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Thin Solid Films 516 (2

Growth and characterisation of electrodeposited ZnO thin films

J.S. Wellings a,⁎, N.B. Chaure a, S.N. Heavens b, I.M. Dharmadasa a

a Solar Energy Group, Materials & Engineering Research Institute, Sheffield Hallam University, Sheffield S1 1 WB, UKb Ionotec Ltd, 14 Berkeley Court, Manor Park, Runcorn, Cheshire WA7 1TQ, UK

Received 14 November 2006; received in revised form 15 June 2007; accepted 15 July 2007Available online 25 July 2007

Abstract

The electrochemical method has been used to deposit zinc oxide (ZnO) thin films from aqueous zinc nitrate solution at 80 °C onto fluorinedoped tin oxide (FTO) coated glass substrates. ZnO thin films were grown between − 0.900 and − 1.025 V vs Ag/AgCl as established byvoltammogram. Characterisation of ZnO films was carried out for both as-deposited and annealed films in order to study the effect of annealing.Structural analysis of the ZnO films was performed using X-ray diffraction, which showed polycrystalline films of hexagonal phase with (002)preferential orientation. Atomic force microscopy was used to study the surface morphology. Optical studies identified the bandgap to be∼ 3.20 eV and refractive index to 2.35. The photoelectrochemical cell signal indicated that the films had n-type electrical conductivity andcurrent–voltage measurements showed the glass/FTO/ZnO/Au devices exhibit rectifying properties. The thickness of the ZnO films was found tobe 0.40 μm as measured using the Talysurf instrument, after deposition for 3 min. Environmental scanning electron microscopy was used to viewthe cross-section of glass/FTO/ZnO layers.© 2007 Elsevier B.V. All rights reserved.

Keywords: Electrodeposition; ZnO; X-ray diffraction; II–VI compound semiconductor

1. Introduction

Zinc oxide (ZnO) is a non-toxic, II–VI, wide bandgap semi-conductor with Eg=3.20 eV and natural n-type electrical con-ductivity. Among various other applications such as light-emitting diodes, laser diodes [1], piezoelectric transducers [2],transistors [3] and phosphors [4], ZnO is used as a transparentbuffer layer in solar cell devices [5]. Various publications to datehave suggested that the addition of a highly resistive intrinsic (i)ZnO layer in copper indium gallium diselenide (CuInGaSe2)solar cell devices drastically improves device efficiency, as itmay prevent the leakage current [6–8]. The i-ZnO layer may alsoact as a pinhole plugging layer or work as an anti-reflectioncoating. ZnO absorbs the UV part of the solar spectrum andallows lower energy photons to travel to the absorber layer.Doping ZnO with group III elements such as B, Al, or Inincreases the n-type conductivity [9,10].

⁎ Corresponding author. Tel.: +44 114 225 5292; fax: +44 114 225 3433.E-mail address: jayne.s.wellings@student.shu.ac.uk (J.S. Wellings).

0040-6090/$ - see front matter © 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.tsf.2007.07.156

Numerous growth techniques have been used to produceZnO, including molecular beam epitaxy (MBE) [11], electronbeam evaporation [12], metal organic chemical vapor deposi-tion (MOCVD) [13], and spray pyrolysis [14]. Drawbacks tousing such techniques are high cost of equipment, only smallmaterial layer production, time consumption and complexity ofoperation. ZnO can also be grown by electrodeposition [9,15],which is a low cost technique that does not require expensive,sophisticated equipment and is simple to operate. Electro-deposition also has the advantage of having low temperatureprocessing, allows various substrate shapes and controllablefilm thickness. This technique also avoids the use of vacuumsystems allowing growth under normal laboratory conditions[16].

In the present work, electrochemical technique was used togrow ZnO thin films on fluorine doped tin oxide (FTO) coatedglass substrates from aqueous solution. Optical, structural, mor-phological, electrical and opto-electrical properties have beenstudied using UV–Vis spectrophotometer, X-ray diffraction(XRD), atomic force microscopy (AFM), environmental scan-ning electron microscopy (ESEM), current–voltage (I–V),

Fig. 2. Voltammogram of 0.1 M zinc nitrate electrolyte with 10 mV s− 1 sweeprate.

