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Nanostructured ZnO films prepared by hydro-thermal chemical deposition and microwave-activated reactive sputtering
SCT-20884; No of Pages 5 Surface & Coatings Technology xxx (2016) xxx–xxx
Contents lists available at ScienceDirect
Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
Nanostructured ZnO films prepared by hydro-thermal chemical deposition and microwave-activated reactive sputtering Yahya Alajlani a,b,⁎, Frank Placido a, Des Gibson a, Hin On Chu a, Shigeng Song a, Liz Porteous a, Sarfraz Moh a a b
Institute of Thin Films, Sensors, and Imaging, Scottish Universities Physics Alliance (SUPA), University of the West of Scotland, Paisley PA1 2BE, UK Department of Physics, Faculty of Science, Jazan University, Jazan, Saudi Arabia
a r t i c l e
i n f o
Article history: Received 24 April 2015 Revised 20 January 2016 Accepted in revised form 21 January 2016 Available online xxxx Keywords: Nanorods Sputtering ZnO Crystallinity
a b s t r a c t Nanostructured, highly porous, films of zinc oxide have been prepared by hydro-thermal chemical deposition and by microwave-activated reactive sputtering for applications in sensors and solar cells. Scanning electron microscopy, X-ray diffraction, optical constant measurements, and Raman spectroscopy are presented demonstrating the pronounced effect of microwave power on the nanostructure of films prepared by microwave-activated reactive sputtering and the marked differences between films grown by the two methods. While the structures obtained by hydro-thermal chemical deposition are highly crystalline and grow as nanorods, the microwaveactivated reactive sputtering films are initially dense with subsequent increase in porosity, leading to unusual cylindrical structures with hemi-spherical caps. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Zinc oxide is a highly functional material in the field of solar cell production given its electrical, optical, and wide bandgap semiconducting properties (approximately 3.4 eV) [1]. Consequently, ZnO nanostructures are commonly used in the production of photovoltaic cells [2].There are numerous methods available for the growth of ZnO nanowire thin films, such as physical vapour deposition (PVD), chemical vapour deposition (CVD), laser ablation, solution method and sputtering [3,4,5]. It is known that the synthesis of ZnO nanowire thin films from hydro-thermal chemical deposition (HCD) is relatively inexpensive, simple, and produces highly-crystalline nanowires [6,7]. However, it is not yet possible to grow nanowires on areas sufficient for large scale industrial production of photovoltaic cells. Sputtering is a commonly used technique in the production of ZnO thin films [8] and can be readily scaled up to large areas. It is therefore of interest to compare the properties of nanostructures made possible by particular variations of sputtering with those of HCD nanowires. The microwave-activated reactive sputtering (MARS) technique is normally known for its ability to produce dense films [9] that are ideally suited to applications in optical filters. This technique is not known to have been used to produce ZnO nanowires. However earlier investigations by the authors of ZnO films using MARS [9] indicated some
⁎ Corresponding author at: Institute of Thin Films, Sensors, and Imaging, Scottish Universities Physics Alliance (SUPA), University of the West of Scotland, Paisley PA1 2BE, UK. E-mail address: [email protected] (Y. Alajlani).
