et

oha

Atomic force microscopy

) thsptart tovem, Aovepo

and substrate temperature of 120 C showed the lowest resistivity (5.1210 cm on PEN substrate,ubstrate) and high average transmittance (N90% in both substrates). AZO lms

) lmsplicationic papanels,applicadO, an

Thin Solid Films 518 (2010) 58605865

Contents lists available at ScienceDirect

Thin Soli

w.eresearch groups have studied materials, ZnO based materials, insteadof ITO due to the toxicity and high cost problems. ZnO is an n-typesemiconductor, with a wide band gap (3.3 eV), large free excitonbinding energy (60 meV) and high mechanical and thermal stabilities[3,5]. Also, it has cost advantage, high resource availability, stability inhydrogen plasma and non-toxicity [6]. When impurity elements ofgroup III (B, Al, Ga, and In), especially Al, are incorporated, ZnO becomesn-type material with the improvement of electrical and opticalproperties because of the increase in the carrier concentration and

easy to carry. Stainless steel or polymer lms can be used as exiblesubstrates for exible devices. Although polymer substrate haslimitation of deposition temperature for maintaining its chemical andmechanical properties, applications of polymer substrate draw muchattention due to lightweight, exibility and transparency contrary tostainless steel substrate. Many researchers deposited AZO lms onpolymer substrates such as polyimide (PI) [17], polyethylene tere-phthalate (PET) [18], polyethylene naphtahlate (PEN) [19], polyethy-lenesulfone [20] and polycarbonate [3]. S. Fernandez et al. and X.T. Japmobility [7]. W. Yang et al. and Lin et al. invZnO) lms which had low resistivity, high a4.6104cm and 90% and 8.4310

Kuroyanagi [10] and Keum [11] group also

Corresponding author.E-mail address: srhee@postech.ac.kr (S.-W. Rhee).

0040-6090/$ see front matter 2010 Elsevier B.V. Adoi:10.1016/j.tsf.2010.05.098d ZnO, etc., must have a3 cm)andgoodoptical[3,4]. Recently, many

low deposition temperature, high reproducibility and good adhesion tothe substrate [16]. Lately, many people are interested in exible andtransparent devices with lightweight and small volume for foldable,large bandgap(N3.2 eV), low resistivity (N10

transmittance (N80%) in the visible region1. Introduction

Transparent conducting oxide (TCOfor transparent and exible device apdisplays, plasma display panels, electroemitting diode, solar cells, touch poptoelectronic devices [1,2]. For theseindium-tin oxide (ITO), In2O3, SnO2, C 2010 Elsevier B.V. All rights reserved.

have beenwidely studiedns such as liquid crystaler displays, organic light-gas sensor and othertions, TCO lms, such as

lms with the resistivity of 1.1103cm at 300 C and 101cmat 200 C. AZO lms can be deposited with several depositiontechniques such as R.F and D.C magnetron sputtering [12], chemicalvapor deposition [6], pulsed laser deposition [13], spray pyrolysis [14],and solgel method [15]. Magnetron sputtering is most widely usedamong all of them. Compared with other deposition techniques,magnetron sputtering technique can deposit a lm on large areas atestigated AZO (Al-dopedverage transmittance of3cm and 86% [8,9].deposited 100 nm AZO

et al. deposited Aand 8.5104possible to depothermal stabilitycolors from yellosolubility [21]. Inexible substratevalues (13 ppm/

ll rights reserved.deposited on PEN substrate showed similar electrical and optical properties like AZO lms deposited on glasssubstrates.Optical properties 3.85103 cm on glass sDeposition of Al-doped ZnO lms on polyfrequency magnetron sputtering

Jung-Min Kim, P. Thiyagarajan, Shi-Woo Rhee System on Chip Chemical Process Research Center, Department of Chemical Engineering, P

a b s t r a c ta r t i c l e i n f o

Available online 1 June 2010

Keywords:Transparent conducting oxidesAl-doped zinc oxidePolyethylene naphthalateFlexible substratesRadio frequency magnetron sputteringX-ray diffraction

100 nm Al-doped ZnO (AZOradio frequency magnetronincluding sputtering power,the sputtering power, targedecreased due to the improincreased from 25 to 120 Cdue to the signicant imprPEN substrates at sputtering

j ourna l homepage: wwhylene naphthalate substrate with radio

ng University of Science and Technology (POSTECH), Pohang 790-784, Republic of Korea

in lms were deposited on polyethylene naphthalate (PEN) substrates withuttering using 2 wt.% Al-doped ZnO target at various deposition conditionsget to substrate distance, working pressure and substrate temperature. Whensubstrate distance and working pressure were decreased, the resistivity wasent of crystallinity with larger grain size. As the substrate temperature was

