Zinc oxide (ZnO) is a wide band gap (~3.37 eV) semiconductor. Thin film ZnO has many attractive applications in optoelectronics and sensors. Recently, nanostructured ZnO (e.g. ZnO quantum dot) has been demonstrated as a hyperbolic material; its dielectric function has opposite signs along different crystal axes within the mid-infrared, making it an interesting material for metamaterials and nanophotonics. Conventional sputtering deposition usually leads to the formation of polycrystalline ZnO films with randomly oriented grains and rough surface. This work demonstrated a solution-based process to grow ZnO thin films with highly oriented nanocrystals. Low-temperature plasmas were employed to modulate the microstructure and optical properties of the films. Such highly anisotropic nanostructured transparent semiconductor films may lead to interesting material properties in developing new optoelectronic devices

Fan, Qi Hua

Neto, Victor

Pokharel, Jyotshna

Shrestha, Maheshwar

Zhou, Li Qin

English

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 Journal of Coating Science and Technology, 2015, 2, 46-50   46 E-ISSN: 2369-3355/15  © 2015 Lifescience Global   Journal of Coating Science and Technology http://www.lifescienceglobal.com/journals/journal-of-coating-science-and-technology  Oriented Zinc Oxide Nanocrystalline Thin Films Grown from  Sol-Gel Solution Jyotshna Pokharel1, Maheshwar Shrestha1, Li Qin Zhou2,3, Victor Neto2,3,* and Qi Hua Fan1,* 1Department of Electrical Engineering and Computer Science, South Dakota State University, Brooking, SD 57007, USA 2Centre for Mechanical Technology and Automation, Department of Mechanical Engineering, University of Aveiro, 3810-193 Aveiro, Portugal 3Aveiro Nanotechnology Institute, University of Aveiro, 3810-193 Aveiro, Portugal Abstract: Zinc oxide (ZnO) is a wide band gap (~3.37 eV) semiconductor. Thin film ZnO has many attractive applications in optoelectronics and sensors. Recently, nanostructured ZnO (e.g. ZnO quantum dot) has been demonstrated as a hyperbolic material; its dielectric function has opposite signs along different crystal axes within the mid-infrared, making it an interesting material for metamaterials and nanophotonics. Conventional sputtering deposition usually leads to the formation of polycrystalline ZnO films with randomly oriented grains and rough surface. This work demonstrated a solution-based process to grow ZnO thin films with highly oriented nanocrystals. Low-temperature plasmas were employed to modulate the microstructure and optical properties of the films. Such highly anisotropic nanostructured transparent semiconductor films may lead to interesting material properties in developing new optoelectronic devices. Received on 21-05-2015 Accepted on 12-08-2015 Published on 14-09-2015  Keywords: Zinc oxide,  sol-gel, oxygen plasma, crystal size. DOI: http://dx.doi.org/10.6000/2369-3355.2015.02.02.2  1. INTRODUCTION* Zinc oxide (ZnO) is a II–VI compound semiconductor with wide band gap ~3.37 eV [1, 2]. Thin film ZnO is highly transparent in the visible spectrum (>90%), with moderate Hall mobility ranging from 0.2 to 7 cm2V-1s-1. These characteristics combined with other interesting properties, such as a strong blue luminescence and a large acoustic velocity (6336 m/s), make ZnO attractive for use in many applications. Owing to their better stability than indium tin oxide in hydrogen plasma, ZnO thin films have been used in the fabrication of hydrogenated amorphous silicon solar cells [3, 4]. ZnO has also been used as transparent electrodes in displays and piezoelectric devices [5-7]. Wide band gap ZnO is a potential material for short‐wavelength optoelectronic devices, such as UV lasers, blue to UV light‐emitting diodes and UV detectors [8, 9], which can be applied to high density data storage systems, solid‐state lighting, secure