samples had unusually porous structures. In this study we report on experiments to reproduce promising film nano-structures that are initially dense, but have subsequent increase in porosity with thickness, leading to unusual cylindrical structures with hemi-spherical caps. As they can be produced in a single processing step, with no post-deposition etching or other intervention, such films show promise for use in both dye sensitised and hetero-junction solar cells, where large interfacial surface areas are desirable. As the MARS technique does not require any substrate heating, there is also potential to produce large area coatings on polymer substrates. The structures obtained by HCD and MARS were analysed by scanning electron microscopy (SEM) in order to determine the surface topography, confirm the thickness, and evaluate the nanostructure; by x-ray diffraction (XRD) in order to determine crystallinity [10]; by optical measurements in order to determine the transmission and reflection of light wavelengths and by Raman spectroscopy, primarily to determine whether the characteristic nanowire peaks were observed at (104, 337, 440 and 539 cm−1) [11,12]. 2. Characterisation Samples were imaged at various magnifications using a Hitachi S4100 field emission SEM. This system has magnification of 40 times, resolution of 1.5 nm, and acceleration voltage for primary electrons of up to 30 kV. The crystalline structure of the ZnO nanowire layers was determined by X-ray diffractometry (XRD) (Siemens D5000) with CuK α radiation (40 kV, 30 mA). The diffraction angle was set between 20° and 60° with 1 scan (count) per second at 0.2 increments. The transmission and reflection spectra of the ZnO films, deposited on micro slide
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Please cite this article as: Y. Alajlani, et al., Surf. Coat. Technol. (2016), http://dx.doi.org/10.1016/j.surfcoat.2016.01.036
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Y. Alajlani et al. / Surface & Coatings Technology xxx (2016) xxx–xxx
Table 1 MARS parameters. Combination
DC pulse power (kW)
Microwave power (kW)
Oxygen PP (Torr)
1 2 3 4
3.00 2.50 2.50 2.50
3.00 2.00 2.00 1.70
7.0 × 10−4 1.5 × 10−3 1.4 × 10–3 1.4 × 10−3
glass substrates by both methods, were measured using an Aquila Instruments' nkd-8000 spectrometer with Pro-Optix software. Samples were all examined in S-polarised light at 10° angle of incidence, both in transmission and reflection, over the 350–1100 nm wavelength range. The data was fitted using a Cauchy model, enabling calculation of refractive index. Raman scattering measurements were taken using a Thermo Scientific DXR Raman Microscope. Samples were focused using an X10 objective, photobleached for 6 s then exposed for 50 s 5 times using a 455 nm laser set to 5.0 mW. A 455 nm filter and fullrange grating was used. Fluorescence corrections and medium cosmic ray thresholds were applied to the data collection.
3. HCD technique 3.1. Experimental procedure 15 mm by 12 mm glass substrates (standard microscope slide cut by a Disco Model DAD 3230 dicing saw) were cleaned using an ultrasonic system (Optimal UCS40). Zinc seed layers of various thicknesses (30 nm, 50 nm, 95 nm, 135 nm, and 170 nm) were sputtered (Nordiko 3750 DC sputtering machine) using argon gas from a zinc metal target (99% pure zinc) onto the glass substrates. As per similar research in this field [7], the glass substrates were then suspended in zinc nitrate hexahydrate solution (20 mM of (Zn(NO3)2·6H2O) in 80 ml of
deionised water) for 6 h at 85 °C. Ammonia hydroxide (35 wt.% NH3 in water 99.99%) was added to adjust the pH of the growth solution (pH ranged from 10.2–10.4). The suspension of the substrates in the solution for different durations, time, and pH results in different growth patterns of ZnO nanowires on the seed layer. All substrates were characterised by SEM and for optical properties. It was found that substrates with 50 nm zinc seed layers produced the best results and these results are noted in the paper for comparison with the MARS technique.
4. MARS technique 4.1. Experimental procedure The MARS technique is based on reactive pulsed DC magnetron sputtering but with the addition of a microwave plasma source. The basic concept has a large rotating drum on which substrates are placed, successively passing one or more magnetrons and a separate plasma region formed by the microwave. Our system (MicroDyn 40,000 Sputtering System) employs a rectangular zinc target (375 × 125 × 12 mm), a 3 kW microwave source, 10 kW pulsed sputtering power supply, argon/oxygen gases, oxygen partial pressure control and a quartz crystal thickness monitor. The nature of the films formed on the substrate differs according to the power of the DC pulse, power of the microwave radiation creating the plasma, and the partial pressure of the oxygen within the argonoxygen environment. For most metal oxide films made by this method (SiO2, ZrO2, TiO2, Nb2O5, ITO, HfO2) the cross-sections show highly dense films with no obvious columnar structure. Unusually, for ZnO films we have found that the nanostructure of the films can be varied from fully dense to highly porous structures. As noted previously, earlier investigations by the authors of ZnO films using MARS indicated some samples had unusually porous structures [9]. It is therefore speculated that the formation of ZnO porous structures in MARS is influenced by
Fig. 1. SEM of ZnO nanowire produced by HCD. Diameters were measured to be 78.3 nm and nominal thickness of 712 nm. Top row shows the surface from top view and the second row is cross section images. Images on the left have a 5 μm scale bar and right side images have 1 μm scale bars.
