CHARACTERISATION OF ELECTRODEPOSITED AND SPUTTERED ZINC OXIDE J.S. Wellings, N.B. Chaure, S.N. Heavens1, P. Warren2, I.M. Dharmadasa Solar Energy Group, Materials & Engineering Research Institute, Sheffield Hallam University, Sheffield, S1 1WB UK Tel: +44 (0)114 225 5292, Fax:+44 (0)114 225 3433, E-mail
[email protected] 1 Ionotec Ltd, 14 Berkeley Court, Manor Park, Runcorn, Cheshire, WA7 1TQ UK 2 Pilkington Group Ltd, Hall Lane, Lathom, Ormskirk, Lancashire, L40 5UF UK ABSTRACT: Electrodeposited zinc oxide (ZnO) thin films have been characterised to compare with bench mark material produced using RF-sputtering by Pilkington Group Ltd. ZnO was electrodeposited from an aqueous zinc nitrate solution at 80°C onto fluorine doped tin oxide (FTO) coated glass substrates. Structural, morphological and optical properties of both electrodeposited and sputtered ZnO was carried out using X-ray diffraction, scanning electron microscopy (SEM) and optical absorbance. Diffractograms showed that electrodeposited has the strongest peak along the (002) plane and the sputtered ZnO highly textures along the (101) plane. The SEM micrographs showed the electrodeposited ZnO to have hexagonal morphology with grain size ~1.0 µm and sputtered ZnO to be nanocrystalline. Optical studies identified the bandgap of electrodeposited and sputtered ZnO to be ~3.22 eV and 3.28 eV respectively. Keywords: Electrodeposited ZnO; Sputtered ZnO; Characterisation & Comparison 1
INTRODUCTION
Zinc oxide (ZnO) is a non-toxic, II-VI, wide bandgap semiconductor with Eg=3.20 eV and natural n-type electrical conductivity. Among various other applications such as laser diodes [1], piezoelectric transducers [2], transistors [3] and phosphors [4], ZnO is used as a transparent buffer layer in copper indium gallium diselenide (CIGS) solar cell devices [5]. Usually, ZnO is deposited using RF-sputtering for solar cell device application [6], due to its relatively low cost of deposition. Drawbacks to using such a technique are high cost of equipment and complexity of operation. ZnO can also be grown by electrodeposition [7-9], which has low equipment and production cost and is simple to operate. Electrodeposition also has the advantage of having low temperature processing, allows various substrate shapes and controllable film thickness. This technique also avoids the use of vacuum systems allowing growth under normal laboratory conditions [10]. In the present work, properties of electrodeposited ZnO thin films were compared with RF-sputtered ZnO in order to assess the quality of electrodeposited ZnO for use in CIGS solar cell devices. The structural, morphological and optical properties of both materials were studied using X-ray diffraction (XRD), scanning electron microscopy (SEM) and UV-vis spectrophotometer respectively. 2
EXPERIMENTIAL
Glass substrates coated with fluorine-doped tin oxide (FTO) having a sheet resistance of 15 Ω/ supplied by Pilkington Group Ltd were used as the working electrode for the electrodeposition of ZnO. Prior to the deposition, substrates were washed thoroughly with boiled, deionised water and rinsed before being cleaned ultrasonically. ZnO thin films were electrodeposited potentiostatically using a conventional three-electrode system, with graphite plate counter electrode and silver/silver chloride (Ag/AgCl) (+220 mV vs normal hydrogen electrode at room temperature) reference electrode. GillAC version 4, computerised potentiostat/galvanostat was employed to
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perform the potentiostatic deposition. The aqueous solution contained 0.1M zinc nitrate, Zn(NO3)2•6H2O dissolved in 150 ml deionised water. The temperature was maintained at 80°C (± 2°C). Samples with dimensions 15 × 30 mm2 were grown at various cathodic potentials between -0.900 and -1.100 V vs Ag/AgCl to study various properties. The thickness of the electrodeposited ZnO was ~100 nm when grown for 1 min. The electrodeposited films were annealed for 15 mins at 550°C. Pilkington Group Ltd supplied RFsputtered ZnO thin films on glass substrates, of thickness 100 nm. This material was also annealed under the same conditions as for electrodeposited ZnO. XRD was carried out using a Philips X-Pert Prodiffractometer with Cu Kα1 source (λ = 1.5406 Å) at a glancing angle of 0.5°. SEM was carried out using Philips XL 30 ESEM-Field-Emission Gun to study the surface morphology. Optical absorption spectra were taken using UV-Vis spectrometer, Unicam UV-2, to determine the absorption edge and bandgap of the ZnO layers. 3
RESULTS AND DISCUSSION
3.1 X-ray Diffraction XRD studies have been carried out on annealed ZnO layers in order to determine phase and crystallographic analysis. Typical XRD of electrodeposited ZnO film is shown in Fig 1a. The diffractogram shows the material to be polycrystalline with hexagonal wurtzite phase and strongest peak is also along the (002) plane. Fig 1b illustrates the diffractogram of sputtered ZnO. The material is highly textured to the (101) plane.
Fig 1. Typical XRD spectra of (a) electrodeposited ZnO and (b) sputtered ZnO.
3.2 Scanning Electron Microscopy SEM was used to investigate the surface morphology. Figure 2a illustrate the electrodeposited layers, showing compact, void-free surfaces, having grain size ~1.0 µm and varying orientation. The micrograph shown in Fig 2b illustrates the appearance of the sputtered ZnO. The grains are highly compact and nanocrystalline. This difference in morphology may be attributed to the difference in substrate. Ideally the substrates should be the same, ZnO has been produced and work is progressing to study this material for comparison.
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CONCLUSION In conclusion the sputtered and electrodeposited ZnO thin films show different properties. The XRD spectra showed electrodeposited ZnO to have the strongest peak arising from the (002) reflection and the sputtered ZnO to be orientated along the (101) plane. SEM illustrated that the electrodeposited ZnO is polycrystalline having hexagonal grains ~1.0 µm in size compared to the nanocrystalline nature of sputtered ZnO. The bandgap of both materials is around ~3.20 eV. The work is continuing to compare the two materials grown on the same substrate (glass/FTO).
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ACKNOWLEDGMENTS The authors would like to acknowledge the UK Government's Department for Trade and Industry (DTI) for providing funding for this research program. Sarfaraz Moh and Ayse Ersoy at Pilkington Group Ltd are thanked for their contributions. Leon Bowen and Matthew Calveley are gratefully acknowledged for their technical support. 6
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Fig 2. SEM micrographs of (a) electrodeposited ZnO and (b) illustrates the nanocrystalline morphology of sputtered ZnO. 3.3 Optical Absorption The optical absorption studies have been carried out to determine the absorption edge and bandgap energy of electrodeposited and sputtered ZnO. Typical absorption spectra of electrodeposited ZnO is shown in Fig 3a. The spectra shows an absorption edge, with bandgap energy of ~3.22 eV. The bandgap of sputtered ZnO is similar at ~3.28 eV (Fig 3b). These values agree with previously reported bandgap for ZnO [11].
Fig 3. Optical absorption spectra of (a) electrodeposited ZnO having bandgap of ~3.22 eV and (b) sputtered ZnO with bandgap ~3.28 eV.
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