Surface & Coatings Technology 202 (2008) 5416–5420

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Surface & Coatings Technology

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Preparation and mechanical properties of aluminum-doped zinc oxide transparentconducting films

Shou-Yi Chang ⁎, Yen-Chih Hsiao, Yi-Chung HuangDepartment of Materials Science and Engineering, National Chung Hsing University, Taichung, 402, Taiwan

⁎ Corresponding author. Tel.: +886 4 22857517.E-mail address: shouyi@dragon.nchu.edu.tw (S.-Y. C

0257-8972/$ – see front matter © 2008 Elsevier B.V. Aldoi:10.1016/j.surfcoat.2008.06.024

A B S T R A C T

A R T I C L E I N F O

Available online 7 June 2008

Keywords:

Aluminum-doped zinc oxidsputtering under differentsubstrate temperatures, the

Transparent conductive filmsMechanical propertiesInterface adhesion

deposited films were constructed by spherical grains. With increasing power andtemperature, the grains became facet with an obvious (002) preferred orientation. The crystallinity and grainsize of the films increased as well, and consequently the electrical resistivity decreased. By nanoindentationtests, the hardness of the deposited films was measured and found to increase from 8 to 10 GPa with highersputtering power and substrate temperature because of higher densification and crystallinity. During

e transparent conducting films were deposited in this study by magnetronsputtering powers and substrate temperatures. At low sputtering powers and

nanoindentation and nanoscratch tests, interface delamination occurred between the films and substrates,and the interface adhesion energy was accordingly obtained. From the measurement of nanoscratch tests, theadhesion energy was found to be improved from 0.49 to 0.86 or 0.79 J/m2, respectively, with highersputtering power or substrate temperature because of the deeper penetration, higher densification andeasier interface reaction of the deposited films.

© 2008 Elsevier B.V. All rights reserved.

1. Introduction

Transparent conducting oxide (TCO) films with good optical andelectrical properties (transparencyN80%, electrical resistivityb10−3Ω·cm)havebeenwidelyapplied tooptoelectronic industry suchas solar cells anddisplays [1]. Typical TCO films include SnO2, In2O3, and ZnO, etc. Minorelements are further doped to obtain better film properties, like themostpopularly used indium-doped tin oxide (ITO). However due to the poorthermal stabilityandhigh cost of ITOfilms, cheap andnon-toxic ZnOfilmshaveattractedmuchattention in theseyears. Especially, aluminum-dopedzinc oxide (ZnO:Al, AZO) films exhibit comparable optical and electricalproperties with ITO films, and have a high potential to replace con-ventional TCO films [2,3]. For the preparation of TCO films, radio-fre-quency (RF) magnetron sputtering, chemical vapor deposition, thermalevaporation, and sol–gel methods have been used. Among them, the RFmagnetron sputtering by which the films are deposited at lowertemperatures with high qualities is more generally adopted [1–3].

However, the mechanical damages of TCO films, such as filmcracking and interface delamination, severely suppress the processingyield and application reliability of the films [4]. Especially for the TCOfilms applied on flexible substrates, repeated flexure stresses duringapplicationmore easily result in film damages. A high resistance to themechanical damages is thus strongly demanded for the TCO films

hang).

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besides good optical and electrical properties, and the mechanicalproperties need to be clarified before practical applications. However,conventional testing tools are not longer suitable for the evaluation ofmechanical properties of thin TCO films. Alternately, nanoindentationand nanoscratch tests have been widely applied for the measurementof the mechanical properties of thin films [5–15]. Besides hardnessand elastic modulus [5,6], more information such as yielding stressand fracture toughness can be extracted to reveal more representativemechanical properties of thin films [7]. Moreover, the nanoindenta-tion and nanoscratch tests are also promising to determine interfaceadhesion strength through film delamination [8–15].

Thus in this study, AZO transparent conducting films have beenprepared on glass substrates by magnetron sputtering. Their micro-structures and basic properties including electrical resistivity andoptical transparency are characterized. Moreover, the mechanicalproperties of the films and the interface adhesion energy between thefilms and substrates are investigated by nanoindentation andnanoscratch tests. The interface delamination behaviors are examinedto evaluate the mechanical reliability of the AZO films. Sputteringpower and substrate temperature are varied to investigate their effectson the properties of the films.

