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Effect of surface morphology of metallic zinc films deposited by ion beam sputter deposition on the formation of ZnO nanowires
Vacuum 86 (2011) 295e298
Contents lists available at ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
Effect of surface morphology of metallic zinc films deposited by ion beam sputter deposition on the formation of ZnO nanowires Liang-Chiun Chao*, Ching-Fu Lin, Chung-Chi Liau Department of Electronic Engineering, National Taiwan University of Science and Technology, Taipei, Taiwan, ROC
a r t i c l e i n f o
a b s t r a c t
Article history: Received 27 September 2010 Received in revised form 10 May 2011 Accepted 22 June 2011
Metallic zinc film with various surface roughnesses was deposited on Si (100) substrates by ion beam sputter deposition utilizing beam energies at 8, 12 and 16 keV. The surface roughness of the metallic zinc film increased as ion beam energy increased and was found to act as a crucial factor for the formation of ZnO nanowires by subsequent thermal oxidation. ZnO nanowires with diameters of w30 nm and average length of w1 mm were obtained from 12 to 16 keV ion beam deposited samples while no ZnO nanowires were found on 8 keV ion beam deposited samples. Photoluminescence study of ZnO nanowires exhibits a strong UV emission at 377.2 nm (3.287 eV) with a full-width at half maximum of 95.0 meV and negligible defect related deep level emission. The ZnO nanowires are grown along the [110] direction and the growth mechanism is likely due to a solid state based-up diffusion process. Field-emission measurement shows a turn-on field of 7.9 MV/m and a field enhancement factor b of 691 is achieved. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: ZnO Nanowire Field emission
1. Introduction ZnO is a well-known wide-band gap semiconductor material that has found various applications such as UVelight-emitting devices [1], cold field emitters [2], solar cell electrodes [3] and chemical sensors [4,5]. ZnO is also one of the materials that exhibit the richest variety of nanostructures and one-dimensional (1-D) ZnO nanostructures are promising candidates for field emission display [2] and nanogenerator applications [6]. 1-D ZnO nanostructures have been prepared by vapor-phase transport [2,4,5,7,8] via vapor-liquid-solid [9] or vapor-solid mechanism [8,10]. An alternative approach of preparing 1-D ZnO nanowires is thermal oxidation of metallic zinc [11e14]. This is a low temperature (<673 K) process [12e14] and is of particular interest due to the fact that extrinsic impurities can be limited and is compatible with silicon processing technology. Ren et al. [12] have prepared 1-D ZnO nanowires by thermal oxidation of sand paper polished bulk zinc. Recently, Chao et al. [13] demonstrated the growth of ZnO nanowrires on metallic zinc foils by ion beam implantation and thermal oxidation. The ZnO nanowires were found grown along the [110] direction while the orientation of the ZnO nanowires were dependent on crystallographic grain orientations of the substrate. Furthermore, Yu et al. [14] deposited metallic nanocrystalline zinc
* Corresponding author. E-mail address: [email protected] (L.-C. Chao). 0042-207X/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2011.06.020
overlayers on copper substrate, while the copper substrate was mechanically polished by various grades of sand papers. 1-D ZnO nanostructures were obtained by thermal oxidation of the zinc/ copper composite layer at 673 K. In this article, metallic zinc films with various surface roughnesses were prepared by ion beam sputter deposition. The effect of zinc film surface morphology on the formation of 1-D ZnO nanowires by low temperature thermal oxidation is presented. 2. Experimental Zn films were deposited on Si (100) substrates by ion beam sputter deposition utilizing a capillaritron ion source. A metallic Zn (99.99%) target was positioned at 30 mm downstream of the capillaritron ion source. Si (100) substrates were positioned at close proximity and parallel to the Zn target. Zn thin films were deposited on Si substrates with beam energy of 8, 12, 16 keV, all with an anode current of 350 mA. The vacuum chamber was first pumped down by a turbo molecular pump to a bass pressure of 4.0 10 4 Pa. The deposition pressure was 0.7 Pa and the deposition time was 1200 s. Annealing was performed at 673 K in flowing oxygen ambient for 3 h in a tube furnace. The surface morphology of 1-D ZnO nanowires were investigated by a field emission scanning electron microscope (FE-SEM, JEOL JSM-6500F) operating at 15 kV. Transmission electron microscope (TEM, Philips Tecnai G2 F20) was employed to investigate the structure property of ZnO nanowires. Photoluminescence (PL) study was performed utilizing a 50 mW
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He-Cd laser at 325 nm as the excitation light source and the spectra were dispersed by a Horiba Jobin Yvon iHR 550 spectrometer and detected by a Symphony CCD detector cooled to 203 K. 3. Results and discussion Fig. 1 shows the morphology of as-deposited metallic Zn film by ion beam sputter deposition utilizing beam energies at 8, 12 and 16 keV. Metallic Zn film deposited by the lowest beam energy shows the smoothest surface consisting of nano-particles with the size of w 50 nm. As beam energy increases to 12 keV, surface
Fig. 1. FE-SEM micrographs of Zn films deposited by ion beam energy of (a) 8, (b) 12 and (c) 16 keV.