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capacitance–voltage (C–V) and photoelectrochemical cell (PEC)measurements respectively.

2. Experimental details

Glass substrates coated with (FTO) having a sheet resistanceof 15 Ω/□ supplied by Pilkington Group Ltd were used as theworking electrode for the electrodeposition of ZnO. Prior to thedeposition, substrates were washed thoroughly with boiled,soapy, deionised water and rinsed before being cleanedultrasonically. ZnO thin films were electrodeposited potentios-tatically using a conventional three-electrode system, withgraphite plate counter electrode and silver/silver chloride (Ag/AgCl) (+ 220 mV vs normal hydrogen electrode) referenceelectrode. GillAC version 4, computerised potentiostat/galvano-stat was employed to perform the potentiostatic deposition. Theaqueous solution contained 0.1 M zinc nitrate, Zn(NO3)2·6H2Odissolved in 150 ml deionised water. The electrolyte was stirredconstantly using a magnetic stirrer throughout deposition andkept at a temperature of 80 °C (± 2 °C). pH of the as-preparedsolution was ∼ 5.0, without any alteration the same bath wasused for the electrochemical experiments.

An air-tight three electrode electrochemical cell wasemployed to electrodeposit the layers. The working and counterelectrodes were held vertically using teflon tape. The referenceelectrode was fixed at one side of the electrochemical cellmaking an angle 45°. This arrangement was used to keep thereference electrode as close as possible to the working electrode.Uniformity of electrodeposited materials is one critical issue,which depends on the current distribution and geometry of theelectrodes. In order to produce a uniform thin film over theentire electrode area, the counter and working electrodes werethe same size and positioned between high and low currentdensity area. The schematic diagram for the electrodepositioncell is shown in Fig. 1.

Fig. 1. Schematic diagram of the electrodeposition set-up.

Voltammogram was used to decide the suitable region ofgrowth potential. Samples with dimensions 15×30 mm2 weregrown at various cathodic potentials between − 0.900 and− 1.100 V vs Ag/AgCl to study various properties. Opticalabsorption spectra were taken using UV–Vis spectrometer,Unicam UV-2, to determine the absorption edge and bandgapof the electrodeposited ZnO layers. XRD was carried out using aPhilips X-Pert Pro-diffractometer with CuKα1 source(λ=1.5406 Å) at a glancing angle of 0.5°. AFM Digital Instru-ment Nanoscope III was used to study the surface morphology.The PEC cell was used to determine the electrical conductivitytype of the electrodeposited ZnO bymonitoring the polarity of theopen circuit voltage (Voc) produced at the liquid/solid junction,

Fig. 3. XRD spectra of as-deposited ZnO films electrodeposited at (a) − 0.900 V,(b)− 0.925V, (c)− 0.975V, (d)− 1.000Vand (e)− 1.025V vsAg/AgCl reference.

Fig. 4. The XRD patterns of ZnO thin films grown at − 0.975 V; (a) as-deposited(b) annealed. The inset shows the solution of the Hull equation.

Table 1Data obtained from XRD of ZnO film grown at − 0.975 V vs Ag/AgCl (a) as-deposited (b) annealed

2θ(deg)

‘d’ spacing (Å) FWHM(degree)

Millerindices(hkl)

Phaseassignment

Standard Observed

As-deposited31.87 2.8179 2.8096 0.36 (100) ZnO hexagonal34.48 2.6049 2.5987 0.33 (002)36.28 2.4786 2.4778 0.27 (101)47.56 1.9128 1.9141 0.29 (102)56.77 1.6269 1.6266 0.24 (110)62.92 1.4784 1.4762 0.33 (103)68.93 1.3799 1.3790 0.40 (112)69.10 1.3601 1.3593 0.42 (201)72.82 1.3024 1.3009 0.30 (004)77.02 1.2393 1.2385 0.24 (202)