2. Experimental details

The AZO films were deposited on Corning 7059 glass substrates byRFmagnetron sputtering at a pressure of 8mTorr using a 98 wt.% ZnO-2 wt.% Al2O3 target. The sputtering power was varied from 50 to200 W (at room temperature, RT), and the substrate temperature was

Fig. 1. XRD patterns of AZO films deposited under (a) different sputtering powers at RTand (b) different substrate temperatures at a sputtering power of 100 W.

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controlled at RT, 100 °C, and 200 °C (at 100 W). The deposition rateincreased from 6.7 to 28.3 nm/min with sputtering power, while itremained at about 14 nm/min under different substrate temperatures.The thickness of the deposited films was controlled at about 500 nmby varying deposition time. The crystal structure of the AZO films wasanalyzed by an X-ray diffractometer (XRD, MAC Science MXP3). Ascanning electron microscope (SEM, JEOL JSM-6700F) was used toobserve the surface morphologies and microstructures of the films.Atomic force microscopy (AFM, SEIKO Instrument SPI3800N) wasapplied to characterize surface roughness. A four-point probe methodwas applied to measure electrical resistivity, and UV/visible spectro-scopy (Hitachi U-3010) was used to measure optical transparency.

The mechanical properties of AZO films were measured by using aUMIS nanoindenter (Based Model, CSIRO) with a Berkovich diamondindenter (tip radius ~100 nm, edge angle 130.6 ) and a nanoscratchtest module. During nanoindentation tests, the load was applied to amaximum value of 1 mN, held for 30 s, and then released under aloading/unloading rate of 0.033 mN/s. The indentation depth wascontrolled at 1/15 to 1/10 of film thickness to avoid a substrate effect.For the measurement of interface adhesion energy between the AZOfilms and glass substrates by nanoindentation tests, a series of in-dentations with varied maximum loads from 110 to 170 mN wereperformed. Under nanoscratch tests, the load was ramped from 0 to50 mN in 30 s, and the scratch length was set as 1500 μm at a movingvelocity of 50 μm/s. An SEM was used to observe interface delami-nation around indent marks and scratch tracks after the nanoindenta-tion and nanoscratch tests.

3. Results and discussion

3.1. Microstructures and basic characterizations of AZO films

Fig. 1 shows the XRD patterns of AZO films deposited under dif-ferent sputtering powers and substrate temperatures. At low sputter-ing powers and substrate temperatures, three main diffraction peaksof ZnO (100), (002) and (101) crystal planes were detected, indicatingno preferred crystalline orientation in the films. With increasingsputtering power to 150 Wor substrate temperature to 100 °C, higherenergy for the regular arrangement of incident atoms promoted thegrowth of the AZO films in a low-energy (002) direction, then en-hancing the formation of more obvious (002) preferred orientation[16]. By using the full widths at half maximum of the (002) diffractionpeaks and the Scherrer equation [17], the grain sizes of the AZO filmscan be calculated. With higher sputtering powers, the grain sizeincreased from 11.0 to 14.1 nm since the higher energy of incidentatoms promoted grain growth. At higher substrate temperatures, thegrain size even increased from 11.2 to 17.5 nm because of a rapid graingrowth.

Fig. 2 shows the SEM surface morphologies and cross-sectionalmicrostructure of AZO films deposited under different sputteringpowers and substrate temperatures. From Fig. 2(a), it was observedthat the AZO films deposited under low sputtering powers and sub-strate temperatures were constructed by spherical particles of about10 nm in size without specific preferred orientation. With increasingsputtering power and substrate temperature, the particles grew andbecame a facet shape as shown in Fig. 2(b) and (c) with an obviouspreferred orientation, in accordance with the XRD analyses shown inFig. 1. From Fig. 2(d), it was observed that the deposited AZO filmswere continuous, dense, and smooth with a uniform thickness ofabout 500 nm. The films possessed a columnar structure. Moreoverfrom the AFM analyses of the AZO films, very small surface roughnesswas measured for most of the films as only about few nanometers.

The electrical resistivity of AZO films was lowered from 0.9 to 0.09and 0.03 Ω·cm as the sputtering power increased to 200 W and thesubstrate temperature to 200 °C, respectively. As aforementioned, thecrystallinity of the deposited films was enhanced with an obvious

(002) preferred orientation when the sputtering power and substratetemperature increased. Moreover, grain growth and film densificationoccurred, reducing defects (grain boundaries, etc.) and thus electricalresistivity. Furthermore from the measurement of optical transpar-ency at different wavelengths, it was observed that all of the AZO filmsdeposited under different sputtering powers and substrate tempera-tures possessed an optical transparency of about 90% at the filmthickness of 500 nm. The bandgap energy Eg of the AZO films de-creased from 3.35 to 3.28 eV with increasing sputtering power, whileit remained at a constant value of about 3.33 eV with varied substratetemperatures.