ZO lms showed lower electrical resistivity and better optical transmittancement of the crystallinity. 2 wt.% Al-doped ZnO lms deposited on glass andwer of 25 W, target to substrate distance of 6.8 cm, working pressure of 0.4 Pa

3

d Films

l sev ie r.com/ locate / ts fZO lmswhich had low resistivity of 1.1103cmcm on PET and PI substrate [17,18]. Although it issit at higher substrate temperature due to higherof PI substrate, it is a highly colored polymer withw to brown and shows poor processability, and lowthe case of PEN substrate, it is a strong candidate fors due to the lowest coefcient of thermal expansionC), high transparency (N85%), low water absorption

(0.14%), the highest tensile strength (275 MPa), high Young's modulus(6.1 GPa) and higher barrier property for oxygen and carbon dioxide[22]. Y.M. Chung et al. investigated the properties of 200 nm AZO lms,the resistivity of 1.2103cm and 80% of transmittance, depositedon PEN substrates with non-reactive pulsed D.C. CHUBM co-magnetronsputtering in order to observe the effect of working pressure [19].

In this study, 100 nmAZOlmwas deposited on PEN substrate usingR.F. magnetron sputtering in order to observe several depositionparameter effects such as sputtering power, target to substrate distance

glass substrate was ultrasonically cleaned in acetone, ethanol, isopropylalcohol, and rinsed in distilled water for 10 min in each case to removeimpurities on the substrate surface. Al-doped ZnO (AZO) lms weredeposited with R.F. magnetron sputtering system using AZO ceramictarget (ZnO 98 wt.%: Al2O3 2 wt.%, 2 in diameters, 99.995%, TASCO). Thebase chamber pressure was 1.3103 Pa obtained by a rotary vacuumpump and a turbomolecular pump. After the evacuation of the chamber,pre-sputtering was executed at 100W, 0.4 Pa for 10 min in order toremove impurities on the target surface. The Ar ow rate was kept at

Fig. 1. AFM images of bare glass substrate (a) and bare PEN substrate (b).

5861J.-M. Kim et al. / Thin Solid Films 518 (2010) 58605865and substrate temperature includingworking pressure. AZO lms werealso deposited on glass substrate for reference to judge properties ofAZO lms deposited on PEN substrate for applications of exible andtransparent devices. To optimize deposition condition of the AZO thinlm as a TCO, their structural, electrical and optical properties wereinvestigated using various analytical tools.

2. Experimental details

Glass and polyethylene naphthalate (PEN) substrates were used assubstrates to compare the material properties on both substrates. TheFig. 2. XRD patterns of AZO lms deposited on glass substrate ((a), (c)) and PEN substrate ((bof 25 W, substrate temperature of 25 C).50 sccm throughout the deposition condition. The substrate was rotatedwith 15 rpm to get uniform AZO lms during deposition. AZO lmsweredeposited at various sputtering powers, target to substrate distances,working pressures and substrate temperatures to nd the optimizeddeposition condition of R.F. magnetron sputtering. The thickness of AZOlms was xed at 100 nm, regardless of substrates and depositionconditions. AZO lms were deposited on glass and PEN substrates atvarious sputtering powers of 25W, 50W, 75W, 100W and 150W,target to substrate distance of 6.8 cm, 9 cm and 11 cm, working pressureof 0.4 Pa, 0.9 Pa, 1.5 Pa, 2.0 Pa and2.7 Pa and the substrate temperatureof25 C, 60 C, 90 C and 120 C. The substrate temperature was limited to), (d)) at various target to substrate distance andworking pressure (at sputtering power