Please cite this article as: Y. Alajlani, et al., Surf. Coat. Technol. (2016), http://dx.doi.org/10.1016/j.surfcoat.2016.01.036
Y. Alajlani et al. / Surface & Coatings Technology xxx (2016) xxx–xxx
3
Fig. 2. SEM images of ZnO nanowires for each combination.
variation of the parameters used when re-sputtering at high microwave powers. Several experiments were undertaken where the DC pulse power, microwave power, or oxygen partial pressure were varied with the remaining two being kept constant. The process was halted once a thickness of 500 nm was achieved.
Most of the combinations studied produced smooth or slightly structured films, however, it was found that four specific combinations of DC pulse power, microwave power, and oxygen partial pressure produced highly porous and nanowire-like structures. These combinations can be seen in Table 1. It can be seen that as the microwave power decreases, porous structures are only obtained when the DC pulse power decreases and the oxygen PP increases.
4.2. MARS setup combinations The default value for DC pulse power was 3 kW, for microwave power was 3 kW and for oxygen partial pressure 7 × 10−4 Torr. Argon gas flow was fixed at 150 sccm for all experiments. When DC power was varied, values of 2 kW, 2.2 kW, 2.5 kW, 2.7 kW, and 3 kW were used. When microwave power was varied, values of 1.7 kW, 2 kW, 2.1 kW, 2.2 kW, 2.5, 2.8 kW, and 3 kW were used. When partial pressure (PP) was varied, values of 7 × 10− 4 Torr, 1.4 × 10−3 Torr, 1.5 × 10−3 Torr, 2.3 × 10−3 Torr, and 3 × 10−3 Torr were used.
5. Results 5.1. SEM results An SEM image of nanowires produced by the HCD process can be seen in Fig. 1. As can be seen from Fig. 1, the nanowires have clear hexagonal cross-sections indicating a highly crystalline structure and porosity that are growing as nanorods of approximately 78.3 nm in diameter.
Fig. 3. XRD results of ZnO nanowire thin film by HCD (a) and MARS (b).
Please cite this article as: Y. Alajlani, et al., Surf. Coat. Technol. (2016), http://dx.doi.org/10.1016/j.surfcoat.2016.01.036
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Y. Alajlani et al. / Surface & Coatings Technology xxx (2016) xxx–xxx
Fig. 4. Optical properties of ZnO nanowires thin film by HCD (a) and MARS (b).
Table 2 Refractive index of ZnO films produced by HCD and MARS. Film
Refractive index (n)
Comb. 1 Comb. 2 Comb. 3 Comb. 4 HCD
1.92 1.98 1.95 1.91 1.38
SEM images for each MARS combination can be seen in Fig. 2. Combination 1 resulted in cylindrical structures with hemi-spherical caps, with a diameter of approximately 57.4 nm, and clear separation, giving a porous surface structure. It can also been seen that the other combination produce similar films but with smaller separation of nanowires and so poorer porosity. 5.2. XRD results XRD results can be seen in Fig. 3. A peak in XRD at 34.51° for the HCD results and 34.46° for the MARS results is indicative in both cases of the presence of ZnO [13] — in general, the greater the intensity of the peak, the greater the crystallinity. It can be seen from Fig. 1 that the HCD process produces a crystalline thin film structure, and from Fig. 3 that each of the four combinations produces a crystalline thin film structure. However, given the differences in intensity, it is presumed that Combination 1 is the most crystalline and Combination 3 the most amorphous. However, the Full Width Half Maximum (FWHM) method of determining crystallinity indicates that HCD nanowires were significantly more crystalline than the MARS nanowires. Coupled with information of the FWHM, Combination
1 should be considered as more crystalline than Combinations 2 and 4. While Combination 3 resulted in a smaller FWHM, a shift in peak position also features a significantly lower peak. This suggests that the nanowires may have different crystal orientations and grain sizes to the other mentioned sputtering parameters. 