roughness increases and particles with the sizes of 100e200 nm can be found throughout the substrate. As beam energy reaches 16 keV, even larger particles with sizes of 0.7e1 mm were found instead. Fig. 1c indicates that the larger particles are due to agglomeration of smaller particles due to higher deposition rates. Metallic zinc films deposited by various beam energies were thermally oxidized at 673 K in flowing oxygen ambient for 3 h Fig. 2 shows FE-SEM micrographs of Zn films after thermal oxidation. Zn films deposited by 8 keV ion beam sputtering shows no formation of ZnO nanowires (Fig. 2a) while Zn films deposited by both 12 and 16 keV ion beam shows ZnO nanowires with an
Fig. 2. FE-SEM micrographs of ZnO nanowires prepared by 673 K oxidation of (a) 8, (b) 12 and (c) 16 keV deposited metallic zinc films.
L.-C. Chao et al. / Vacuum 86 (2011) 295e298
297
Fig. 3. A TEM image of a ZnO nanowire, insert is a SADP image indicating that ZnO nanowire is grown along the [110] direction.
average length of w1 mm evenly distributed across the substrate. Comparing Fig. 2b and c, the density of ZnO nanowires in Fig. 2b is slightly higher than that in Fig. 2c. From Fig. 3, the diameter of the nanowire decreases from 40 nm at the bottom to 20 nm at the tip, indicating that the growth mechanism is a solid state based-up diffusion process [14]. Selective area diffraction pattern (SADP) (insert of Fig. 3) indicates that the ZnO nanowire is grown along the [110] direction. The ZnO nanowires were further studied by photoluminescence (PL). Fig. 4 shows that oxidation of 12 keV deposited samples exhibit a strong near-band-edge (NBE) emission centered at 377.2 nm (3.287 eV) with a full-width at half maximum (FWHM) of 94.5 meV. ZnO nanowires obtained by oxidation of 16 keV deposited Zn films exhibits similar NBE emission peak at 377.4 nm (3.286 eV) with an FWHM of 95.0 meV and negligible defect related deep level emission. However, ZnO obtained by oxidation of 8 keV deposited Zn films results an NBE peak emission at 382.5 nm (3.242 eV) with an FWHM of 172.2 meV. Beside the red-shifted NBE emission, a strong defect-related deep level emission centered at w520 nm (2.385 eV) was also observed, indicating the presence of
Fig. 5. Field emission properties and F-N plots of ZnO nanowires prepared by thermal oxidation Zn films deposited with (a) 12 and (b) 16 keV ion beam.