reduction of the collisions. This leads to the improvement of crystallinityand the increase of grain size as shown in XRD result, thereby, the carrierconcentration and mobility can be increased because of the decrease ofimpurity scattering and grain boundary scattering [24,26]. Fig. 4 showsXRD patterns of AZO lms deposited on the glass and PEN substrates atdifferent sputtering powers with xing target to substrate distance of6.8 cm, working pressure of 0.4 Pa and substrate temperature of 25 C.AZO lms have the strong (002) peaks around 234.36 on glasssubstrate and 234.3 on PEN substrates at 25W. Thismeans that AZOthin lms are hexagonal structures and have preferred orientation withthe c-axis perpendicular to the substrate [24]. There are no metallicaluminum ormetallic zinc peaks from the XRD patterns. The (002) peakposition was continuously shifted to lower angle with the increase ofsputtering power because of the stress of AZO lms. Generally, a peakshift to a smaller angle indicates an increase in the lattice d-spacingwithcompressive stress, while a shift to a larger angle indicates a decrease inthe lattice d-spacing with tensile stress [27]. Therefore, AZO lmsdeposited on glass and PEN substrate got compressive stress assputtering power increases. In the case of AZO lms deposited on PENsubstrate, the variation of peak intensity and shift was larger. AZO lmsdeposited on PEN substrate probably get higher damage at highsputtering power because PEN substrate is softer than glass substrate.When the sputtering power increases, the (002) peak intensity of AZOlms deposited on glass and PEN substrates becomes lower. This meansthat the crystallinity becomes poorwith increasing sputtering power. Asthe sputtering power increased from25W to 150W, the deposition rate

Fig. 3. The resistivity of AZO lms deposited on glass and PEN substrates at varioustarget to substrate distance (a) and working pressure (b).

5862 J.-M. Kim et al. / Thin Solid Films 518 (2010) 58605865120 C because Tg of PEN substrate was 122 C. AZO lms deposited onglass and PEN substrates were investigated with several analysismethods. The thickness of AZO lm was measured using scanningelectronmicroscope (JEOL JSM-840A). The resistivity of theAZO thinlmwasmeasured using 4-point probe (KEITHLEY 2400). The crystallinity ofthe AZO thin lm was analyzed with X-ray diffraction (XRD, modiedPhilips-1880) using CuK radiation. The surface morphology and theroughness of the AZO thin lm were analyzed using Atomic ForceMicroscope (AFM, DimensionTM 3100). The transmittance of the AZOthin lm was measured using UVVis-Spectroscopy (JASCO, V-530).

3. Results and discussion

The surface morphology of the glass and PEN substrate wascompared to observe the effect of the substrate roughness to theproperties of the lm. Fig. 1 shows AFM images of bare glass and barePEN substrate. Bare glass surface shows smooth surface with itsroughness of about 0.28 nm. On the other hand, bare PEN substratesurfacemorphology reveals the crest and troughwith its average surfaceroughness of 1.94 nm.Rougher surface substrate leads to lower electricalproperties of the lms by carrier scattering [18], therefore, the lmsdeposited on PEN substrates show to be more resistive than thosedeposited on glass substrate following our results. A. Miyake et al. groupalso reported thatGZOlmsdeposited on cyclo-olenpolymer substrateshowed lower resistivity and transmittance than that on glass substratebecause of the deterioration in the crystallinity of GZO lm [23]. Aftercomparison of the substrate surface morphologies, AZO lms weredeposited on glass and PEN substrates at various target to substratedistances and working pressures in order to observe their effect. Otherconditions were xed at sputtering power of 25W and substratetemperature of 25 C.When the target to substrate distanceandworkingpressure decrease from 11 cm to 6.8 cm and from 2.7 Pa to 0.4 Pa, thedeposition rates increase from 1.46 nm/min to 2.81 nm/min and1.94 nm/min to 2.81 nm/min. When the target to substrate distanceand working pressure decrease, the collisions of between Ar ions andsputtered atoms decrease. This leads to the sputtered atoms havingenough kinetic energy for arriving at the substrate, and diffusive abilityof the atoms increases [24], thereby, the deposition rate increased withdecreasing target to substrate distance and working pressure. XRDpatterns of AZO lms deposited on glass and PEN substrates of differenttarget to substrate distance andworking pressure are shown in Fig. 2. Inall cases, strong (002) peak of AZO lms is observed at 2=34.36 onglass substrate and 2=34.3 on PEN substrate. This indicates that AZOlm deposited on glass and PEN substrates is the hexagonal wurzitestructure and has a preferred orientation with the c-axis perpendicularto the substrate, regardless of substrate materials [24]. There is no peakshiftwith changing target to substrate distanceandworkingpressure. Asthe target to substrate distance and working pressure decrease, the(002) peak intensity of AZO lms improves. The indicates that thecollision decreases between Ar ions and sputtered atoms withdecreasing target to substrate distance and working pressure, thereby,improving the crystallinity and making larger grain with higher kineticenergy [25,26]. L. Chen et al. and Y.K. Moon et al. showed similar resultswith changing target to substrate distance andworkingpressure [25,26].When the peak intensity and peak variation of AZO lms deposited onglass andPEN substratewere compared, those of AZOlms deposited onglass substrate are larger. The reason probably is that glass substrate isharder and smoother which means glass substrate can support to formgood crystallinity of AZO lms than PEN substrates. Fig. 3 shows theresistivity of AZO thin lms deposited on glass and PEN substrates atvarious target to substrate distance and working pressure. When AZOlms were deposited on both substrates at the target to substratedistance of 6.8 cmandworking pressure of 0.4 Pa, AZOlms show lowerresistivity, 1.8102cm on glass substrate and 3.2102cm onPEN substrate. When the target to substrate distance and working

pressure decrease, the sputtered atoms can get higher energy due to the also increased from 2.81 nm/min to 14.74 nm/min, because the number

