Growth of ZnO:Ga thin films at room temperature on polymeric substrates: thickness dependence

Thin Solid Films 442(2003) 121–126

0040-6090/03/$ - see front matter� 2003 Elsevier B.V. All rights reserved.doi:10.1016/S0040-6090(03)00958-1

Growth of ZnO:Ga thin films at room temperature on polymericsubstrates: thickness dependence

Elvira Fortunato*, Alexandra Goncalves, Vitor Assuncao, Antonio Marques, Hugo Aguas,´˜ ´¸ ¸Luıs Pereira, Isabel Ferreira, Rodrigo Martins´

Department of Materials ScienceyCENIMAT, FCT-UNL and CEMOP-UNINOVA, Campus da Caparica, Caparica 2829-516, Portugal

Abstract

In this paper, we present results concerning the thickness dependence(from 70 to 890 nm) of electrical, structural, morphologicaland optical properties presented by gallium-doped zinc oxide(GZO) deposited on polyethylene naphthalate(PEN) substrates byr.f. magnetron sputtering at room temperature. For thicknesses higher than 300 nm an independent correlation between theelectrical, morphological, structural and optical properties are observed. The lowest resistivity obtained was 5=10 V cm withy4

a sheet resistance of 15Vyh and an average optical transmittance in the visible part of the spectra of 80%. It is also shown thatby passivating the surface of the polymer by depositing a thin silicon dioxide layer the electrical and structural properties of thefilms are improved nearly by a factor of two.� 2003 Elsevier B.V. All rights reserved.

Keywords: Zinc oxide; Sputtering; Electrical properties and measurements; Polymers

1. Introduction

Plastic materials are replacing glass substrates in avariety of applications due to its lower cost associatedwith its lighter weight. Everywhere we look there aretouch-input devices ranging from personal digital assis-tants(PDAs) to automatic telling machines(ATMs) andpoint of sale (POS) cash registersw1x. The commonfeature of all these devices is the utilisation of atransparent conductive oxide(TCO) used as electrode.Indium tin oxide (ITO) has been the predominantlyTCO used for a number of years mainly for the flatpanel display industryw2x. Nevertheless, recently theutilisation of alternatively TCOs like zinc oxide dopedthin films have generated much attention, because oftheir material low cost, relatively low deposition tem-perature, non-toxicity and its stability in hydrogen plas-ma. The authors present the recently best-knownoptoelectronic results concerning gallium-doped zincoxide deposited on glass substrates at room temperatureby r.f. magnetron sputteringw3x. However, when flexibledevices are required, a polymeric substrate must beused. Nevertheless, these substrates present certain chal-

*Corresponding author..E-mail address: [email protected](E. Fortunato).

lenges such as considerably lower working temperatureand rougher surfaces as compared to glass substrates. Inorder to overcome these limitations, we have optimisedthe r.f. magnetron sputtering process to be able toproduce highly transparent and highly conductive GZOfilms exhibiting a low film stress and with a smoothsurfacew4,5x. The films were grown by r.f. magnetronsputtering at room temperature on commercially avail-able polymeric substrates(Kaladex 1020, 75-mm thick,�

from DuPont). PEN is a biaxially oriented film withhigh stiffness and mechanical strength, low thermalshrinkage and high chemical resistance.In this paper a detailed description of the preparation

conditions as well as on the electrical(Hall effect andresistivity as a function of temperature), optical (trans-mittance), structural(X-ray diffraction) and morpholog-ical (FE SEM) properties will be presented.

2. Experimental procedures

The GZO thin films with different thickness weredeposited on PEN(polyethylene naphthalate) substrateswith a thickness of 75mm, supplied by DuPont(Kaladex 1020). Prior to the deposition of the GZO�

films, the polymeric substrates were ultrasonically

122 E. Fortunato et al. / Thin Solid Films 442 (2003) 121–126

Fig. 1. Scheme used for the deposition of the GZO thin film. Themaximum allowed temperature during the deposition was 408C.

Fig. 2. Dependence of electrical resistivity on the film thickness.

Fig. 3. Carrier concentration(h) and Hall mobility(j) as a functionof film thickness.

cleaned in isopropyl alcohol. The GZO films weredeposited by r.f.(13.56 MHz) magnetron sputteringusing a ceramic oxide target ZnOyGa O (95.5 wt.%; 52 3

cm diameter) with a purity of 99.99%. The sputteringwas carried out under room temperature and with theoptimum conditions for the system usedw3x: argon fluxof 20 sccm and a deposition pressure of 0.15 Pa. Thedistance between the substrate and the target was 10 cmand the r.f. power was maintained constant at 175 W.The growth rate for this deposition conditions is 28 nmymin. In order to avoid the undesirable heating of thesubstrate due to the ion bombardment of the substratethe thicker samples were grown by steps(maximum ofthree) that allow keeping the substrate temperaturebelow 408C (313 K), as depicted in the sketch of Fig.1.In order to optimise the interface between the poly-

meric substrate and the GZO film a thin silicon dioxide(SiO ) film with a thickness not exceeding 100 nm was2