oxygen vacancy defects near the surface. Fig. 5a shows fieldemission properties of ZnO nanowires prepared by thermal oxidation of Zn film deposited by 12 keV ion beam. The turn-on field and field enhancement factor are 7.93 MV/m and 691, respectively. ZnO nanowires prepared by thermal oxidation of metallic zinc films deposited by 16 keV (Fig. 5b) ion beam exhibit a higher turn-on field of 10.4 MV/m a slightly lower field enhancement factor of 500. Our results indicate that the formation of ZnO nanowire is dependent on the metallic zinc particle sizes. As ZnO film forms on metallic zinc surface, an electric field between ZneZnO and Znoxygen builds up [14,15]. This electric field and the appropriate oxidation temperature cause Zn ions to diffuse into the ZnO layer causing Zn-rich regions that form nucleation sites [16,17]. As Zn ions continue to diffuse through the Zn-rich nucleation sites they react with oxygen that results in the formation of ZnO nanowires. The interface electric field is higher on structures with larger curvatures. The surface of metallic zinc films deposited by 8 keV ion beam is relatively smooth, resulting in a smaller interface electric field. As surface roughness increases due to increased beam energy, increased interface electric field causes enhanced diffusion of zinc ions that facilitates the formation of ZnO nanwires. 4. Conclusion
Fig. 4. PL spectra of ZnO nanowires prepared by thermal oxidation of metallic zinc films deposited with ion beam energy at 8, 12 and 16 keV. The spectra are shifted vertically for clarity.
ZnO nanowires have been synthesized by thermal oxidation of metallic zinc films at 673 K without using catalysts. The
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surface roughness of the metallic zinc film is dependent on ion beam energy that acts as a crucial factor on the formation of ZnO nanowires. High surface roughness results in higher electric field between ZneZnO and Zn-oxygen boundaries that facilitates the out diffusion of Zn ions and the growth of ZnO nanowires. ZnO nanowire with diameter of w30 nm and length of w1 mm can be achieved and the ZnO nanowires were growing along the [110] direction. A turn-on field 7.93 MV/m and a field enhancement factor of 691 were achieved by thermal oxidation of Zn film deposited with 12 kV beam energy. This process allows preparation of large area ZnO nanowires on rigid mechanical support which is valuable for cold field emission applications.
Acknowledgement This research work was supported by National Science Council of Taiwan under contract number NSC 98-2112-M-011-MY3.
References [1] Tsukazaki A, Ohtomo A, Onuma T, Ohtani M, Mankino T, Sumiya M, et al. Nat Mater 2005;4:42. [2] Zhu YW, Zang HZ, Sun XC, Feng SQ, Xu J, Zhao Q, et al. Appl Phys Lett 2003;83: 144. [3] Rensmo H, Keis K, Lindstreöm H, Södergren S, Solbrand A, Hagfeldt A, et al. J Phys Chem B 1997;101:2598. [4] Wan Q, Li QH, Chen YJ, Wang TH, He XL, Li JP, et al. Appl Phys Lett 2004;84: 3654. [5] Li LM, Li CC, Zhang J, Du ZF, Zou BS, Yu HC, et al. Nanotechnology 2007;18: 225504. [6] Wang ZL. Mater Today 2007;10:20. [7] Huang MH, Wu Y, Feick H, Tran N, Weber E, Yang P. Adv Mater 2001;13:113. [8] Park WI, Kim DH, Jung SW, Yi GC. Appl Phys Lett 2002;80:4232. [9] Wagner RS, Ellis WC. Appl Phys Lett 1964;4:89. [10] Chen Y, Qu B, Barnakov YA, Tang Q, Chen J. J Mater Sci. Mater Electron 2009; 20:328. [11] Gui Y, Xie C, Zhang Q, Hu M, Yu J, Weng Z. J Cryst Growth 2006;289:663. [12] Ren S, Bai YF, Chen J, Deng SZ, Xu NS, Wu QB, et al. Mater Lett 2007;61:666. [13] Chao LC, Lee JW, Liau CC. J Phys D Appl Phys 2008;41:115405. [14] Yu W, Pan C. Mater Chem Phys 2009;115:74. [15] Ocal C, Ferrer S, Garcia N. Surf Sci 1985;163:335. [16] Wang N, Fung KK, Wang S, Yang S. J Cryst Growth 2001;233:226. [17] Wang ZL, Kong XY, Zuo M. Phys Rev Lett 2003;91:185502.
