Thin Solid Films 520 (2011) 1174–1177

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The effect of dopants on the morphology, microstructure and electrical properties oftransparent zinc oxide films prepared by the sol-gel method

Shane O'Brien a, Mehmet Çopuroglu a, Paul Tassie b, Mark G. Nolan a, Jeff A. Hamilton a, Ian Povey a,Luis Pereira c, Rodrigo Martins c, Elvira Fortunato c, Martyn E. Pemble a,⁎a Advanced Materials and Surfaces Group, Tyndall National Institute, Lee Maltings, Cork, Irelandb Central Fabrication Laboratory, Tyndall National Institute, Lee Maltings, Cork, Irelandc Materials Science Department, CENIMAT I3N and CEMOP/UNINOVA, FCT-UNL, Campus de Caparica, 2829-516 Caparica, Portugal

⁎ Corresponding author. Tel.: +353 21 4904456.E-mail address: martyn.pemble@tyndall.ie (M.E. Pem

0040-6090/$ – see front matter © 2011 Elsevier B.V. Adoi:10.1016/j.tsf.2011.04.210

a b s t r a c t

a r t i c l e i n f o

Available online 5 May 2011

Keywords:Zinc oxideTransparent conducting oxideSol-gelResistivityMorphologyCrystallinity

The influence of doping on the morphology, physical and electrical properties of zinc oxide produced by thesol-gel method was examined. Undoped zinc oxide was observed to form relatively porous films. Addition ofan Al dopant influenced the sheet resistance, but did not result in a change in morphology, examined byatomic force microscopy when compared to undoped films. In the case of electrical measurements, undopedZnO films were extremely resistive. A minimum dopant concentration of 2 at.%. Al was required to producematerials which were more conductive, as observed by sheet resistance measurements, which were shown tovary with annealing temperature. The versatile nature of sol-gel processing was demonstrated by selectiveink-jet deposition of sol-gel droplets which were annealed to form oxide materials.

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© 2011 Elsevier B.V. All rights reserved.

1. Introduction

Transparent conductive oxides (TCO) are widely used as elec-trodes on flat panel displays, sensors and solar cells. To date, indiumtin oxide (ITO) has been the material of choice. However, areplacement for ITO is now required due to both cost andenvironmental issues. Zinc oxide is attracting significant attention inthis regard as it is a wide direct band-gap oxide semiconductor thathas been identified as having significant potential application inelectronic, optoelectronic and information technology device plat-forms due to its electrical and optical properties [1–3]. Several thin-film deposition techniques have been demonstrated to produce pureZnO films, including sputtering [4], molecular beam epitaxy [5],metal-organic chemical vapour deposition [6], pulsed laser deposition[7], spray pyrolysis [8] and the sol-gel process [9–16]. The sol-gelmethod has several distinct potential advantages over its counter-parts, due to its ability to tune microstructure via sol-gel chemistry,conformal deposition ability, mouldability and compositional controllarge surface area coating capability in addition to suitability forselective deposition by inkjet printing.

In this paper, the influence of aluminium doping on themorphology, physical and electrical properties of zinc oxide isexamined. Also, in order to demonstrate the versatility of sol-gelmaterials and their relevance to processing technologies, a test array

of aluminium doped zinc oxide transparent contacts was produced byinkjet printing.

2. Experimental details

2.1. Synthesis, and film preparation and processing

A schematic diagram showing the various steps in film productionis given in Fig. 1. The ZnO sol-gel was prepared as follows: zinc acetate(Zn(C2H3O2)2, 99.99% chemical purity) was first dissolved inisopropanol ((CH3)2CHOH) at room temperature. Monoethanolamine(H2NCH2CH2OH, MEA, AR) was then added as a sol stabiliser. Themolar ratio of MEA to zinc acetate was maintained at 1.0 and the finalconcentration of zinc acetate was 0.2 mol/L. The resulting mixturewas then stirred at 50 °C for 1 h to form a clear and transparenthomogeneous mixture. Aluminium nitrate was introduced in therequired amounts, to achieve a sol containing 2 and 3 at.% Al,dissolved in ethanol. Prior to use, the solution was syringe filteredusing a 0.45 μm membrane filter and aged for 24 h at roomtemperature. The sol-gel synthesis and thin film process is outlinedin Fig. 1. Inkjet writing of sol-gel materials was performed using aMicrofab Jetfirst printing system.

