Materials Research Bulletin 46 (2011) 2147–2154

Electrodeposited ZnO films with high UV emission properties

Elena Matei, Ionut Enculescu *

National Institute of Materials Physics, PO Box MG 7, 77125 Magurele, Ilfov, Romania

A R T I C L E I N F O

Article history:

Received 10 November 2010

Received in revised form 18 April 2011

Accepted 26 April 2011

Available online 5 May 2011

Keywords:

A. Oxides

A. Semiconductors

A. Optical materials

D. Optical properties

A B S T R A C T

We report here our results in the preparation of ZnO films with high UV band to band characteristic

luminescence emission by potentiostatic electrodeposition. Zinc nitrate aqueous baths with different

concentration and additives were employed for the preparation of the films on platinum substrates. We

focused our research in determining how the electrodeposition bath composition, i.e. zinc nitrate

concentration and addition of KCl or polyvinyl pyrolidone and applied overpotential influence the

morphological and optical properties of the oxide films. Scanning electron microscopy was employed for

characterizing the films in terms of morphology. Optical reflection, photoluminescence spectroscopy and

cathodoluminescence were used for determining the optical characteristics of the samples. The

morphology of the deposit varies from hexagonal prisms to platelets as a function of the deposition rate.

This experimental parameter also influences the luminescence properties. We found that at low

deposition rates high UV luminescent material is obtained.

� 2011 Elsevier Ltd. All rights reserved.

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Materials Research Bulletin

jo u rn al h om ep age: ww w.els evier .c o m/lo c ate /mat res b u

1. Introduction

Electrochemical deposition becomes an interesting approachfor fabricating semiconductor films. It is well known thatelectroplating is widely employed for depositing metals, being amajor, large scale, industrial technique. However, the electrode-position of semiconductors attracted more and more attention justduring the last decade due to its major advantages namely low costand scalability, which can be crucial in applications as e.g. solarcells. The possibility of 3 dimensional deposition, such as platingthrough holes, is another important factor in comparing electro-plating with classical approaches.

Zinc oxide is an extremely interesting material due to the widefield of potential applications. These range from cheap transparentelectrodes for solar cells to ultraviolet lasing material or as catalyst[1–3]. Moreover, an extremely interesting variety of ZnOnanostructures with morphologies varying from hexagonal rodsto combs or pods were fabricated by an array of methods [4–7].Recent literature shows that nanostructured zinc oxide presentsexotic properties such as room temperature UV lasing [8].Moreover, a new research field, namely spintronics, representsanother major challenge for researchers focusing on zinc oxide. Itwas theoretically predicted that transition metal doped zinc oxidemay present ferromagnetism at room temperature being one of the

* Corresponding author at: National Institute of Materials Physics, Laboratory of

Multifunctional Materials and Structures, Atomistilor 105 bis, Magurele, Romania.

Tel.: +40 723012084; fax: +40 213575077.

E-mail address: encu@infim.ro (I. Enculescu).

0025-5408/$ – see front matter � 2011 Elsevier Ltd. All rights reserved.

doi:10.1016/j.materresbull.2011.04.025

materials named diluted magnetic semiconductors. Achieving thismay represent a revolution in today’s electronics [9,10].

Electrochemical deposition of ZnO, which was first proved byIzaki and coworkers during the 1990s [11,12], becomes more andmore attractive, the number of reports increasing steadily in thelast decade. Extensive studies are dedicated to both electrodepo-sition and properties of the plated material [13–18]. A major aspectof the research is given by the fact that in ZnO case there is a verystrong influence of the preparation conditions on the properties ofthe material. Even minute changes in preparation conditions leadto major differences in morphological, structural, optical orelectrical properties of the material.

In the present paper we report our results regarding thefabrication of zinc oxide nanostructured thin films by electrode-position from a nitrate bath. We focused our research on the way inwhich materials optical properties and morphology are influencedby the composition of the electrochemical bath and by the value ofthe applied overpotential. We succeeded in finding the conditionsfor fabricating films with strong, room temperature ultravioletemission and low, defect related emission. The samples werecharacterized by means of scanning electron microscopy, photo-luminescence and reflection UV–vis spectroscopy and cathodolu-minescence.