3.2. Nanomechanical properties of AZO films

From the load-penetration depth curves of nanoindentation testsof AZO films, it was found that, under small indentation depths below15 nm, the loading curves matched to the elastic unloading curvesin accordance with a “Hertzian elastic relation” [18], indicating theelastic deformation of the AZO films. Beyond the depth, the curvesbegan to deviate from the “Hertzian response”, and the permanentlyplastic deformation of the AZO films was expected to occur. From theOliver–Pharr relation [5], the hardness and elastic modulus of the AZOfilms deposited under different experimental conditions were ob-tained as plotted in Fig. 3. It was found that, although the deposition

Fig. 2. SEM surface morphologies of AZO films deposited under different sputtering powers and substrate temperatures, (a) 100 W at RT, (b) 200 W at RT, and (c) 100 W at 200 °C.(d) SEM cross-sectional microstructure of AZO film deposited under 100 W at RT.

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parameters varied, the elastic modulus of the AZO films basicallyremained at the level of about 110 GPa because the elastic moduluswas an intrinsic property of materials. In comparison, the hardnessincreased from 8 to 10 GPa with higher sputtering powers and sub-strate temperatures. As aforementioned, the stronger bombardmentsunder higher powers and the higher energy for crystallization athigher temperatures induced higher densification (less porosity) andcrystallinity of the deposited films, thus enhancing the hardness [19].

3.3. Interface adhesion measured by nanoindentation test

Nanoindentation introduced localized mechanical deformation toAZO films, and shear stresses began to accumulate at the interfacebetween the AZO films and glass substrates due to strain mismatches.Once the accumulated stresses exceeded interface adhesion strength,the interface would then delaminate. Fig. 4(a) shows the SEM surfacemorphology of the AZO film deposited under a sputtering power of100 W and a substrate temperature of 200 °C after nanoindentationtest at a maximum applied load of 130 mN. When the load of 120 mNwas applied, unclear film blister and buckling just formed around theindented region, indicating the occurrence of interface decohesion.Under higher applied loads than 130 mN, film blister and bucklingbecamemuchmore obvious, and the buckled filmwas further pressedby the indenter tip and then slightly cracked.

By using the following equation [8–13], the fracture energy releaserate Gc, i.e. the adhesion energy per unit area, for interface delami-nation between AZO films and substrates can be then obtained.

Gc ¼ σ2rxtEf

2 1−�2f� �

1þ �f þ 1−�f� �

a=xð Þ2h i2 ð1Þ

where σrx is the stress acting on the AZO films, equal to P/A (P: appliedload, A: contact area). The thickness t of the films is about 500 nm, andthe elastic modulus Ef is 110 GPa. The νf denotes the Poisson's ratio,approximate 0.3. The crack length of interface delamination a wasmeasured under SEM observations, and the contact length of theindenter tip x was determined by the instrument. After calculation,the interfacial adhesion energy between the substrate and AZO filmdeposited at 100 W and 200 °C was obtained as plotted in Fig. 4(b)with different maximum applied loads from 110 to 170 mN. It wasfound that the interface adhesion energy drastically decreased from25.4 J/m2 and then stabilized at about 2.4 J/m2 with increasing appliedloads. When low indentation loads (110 and 120 mN) were applied,the adhesion energy was inaccurately estimated as larger because filmblister and buckling was unclear to identify; whereas with higherapplied loads, the blister and buckling became much obvious, and theadhesion energy was more accurately determined as a stable value.

3.4. Interface adhesion measured by nanoscratch test

From the load-distance and indentation depth-distance curves ofnanoscratch tests of AZO films, a variation was found as the appliedload was raised to about 20–30 mN (scratch distance ~900–1100 μm).Afterwards, a more drastic fluctuation in the curves occurred. Bycomparing the surface morphologies of the films along the scratchtrack as shown in Fig. 5, the fracture behaviors of the films at differentscratch stages were realized. Firstly at the early stage of the scratchtests, the AZO films were just pressed, and only a slight scratch tracewas observed as shown at the left side of Fig. 5(a). When the load wasapplied to a critical value Pc, the interfaces between the AZO films andsubstrates began to delaminate, where clear film buckling and peelingwere observed as shownat the right side of Fig. 5(a). During the scratch

Fig. 4. (a) SEM morphology and (b) interface adhesion energy of AZO film depositedunder a sputtering power of 100 W and a substrate temperature of 200 °C measured bynanoindentation tests.