5.3. Optical properties Optical property results can be seen in Fig. 4. It can be seen from Fig. 4a that the ZnO nanowires show no optical interference fringes with transmission increasing towards 1100 nm. This could be due to thin films produced by HCD being very inhomogeneous with a rough surface topography as shown by the SEM image. Also, the specular reflectance is very low (below 0.05 over this range of wavelengths) due to light scattering. As expected for ZnO thin films, there was virtually no light transmission below 350 nm. It can be seen from Fig. 4b that for all combinations, the MARS ZnO films show interference fringes. The films are highly transmissive between 400 nm and 1100 nm with low absorption/scattering. As with the HCD films, there was virtually no light transmission below 350 nm. The refractive indexes of the films produce by MARS and HCD were measured and can be seen in Table 2. It can be seen that the observed refractive index of the films produced by HCD is lower than the films produced by MARS indicating the more porous nature of film produced by HCD. 5.4. HCD Raman spectroscopy results Raman spectroscopy results can be seen in Fig. 5. As can be seen from Fig. 5a, ZnO nanowires by HCD feature peaks in the Raman spectroscopy that is indicative of the presence of ZnO nanowires (peaks at 104, 337 and 440 cm−1 wavenumber).
Fig. 5. Raman spectroscopy of ZnO nanowire thin film by HCD (a) and MARS (b).
Please cite this article as: Y. Alajlani, et al., Surf. Coat. Technol. (2016), http://dx.doi.org/10.1016/j.surfcoat.2016.01.036
Y. Alajlani et al. / Surface & Coatings Technology xxx (2016) xxx–xxx
As can be seen from Fig. 5b, all combinations feature a peak in the Raman spectroscopy that is indicative of the presence of ZnO nanowires (104, 440 and 539 cm−1 wavenumber). There is no peak at 337 and one instead at 539. This again is indicative of the presence of ZnO nanowires and the difference in the two peaks could be due to crystal orientation. 6. Conclusions The aim of this research is to find a fast and clean method producing a highly porous crystalline ZnO nanowire thin film that can cover large areas for the potential production of solar cells and metal oxide sensors. ZnO nanowire thin films were found to be produced by both the MARS technique and the HCD process as confirmed by Raman spectroscopy. The thin films produced were very different in terms of optical properties but with similar crystallinity (as analysed by XRD). However, SEM showed that the ZnO nanowires produced by the HCD process appeared to be more crystalline in structure, with hexagonal cross-sections whereas those produced by the MARS technique appear to have cylindrical cross-sections with hemi-spherical caps. The MARS nanowires were also potentially good (depending on combination), but less porous than the nanowires produced by the HCD process.
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[2] J.B. Baxter, E.S. Aydil, Nanowire-based dye-sensitized solar cells, Appl. Phys. Lett. 86 (5) (2005) 053114/1–053114/3. [3] Ü. Özgür, Y.I. Alivov, C. Liu, A. Teke, M.A. Reshchikov, S. Doğan, et al., A comprehensive review of ZnO materials and devices, J. Appl. Phys. 98 (4) (2005) 041301. [4] L.E. Greene, M. Law, D.H. Tan, M. Montano, J. Goldberger, G. Somorjai, et al., General route to vertical ZnO nanowire arrays using textured ZnO seeds, Nano Lett. 5 (7) (2005) 1231–1236. [5] J.S. Lee, K. Park, M.I. Kang, I.W. Park, S.W. Kim, W.K. Cho, et al., ZnO nanomaterials synthesized from thermal evaporation of ball-milled ZnO powders, J. Cryst. Growth 254 (3) (2003) 423–431. [6] I. Gonzalez-Valls, M. Lira-Cantu, Vertically-aligned nanostructures of ZnO for excitonic solar cells: a review, Energy Environ. Sci. 2 (1) (2009) 19–34. [7] Y.T. Yong, Controlled growth of well-aligned ZnO nanorod array using a novel solution method, J. Phys. Chem. B (2005) 19263–19269. [8] L.P. Schule, Properties and Characterisation of Sputtered ZnOPhD Thesis University of Canterbury, Christchurch, New Zealand, 2008. [9] S. Moh, Microwave Assisted Sputtered CoatingsPhD Thesis University of the West of Scotland, Paisley, Scotland, UK, 2012. [10] J. Lindsay, Growth of ZnO nanorod arrays on flexible PDMS substrates, Prod. Finish. (Cincinnati) 76 (6) (2012) 14-14. [11] R. Zhang, P.-G. Yin, N. Wang, L. Guo, Photoluminescence and Raman scattering of ZnO nanorods, Solid State Sci. 11 (2009) 865–869. [12] K.A. Alim, V.A. Fonoberov, M. Shamsa, A.A. Balandin, Micro-Raman investigation of optical phonons in ZnO nanocrystals, J. Appl. Phys. 97 (2005) 124313. [13] M. Saleem, L. Fang, A. Wakeel, M. Rashad, C.K. Kong, Simple preparation and characterisation of nano-crystalline zing oxide thin films sol–gel method on glass substrate, World J. Condens. Matter Phys. 2 (2012) 10–15.