Contents lists available at ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
Effect of surface morphology of metallic zinc films deposited by ion beam sputter deposition on the formation of ZnO nanowires Liang-Chiun Chao*, Ching-Fu Lin, Chung-Chi Liau Department of Electronic Engineering, National Taiwan University of Science and Technology, Taipei, Taiwan, ROC
a r t i c l e i n f o
a b s t r a c t
Article history: Received 27 September 2010 Received in revised form 10 May 2011 Accepted 22 June 2011
Metallic zinc film with various surface roughnesses was deposited on Si (100) substrates by ion beam sputter deposition utilizing beam energies at 8, 12 and 16 keV. The surface roughness of the metallic zinc film increased as ion beam energy increased and was found to act as a crucial factor for the formation of ZnO nanowires by subsequent thermal oxidation. ZnO nanowires with diameters of w30 nm and average length of w1 mm were obtained from 12 to 16 keV ion beam deposited samples while no ZnO nanowires were found on 8 keV ion beam deposited samples. Photoluminescence study of ZnO nanowires exhibits a strong UV emission at 377.2 nm (3.287 eV) with a full-width at half maximum of 95.0 meV and negligible defect related deep level emission. The ZnO nanowires are grown along the [110] direction and the growth mechanism is likely due to a solid state based-up diffusion process. Field-emission measurement shows a turn-on field of 7.9 MV/m and a field enhancement factor b of 691 is achieved. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: ZnO Nanowire Field emission
1. Introduction ZnO is a well-known wide-band gap semiconductor material that has found various applications such as UVelight-emitting devices [1], cold field emitters [2], solar cell electrodes [3] and chemical sensors [4,5]. ZnO is also one of the materials that exhibit the richest variety of nanostructures and one-dimensional (1-D) ZnO nanostructures are promising candidates for field emission display [2] and nanogenerator applications [6]. 1-D ZnO nanostructures have been prepared by vapor-phase transport [2,4,5,7,8] via vapor-liquid-solid [9] or vapor-solid mechanism [8,10]. An alternative approach of preparing 1-D ZnO nanowires is thermal oxidation of metallic zinc [11e14]. This is a low temperature (<673 K) process [12e14] and is of particular interest due to the fact that extrinsic impurities can be limited and is compatible with silicon processing technology. Ren et al. [12] have prepared 1-D ZnO nanowires by thermal oxidation of sand paper polished bulk zinc. Recently, Chao et al. [13] demonstrated the growth of ZnO nanowrires on metallic zinc foils by ion beam implantation and thermal oxidation. The ZnO nanowires were found grown along the [110] direction while the orientation of the ZnO nanowires were dependent on crystallographic grain orientations of the substrate. Furthermore, Yu et al. [14] deposited metallic nanocrystalline zinc
* Corresponding author. E-mail address: [email protected] (L.-C. Chao). 0042-207X/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2011.06.020
overlayers on copper substrate, while the copper substrate was mechanically polished by various grades of sand papers. 1-D ZnO nanostructures were obtained by thermal oxidation of the zinc/ copper composite layer at 673 K. In this article, metallic zinc films with various surface roughnesses were prepared by ion beam sputter deposition. The effect of zinc film surface morphology on the formation of 1-D ZnO nanowires by low temperature thermal oxidation is presented. 2. Experimental Zn films were deposited on Si (100) substrates by ion beam sputter deposition utilizing a capillaritron ion source. A metallic Zn (99.99%) target was positioned at 30 mm downstream of the capillaritron ion source. Si (100) substrates were positioned at close proximity and parallel to the Zn target. Zn thin films were deposited on Si substrates with beam energy of 8, 12, 16 keV, all with an anode current of 350 mA. The vacuum chamber was first pumped down by a turbo molecular pump to a bass pressure of 4.0 10 4 Pa. The deposition pressure was 0.7 Pa and the deposition time was 1200 s. Annealing was performed at 673 K in flowing oxygen ambient for 3 h in a tube furnace. The surface morphology of 1-D ZnO nanowires were investigated by a field emission scanning electron microscope (FE-SEM, JEOL JSM-6500F) operating at 15 kV. Transmission electron microscope (TEM, Philips Tecnai G2 F20) was employed to investigate the structure property of ZnO nanowires. Photoluminescence (PL) study was performed utilizing a 50 mW
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L.-C. Chao et al. / Vacuum 86 (2011) 295e298
He-Cd laser at 325 nm as the excitation light source and the spectra were dispersed by a Horiba Jobin Yvon iHR 550 spectrometer and detected by a Symphony CCD detector cooled to 203 K. 3. Results and discussion Fig. 1 shows the morphology of as-deposited metallic Zn film by ion beam sputter deposition utilizing beam energies at 8, 12 and 16 keV. Metallic Zn film deposited by the lowest beam energy shows the smoothest surface consisting of nano-particles with the size of w 50 nm. As beam energy increases to 12 keV, surface
Fig. 1. FE-SEM micrographs of Zn films deposited by ion beam energy of (a) 8, (b) 12 and (c) 16 keV.