2. Experimental

Electrochemical deposition was employed for preparing the filmsof zinc oxide. The electrodeposition process was carried out in atypical three electrode set-up. The conductive substrates (workingelectrode) employed were platinum plates 5 mm � 10 mm. The

Fig. 1. Electrochemical polarization curves for three different growth solutions: (a)

0.05 M Zn(NO3)2, 0.1 M KCl, 0.1% PVP, (b) 0.1 M Zn(NO3)2, (c) 0.1 M Zn(NO3)2, 0.1%

PVP.

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reference electrode was a commercial saturated calomel electrode.The counter electrode was a platinum electrode of approximately1 cm2 surface. A potentiostat was employed for controlling thedeposition parameters. We performed the electrochemical deposi-tion at a temperature of 90 8C, in a double walled electrochemicalcell.

The electrochemical bath is based on zinc nitrate in differentconcentrations varying from 0.05 M to 2 M. Additionally, thedeposition solution contains, in some cases different concentra-tions of KCl and polyvinylpyrrolidone (PVP). The chemicalsemployed were ACS grade purchased from Aldrich and usedwithout further purification. For preparing the electrochemicalbath Millipore ultrapurewater 18.2 MV cm was employed.

After deposition the samples were extensively characterizedby means of optical spectroscopy and scanning electron micros-copy. A Perkin Elmer Lambda 40 spectrophotometer with anintegrating sphere was employed for measuring the opticalreflection spectra. The measurement was performed using asample holder which oriented the sample surface at an angle of 88versus the incident beam, in this way the total reflection beingmeasured, i.e. both specular and diffuse reflection. This wasnecessary since the films were strongly nanostructured. Theluminescence spectroscopy measurements were performed usingan Edinburgh LS 900 photoluminescence spectrometer withdouble monochromators and a 450 W Xe lamp as excitationsource. The luminescence measurements were performed in afront surface mode. A Zeiss Evo 50 scanning electron microscopewas used for the morphological and cathodoluminescencecharacterization.

3. Results and discussion

We focused our studies on determining how the composition ofthe deposition bath affects the morphology and optical propertiesof the nanostructured zinc oxide thin films. Typical ZnOelectrodeposition baths are either zinc nitrate or oxygen saturatedzinc chloride. Our report is focused on the nitrate bath, with theaim of fabricating high UV emission, nanostructured films.

The cathode reactions corresponding to the ZnO depositionprocesses are the following:

2e� þ NO3� þ H2O ! NO2

� þ 2OH� (1)

Zn2þ þ 2OH� ! ZnðOHÞ2 ! ZnO # þ H2O (2)

or the global reaction:

ZnðNO3Þ2þ 2e� ! ZnO # þ NO3� þ NO2

� (3)

Practically the cathodic reduction reaction is (1) representingthe nitrate to nitrite reduction. Reaction (2) represents thedecomposition of zinc hydroxide with the precipitation of zincoxide and does not present a charge transfer from the electrode. Asa consequence only the reaction (1) can be monitorized throughthe deposition current.

We noticed that agitation or stirring of the solution during thedeposition process leads to low uniformity layers, probably due tointerfering with the second reaction of precipitation. In order toavoid this, the solution was kept still during the process.

The first step in our study was to perform a set of polarizationmeasurements for each of the solutions used. In Fig. 1 three sets ofplots corresponding to such measurements are shown. Two typesof behaviour were identified. Thus, in the first case, where KCl wasadded to the growth solution one can notice a peak appearing at apotential of approximately �900 mV in the polarization curvewhile for the samples without KCl the peak is not visible. Thisbehaviour was noticed for all solutions regardless other param-

eters such as the presence of PVP or zinc nitrate concentration. Wepreviously show that the addition of PVP shifts the potential forwhich the electrochemical process of nitrate reduction with about100 mV towards more electronegative potential [15]. We noticedthis time that the addition of KCl shifts the deposition potential in

Fig. 2. Chronoamperic curves for the deposition of samples obtained in different experimental conditions: (a) 0.05 M Zn(NO3)2, 0.1 M KCl, 0.1% PVP at �850 mV dashed and

�900 mV continuous; (b) deposition at �900 mV: continuous curve 0.05 M Zn(NO3)2, 0.1 M KCl and 0.1% PVP while the dashed curve 0.2 M Zn(NO3)2, 0.1 M KCl and 0.1% PVP;

(c) deposition at �900 mV continuous 0.2 M Zn(NO3)2, 0.1 M KCl, 0.1% PVP dashed 0.2 M Zn(NO3)2, 0.1% PVP (d) continuous: 0.1 M Zn(NO3)2 dashed 0.1 M Zn(NO3)2 0.1% PVP,

deposition performed at �900 mV.