communications and bio‐detection. Of particularly interesting, ZnO nanostructures (e.g. ZnO quantum dots) have been                                                 *Department of Electrical Engineering and Computer Science, South Dakota State University, Brooking, SD 57007, USA; Tel: 605-688-5910;  Fax: 605-688-4401; E-mail: qihua.fan@sdstate.edu Centre for Mechanical Technology and Automation, Department of Mechanical Engineering, University of Aveiro, 3810-193 Aveiro, Portugal;  E-mail: vneto@ua.pt demonstrated as a hyperbolic material; its dielectric function has opposite signs along different crystal axes within the mid-infrared, making it an attractive metamaterial and promising for nanophotonics [10]. The wurtzite crystal structure of ZnO is thermodynamically stable with oxygen atoms on hexagonal sites and zinc atoms on tetrahedral sites [11]. It exists with non-polar (1011) surface termination, polar (0001) O-termination, and polar (0001) Zn-terminated surfaces [12], resulting in termination-dependent chemical reactivity. ZnO crystallites with preferential orientation are desirable for applications where crystallographic anisotropy is a prerequisite as in UV-diode, piezoelectric surface acoustic wave or acousto-optic devices [13]. Various methods have been used to fabricate ZnO films. The common methods include pulsed laser deposition, RF magnetron sputtering, chemical vapor deposition, and spray pyrolysis [14, 15]. Polycrystalline ZnO films fabricated from these processes usually consist of randomly oriented crystals. Furthermore, vacuum-based processes are generally expensive. Solution-based sol-gel method offers simple, easy and low-cost deposition of ZnO, which can be integrated into roll-to-Journal of Coating Science and Technology, 2015, Volume 2, No. 2  Oriented Zinc Oxide Nanocrystalline Thin Films Grown 47 roll fabrication [16-18]. ZnO films prepared by solution processes usually have high resistivity. Post annealing at moderate temperatures in a proper environment is needed to reduce the resistivity. This work aims to produce nanostructured ZnO thin films with highly oriented crystals using a solution-based process. As discussed above, such films may find a broad variety of applications. Post plasma treatment is employed to verify whether or not it can modulate the film structures and optical properties at low temperatures. 2. EXPERIMENTAL PROCEDURE ZnO nanostructured thin films were deposited by sol-gel spin coating onto glass substrate. Zinc acetate [Zn(CH3COO)-2H2O],2-methoxethanol (MEA), and ethanolamine were used as the starting material, solvent, and stabilizer, respectively. The molar ratio of MEA to zinc acetate dehydrate was maintained at 1.0 and the concentration of zinc acetate was 0.35 M. The solution was stirred at 500 RPM for two hours at room temperature followed by stirring at 80o C for 1 hr to result in a clear homogeneous solution. The glass substrates were cleaned with a soapy water, DI water, acetone and IPA in sequence using a Fisher Scientific FS60D ultrasonic cleaner, each step for 10 min, respectively. The zinc oxide solution was then spin-coated on the glass substrates at 2500 RPM for 30 seconds using a LAURELL WS-650MH-23NPP/LITE spin coater. After the deposition, the film was dried at different temperatures from 200o C to 500o C for 1 hour in a furnace to remove organic residuals. The routine was repeated from coating to drying for five times to give a final film thickness ~200 nm. After the films were deposited, the samples were processed with oxygen plasma generated with a capacitively coupled RF discharge, during which the oxygen gas pressure was maintained at ~2 Torr. No external heating was applied and the substrate temperature was below 150o C during the plasma treatment. The thickness of the film was measured using VEECO DEKTAK 150 surface profiler. The transmittance measurement was carried out using FILMETRICS F20-UVX spectrophotometer. X-ray diffraction (XRD) was taken using RIGAKU SMARTLAB system. 