Annealed31.84 2.8179 2.8095 0.20 (100) ZnO hexagonal34.50 2.6049 2.5993 0.22 (002)36.28 2.4786 2.4763 0.26 (101)47.64 1.9128 1.9113 0.29 (102)56.72 1.6269 1.6230 0.28 (110)62.98 1.4784 1.4719 0.39 (103)68.10 1.3799 1.3793 0.52 (112)69.28 1.3601 1.3563 0.48 (201)72.82 1.3024 1.2980 0.40 (004)77.12 1.2393 1.2372 0.56 (202)

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under illumination. Philips XL 30 ESEM–Field-Emission Gunwith accelerating voltage of 20 kV was used to study the cross-section of the glass/FTO/ZnO. The thickness of the ZnO layer wasobtained by Taylor-Hobson Tallysurf 1204 L instrument andcompared with the ESEM cross-section.

Selected samples were annealed to determine the effect ofpost-growth heat treatment. Annealing was achieved using aBio-Rad furnace operating at a temperature of 550 °C for 15 minin air atmosphere. Annealed samples were metallised with gold(Au) contacts to produce devices of 2 mm in diameter. Theelectrical properties were studied using I–VandC–V techniques.

3. Results and discussion

3.1. Voltammogram

The electrochemical behaviour of ZnO was studied usingvoltammetry to optimise the growth potential, in conjunction withthe X-ray diffraction technique. Fig. 2 shows the voltammogramfor the solution containing Zn(NO3)2 at ∼ pH 5.0 with moderatecontinuous stirring vs Ag/AgCl reference electrode. The sweeprate was 10 mV s− 1. For low cathodic potential the currentincreases slowly first and shows a rapid increase from about− 0.650 V. A plateau is observed between − 0.800 to − 1.000 Vwhere the ZnO deposition is expected by the followingmechanisms.

ZnðNO3Þ2→Zn2þ þ 2NO−3 ð1Þ

NO−3 þ H2O þ 2e−→NO−

2 þ 2OH− ð2Þ

Zn2þ þ 2OH−→ZnO þ H2O ð3Þ

The sharp rise in cathodic current observed beyond− 1.050 V could be due to the metallic deposition of Zn andhydrogen (H2) evolution reaction.

3.2. X-ray diffraction (XRD)

XRD studies have been carried out on ZnO layers in order todetermine phase and crystallographic analysis. The films de-posited at various potentials, shown in Fig. 3 (a–e) arepolycrystalline with hexagonal phase and (002) preferentialorientation, except sample (b) deposited at − 0.925 V vs Ag/AgCl. The appearance of the film grown at − 0.975 V wastransparent, compact and uniform. Moreover, improved crys-tallinity was observed for the film grown at − 0.975 V comparedto the other growth potentials.

The effect of annealing has been studied on the film grown at− 0.975 V cathode potential, with annealing temperature of550 °C for 15 min. Typical XRD spectra for the as-depositedand annealed film is shown in Fig. 4 (a and b). Table 1 illustratesthe XRD data obtained for both as-deposited and annealed ZnOfilms. No noticeable change was observed in XRD spectra forthe as-deposited and annealed sample. The data obtained forXRD analysis was used to determine the mean grain size (t) andstrain (ε) using the Hull Eq. (4).

bcosh ¼ ktþ esinh ð4Þ

where β is full width at half maximum (FWHM) intensity, θ isthe Bragg angle and λ is the wavelength. The plot of βcosθ

Fig. 5. AFM micrograph of ZnO film grown at − 0.975 V vs Ag/AgCl (a) as-deposited (b) annealed at 550 °C for 15 min.

