Current Applied Physics 10 (2010) S290–S293

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Current Applied Physics

journal homepage: www.elsevier .com/locate /cap

Effects of O2 plasma pre-treatment on ZnO thin films grownon polyethersulfone substrates at various deposition temperaturesby atomic layer deposition

J.Y. Lee a, C.M. Shin a, J.H. Heo a, C.R. Kim a, J.H. Park a, T.M. Lee a, H. Ryu a,*, C.S. Son b, B.C. Shin c, W.J. Lee c

a Department of Nano Systems Engineering, Inje University, Gimhae 621-749, Republic of Koreab Department of Electronic Materials Engineering, Silla University, Busan 617-736, Republic of Koreac Department of Nano Engineering, Dong-Eui University, Busan 614-714, Republic of Korea

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

Article history:Received 29 December 2008Accepted 21 July 2009Available online 12 November 2009

Keywords:Atomic layer depositionZinc oxideFlexible substrate

1567-1739/$ - see front matter � 2009 Elsevier B.V. Adoi:10.1016/j.cap.2009.07.032

* Corresponding author. Fax: +82 55 320 3631.E-mail address: hhryu@inje.ac.kr (H. Ryu).

ZnO thin films were deposited on polyethersulfone (PES) substrates by atomic layer deposition (ALD). Weinvestigated the effects of O2 plasma substrate pre-treatment power for various deposition temperaturesfrom 100 �C to 250 �C. X-ray diffraction (XRD) measurements revealed that the ZnO thin films preferc-axis orientation when the deposition temperature is increased. In addition, the structural propertiesof ZnO thin films varied with plasma pre-treatment power.

� 2009 Elsevier B.V. All rights reserved.

1. Introduction

ZnO have been considered as one of the most important semi-conductor materials due to its potential applications [1,2]. ZnOhas a hexagonal wurtzite structure with a wide band gap of3.37 eV and a large exciton binding energy of 60 meV. Hence, itis a promising material for short-wavelength (green, blue, ultravi-olet) optoelectronic applications. Glass substrates are currentlyused for display applications, although glass is too heavy and brit-tle to be easily deformed. Deformation, however, is a requirementfor certain display applications, such as smart cards, electronicmaps and flat panel displays [3,4]. To overcome these problems,many researchers have grown ZnO on flexible substrates such aspolypropylene adipate (PPA), polyisocyanate (PI), and polyethyleneterephthalate (PET). Thin films of ZnO on such substrates havebeen produced by several techniques, including atomic layer depo-sition (ALD) [5], laser molecular beam epitaxy [6], metal organicchemical vapor deposition [7] and radio frequency magnetronsputtering [8]. Among them, the ALD process is best suited forthe growth of thin films with high-precision thickness controland excellent uniformity. Notably, it is also possible to use polymersubstrates for flexible devices. However, there are few reports onthe growth of ZnO thin films on PES substrates by ALD [9].

ll rights reserved.

In this study, we investigated the properties of ZnO thin filmsgrown by ALD on pre-treated PES substrates at 100–250 �C withvarious O2 plasma powers. The surface morphology and structuralproperties were analyzed by atomic force microscopy (AFM) andX-ray diffraction (XRD), respectively. Also, UV/Vis spectrome-ter was carried out to characterize the transmittance of ZnO thinfilms.

2. Experiments

ZnO thin films were deposited on PES substrates by remoteplasma ALD. Diethylzinc (DEZn; carried by high purity nitrogen,99.9999%) and oxygen (O2) gas were used as the Zn and O precur-sors, respectively. The DEZn source was kept at 10 �C in a temper-ature-controlled bath. A typical pulse time of the precursor for thereactants was 3 s, and the N2 purge time for evacuation betweenthe precursor injection was 6 s. The flow rates of the DEZn, O2

and N2 purge gases were 15, 35 and 100 sccm, respectively. ThePES substrate was ultrasonicated in isopropylalcohol (IPA) for10 min at 50 �C, rinsed with deionized water and dried with N2

gas. Before deposition, the substrate was treated by O2 plasma withvarious powers. Subsequently, ZnO thin films were deposited atgrowth temperatures of 100, 150, 200, and 250 �C. The surfacemorphology and structural properties were measured by atomicforce microscopy (AFM) and X-ray diffraction (XRD), respectively.The transmittance was analyzed by a UV/Vis spectrometer.

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3. Results and discussion

Fig. 1 shows the AFM morphologies of O2 plasma pre-treated PESsubstrates. The scan area is 3 � 3 lm for all samples. The root-mean-square (RMS) values of the PES substrate surface roughness are10.5 nm at 0 W, 14.8 nm at 25 W, 17.3 nm at 50 W, 20.9 nm at100 W, and 26.8 nm at 200 W. It was found that the RMS values in-creased with increasing plasma power. Feng et al. have reported thatPES substrate surfaces become more rough with increasing O2 plas-ma treatment time [10]. This is similar to our result that the surfacesbecome more rough with the increase of O2 plasma power.