previously deposited in one sample by an electron gun.The main objective of this layer is to prevent degradationof the polymeric surface during the deposition of theGZO thin film by r.f. magnetron sputtering as well asto prevent diffusion or some kind of chemical reactionsat the polymer–film interface.The film thickness(d) was measured using a surface

profilometer(Dektak 3D from Sloan Tech.). The elec-trical resistivity (r), free carrier concentration(n) andHall mobility (m) were determined from Hall effectmeasurements using the Van der Pauw geometry(BioradHL5500) at a constant magnetic field of 0.5 T. Theoptical transmittance was measured using a UV–VIS–NIR double beam spectrophotometer(UV-3100 PC,Shimadzu) in the wavelength range from 300 to 2500nm. The structural properties of the films were deter-mined using X-ray diffraction measurements with Cu–

K radiation(Rigaku DMAX III-C series). The surfacea

morphologies were analysed using a field effect scan-ning electron microscope(FE-SEM, S-1400 Hitachi).

3. Results and discussion

The deposited films were physically stable and presentvery good adherence to the polymer substrates. No crackor peel off of the films was observed after deposition.Fig. 2 shows the electrical resistivity as a function of

the GZO film thickness. With increasing thickness upto a value approximately 300 nm, the resistivity decreas-es to a minimum value of 6=10 V cm. After thaty4

point r is kept almost constant, independently of thethickness of the film used. This means that up toapproximately 300 nm the film properties are highlydependent of the surface(two-dimensional behaviour),while for thicker films r is clearly dependent on the

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Fig. 4. XRD pattern for GZO films deposited on PEN substrates as afunction of film thickness.

Fig. 5. Optical transmittance as a function of wavelength for GZOfilms with different thicknesses on PEN substrates.

bulk properties of the material(three-dimensionalbehaviour).Fig. 3 presents the variation of the carrier concentra-

tion and Hall mobility as a function ofd. As can beseen, the carrier concentration increases from 3.5=1020

cm to a stabilised value approximately 8=10y3 20

cm for dG300 nm. However, the Hall mobility seemsy3

to increase withd, this increase being more evident ford-300 nm. For thicker filmsm stabilises by approxi-mately 12 cmyVs. These data agree with ther data2

shown in Fig. 2.Overall, the data show that the lowr and the apparent

increase ofm for d)300 nm are mainly due to theimproved bulky crystallinity and to the enhancement ofthe crystallite sizes that weakens inter-crystallite bound-ary scattering and increases carrier lifetime. Consequent-ly, an improvement of the mobility is observed insteadof an increase associated to the free carrier concentra-tion. Similar results have been obtained by Hao et al.w6x on Al-doped ZnO films deposited on polypropylenesubstrates.Fig. 4 shows the X-ray diffraction pattern for GZO

films as a function of film thickness. For all the filmsonly the ZnO (002) peak at 2u;34.18 is observed,revealing that the films are polycrystalline with a hex-agonal structure and a preferred orientation with thec-axis perpendicular to the substrate. For comparisonpurposes also the XRD pattern for the PEN substrate isindicated (black curve). Fig. 4 also shows that asdincreases the diffraction peaks become sharper and theirintensity is enhanced(as expected) while its locationdoes not change significantly. This behaviour means thatno significant residual stresses are generated during thedeposition process and that the crystallite size isenhanced withd, in line with the Hall mobility datashown in Fig. 3.Fig. 5 presents the optical transmittance in the wave-

length range 300–2500 nm for GZO films with thick-

nesses in the range between 70 and 420 nm, using airas reference. The average transmittance in the visiblepart of the spectra(400–700 nm) was on the average80%, for all the samples analysed. The transmittance ofthe PEN substrate with 75mm thickness, for the samewavelength range is 85%.The near-infrared transmittance decrease asd increas-

es. These changes of the optical properties are consistentwith the changes observed in the electrical properties,mainly the ones associated to the increase of the carrierconcentration, which leads to a lower value associatedto the plasma frequencyw7x. Besides that the existenceof two regions (A and B) is clearly seen near theinfrared region. These two different regions are ascribedto the surface morphologies observed by SEM analysis(see Fig. 6) and also evidenced by the electricalproperties.For films with a thickness lower than 300 nm there

are some memory effects of the roughness of thepolymeric substrate. For higher thicknesses this effectdisappears leading to very uniform surface morpholo-gies, similar to that ones obtained on glass substrates.As it was mentioned in the experimental details, the

deposition of thicker samples was performed in a dis-continuous manner to avoid the heating of the polymersurface. Otherwise we obtain cracks in the films due tothe different thermal expansion coefficient of the PENsubstrate and that one of GZO film, originated from thelarger thermal stresses developed. By analysing one ofthis cracks at the microscope Fig. 7, it is possible toobserve the cross sectional view of the film, where ahigh compact and dense columnar structure is clearlyseen.As the electrical performances of the GZO films

deposited on PEN substrates are slightly worse than the

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Fig. 6. Scanning electron micrographs for GZO film with different thickness deposited on PEN substrates. The thickness is indicated inside themicrographs.