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against sinθ inset in Fig. 4 (a and b) was found to be linear. Thevalues obtained for strain and mean grain size for the as-deposited and annealed ZnO films were 9.32×10− 3and42.7 nm and 2.09×10− 3 and 55.0 nm respectively. Note that

Fig. 6. Typical optical absorption curve for (a) as-deposited and (b) annealed at550 °C for 15 min of ZnO layer electrodeposited at − 0.975 V vs Ag/AgCl.

the strain reduces after annealing the sample, which could bedue to the restructuring of the crystal lattice. The latticeparameter of ‘a’ and ‘c’ calculated using Eq. (5) for the as-deposited and annealed ZnO films grown at − 0.975 Vare closeto the standard values a=3.253 Å, c=5.209 Å [17,18].

1d2

¼ 43

h2 þ k2 þ hka2

� �þ l2

c2ð5Þ

3.3. Atomic force microscopy

AFM technique was used to investigate the surface morphol-ogy of as-deposited and annealed ZnO thin films grown at− 0.975 V vs Ag/AgCl. As-deposited samples show compact,uniform and void-free surfaces as shown in Fig. 5 (a). Smallclusters of size 100 nm formed by agglomeration of smallerindividual grains of size ∼ ±40 to 50 nm. After annealing at550 °C for 15 min the morphology of the deposit changed andcan be seen in Fig. 5 (b). The grains have formed large clustersof various sizes ranging from ∼ 200 nm to ∼ 1.5 μm size. Therms roughness values obtained for as-deposited and annealedZnO films were 41.2 nm and 49.5 nm. These results obtainedusing AFM are in agreement with Nunes et. al. [19].

3.4. Optical absorption

The optical absorption studies have been carried out onselected ZnO films deposited at various growth potentials. Themost uniform and transparent films were observed to be grownat − 0.975 V vs Ag/AgCl. The film deposited at − 0.975 Vshowed a sharp optical absorption edge, with bandgap for as-deposited film of 3.22 eV as illustrated by Fig. 6 (a). Afterannealing at 550 °C for 15 min the absorption edge was redshifted to ∼ 3.20 eV as shown by Fig. 6 (b). This agrees withpreviously reported bandgap for ZnO [20]. High opticaltransparency is essential for ZnO if it is to be used as a bufferlayer in solar cell devices. In addition the refractive index of

Fig. 7. Typical I–V curve for glass/FTO/ZnO/Au device showing rectifyingbehaviour.

Fig. 8. A typical 1 /C2 vs voltage curve for glass/FTO/ZnO/Au structures. Fig. 9. Variation of thickness of ZnO thin films grown at − 0.975 V vs Ag/AgClas a function of deposition time. Note the linear growth up to ∼ 10 min and areduced growth rate beyond this limit.

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zinc oxide was calculated using Eq. (6) [21] to be 2.35, which isclose to the value in the literature [22,23].

ðn2 � 1Þðn2 þ 2Þ ¼ 1�

ffiffiffiffiffiffiffiffiffiffiffiffiffiEg=20

qð6Þ

3.5. Electrical properties

The dark current–voltage (I–V) characteristics of glass/FTO/ZnO/Au structures were measured using fully automated systemand a typical curve is shown in Fig. 7. These structures showedgood rectification properties with rectification factor of∼ 103 at1.0 V, ideality factor of ∼ 3.0 and barrier height of N 1.08 eV.Since the diodes are non-ideal, accurate barrier height cannot beestimated. The series resistance (Rs) of the structure wasestimated using the high forward biased region of the I–V curveand this was found to be about 1.6 kΩ. Making use of theknown contact area and the film thickness, the bulk resistivity(ρ) and the electrical conductivity (σ) were found to be1.67×106 (Ωcm) and 5.98×10− 7 (Ωcm)− 1 respectively.

In order to accurately estimate the electrical properties ofthese films a low work function (Φm) metal, silver (Ag) wasused to make electrical contacts, expecting ohmic behaviour.However the I–V curves of glass/FTO/ZnO/Ag also exhibitrectifying characteristics similar to those of Au contacts. Thisindicates that ideal Schottky behaviour is not observed for ZnO/metal interfaces.

