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    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: (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


    d Films

    l sev ie 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 an