roughness increases and particles with the sizes of 100e200 nm can be found throughout the substrate. As beam energy reaches 16 keV, even larger particles with sizes of 0.7e1 mm were found instead. Fig. 1c indicates that the larger particles are due to agglomeration of smaller particles due to higher deposition rates. Metallic zinc films deposited by various beam energies were thermally oxidized at 673 K in flowing oxygen ambient for 3 h Fig. 2 shows FE-SEM micrographs of Zn films after thermal oxidation. Zn films deposited by 8 keV ion beam sputtering shows no formation of ZnO nanowires (Fig. 2a) while Zn films deposited by both 12 and 16 keV ion beam shows ZnO nanowires with an
Fig. 2. FE-SEM micrographs of ZnO nanowires prepared by 673 K oxidation of (a) 8, (b) 12 and (c) 16 keV deposited metallic zinc films.
L.-C. Chao et al. / Vacuum 86 (2011) 295e298
297
Fig. 3. A TEM image of a ZnO nanowire, insert is a SADP image indicating that ZnO nanowire is grown along the [110] direction.
average length of w1 mm evenly distributed across the substrate. Comparing Fig. 2b and c, the density of ZnO nanowires in Fig. 2b is slightly higher than that in Fig. 2c. From Fig. 3, the diameter of the nanowire decreases from 40 nm at the bottom to 20 nm at the tip, indicating that the growth mechanism is a solid state based-up diffusion process [14]. Selective area diffraction pattern (SADP) (insert of Fig. 3) indicates that the ZnO nanowire is grown along the [110] direction. The ZnO nanowires were further studied by photoluminescence (PL). Fig. 4 shows that oxidation of 12 keV deposited samples exhibit a strong near-band-edge (NBE) emission centered at 377.2 nm (3.287 eV) with a full-width at half maximum (FWHM) of 94.5 meV. ZnO nanowires obtained by oxidation of 16 keV deposited Zn films exhibits similar NBE emission peak at 377.4 nm (3.286 eV) with an FWHM of 95.0 meV and negligible defect related deep level emission. However, ZnO obtained by oxidation of 8 keV deposited Zn films results an NBE peak emission at 382.5 nm (3.242 eV) with an FWHM of 172.2 meV. Beside the red-shifted NBE emission, a strong defect-related deep level emission centered at w520 nm (2.385 eV) was also observed, indicating the presence of
Fig. 5. Field emission properties and F-N plots of ZnO nanowires prepared by thermal oxidation Zn films deposited with (a) 12 and (b) 16 keV ion beam.