5863J.-M. Kim et al. / Thin Solid Films 518 (2010) 58605865of atoms is sputtered from the target and gets a higher energy thatcontributes to the lm growth with increasing sputtering power.However, when the deposition rate is too fast at higher sputteringpower, there is not enough time to form good crystallinity and compactAZO lms [28]. In addition, sputtered atoms having higher energy athigher sputtering power impact to form the lms, thereby, causing thebombardment electron to lose energy and promoting non-uniform lmgrowth because of the creation of stress [28]. High sputtering powereffect also was conrmed with AFM images. Fig. 5 shows the

Fig. 4. XRD patterns of AZO lms as a function of sputtering powers on glass substrate(a) and PEN substrate (b) (at target to substrate distance of 6.8 cm, working pressure of0.4 Pa, substrate temperature of 25 C).morphologies of AZO lms deposited on PEN substrate at varioussputtering powers.Morphology of AZO lms becomes poor and rougherwith lowerlmdensity as sputteringpower increases. This result relatedto XRD patterns as shown in Fig. 4. For increasing the kinetic energy ofsputtered atoms to form good crystallinity, the substrate temperaturewas increased until 120 C at sputtering power of 25W, target tosubstrate distance of 6.8 cmandworking pressure of 0.4 Pa. Fig. 6 showsthe XRD patterns of AZO lms deposited on glass and PEN substrate atvarious substrate temperatures. The limitation of substrate temperaturewas 120 C, because the glass transition temperature (Tg) of PEN was122 C. All AZO lms have strong (002) peak which indicates that AZO

Fig. 5. AFM images (1 m1 m) of AZO lms deposited onlmsarepolycrystallinewith thehexagonalwurzite structure andhaveapreferred orientation with the c-axis perpendicular to the substrates[24].When the substrate temperature increases, the(002)peakbecomes

PEN substrates at various sputtering power ((a)(c)).

Fig. 6. XRD patterns and grain size of AZO lms deposited at various substratetemperatures on glass substrate (a), PEN substrate (b) and grain size (c) (at sputteringpower of 25 W, target to substrate distance of 6.8 cm, working pressure of 0.4 Pa).

more intense and sharper, obviously. This means that the crystallinityand grain size are signicantly improved due to increasing surfacediffusion of sputtered atoms which have higher kinetic energy from thesubstrate [19]. On the glass substrate, the enhancement of thecrystallinity of AZO lm was better than that on PEN substrates. Grainsize of AZO lms at various substrate temperatures was calculated usingDebyeScherrer formula as shown in Fig. 6(c). Scherrer's formula [28] is:

D =0:9B cos

where 0.9 is correction factor, is thewavelength of X-ray source, is theBragg diffraction angle in degree, and B is the diffraction peak widthproportional to FWHM. When the substrate temperature increases, thegrain size becomes larger from 4.54 nm to 6.01 nm and from 3.57 nm to5.09 nm on glass and PEN substrates, respectively. This result isconsistent with the XRD observations. As the substrate temperatureincreases, the (002) position shifts to higher angle from 2=34.36 to34.48 on glass substrate, from2=34.3 to 34.48 on PEN substrate. The