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the opposite direction namely towards more electropositivevalues.

In Fig. 2(a) a set of chronoampermetric curves for two differentoverpotentials are presented. The deposition was performed forthe two cases for the same deposition solution containing 0.05 MZn(NO3)2, 0.1 M KCl and 0.1% PVP. As one can notice, the currentstrongly increases with increasing the deposition potential, whichis to be expected. This makes possible to adjust the deposition ratejust by adjusting a simple experimental parameter. In Fig. 2(b) weplotted the chronoamperic curves for two samples grown at thesame overvoltage from solutions containing different amounts ofzinc nitrate. The continuous curve corresponds to the layerelectrodeposited from a solution containing 0.05 M Zn(NO3)2,0.1 M KCl and 0.1% PVP while the dashed curve corresponds to thelayer which was deposited from 0.2 M Zn(NO3)2, 0.1 M KCl and0.1% PVP. The ratio of the two currents was of about 2.5 thussmaller than 4, the ratio of the two concentrations of zinc nitrate inthe bath. In Fig. 2(c) the represented plots correspond to thedeposition of two ZnO layers from solutions containing the sameamount of zinc nitrate and PVP. Thus the continuous curve wasmeasured during the deposition from a solution containing 0.2 MZn(NO3)2, 0.1 M KCl, 0.1% PVP while the dashed curve correspondsto a solution containing 0.2 M Zn(NO3)2, 0.1% PVP. The depositionwas performed at �900 mV. It is interesting to notice that thecurrent from the solution containing KCl was half the value of the

current for the bath which was prepared without, even if theconductivity of the first bath was higher. In the last plot, Fig. 2(d)the deposition current for a bath with similar compositions (0.1 MZn(NO3)2) but with (dashed curve) and without (continuous curve)0.1% polyvinylpirrolidone. One can notice a two fold increase of thecurrent as the PVP was added to the growth solution.

As we already mentioned, particularly difficult for the growthof ZnO is the fact that the current value is just indirectly relatedto the deposition process and thus the measured current value isnot directly related to the growth rate. Further, we intended tosee how the deposition overpotential and the composition of thebath influence the morphological properties of the film. In Fig. 3one can observe the morphology of two samples deposited fromthe same solution at two different overpotentials (experimentsfor which the chronoampermetric curves are presented inFig. 2(a)). The top images correspond to the deposition at�850 mV while the others correspond to deposition at �900 mV.

The morphology for the two layers differs as a consequence ofthe different deposition rate. Thus, one can notice that, for thelowest rate, an array of submicron hexagonal prisms is obtained.Also, one can notice that the dimension dispersion of the prisms isrelatively low and that the structures have similar orientationalong the c axis. In the images presented in the lower row themorphology presented corresponds to an increase of only 50 mV inthe deposition potential.

Fig. 3. Morphology of electrodeposited ZnO as a function of deposition potential: (a) and (b) hexagonal prisms deposited at �850 mV, (c) and (d) hexagonal prisms and large

platelets deposited at �900 mV.

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The same hexagonal prisms nanostructures can be observed,but besides these, a new set of structures, with differentmorphology, namely larger platelets, appear. These platelets arealso hexagonal but this time the c axis appears to be parallel tothe substrate. One can also notice, from the images taken atlower magnification, that the layers are uniform over largeareas.

The concentration of the deposition bath was furtherincreased, in Fig. 4 being presented a set of images obtainedfor a solution similar to the one employed previously but withthe amount of zinc nitrate increased four folds to a value of0.2 M. One can notice that even at the lowest depositionpotential where a layer could be obtained (�800 mV) thecombination prisms – platelets exists. As the deposition rate isfurther increased by increasing the deposition potential thenumber of platelets increases as can be noticed in Fig. 4(b) and(c). It is interesting that the morphology goes from predominantprisms to predominant platelets in a quite narrow range of thedeposition overpotential.