References [1] L. Schmidt-Mende, J.L. MacManus-Driscoll, ZnO—nanostructures, defects, and devices, Mater. Today 10 (5) (2007) 40–48.
Please cite this article as: Y. Alajlani, et al., Surf. Coat. Technol. (2016), http://dx.doi.org/10.1016/j.surfcoat.2016.01.036
Contents lists available at ScienceDirect
Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
Nanostructured ZnO films prepared by hydro-thermal chemical deposition and microwave-activated reactive sputtering Yahya Alajlani a,b,⁎, Frank Placido a, Des Gibson a, Hin On Chu a, Shigeng Song a, Liz Porteous a, Sarfraz Moh a a b
Institute of Thin Films, Sensors, and Imaging, Scottish Universities Physics Alliance (SUPA), University of the West of Scotland, Paisley PA1 2BE, UK Department of Physics, Faculty of Science, Jazan University, Jazan, Saudi Arabia
a r t i c l e
i n f o
Article history: Received 24 April 2015 Revised 20 January 2016 Accepted in revised form 21 January 2016 Available online xxxx Keywords: Nanorods Sputtering ZnO Crystallinity
a b s t r a c t Nanostructured, highly porous, films of zinc oxide have been prepared by hydro-thermal chemical deposition and by microwave-activated reactive sputtering for applications in sensors and solar cells. Scanning electron microscopy, X-ray diffraction, optical constant measurements, and Raman spectroscopy are presented demonstrating the pronounced effect of microwave power on the nanostructure of films prepared by microwave-activated reactive sputtering and the marked differences between films grown by the two methods. While the structures obtained by hydro-thermal chemical deposition are highly crystalline and grow as nanorods, the microwaveactivated reactive sputtering films are initially dense with subsequent increase in porosity, leading to unusual cylindrical structures with hemi-spherical caps. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Zinc oxide is a highly functional material in the field of solar cell production given its electrical, optical, and wide bandgap semiconducting properties (approximately 3.4 eV) [1]. Consequently, ZnO nanostructures are commonly used in the production of photovoltaic cells [2].There are numerous methods available for the growth of ZnO nanowire thin films, such as physical vapour deposition (PVD), chemical vapour deposition (CVD), laser ablation, solution method and sputtering [3,4,5]. It is known that the synthesis of ZnO nanowire thin films from hydro-thermal chemical deposition (HCD) is relatively inexpensive, simple, and produces highly-crystalline nanowires [6,7]. However, it is not yet possible to grow nanowires on areas sufficient for large scale industrial production of photovoltaic cells. Sputtering is a commonly used technique in the production of ZnO thin films [8] and can be readily scaled up to large areas. It is therefore of interest to compare the properties of nanostructures made possible by particular variations of sputtering with those of HCD nanowires. The microwave-activated reactive sputtering (MARS) technique is normally known for its ability to produce dense films [9] that are ideally suited to applications in optical filters. This technique is not known to have been used to produce ZnO nanowires. However earlier investigations by the authors of ZnO films using MARS [9] indicated some
⁎ Corresponding author at: Institute of Thin Films, Sensors, and Imaging, Scottish Universities Physics Alliance (SUPA), University of the West of Scotland, Paisley PA1 2BE, UK. E-mail address: [email protected] (Y. Alajlani).