ones of the films deposited on glass substrates using thesame deposition conditions, some problem should existbetween the interface of the film–polymeric substrate.In order to optimise this interface, a thin SiO layer2

(100 nm) was previously deposited on the PEN. Theaim was to prevent degradation of the polymeric surfaceduring the deposition process of the GZO thin film andto minimize the residual thermal stresses, as well as toprevent diffusion or some kind of chemical reactions atthe polymer–film interface.Table 1 summarises the electrical properties deter-

mined from Hall effect measurements for the GZOdeposited on PEN with and without the SiO barrier.2

For comparison purposes the electrical properties forthe same GZO film deposited onto soda lime glass arealso indicated. The results clearly show that the electricalresistivity of the GZO film was improved on the PENsubstrate coated with SiO film. Nevertheless, the opti-2

mum values for the electrical properties of GZO depos-ited onto soda lime glass were not yet reached. Themain reason is related to the lower value obtained forthe free carrier concentration. This behaviour is due tothe physical, chemical and mechanical differencesbetween the polymer substrate and the glass substrate.In opposition to glass substrates, the polymers have alow thermal resistance, weak mechanical properties and

125E. Fortunato et al. / Thin Solid Films 442 (2003) 121–126

Fig. 7. GZO crack due to the different thermal expansion coefficient of GZO and the PEN substrate. The film was deposited continuously.(a)Low magnification(6 k); (b) High magnification(60 k).

Table 1Electrical properties exhibited by the GZO thin film deposited on different substrates with an average thickness of 350 nm

Sample Sheet resistance,rs Electrical resistivity,r Hall mobility, m Carrier concentration,n(V) (V cm) (cm yVs)2 (cm )y3

GZOyPEN 26 9.0=10y4 8.6 8.0=1020

GZOySiO yPEN2 15 5.3=10y4 13.7 8.6=1020

GZOyglass 5 1.7=10y4 14.7 2.4=1021

Fig. 8. X-Ray diffraction patterns for GZO thin film, deposited on SiOyPEN and PEN substrates.2

a high thermal expansion coefficient(20=10 yK w8x).y6

The different values of the thermal expansion coefficientof the polymer and that of zinc oxide(4.75=10 yKy6

w9x), induces defects(for example residual thermalstresses) and consequently deteriorates(mainly) theelectrical properties of the conducting film. In the limit,these stresses are responsible for the appearance ofcracks in the film(see Fig. 7).

Another problem is related to the gas and moistureabsorption of the polymers. Even by using the SiO2

barrier the polymer exhibit some absorption comparedto glass substrates. The oxygen and water vapour per-meation values for similar barrier films are known to be0.2 cm m day atm and 5 g day , respec-3 y2 y1 y1 y2 y1

tively, w10x. Besides that, the polymeric substrates havehigh oxygen content within their matrix structure and

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so, oxygen may flow out, resulting in a non-desirablepartial pressure of oxygen during the deposition process.This extra oxygen is also responsible for the deteriora-tion of the electrical properties of GZO deposited ontoPEN.In Fig. 8, the X-ray diffraction patterns of GZO film

deposited on PEN and SiOyPEN, respectively, are2

presented. The(002) peak location is at 2us34.078 and34.168, respectively, for GZO deposited on PEN sub-strate and on SiOyPEN substrate, respectively. The2

intensity of the peak is larger for the film depositedonto SiOyPEN substrate. This is due to the higher2

degree of crystallinity and an increase of the crystallitesize. By using the Scherrer formulaw11x we estimateda value for the crystallite size of 20 nm and 23 nm,respectively, for the GZO deposited onto PEN substrateand SiOyPEN substrate. This is in line with the values2

obtained for the Hall mobility.Concerning the morphological properties no differ-

ences were observed by using the intermediate SiO2

layer.

4. Conclusions

In this paper we present results concerning the thick-ness dependence(from 70 to 890 nm) of electrical,structural, morphological and optical properties present-ed by GZO deposited on polyethylene naphthalate(PEN) substrates by r.f. magnetron sputtering at roomtemperature. For thicknesses higher than 300 nm anindependent correlation between the electrical, morpho-logical, structural and optical properties are observed,meaning that the properties obtained are not dependenton the thickness of the film. The lowest resistivityobtained was 5=10 V cm with a sheet resistance ofy4

15 Vyh and an average optical transmittance in thevisible part of the spectra of 80%. It was also shownthat by passivating the polymer surface with a thin

SiO layer, the electrical and structural properties of the2

films are improved nearly by a factor of two. Theobtained results are in accordance with the target-specifications required for TCO coated flexiblesubstrates.

Acknowledgments

The authors would like to acknowledge AugustoLopes for the SEM analysis and Rui Martins for theXRD measurements. This work was supported by the‘Fundacao para a Ciencia e a Tecnologia’ through˜ ˆ¸Pluriannual Contracts with CENIMAT and by the pro-jects: POCTIy1999yESEy35578, POCTIy1999yCTMy35440 and POCTIy2001yCTMy38924.

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