Fig. 3. Hardnesses and elastic moduli of AZO films deposited under (a) differentsputtering powers at RT and (b) different substrate temperatures at a sputtering powerof 100 W measured by nanoindentation tests.

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tests, shear stresses accumulated at the interfaces when the tip in-dented and scratched. Under sufficient accumulation of shear stresseshigher than the interface adhesion strengths, the interfaces then de-laminated. As the indenter tip moved forward, the delaminated andbuckled film under the tip cracked as shown in Fig. 5(b). The repeatedfilm delamination, buckling, and cracking resulted in stress accumula-tion and release, leading to the fluctuation of the curves.

By using the following equation and introducing critical scratchtrack widths dc as the interface delaminated, the critical stresses σc

for interface delamination (adhesion strength) between AZO films andsubstrates were obtained as plotted in Fig. 6 [11–15].

σ c ¼ 2Pcπd2c

� �4þ �fð Þ3πμ

8− 1−2�fð Þ

� ð2Þ

inwhich μ is themeasured friction coefficient of indenter sliding givenby the nanoscratch tester as about 0.035. Afterwards by using thefollowing equation, with the thickness t and elastic modulus Ef of theAZO films, the fracture energy release rates Gc for the interface de-lamination (adhesion energy) between the films and the substrateswas then obtained as plotted in Fig. 6 as well [11–15].

Gc ¼ σ2c t

2Efð3Þ

From Fig. 6(a), it was found that the interface adhesion energy in-creased from0.49 to 0.86 J/m2with sputteringpower because the strongbombardments of deposited atoms under high powers induced deep

penetration as well as high densification [19]. With higher substratetemperatures, the interface adhesion energy increased aswell from0.49to 0.79 J/m2 as seen in Fig. 6(b) since it would be easier for the depositedAZO films to react with or to interdiffuse into the substrates to formchemical bonding at the interface at higher temperatures [19].

However, the interface adhesion energy measured by a nanoin-dentation test was higher than that obtained by a nanoscratch test.The most important difference between these two tests was that thescratch test, besides a normal compressive stress, also provided lateralshear and even tensile stresses moving forward with the indenter tip,thus causing earlier interface delamination. Therefore consisting withthe “mode mixity (phase angle)” effect reported by Volinsky [8], therequired energy for interface delamination measured under thescratch test was lower and much closer to the theoretical work ofadhesion.

4. Conclusions

In this study, dense and smooth AZO transparent conducting filmswith a columnar structure were deposited by magnetron sputtering.The AZO films deposited at low sputtering powers and substratetemperatures were constructed by spherical grains. As the power andtemperature increased, the grains grew and became facet with a (002)preferred orientation. The electrical resistivity of the AZO filmsdecreased to 0.03 Ω·cm with higher sputtering powers and substratetemperatures. At a thickness of 500 nm, the AZO films possessed anoptical transparency of about 90% and bandgap energy of 3.33 eV. Byusing nanoindentation tests, the hardness and elastic modulus of theAZO filmsweremeasured as about 8 and 110 GPa, respectively, and the

Fig. 6. Critical stresses for interface delamination and interface adhesion energy AZOfilms deposited under (a) different sputtering powers at RT and (b) different substratetemperatures at a sputtering power of 100 W, measured by nanoscratch tests.

Fig. 5. SEM fractographies of AZO film deposited under a sputtering power of 100Wanda substrate temperature of RT at different stages of nanoscratch test, (a) early-stageinterface delamination and (b) middle- and late-stage severe film cracking.

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hardness increased to 10 GPa with higher sputtering powers andsubstrate temperatures because of higher densification and crystal-linity. During nanoindentation and nanoscratch tests, the interfacesbetween the AZO films and substrates delaminated, and the interfaceadhesion energy was accordingly obtained. By the nanoscratch tests,the adhesion energy increased from 0.49 to 0.86 and 0.79 J/m2, re-spectively, with increasing sputtering power and substrate temper-ature because of the deeper penetration, higher densification andeasier interface reaction of the deposited films.

Acknowledgements

The authors gratefully acknowledge the financial supports for thisresearch by the National Science Council, Taiwan, under Grant No.NSC-96-2221-E-005-007, and in part by the Ministry of Education,Taiwan, under the ATU plan.

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