the spheres are surrounded by burs of nanorods and nanoneedles. The nanorod- and nanoneedle chestnuts are both about 10 μm in diameter, while a single rod and a single needle have diameters of about 300 and 100 nm, respectively. Based on observations of the boundary line between the nanochestnut and
Fig. 2. FE-SEM images of (a) a GaN nanorod chestnut, (b) a cross-sectional image of a nanorod chestnut, (c) a nanoneedle chestnut, and (d) a cross-sectional image of a nanoneedle chestnut.
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the substrate, we conjecture that the nanochestnut did not grow directly from the substrate; instead, it grew from the nanorods or nanoneedles that had grown vertically on the substrate. The growth rate of the upper part of the nanochestnut is higher than that of the lower part after the nanochestnut had fallen onto the vertical nanorods or nanoneedles. Thus, nanochestnuts do not have completely circular cross-sections. HR-TEM observations were performed for a nanorod chestnut to determine its growth mechanism. Fig. 3(a) shows that coaxial stacking layers surround the core and omnidirectionally grown nanorods are on the outside. Fig. 3(b) shows a schematic diagram of the nanorod chestnut. The diameters of the core, the stacking layers, and the whole chestnut are 0.85, 4.3, and 10 μm, respectively. Fig. 3(c) and (d) shows a HR-TEM lattice image and a selected-area electron diffraction pattern, respectively, which were obtained from the area indicated by the white solid circle in Fig. 3(a) (point 1 in Fig. 3(b)). Fig. 3(c) and (d) reveals its polycrystalline nature. Fig. 3(e) and (g) was obtained from the middle (point 2 in Fig. 3(b)) and the periphery
(point 3 in Fig. 3(b)) of a nanochestnut, respectively. Planes (0004) and (112̄0) in Fig. 3(f) confirm that the area indicated by the white circle in Fig. 3(e) consists of two single crystal planes, and plane (0004) in Fig. 3(h) confirms that the area indicated by the white circle in Fig. 3(g) is composed of a single crystal. Initially, the nanochestnut grows radially as a single crystal. Subsequently, GaN fills the air gaps in the lower regions between the nanorods in a regular sequence to form a polycrystal; as this process continues, the nanochestnut grows in size. Fig. 4 shows CL spectra of nanorod- and nanoneedle chestnuts at room temperature. Characteristic emission peaks at 380 and 390 nm are observed in the spectra of nanorod- and nanoneedle chestnuts, respectively. The peak at 380 nm is due to recombination of free electrons with holes bound at acceptors (eA transitions) [17]. The peak at 390 nm in the nanoneedle chestnut spectra is also due to eA transitions, but it is shifted by 10 nm relative to the peak in the nanorod chestnut spectra. The energy band gap increases when compressive strain is present and decreases when residual tensile
Fig. 3. HR-TEM images of a nanorod chestnut, showing the (a) core and stacking layers, (e) middle, and (g) periphery regions of a nanorod chestnut. (c) and (d) show the HR-TEM lattice image and electron diffraction pattern obtained for the area indicated by the white circle in panel (a). The inset figures (f) and (h) show HR-TEM lattice images obtained for the areas indicated by white circles in panels (e) and (g), respectively. (b) Schematic diagram of a nanorod chestnut.
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the nanostructure to grow until its mass exceeds the lift-off force. It then falls and grows larger together with the nanostructures under it. Cross-sectional images of nanochestnuts reveal a core, concentric circular stacking layers, and omnidirectionally grown nanoburs on all sides. Both nanorod- and nanoneedle chestnuts have eA transitions, but the peak in the CL spectra of nanoneedle chestnuts is shifted because of residual tensile strain in the nanoneedles.
References
Fig. 4. CL spectra of a nanorod- and a nanoneedle chestnut measured at room temperature.
strain exists [18]. Thus, the observed energy shift is caused by residual tensile strain in nanoneedle chestnuts, which has thinner burs than do nanorod chestnuts. The CL spectra also exhibit a broad yellow luminescence peak, which is probably attributable to intrinsic point defects, such as Ga vacancies or impurities [19]. 4. Conclusions We synthesized spherical nanorod- and nanoneedle chestnuts using HVPE. A possible growth mechanism is that GaN nuclei cause
[1] Duan X, Huang Y, Agarwal R, Lieber CM. Nature 2003;421:241–5. [2] Polenta L, Rossi M, Cavallini A, Calarco R, Marso M, Meijers R, et al. ACSNANO 2008;2:287–92. [3] Yi SN, Na JH, Lee KH, Jarjour AF, Taylor RA, Park YS, et al. Appl Phys Lett 2007;90: 101901–3. [4] Mahan GD, Gupta R, Xiong Q, Adu CK, Eklund PC. Phys Rev B 2003;68:073402–4. [5] Duan X, Lieber CM. J Am Chem Soc 2000;122:18189. [6] Nakamura S. Science 1998;281:956–61. [7] Wang D, Qian F, Yang C, Zhong ZH, Lieber CM. Nano Lett 2004;4:871–4. [8] Lan ZH, Liang CH, Hsu CW, Wu CT, Lin HM, Dhara S, et al. Adv Funct Mater 2004;14: 233–7. [9] Sharma S, Sunkara MK. J Am Chem Soc 2002;124:12288–93. [10] Jiang CY, Sun XW, Lo GQ, Kwong DL, Wang JX. Appl Phys Lett 2007;90:263501–3. [11] Han W, Redlich P, Ernst F, Ruhle M. Appl Phys Lett 2000;76:652–4. [12] Khanderi J, Wohlfart A, Parala H, Devi A, Hambrock J, Birkner A, et al. J Mater Chem 2003;13:1438–46. [13] Kim YH, Lee JY, Lee SH, Oh JE, Lee HS. Appl Phys A 2005;80:1635–9. [14] Liu BD, Bando Y, Tang CC, Golberg D, Xie RG, Sekiguchi T. Appl Phys Lett 2005;86: 083107-3. [15] Moon JY, Kwon HY, Choi YJ, Shin MJ, Yi SN, Yun YJ, et al. J Alloys Compd 2009;480: 853–6. [16] Kwon HY, Shin MJ, Choi YJ, Moon JY, Ahn HS, Yi SN, et al. J Cryst Growth 2009;311: 4146–51. [17] Turnbull DA, Li X, Gu SQ, Reuter EE, Coleman JJ, Bishop SG. J Appl Phys 1996;80: 4609–14. [18] Nasr FB, Matoussi A, Salh R, Guermazi S, Fitting H-J, Fakhfakh Z. Physica E 2009;41: 454–9. [19] Reshchikov MA, Morkoc HJ. Appl Phys 2005;97:06130101–95.