3. RESULTS AND DISCUSSION The transmission spectra of the ZnO films fabricated at different temperatures are shown in Figure 1. The ZnO film prepared at 200oC showed good transmittance in 380-750nm range with an average transmittance above 90%. Increasingthe temperature from 200oC to 400oC, the transmittance increased slightly. The maximum point of transmittance shifted to shorter wavelength with the increase in the temperature, indicating that the film optical thickness was reduced. This was due to the removal of organic residuals at higher temperatures. Further increase the fabrication temperature to 500o C led to decrease in the transmittance. Two possible reasons contributed to this reduction in transparency: surface roughening that led to light scattering and oxygen loss that created vacancies at hightemperatures. Similar reduction in optical transparency was observed when the annealing temperature of ZnO films was increased [19].  Figure 1: Optical transmittance spectra of ZnO films prepared at different temperature. Figure 2 shows the XRD pattern of the ZnO films prepared at different temperatures. The ZnO films prepared at temperatures above 400oCshowed oriented crystal growth along (002) plane. This oriented growth confirmed that the highest density of Zn atoms was in the (002) planes as it was kinetically favorable [20]. At sufficient temperatures, the excess energy acquired by the ZnO crystallites allowed them to orient themselves along the (002) plane where the surface energy was minimum [19]. The ZnO films prepared at temperatures below 300oC did not exhibit obvious crystallization. The films weremostly amorphous. An increase in the (002) peak intensity was observed when the films were prepared at 300oC, 400oC, and 500oC, as shown in Figure 2. The gain in peak intensity was obviously due to an increase in the crystallinity with temperature.  Figure 2: XRD patterns of ZnO films prepared at different temperatures. Journal of Coating Science and Technology, 2015, Volume 2, No. 2  48 Pokharel et al. Figure 3 shows the effect of temperature on the crystal size of the ZnO films. The average crystal size increased with the process temperature. The size of crystallite was estimated from the full-width at half-maximum (FWHM) of (002) peak using the Scherrer formula [21], d =0.9Bcos B,  where  is the X-ray wavelength of 1.54 Å, B is Bragg diffraction angle, and B is the FWHM of the XRD diffraction peak. The crystal size increased from 5.9 nm at 300oC to 23.8 nm at 500oC. The increase in the temperature caused coalescence/merging of small grains by grain boundary diffusion, resulting in grain growth [22].  Figure 3: Effect of temperature on the crystal size of the ZnO films. Figure 4 compares the optical transmittance of the ZnO films prepared at 500oC followed by oxygen plasma treatment for 5, 10, 20 and 40 minutes. Overall, the transmittance slightly increased after the plasma treatment. 20-minute oxygen plasma treatment appeared sufficient, while prolonged plasma treatment (e.g. 40 minutes) led to slight decreasein the transmittance. The variation in the transmittance with the plasma treatment time was attributed to two opposite effects: 1) oxygen plasma reduced oxygen vacancies and led to the increase in the transmittance; and 2) the bombardment of charged ions on the ZnO film created defects and roughened the film surface, leading to a decrease in the transmittance. Figure 5 compares XRD spectra of the ZnO films processed at 500oC followed by oxygen plasma treatment for 5, 10, 20 and 40 minutes. The FWHM values for the XRD spectra are presented in Table 1. The FWHM value for the as-deposited ZnO film was 0.3639, while the lowest FWHM value was obtained for 20 min O2 plasma treatment. However, sharpening of the diffraction peak or lowering of the FWHM value was not seen inthe films exposed to O2 plasma beyond 20 minutes. This result was further supported by analyzing the crystal size of the ZnO films treated by O2 plasma for different time.  