shows the lower resistivity due to the reduction in the scattering of thecarriers at the grain boundaries and crystal defects, which increased theapparent carriermobility [30]. Grain boundary scattering is reducedwithimproving crystallinity and grain size, the carrier concentration andmobility increase [17]. Therefore, the resistivity of AZO lms deposited athigher substrate temperature becomes low. From this electricalresistivity results, the structural property is a powerful effect to improveelectrical properties. Kim's group also reported that the electricalproperties strongly depend on the crystallinity of AZO lms [31].Comparison of AZO lms deposited on glass and PEN, AZO lmsdeposited on glass substrates have lower resistivity than AZO lmsdeposited on PEN substrate due to better crystallinity and grain sizes.Fig. 8 shows the transmittance of AZO thin lms deposited on glass andPEN substrate as a function of substrate temperature. All of AZO lmsdeposited on glass and PEN substrate exhibited a transmission of higherthan 80% in the visible region. The average transmittance of AZO lmsdeposited on glass substrate increases from 87.46% to 90.23%, and thatdeposited on PEN substrate also increases from 87.36% to 90.12% withincreasing substrate temperature from 25 C to 120 C. It is probably thatthe improvement of crystallinity at higher temperature as XRD results. S.Fernandez et al. also got the similar trend result [18]. The highest averagetransmittance of AZOlms deposited on glass and PEN substrate is above90% at 120 C, regardless of substrate materials. The absorption edge ofAZO lms deposited on glass substrate slightly shifted to shortwavelength than that of AZO lms deposited on PEN substrate. Itmeans that the band gap of AZO lms deposited on glass substrate iswider [18]. ThebandgapsofAZOlmswere calculatedby theTaucmodel

5864 J.-M. Kim et al. / Thin Solid Films 518 (2010) 58605865reason is that the compressive stress of AZO lms becomes reduced.When the AZO lms were deposited at lower substrate temperature, thestress is generated due to the freezing in the structure defects [19].However, the increasing substrate temperature in a suitable rangeincreases the atomic mobility and reduces the structural defects, thus arelaxation of AZO lms is observed [19]. Compared with the lmsdeposited on PEN substrates, the intensity of diffraction peaks of AZOlms deposited on glass substrates is more intense and sharper becauseof better crystallinity and grain sizes of AZO lms deposited on glasssubstrates. Glass substrate is probably harder and smoother than PENsubstrate, it can support well to growAZO lm on the substrate, thereby,AZO lms deposited on glass substrate can have better crystallinity. Asthe substrate temperature increased, the surface roughness of lm onboth substrates, glass and PEN, also increased from 1.47 nm to 2.62 nmand from1.87 nmto2.70 nmbecauseof largegrain size, respectively.Ourlaboratory already reported the effect of substrate temperature on AZOlms elsewhere [29]. Fig. 7 shows the resistivity of AZO lms depositedonglass andPEN substrates as a functionof substrate temperature.Whenthe substrate temperature increases from 25 C to 120 C, the resistivityof AZO lm deposited on glass substrate decreases from1.8102 cmto 3.85103 cm, and the resistivity of AZO lm deposited on PENsubstrate decreases from 3.2102 cm to 5.12103 cm due tosignicant improvement of crystallinity and grain size as shown in XRDresults. The crystallinity and grain size are improved with increasingsubstrate temperature because the sputtered atoms can get higherenough energy from the substrate temperature. The higher crystallinity

Fig. 7. The resistivity of AZO lms deposited on glass and PEN substrates at various

substrate temperatures.Fig. 8. The transmittance of AZOlms deposited on glass (a) and PEN substrate (b) at various

substrate temperatures.

[32]. The band gaps of AZO lms deposited changed from 3.47 eV to3.51 eV on glass substrate and 3.40 eV to 3.43 eV on PEN substrate assubstrate temperature increases from 25 C to 120 C. The improvementof crystallinity of AZO lms with increasing substrate temperature leadsto the increase of carrier concentration andmobility due to the reductionof grain boundary scattering [30]. As the carrier concentration increases,the Fermi level in the conduction band of AZO lms increases, thereby,the band gap becomes wider with the lowest resistivity according toBursteinMoss effect [3,33].

4. Conclusion

To optimize deposition condition of AZO lm as a TCO, we havestudied AZO lms deposited on glass and PEN substrates using R.F.magnetron sputtering systemwith various deposition parameters suchas sputtering power, target to substrate distance, working pressure andsubstrate temperature.When the AZO lms deposited on glass and PENsubstrates at low sputtering power of 25W, short target to substratedistance of 6.8 cm, low working pressure of 0.4 Pa and the highsubstrate temperature 120 C, they were shown to have the bestcrystallinity, the lowest resistivity (3.85103 cm on glass substrateand 5.12103 cm on PEN substrate) and the highest transmittance(N90%). Besides, AZO lms deposited on PEN substrates showcomparable properties like lms deposited on glass substrates, andAZO lms deposited at optimized condition can be used for the exibledevices using transparent and exible substrates.

Acknowledgments

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Deposition of Al-doped ZnO films on polyethylene naphthalate substrate with radio frequency magnetron sputteringIntroductionExperimental detailsResults and discussionConclusionAcknowledgmentsReferences