samples had unusually porous structures. In this study we report on experiments to reproduce promising film nano-structures that are initially dense, but have subsequent increase in porosity with thickness, leading to unusual cylindrical structures with hemi-spherical caps. As they can be produced in a single processing step, with no post-deposition etching or other intervention, such films show promise for use in both dye sensitised and hetero-junction solar cells, where large interfacial surface areas are desirable. As the MARS technique does not require any substrate heating, there is also potential to produce large area coatings on polymer substrates. The structures obtained by HCD and MARS were analysed by scanning electron microscopy (SEM) in order to determine the surface topography, confirm the thickness, and evaluate the nanostructure; by x-ray diffraction (XRD) in order to determine crystallinity [10]; by optical measurements in order to determine the transmission and reflection of light wavelengths and by Raman spectroscopy, primarily to determine whether the characteristic nanowire peaks were observed at (104, 337, 440 and 539 cm−1) [11,12]. 2. Characterisation Samples were imaged at various magnifications using a Hitachi S4100 field emission SEM. This system has magnification of 40 times, resolution of 1.5 nm, and acceleration voltage for primary electrons of up to 30 kV. The crystalline structure of the ZnO nanowire layers was determined by X-ray diffractometry (XRD) (Siemens D5000) with CuK α radiation (40 kV, 30 mA). The diffraction angle was set between 20° and 60° with 1 scan (count) per second at 0.2 increments. The transmission and reflection spectra of the ZnO films, deposited on micro slide
http://dx.doi.org/10.1016/j.surfcoat.2016.01.036 0257-8972/© 2016 Elsevier B.V. All rights reserved.
Please cite this article as: Y. Alajlani, et al., Surf. Coat. Technol. (2016), http://dx.doi.org/10.1016/j.surfcoat.2016.01.036
2
Y. Alajlani et al. / Surface & Coatings Technology xxx (2016) xxx–xxx
Table 1 MARS parameters. Combination
DC pulse power (kW)
Microwave power (kW)
Oxygen PP (Torr)
1 2 3 4
3.00 2.50 2.50 2.50
3.00 2.00 2.00 1.70
7.0 × 10−4 1.5 × 10−3 1.4 × 10–3 1.4 × 10−3
glass substrates by both methods, were measured using an Aquila Instruments' nkd-8000 spectrometer with Pro-Optix software. Samples were all examined in S-polarised light at 10° angle of incidence, both in transmission and reflection, over the 350–1100 nm wavelength range. The data was fitted using a Cauchy model, enabling calculation of refractive index. Raman scattering measurements were taken using a Thermo Scientific DXR Raman Microscope. Samples were focused using an X10 objective, photobleached for 6 s then exposed for 50 s 5 times using a 455 nm laser set to 5.0 mW. A 455 nm filter and fullrange grating was used. Fluorescence corrections and medium cosmic ray thresholds were applied to the data collection.
3. HCD technique 3.1. Experimental procedure 15 mm by 12 mm glass substrates (standard microscope slide cut by a Disco Model DAD 3230 dicing saw) were cleaned using an ultrasonic system (Optimal UCS40). Zinc seed layers of various thicknesses (30 nm, 50 nm, 95 nm, 135 nm, and 170 nm) were sputtered (Nordiko 3750 DC sputtering machine) using argon gas from a zinc metal target (99% pure zinc) onto the glass substrates. As per similar research in this field [7], the glass substrates were then suspended in zinc nitrate hexahydrate solution (20 mM of (Zn(NO3)2·6H2O) in 80 ml of
deionised water) for 6 h at 85 °C. Ammonia hydroxide (35 wt.% NH3 in water 99.99%) was added to adjust the pH of the growth solution (pH ranged from 10.2–10.4). The suspension of the substrates in the solution for different durations, time, and pH results in different growth patterns of ZnO nanowires on the seed layer. All substrates were characterised by SEM and for optical properties. It was found that substrates with 50 nm zinc seed layers produced the best results and these results are noted in the paper for comparison with the MARS technique.