2.2. Characterisation

A Philips (PW3719) X'pert Materials Research X-ray diffractom-eter, operated at 40 kV and 35 mA, with a Cu–Kα radiation source,was employed for X-ray diffraction (XRD). Planar and cross-sectional

Zinc acetate + Isopropylalcohol

Al-nitrate/ethanol Monoethanolamine

Sol

Spin coating 2000 RPM, 30s Ink-jet Deposition

Hotplate Drying

Furnace annealing

Al-ZnO film

Fig. 1. Schematic of thin film fabrication.

1175S. O'Brien et al. / Thin Solid Films 520 (2011) 1174–1177

scanning electron microscope (SEM) images of the films werecollected using a Hitachi S-4000 Field Effect SEM instrument, whoseoperating voltage was 20 keV. Film thickness was estimated from thecross-sectional SEM images. UV/vis spectroscopy was performed witha Shimadzu 2401 PC UV/vis spectrophotometer. The surface mor-phology of the films was determined by atomic force microscopy(AFM) using an Asylum Research MFP-3D™ AFM, operating in thetapping mode with a 10 nm diameter silicon tip (Asylum AC160TS)over an area of 1 μm2 and scan rate of 1 Hz. Electrical resistivitymeasurements were carried out using a Keithley 617 electrometerevaporating two coplanar Al contacts with a width of 4 mm and aseparation of 1 mm, in vacuum, at ambient temperature.

3. Results and discussion

Initial micro structural characterisation of doped and undopedfilms has been reported previously [17]. In this study it was observedthat undoped films were polycrystalline in nature with a hexagonalwurtzite structure as evidenced by the detection of the (100), (002)and (101) peaks using X-ray diffraction. It was also found that the Al-doped films formed were preferentially oriented in the c-axis, or(002) plane.

Fig. 2 reveals that in this present work, highly transparent Al–ZnOfilms were obtained with absorbance of 0.04 in the visible region ofthe spectrum. Differences in the 320–400 nm spectral region areattributed to increased thickness as a function of annealing temper-ature. All films were the same thickness, 120 nm (±10 nm).

Fig. 2. UV–vis spectra of 2 at.% Al–ZnO samples and glass substrate.

X-ray diffraction data for these samples are shown in Fig. 3displayed features associated with the (100), (002), (101), (102) and(110) planes. As noted earlier, previously it was found that Al dopingwas found to result in preferred (002) orientation when films weredried on a hotplate at 275 °C [17]. However, in this study, whensamples were dried on a hotplate at the lower temperature of 60 °C,films were not preferentially orientated, as peaks assigned to (100),(002) and (101) planes were approximately equal intensity.(It shouldbe noted in Fig. 3, for ease of viewing, XRD patterns were stacked byaddition of a separation factor prior to plotting. All XRD patterns are ofsimilar intensity).

The morphology of samples was examined by AFM, as shown inFig. 4. Spherical shaped grains were observed in all samples. For the2 at.% Al–ZnO samples annealed at 425 °C (Fig. 4(a)), particle sizesranged from 40 to 60 nm. In the case of the 2 at.% Al–ZnO samplesannealed at 475 °C (Fig. 4(b)), particle sizes ranged from 60 to 70 nm.Pores were also clearly observed in 2 at.% Al–ZnO samples annealed at425 and 475 °C, similar to those observed in undoped ZnO andreported in a previous study [18]. Such behaviour differs fromobservations by authors such as Zhou [16] where addition of Aldopants resulted in reduction of porosity.

Electrical characterisation data for Al-doped materials are provid-ed in Table 1. Note that undoped ZnO films were too resistive toaccurately and reproducibly measure their respective sheetresistances.