oxygen vacancy defects near the surface. Fig. 5a shows fieldemission properties of ZnO nanowires prepared by thermal oxidation of Zn film deposited by 12 keV ion beam. The turn-on field and field enhancement factor are 7.93 MV/m and 691, respectively. ZnO nanowires prepared by thermal oxidation of metallic zinc films deposited by 16 keV (Fig. 5b) ion beam exhibit a higher turn-on field of 10.4 MV/m a slightly lower field enhancement factor of 500. Our results indicate that the formation of ZnO nanowire is dependent on the metallic zinc particle sizes. As ZnO film forms on metallic zinc surface, an electric field between ZneZnO and Znoxygen builds up [14,15]. This electric field and the appropriate oxidation temperature cause Zn ions to diffuse into the ZnO layer causing Zn-rich regions that form nucleation sites [16,17]. As Zn ions continue to diffuse through the Zn-rich nucleation sites they react with oxygen that results in the formation of ZnO nanowires. The interface electric field is higher on structures with larger curvatures. The surface of metallic zinc films deposited by 8 keV ion beam is relatively smooth, resulting in a smaller interface electric field. As surface roughness increases due to increased beam energy, increased interface electric field causes enhanced diffusion of zinc ions that facilitates the formation of ZnO nanwires. 4. Conclusion
Fig. 4. PL spectra of ZnO nanowires prepared by thermal oxidation of metallic zinc films deposited with ion beam energy at 8, 12 and 16 keV. The spectra are shifted vertically for clarity.
ZnO nanowires have been synthesized by thermal oxidation of metallic zinc films at 673 K without using catalysts. The
298
L.-C. Chao et al. / Vacuum 86 (2011) 295e298
surface roughness of the metallic zinc film is dependent on ion beam energy that acts as a crucial factor on the formation of ZnO nanowires. High surface roughness results in higher electric field between ZneZnO and Zn-oxygen boundaries that facilitates the out diffusion of Zn ions and the growth of ZnO nanowires. ZnO nanowire with diameter of w30 nm and length of w1 mm can be achieved and the ZnO nanowires were growing along the [110] direction. A turn-on field 7.93 MV/m and a field enhancement factor of 691 were achieved by thermal oxidation of Zn film deposited with 12 kV beam energy. This process allows preparation of large area ZnO nanowires on rigid mechanical support which is valuable for cold field emission applications.
Acknowledgement This research work was supported by National Science Council of Taiwan under contract number NSC 98-2112-M-011-MY3.
References [1] Tsukazaki A, Ohtomo A, Onuma T, Ohtani M, Mankino T, Sumiya M, et al. Nat Mater 2005;4:42. [2] Zhu YW, Zang HZ, Sun XC, Feng SQ, Xu J, Zhao Q, et al. Appl Phys Lett 2003;83: 144. [3] Rensmo H, Keis K, Lindstreöm H, Södergren S, Solbrand A, Hagfeldt A, et al. J Phys Chem B 1997;101:2598. [4] Wan Q, Li QH, Chen YJ, Wang TH, He XL, Li JP, et al. Appl Phys Lett 2004;84: 3654. [5] Li LM, Li CC, Zhang J, Du ZF, Zou BS, Yu HC, et al. Nanotechnology 2007;18: 225504. [6] Wang ZL. Mater Today 2007;10:20. [7] Huang MH, Wu Y, Feick H, Tran N, Weber E, Yang P. Adv Mater 2001;13:113. [8] Park WI, Kim DH, Jung SW, Yi GC. Appl Phys Lett 2002;80:4232. [9] Wagner RS, Ellis WC. Appl Phys Lett 1964;4:89. [10] Chen Y, Qu B, Barnakov YA, Tang Q, Chen J. J Mater Sci. Mater Electron 2009; 20:328. [11] Gui Y, Xie C, Zhang Q, Hu M, Yu J, Weng Z. J Cryst Growth 2006;289:663. [12] Ren S, Bai YF, Chen J, Deng SZ, Xu NS, Wu QB, et al. Mater Lett 2007;61:666. [13] Chao LC, Lee JW, Liau CC. J Phys D Appl Phys 2008;41:115405. [14] Yu W, Pan C. Mater Chem Phys 2009;115:74. [15] Ocal C, Ferrer S, Garcia N. Surf Sci 1985;163:335. [16] Wang N, Fung KK, Wang S, Yang S. J Cryst Growth 2001;233:226. [17] Wang ZL, Kong XY, Zuo M. Phys Rev Lett 2003;91:185502.