Figure 5: Comparison of XRD spectra of ZnO films processed at 500oC followed by oxygen plasma treatment for different times. Table 1: FWHM of XRD (002) Peak for the ZnO Films Treated with O2 Plasma O2 Plasma Treatment FWHM As-deposited 0.3639 5 min 0.3585 10 min 0.3603 20 min 0.3567 40 min 0.3652  Figure 6 shows the crystal size of the ZnO films treated by oxygen plasma. The average crystal size increased slightly with plasma treatment time till 20 minutes, after which thecrystal size decreased. The crystal size increased from 23.80 nm (as-deposited) to 24.35 nm (20-minute oxygen  Figure 4: Comparison of optical transmission spectra of ZnO films processed at 500oC followed by oxygen plasma treatment for different times. Journal of Coating Science and Technology, 2015, Volume 2, No. 2  Oriented Zinc Oxide Nanocrystalline Thin Films Grown 49 plasma). The crystal size after 40-minute oxygen plasma treatment dropped to 23.77 nm. These results indicated that oxygen plasma treatment increased the crystallinity of the ZnO films by increasing the size of the crystallites. An optimum treatment time was 20 minutes. This result was in agreement with the transmittance variation.As discussed previously, this happened due to the diffusion of oxygen from the plasma into the ZnO film and reduced oxygen vacancies. However, further increase in oxygen plasma treatment time led to excessive ion bombardment, which broke ZnO bonds.  Figure 6: Effects of oxygen plasma treatment time on the crystal size of ZnO films. 4. CONCLUSIONS Highly oriented (002) ZnO nanocrystalline thin films was produced by a solution-based process. The crystallinity of the ZnO films increased with the process temperature. The transmittance decreased slightly as the process temperature approached 500o C. Oxygen plasma treatment led to increased transmittance. However, prolonged oxygen plasma treatmentled to reduced transmittance. The variation in the transmittance was in agreement with the change in the crystal size deduced from the FWHM of the XRD (002) peak. ACKNOWLEDGEMENTS This work was supported by the National Science Foundation, award CMMI-1462389. The authors also would like to thank the support from FCT Portugal Grant No. PTDC/CTM-CER/121440/2010 and PEst-C/EME/UI0481/ 2013. REFERENCES [1] Ilican S, Caglar Y, Caglar M. Preparation and characterization of ZnO thin films deposited by sol-gel spin coating method. J Optoelectron Adv Mater 2008; 10: 2578-83. [2] Meena JS, Chu M-C, Chang Y-C, et al. Effect of oxygen plasma on the surface states of ZnO films used to produce thin-film transistors    on soft plastic sheets. J Mater Chem C 2013; 1: 6613-22. http://dx.doi.org/10.1039/c3tc31320d [3] Li H, Wang J, Liu H, et al. Sol–gel preparation of transparent zinc oxide films with highly preferential crystal orientation. Vacuum 2004; 77: 57-62. http://dx.doi.org/10.1016/j.vacuum.2004.08.003 [4] Zhou Z, Kato K, Komaki T, et al. Effects of dopants and hydrogen on the electrical conductivity of ZnO. J Eur Ceram Soc 2004; 24: 139-46. http://dx.doi.org/10.1016/S0955-2219(03)00336-4 [5] Wang D, Zhao D, Wang F, Yao B, Shen D. Nitrogen‐doped ZnO obtained by nitrogen plasma treatment. Physica Status Solidi A 2015; 212: 846-50. http://dx.doi.org/10.1002/pssa.201431779 [6] Van de Walle CG. Hydrogen as a cause of doping in zinc oxide. Phys Rev Lett 2000; 85: 1012. http://dx.doi.org/10.1103/PhysRevLett.85.1012 [7] Kumar NS, Bangera KV, Shivakumar G. Effect of annealing on the properties of zinc oxide nanofiber thin films grown by spray pyrolysis technique. Appl Nanosci 2014; 4: 