Similar measurements were performed using depositionbaths which do not contain potassium chloride. In this casewe observe the same tendency towards hexagonal structuring ofthe layer but the structures are not so well aligned. In Fig. 5 onecan observe the morphology of a set of samples deposited from abath containing 0.1 M zinc nitrate at three different over-potentials. The deposits have a morphology based on hexagonalstructures. At the lowest deposition potential employed, namely�800 mV the structures obtained are hollow. As the potential isincreased to �900 mV the films become more compact, withhexagonal prism shaped crystallites. Further increase of the

deposition potential leads to the formation of the same hexagonprisms but the deposit becomes less compact.

In Fig. 6 one can observe the morphology of deposition for a setof samples prepared from baths containing 0.1 M Zn(NO3)2 and0.1% PVP and 0.2 M Zn(NO3)2 and 0.1% PVP respectively.

Again hexagonal structures are predominant but the aspectof the films is different compared to the previous cases. Thus,the lateral dimensions of the structures are much larger whencompared to the array of prisms presented in Fig. 3. Moreover,the differences between the samples fabricated at differentovervoltages and with different bath compositions are minimal.

The next step was related to the investigation of the opticalproperties of the zinc oxide films. In Fig. 7(a) the optical reflectionspectra for four different films are presented. Thus, one can noticethe similar position of the films cut-off due to similar bandgap valuefor the three cases. In Fig. 7(b) the Kubelka Munk function isrepresented as a function of photon energy, the plot being employedfor calculating the band-gap. We obtained a value of 3.20 � 0.05 eVfor all the samples measured which leads us to the conclusion that thedeposition conditions do not affect this material parameter.

Photoluminescence is intensely studied due to the fact thatmaterial parameters make zinc oxide extremely interesting forapplications in the fields of lasers or optoelectronic devices. The lightemission is either due to near band to band recombination (excitonicpeak centered at approximately 375 nm) or related to point defectssuch as zinc or oxygen vacancies or interstitials. This is the reason forwhich a strong UV emission becomes the synonym to a high qualitymaterial with low defect concentration. In Fig. 8 we plotted theemission spectra for several sets of samples. Each plot contains thecurves of samples deposited at different overvoltages from the same

Fig. 4. Films of ZnO electrodeposited at (a) �800 mV; (b) �900 mV; (c) �1000 mV from a bath containing 0.2 M ZnNO3, 0.1 M KCl, 0.1% PVP.

Fig. 5. Zinc oxide nanostructured films obtained from a solution containing 0.1 M Zn(NO3)2 at: (a) �800 mV, (b) �900 mV, (c) �1100 mV.

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electrochemical bath. In Fig. 8(a) (samples grown from a bath

Fig. 6. Films deposited (a) at �800 mV from a bath containing 0.1 M Zn(NO3)2 and 0.1% PVP, (b) at �800 mV from a bath containing 0.2 M Zn(NO3)2 and 0.1% PVP and (c)

�1000 mV from a bath containing 0.2 M Zn(NO3)2 and 0.1% PVP.

Fig. 7. (a) Reflection spectra for a set of samples grown in different conditions; (b) representation of the Kubelka Munk functions employed for determining the band-gap

value.

containing 0.1 M Zn(NO3)2 and 0.1% PVP) the luminescence presentsonly one very strong emission maximum, namely the broad peakcentered at 570 nm corresponding to oxygen interstitials. Thisemission is characteristic to ZnO grown in wet oxygen rich media. Itis interesting to notice that in this case the peak corresponding to thenear band to band transition is not visible at all, meaning that thequality of the material is low, the material being rich in point defects.

Fig. 8(b) and (c) presents similar behaviour in terms of

luminescence i.e. the samples which were grown at higherovervoltages from each set, thus at higher deposition rate, presentonly the luminescence peak corresponding to defects, while thesamples deposited at low deposition rates (�800 mV) present alsothe band to band luminescence peak. This leads to the conclusionthat high quality material can be obtained just for the case whenthe deposition rate is sufficiently low. Also one can notice that the

Fig. 8. Luminescence emission measured for excitation at �365 nm: continuous curve deposited at �800 mV, dashed �900 mV, dotted at �1000 mV (a) 0.1 M Zn(NO3)2, 0.1%

PVP, (b) 0.1 M Zn(NO3)2, (c) 0.2 M Zn(NO3)2, 0.1 M KCl, 0.1% PVP, (d) 0.05 M Zn(NO3)2, 0.1 M KCl, 0.1% PVP.