It has previously been reported by Nunes [19] that 1 at.% was theoptimum doping concentration for Al in ZnO, however, this was notobserved to be the case here, most likely due to the differences inmicro structure and processing conditions associated with our use ofdifferent thin film fabrication methods. In this work, as shown inTable 1, it was observed that there is an optimal annealingtemperature for a given dopant concentration with respect tominimum sheet resistance. Undoped films were extremely resistiveand perhaps would be better described as highly insulating. 1% Aldoping did not change this as no effect of doping on sheet resistancewas observed for doping at this level. 2 at.% Al–ZnO resulted in thelowest value of sheet resistance obtained (1.4 MΩ/□), when the filmwas annealed at 475 °C. In the case of 3 at.% Al–ZnO, the lowest sheetresistance was obtained (4.3 MΩ/□), when films were annealed at425 °C. Although the data do not follow an obvious trend here, wenote that similar variations in sheet resistance annealing temperaturehave been observed by Rozati for films prepared by spray pyrolysis[20]. We suggest here that some segregation of the doping may occurduring annealing such that it may be necessary to optimise annealingtime as well as annealing temperature, for a given level of aluminiumdoping, in order to repeatedly obtain the optimum values of sheet

Fig. 3. XRD patterns of 2 at.% Al–ZnO films annealed at 425 and 475 °C for 1 h.

Fig. 4. AFM image of 2 at.% Al doped ZnO film, annealed at (a) 425 °C and (b) 475 °C for1 h.

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resistance. However, more data are required in order to be able toconfirm or deny this hypothesis.

Finally, in order to demonstrate the suitability of these materialsfor use in additive processing for TCO applications, sol-gel dropletswere deposited onto glass substrates at room temperature. Samples

Table 1Sheet resistance values for 2% and 3 at.% Al-doped ZnO annealed at 375, 425 and 475 °Cfor 1 h.

Sample, annealing temperature Sheet resistance (MΩ/□)

Undoped ZnO 375 °C 8×106

Undoped ZnO 425 °C 8×106

Undoped ZnO 475 °C 8×106

1% Al–ZnO 375 °C 8×106

1% Al–ZnO 375 °C 8×106

1% Al–ZnO 375 °C 8×106

2% Al–ZnO, 375 °C 26.72% Al–ZnO, 425 °C 1002% Al–ZnO, 475 °C 1.43% Al–ZnO, 375 °C 5803% Al–ZnO, 425 °C 4.33% Al–ZnO, 475 °C 20.7

were dried and then annealed at 400 °C for 1 h. As shown in Fig. 5,uniform discs of oxide material were obtained, approximately 40 μmin diameter. Recently there has been significant interest in printedelectronics processing by ink-jet technology. This is because it offersmany advantages over traditional materials processing methodsinvolving total substrate coverage and photo-lithographic processingto leave material in the required areas [21]. As an additive process(material only placed where required) ink-jet writing offers thebenefit of reduced material consumption and waste, reduction inprocessing time and costs (due to the reduced number of processingsteps) and lends itself to rapid substrate throughput as required inindustrial processing.

While current solution methods to produce conductive oxides donot result in materials with the same properties as those produced bymethods such as magnetron sputtering, interest remains in solutionbased processing methods due to their potential application in low-cost industrial processing. Fabrication of dense ceramic discs of Al–ZnO, as demonstrated here is a significant step towards that end-goal.Further work, however, is required in post-processing of solutiondeposited oxide material (such as forming gas annealing) in order toobtain materials with lower sheet resistance which would be suitablefor use in transparent display technology.

4. Conclusions

We have performed a preliminary study of the influence of dopingon the morphology, physical and electrical properties of zinc oxide,produced by the sol-gel method. For the particular methods employedit was found that both undoped and aluminium doped zinc oxidewere observed to form relatively porous films. In terms of theelectrical properties of the films it was found that the porous undopedZnO films were highly resistive, as expected. Also as expected, thealuminium doped ZnO films had significantly lower sheet resistance.It was observed that sheet resistance was annealing temperaturedependent, with lowest sheet resistance (1.4 MΩ/□) obtained whenfilms were annealed at 475 °C, although the relationship betweensheet resistance and annealing temperature for a given level of dopingis not an obvious one. Lower sheet resistance values for thesematerials have been reported in the literature. These are generallyproduced by other methods, such as CVD or sputtering and havedifferent microstructures [4,18]. However, due to the versatile natureof sol-gel processing, as shown by selective ink-jet deposition, sol-gelmaterials are still technologically relevant, although further work isrequired (e.g. dopant type, annealing and post-processing) to reducesheet resistance values.

Fig. 5. SEM image of 2 at.% Al–ZnO sol-gel droplets deposited onto glass by inkjetprinting.

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Acknowledgments

This work was funded by the EU Strategic Research Project‘Multiflexioxides’ and Science Foundation Ireland Principle Investiga-tor grant no. 07/IN.1/1787.

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