4. MARS technique 4.1. Experimental procedure The MARS technique is based on reactive pulsed DC magnetron sputtering but with the addition of a microwave plasma source. The basic concept has a large rotating drum on which substrates are placed, successively passing one or more magnetrons and a separate plasma region formed by the microwave. Our system (MicroDyn 40,000 Sputtering System) employs a rectangular zinc target (375 × 125 × 12 mm), a 3 kW microwave source, 10 kW pulsed sputtering power supply, argon/oxygen gases, oxygen partial pressure control and a quartz crystal thickness monitor. The nature of the films formed on the substrate differs according to the power of the DC pulse, power of the microwave radiation creating the plasma, and the partial pressure of the oxygen within the argonoxygen environment. For most metal oxide films made by this method (SiO2, ZrO2, TiO2, Nb2O5, ITO, HfO2) the cross-sections show highly dense films with no obvious columnar structure. Unusually, for ZnO films we have found that the nanostructure of the films can be varied from fully dense to highly porous structures. As noted previously, earlier investigations by the authors of ZnO films using MARS indicated some samples had unusually porous structures [9]. It is therefore speculated that the formation of ZnO porous structures in MARS is influenced by
Fig. 1. SEM of ZnO nanowire produced by HCD. Diameters were measured to be 78.3 nm and nominal thickness of 712 nm. Top row shows the surface from top view and the second row is cross section images. Images on the left have a 5 μm scale bar and right side images have 1 μm scale bars.
Please cite this article as: Y. Alajlani, et al., Surf. Coat. Technol. (2016), http://dx.doi.org/10.1016/j.surfcoat.2016.01.036
Y. Alajlani et al. / Surface & Coatings Technology xxx (2016) xxx–xxx
3
Fig. 2. SEM images of ZnO nanowires for each combination.
variation of the parameters used when re-sputtering at high microwave powers. Several experiments were undertaken where the DC pulse power, microwave power, or oxygen partial pressure were varied with the remaining two being kept constant. The process was halted once a thickness of 500 nm was achieved.
Most of the combinations studied produced smooth or slightly structured films, however, it was found that four specific combinations of DC pulse power, microwave power, and oxygen partial pressure produced highly porous and nanowire-like structures. These combinations can be seen in Table 1. It can be seen that as the microwave power decreases, porous structures are only obtained when the DC pulse power decreases and the oxygen PP increases.
4.2. MARS setup combinations The default value for DC pulse power was 3 kW, for microwave power was 3 kW and for oxygen partial pressure 7 × 10−4 Torr. Argon gas flow was fixed at 150 sccm for all experiments. When DC power was varied, values of 2 kW, 2.2 kW, 2.5 kW, 2.7 kW, and 3 kW were used. When microwave power was varied, values of 1.7 kW, 2 kW, 2.1 kW, 2.2 kW, 2.5, 2.8 kW, and 3 kW were used. When partial pressure (PP) was varied, values of 7 × 10− 4 Torr, 1.4 × 10−3 Torr, 1.5 × 10−3 Torr, 2.3 × 10−3 Torr, and 3 × 10−3 Torr were used.
5. Results 5.1. SEM results An SEM image of nanowires produced by the HCD process can be seen in Fig. 1. As can be seen from Fig. 1, the nanowires have clear hexagonal cross-sections indicating a highly crystalline structure and porosity that are growing as nanorods of approximately 78.3 nm in diameter.
Fig. 3. XRD results of ZnO nanowire thin film by HCD (a) and MARS (b).
Please cite this article as: Y. Alajlani, et al., Surf. Coat. Technol. (2016), http://dx.doi.org/10.1016/j.surfcoat.2016.01.036
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Fig. 4. Optical properties of ZnO nanowires thin film by HCD (a) and MARS (b).