Table 2PEC signal of ZnO films illustrating n-type electrical conductivity of the layers

Material Depositionpotential (V)

Vdark

(mV)Vlight

(mV)ΔV(mV)

Conductivitytype

n-Si Standard sample − 0.390 − 0.530 − 140 np-Si Standard sample − 0.360 − 0.320 40 pZnO − 0.900 − 0.305 − 0.356 − 51 nZnO − 0.925 − 0.315 − 0.350 − 35 nZnO − 0.975 − 0.340 − 0.428 − 88 nZnO − 1.000 − 0.364 − 0.410 − 46 nZnO − 1.025 − 0.375 − 0.455 − 80 n

Capacitance–voltage measurements of glass/FTO/ZnO/Audevices were taken as a function of bias voltage with a detectionsignal frequency of 1 MHz. In order to estimate doping con-centration of ZnO, 1 /C2 vs V plots were generated and a typicalcurve is shown in Fig. 8. Using the gradient of the curve andtaking the dielectric permittivity of ZnO to be εs=9.26 [24]yields 2.8×1012 cm− 3 for donor concentrations. It should benoted that the C–V measurements are prone to effects of defectsand hence produce doping concentrations away from accuratevalues. Assuming the above value for doping concentration andelectrical conductivity of 5.98 10− 7 (Ωcm)− 1, 1.25 cm2/V s hasbeen obtained for the mobility of electrons in the electro-deposited ZnO layers.

3.6. Opto-electrical properties

Electrical conductivity type was determined by PEC mea-surements. The glass/FTO/ZnO substrate and a graphite rodwere immersed in a sodium chloride, NaCl solution and thedifference between the observed dark and light voltages wasrecorded using a voltmeter. The polarity of the PEC signaldemonstrates the electrical conductivity type of the material.

Fig. 10. ESEM cross-section of glass/FTO/ZnO structures showing ZnO layerthickness of ∼ 0.4 μm after growth for 3 min.

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The magnitude of the signal indicates the suitability of thedoping concentration (Nd) of the semiconductor layers fordevice application. The PEC system was standardised usingknown samples of n and p-type silicon. All ZnO films demon-strated n-type electrical conductivity as summarised in Table 2.

3.7. Thickness measurements

Talysurf measurements were carried out on as-depositedZnO films grown at − 0.975 V for different growth periods todetermine the thickness. Fig. 9 illustrates an almost linearrelationship up to 10 min and reduced growth rate there-after.Reduction in deposition rate could arise from dissolution of theelectrodeposited ZnO film in the acidic electrolyte. The infor-mation has vital importance in depositing ZnO layers withdesired thicknesses for device structures. Electrodepositiontherefore provides an excellent method of depositing sub-micron thick ZnO layers within a few minutes using normallaboratory conditions, in order to reduce underlying cost of solarcells.

To confirm the film thickness determined by the Talysurfmeasurement the glass/FTO/ZnO cross-section was examinedby ESEM as shown in Fig. 10. The thickness of the ZnO layerelectrodeposited for 3 min was shown to be 0.4 μm, inagreement with Fig. 9.

4. Conclusion

Polycrystalline, hexagonal ZnO layers with (002) preferen-tial orientation have been deposited from aqueous solutionsusing an electrochemical technique onto FTO-coated glass. Thedeposition potential was optimised to be − 0.975 V vs Ag/AgClresulting in smooth, compact, uniform and well adhered films.The grain size calculated by Hull equation for as-deposited andannealed sample was 42.7 and 55 nm, which are close to thevalues obtained by AFM. The PEC cell measurements confirmn-type conductivity of the ZnO films. Optical absorptionindicated the bandgap to be ∼ 3.20 eV and refractive index tobe 2.35. The thickness of the ZnO layer had a linear relationshipfor shorter periods of growth and saturated when grown forlonger periods.

Acknowledgment

The authors would like to acknowledge the UK Govern-ment's Department of Trade and Industry (DTI) for providing

funding for this research. Pilkington Group Ltd are acknowl-edged for their contribution to this project. Mr Leon Bowen isalso gratefully acknowledged for his technical support.

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