209-16. http://dx.doi.org/10.1007/s13204-012-0190-9 [8] Ozgur U, Hofstetter D, Morkoc H. ZnO devices and applications: a review of current status and future prospects. Proc IEEE 2010; 98: 1255-68. http://dx.doi.org/10.1109/JPROC.2010.2044550 [9] Znaidi L. Sol–gel-deposited ZnO thin films: a review. Mater Sci Eng B 2010; 174: 18-30. http://dx.doi.org/10.1016/j.mseb.2010.07.001 [10] Fonoberov VA, Balandin AA. Polar optical phonons in wurtzite spheroidal quantum dots: theory and application to ZnO and ZnO/MgZnO nanostructures. J Phys Condens Matter 2005; 17: 1085-97. http://dx.doi.org/10.1088/0953-8984/17/7/003 [11] Hsu C-W, Cheng T-C, Yang C-H, Shen Y-L, Wu J-S, Wu S-Y. Effects of oxygen addition on physical properties of ZnO thin film grown by radio frequency reactive magnetron sputtering. J Alloys Compd 2011; 509: 1774-6. http://dx.doi.org/10.1016/j.jallcom.2010.10.037 [12] Kuo F-L, Li Y, Solomon M, Du J, Shepherd ND. Work function tuning of zinc oxide films by argon sputtering and oxygen plasma: An experimental and computational study. J Phys D Appl Phys 2012; 45: 065301. http://dx.doi.org/10.1088/0022-3727/45/6/065301 [13] Znaidi L. Sol–gel-deposited ZnO thin films: a review. Materi Sci Eng B 2010; 174: 18-30. http://dx.doi.org/10.1016/j.mseb.2010.07.001 [14] Elilarassi R, Chandrasekaran G. Effect of annealing on structural and optical properties of zinc oxide films. Mater Chem Phys 2010; 121: 378-84. http://dx.doi.org/10.1016/j.matchemphys.2010.01.053 [15] Ng Z-N, Chan K-Y, Sin Y-K, Hoon J-W, Ng S-S. Influence of post-annealing condition on the properties of ZnO films. Ceram Int 2013; 39: S263-S7. http://dx.doi.org/10.1016/j.ceramint.2012.10.074 [16] Sengupta J, Sahoo R, Mukherjee C. Effect of annealing on the structural, topographical and optical properties of sol–gel derived ZnO and AZO thin films. Mater Lett 2012; 83: 84-7. http://dx.doi.org/10.1016/j.matlet.2012.05.130 [17] Ng Z-N, Chan K-Y, Tohsophon T. Effects of annealing temperature on ZnO and AZO films prepared by sol–gel technique. Appl Surf Sci 2012; 258: 9604-9. http://dx.doi.org/10.1016/j.apsusc.2012.05.156 [18] Zak AK, Abrishami ME, Majid WA, Yousefi R, Hosseini S. Effects of annealing temperature on some structural and optical properties of ZnO nanoparticles prepared by a modified sol–gel combustion method. Ceram Int 2011; 37: 393-8. http://dx.doi.org/10.1016/j.ceramint.2010.08.017 [19] Sengupta J, Sahoo R, Bardhan K, Mukherjee C. Influence of annealing temperature on the structural, topographical and optical properties of sol–gel derived ZnO thin films. Mater Lett 2011; 65: 2572-4. http://dx.doi.org/10.1016/j.matlet.2011.06.021 [20] Zhai J, Zhang L, Yao X. The dielectric properties and optical propagation loss of c-axis oriented ZnO thin films deposited by sol–gel process. Ceram Int 2000; 26: 883-5. http://dx.doi.org/10.1016/S0272-8842(00)00031-6   Journal of Coating Science and Technology, 2015, Volume 2, No. 2  50 Pokharel et al. [21] Chen X, Zhou J, Wang H, Xu P, Pan G. In situ high temperature X-ray diffraction studies of ZnO thin film. Chinese Phys B 2011; 20: 096102. http://dx.doi.org/10.1088/1674-1056/20/9/096102  [22] Caglar Y, Ilican S, Caglar M, et al. Influence of heat treatment on the nanocrystalline structure of ZnO film deposited on p-Si. J Alloys Compd 2009; 481: 885-9. http://dx.doi.org/10.1016/j.jallcom.2009.03.140  © 2015 Pokharel et al.; Licensee Lifescience Global. This is an open access article licensed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0/) which permits unrestricted, non-commercial use, distribution and reproduction in any medium, provided the work is properly cited.    

Oriented Zinc Oxide Nanocrystalline Thin Films Grown from Sol-Gel Solution

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Oriented Zinc Oxide Nanocrystalline Thin Films Grown from Sol-Gel Solution

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