Table 2 Refractive index of ZnO films produced by HCD and MARS. Film
Refractive index (n)
Comb. 1 Comb. 2 Comb. 3 Comb. 4 HCD
1.92 1.98 1.95 1.91 1.38
SEM images for each MARS combination can be seen in Fig. 2. Combination 1 resulted in cylindrical structures with hemi-spherical caps, with a diameter of approximately 57.4 nm, and clear separation, giving a porous surface structure. It can also been seen that the other combination produce similar films but with smaller separation of nanowires and so poorer porosity. 5.2. XRD results XRD results can be seen in Fig. 3. A peak in XRD at 34.51° for the HCD results and 34.46° for the MARS results is indicative in both cases of the presence of ZnO [13] — in general, the greater the intensity of the peak, the greater the crystallinity. It can be seen from Fig. 1 that the HCD process produces a crystalline thin film structure, and from Fig. 3 that each of the four combinations produces a crystalline thin film structure. However, given the differences in intensity, it is presumed that Combination 1 is the most crystalline and Combination 3 the most amorphous. However, the Full Width Half Maximum (FWHM) method of determining crystallinity indicates that HCD nanowires were significantly more crystalline than the MARS nanowires. Coupled with information of the FWHM, Combination
1 should be considered as more crystalline than Combinations 2 and 4. While Combination 3 resulted in a smaller FWHM, a shift in peak position also features a significantly lower peak. This suggests that the nanowires may have different crystal orientations and grain sizes to the other mentioned sputtering parameters. 5.3. Optical properties Optical property results can be seen in Fig. 4. It can be seen from Fig. 4a that the ZnO nanowires show no optical interference fringes with transmission increasing towards 1100 nm. This could be due to thin films produced by HCD being very inhomogeneous with a rough surface topography as shown by the SEM image. Also, the specular reflectance is very low (below 0.05 over this range of wavelengths) due to light scattering. As expected for ZnO thin films, there was virtually no light transmission below 350 nm. It can be seen from Fig. 4b that for all combinations, the MARS ZnO films show interference fringes. The films are highly transmissive between 400 nm and 1100 nm with low absorption/scattering. As with the HCD films, there was virtually no light transmission below 350 nm. The refractive indexes of the films produce by MARS and HCD were measured and can be seen in Table 2. It can be seen that the observed refractive index of the films produced by HCD is lower than the films produced by MARS indicating the more porous nature of film produced by HCD. 5.4. HCD Raman spectroscopy results Raman spectroscopy results can be seen in Fig. 5. As can be seen from Fig. 5a, ZnO nanowires by HCD feature peaks in the Raman spectroscopy that is indicative of the presence of ZnO nanowires (peaks at 104, 337 and 440 cm−1 wavenumber).
Fig. 5. Raman spectroscopy of ZnO nanowire thin film by HCD (a) and MARS (b).
Please cite this article as: Y. Alajlani, et al., Surf. Coat. Technol. (2016), http://dx.doi.org/10.1016/j.surfcoat.2016.01.036
Y. Alajlani et al. / Surface & Coatings Technology xxx (2016) xxx–xxx
As can be seen from Fig. 5b, all combinations feature a peak in the Raman spectroscopy that is indicative of the presence of ZnO nanowires (104, 440 and 539 cm−1 wavenumber). There is no peak at 337 and one instead at 539. This again is indicative of the presence of ZnO nanowires and the difference in the two peaks could be due to crystal orientation. 6. Conclusions The aim of this research is to find a fast and clean method producing a highly porous crystalline ZnO nanowire thin film that can cover large areas for the potential production of solar cells and metal oxide sensors. ZnO nanowire thin films were found to be produced by both the MARS technique and the HCD process as confirmed by Raman spectroscopy. The thin films produced were very different in terms of optical properties but with similar crystallinity (as analysed by XRD). However, SEM showed that the ZnO nanowires produced by the HCD process appeared to be more crystalline in structure, with hexagonal cross-sections whereas those produced by the MARS technique appear to have cylindrical cross-sections with hemi-spherical caps. The MARS nanowires were also potentially good (depending on combination), but less porous than the nanowires produced by the HCD process.
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Please cite this article as: Y. Alajlani, et al., Surf. Coat. Technol. (2016), http://dx.doi.org/10.1016/j.surfcoat.2016.01.036