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defect related emission peak increases in intensity as thedeposition rate increases.

In Fig. 8(d) corresponding to the deposition from the bathcontaining 0.05 M Zn(NO3)2, 0.1 M KCl, 0.1% PVP at �850 and �900respectively one can notice the strong increase of the excitonicpeak intensity compared to the previous cases. As one can interferefrom these plots and from the morphology of the samples itappears that for the case when we deal with oriented hexagonalprisms we get the highest quality material with strong excitonicemission while for the case of platelets the emission becomesdominated by the defect related band.

In order to check the uniformity of the emission we performedcathodoluminescence measurements. Two types of measurementswere performed, either point measurements of emission spectra oremission mapping. The mapping was performed either by collectingintegral light on the photomultiplier, or by collecting monochro-mated light. The emission spectra were, as expected, quasi identicalto the ones measured using the Xe lamp excitation.

Mapping revealed that in most of the cases uniform emission isobtained from the samples. Thus, in Fig. 9a–c one can noticemapping performed on samples obtained in different conditions.However the most interesting case is the one where the two typesof structures, platelets and prisms coexist. When monochromatedat 570 nm one can notice that the emission intensity is muchhigher from the platelets than from the prisms (Fig. 9(d)). Thismeans that the point defects concentration is much higher on theplatelets than on the prisms.

EDX analysis was performed an the samples and besides the Znand O content traces of chloride of less than 1% were observed inthe samples grown from baths containing KCl as an additive. The Xray diffraction data show for all the samples wurtzite structurewith a slight (0 0 2) texture.

4. Conclusions

In the present study we investigated the influence of depositionconditions on the properties of electrodeposited ZnO films. Wefocused our research on how the composition of the electrodepo-sition bath, i.e. concentration of zinc nitrate and addition of KCl orPVP and the overpotential influence the morphological andphotoluminescence properties of the material.

The typical morphology consists in hexagonal prisms andplatelets. For the same bath composition we found that themorphology can be changed from hexagonal prisms to hexagonalplatelets with different orientations just by increasing thedeposition overpotential.

At high deposition voltages or at high concentration of zincnitrate the electrodeposition process occurs with a high rate andleads to the formation of material with high concentration ofdefects as proved by the strong photoluminescence defect relatedband centered at 570 nm.

Cathodoluminescence measurements performed on hybrid sam-ples containing both prisms and platelets proved that the plateletsposses a much stronger defect related luminescence than the prisms.

Fig. 9. Split images with cathodoluminescence mapping (left)/secondary electrons scanning (right): (a) 0.05 M Zn(NO3)2, 0.1 M KCl, 0.1% PVP at �850 mV monochromated at

380 nm; (b) 0.1 M Zn(NO3)2 at �900 mV integral emission; (c) 0.2 M Zn(NO3)2, 0.1% PVP at �900 mV integral emission and (d) 0.05 M Zn(NO3)2, 0.1 M KCl, 0.1% PVP at

�850 mV monochromated at 550 nm.

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Addition of KCl and PVP in the bath strongly influences thegrowth result in terms of morphology and defect content. Bothadditives appear to increase the uniformity of the deposit. Themost interesting results were obtained with the bath with lowestconcentration of zinc nitrate and with both KCl and PVP addition.In this case films of uniform hexagonal prisms with high ultravioletemission were obtained for deposition potentials of up to �900 mVversus SCE.

The studies presented here confirm the fact that nitrate bathdeposition is a promising route for obtaining high qualitynanostructured zinc oxide films suited for a wide field ofapplications especially in optoelectronics. The technique has asa major advantage the low cost and the scalability, qualities whichare extremely important for high output demands.

Acknowledgements

The authors acknowledge the financial support of the RomanianEducation and Research Ministry through the Nucleu Programmeand the PN II D 11 060 contract.

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