The AFM morphologies of ZnO thin films grown on PES sub-strates at various temperatures are shown in Fig. 2. The scan areais 1.5 � 1.5 lm for all samples and it can be seen that the ZnO thin

Fig. 1. AFM images of O2 plasma pre-treated PES subs

Fig. 2. AFM images of as-grown ZnO thin films on PES substrates deposit

films are crack-free. There is no significant difference in surfacemorphology between the ZnO thin films deposited at differenttemperatures. The RMS values were 1.8, 0.72, 3.3 and 2.65 nm atdeposition temperatures of 100, 150, 200 and 250 �C, respectively.

Fig. 3 presents the optical transmittance of the ZnO thin filmsgrown on PES substrates with various deposition temperatures.The transmittance spectra of ZnO thin films were measured inthe wavelength range from 300 to 800 nm, which is the visiblerange. The average transmittances of the ZnO thin films were83.6% at 100 �C, 80.64% at 150 �C, 79% at 200 �C, and 76.6% at250 �C. It is seen that the optical transmittance decreases withincreasing deposition temperature.

Fig. 4 shows XRD patterns of the ZnO thin films grown on O2

plasma pre-treated PES substrates deposited at (a) 100, (b) 150,

trates: (a) 0, (b) 25, (c) 50, (d) 100 and (e) 200 W.

ed at various temperatures: (a) 100, (b) 150, (c) 200 and (d) 250 �C.

Fig. 3. Optical transmission spectra of as-grown ZnO thin films on PES substrate invisible range deposited at various temperatures.

S292 J.Y. Lee et al. / Current Applied Physics 10 (2010) S290–S293

(c) 200 and (d) 250 �C. ZnO diffraction peaks were not observed at100 �C regardless of the plasma pre-treatment power. This indi-cates that crystalline ZnO thin films had not been formed. So, itcan be inferred that the ZnO thin films are amorphous at 100 �C,because the temperature does not provide enough thermal energyfor the formation of crystalline ZnO [11]. Bae et al. reported thatamorphous indium zinc oxide (IZO) thin films formed at lowdeposition temperatures of 50 �C [12]. We observed the (0 0 2) dif-fraction peak at 2h values of about 34� for all ZnO thin films exceptfor films deposited at 100 �C. The ZnO thin films also exhibit otherpeaks originating from (1 0 0) at about 31� and (1 0 1) at about 36�.ZnO thin films deposited at 150, 200 and 250 �C are polycrystallinewith a hexagonal structure and exhibit preferred orientation withthe c-axis, perpendicular to the substrate.

Fig. 4. XRD patterns of the ZnO tin films grown on O2 plasma pr

The structural properties of grown ZnO thin films also variedwith O2 plasma substrate-pre-treatment before deposition. The(0 0 2) peak intensity of the ZnO thin films increased with increas-ing deposition temperature. At the same time, the pre-treatmentpower required to have the strongest (0 0 2) peak intensity de-creased when increasing the deposition temperature. ZnO thinfilms that were deposited on substrates pre-treated by O2 plasmaat 50 W had the strongest (0 0 2) peak intensity when the deposi-tion temperature was 150 �C. However, for deposition tempera-tures of 200 �C and 250 �C, the ZnO thin films that were pre-treated at 25 W and 0 W, respectively, had the strongest (0 0 2)peak intensities.

The oxygen plasma treatment can lead to surface cleaning ef-fects and increased surface roughness, improving the adhesion ofpolymers [10,13]. Hence, these effects can improve the structuralproperties of ZnO thin films. As shown in Fig. 4, high growth tem-peratures do not need plasma pre-treatment or need low plasmapower and low growth temperatures need some plasma treatment.This is because high growth temperatures can supply more ther-mal energy than low growth temperatures. On the other hand,Fig. 4 also shows that when the plasma power is increased beyondthe value that produces the strongest (0 0 2) peak, the (0 0 2) peakintensities decrease. This degradation of structural properties canbe possibly attributed to substrate surface damage by the bom-bardment of more energetic particles when the plasma power is in-creased [14].

The (0 0 2) peak intensity and FWHM of the ZnO thin filmsgrown on PES substrates deposited at various temperatures areshown in Fig. 5. As the deposition temperature was increased,the (0 0 2) peak intensity also increased and the FWHM of the(0 0 2) peak decreased, which is indicative of good crystalline qual-ity. It can be said that ZnO thin films grow along the (0 0 2) planewith the lowest surface energy when the deposition temperature isincreased.

e-treated PES substrates deposited at various temperatures.

Fig. 5. (0 0 2) Peak intensity and FWHM of as-grown ZnO thin films on PESsubstrates deposited at various temperatures.

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4. Conclusions

ZnO thin films were grown on O2 plasma pre-treated PES sub-strates at deposition temperatures from 100 �C to 250 �C. AFManalysis showed that the surface roughness of the PES substratesincreased when O2 plasma pre-treatment power was increased.The optical transmittance was 84% at a deposition temperatureof 100 �C and decreased to 77% when the deposition temperaturewas increased to 250 �C. XRD measurements revealed that theZnO thin films had preferred orientation with the c-axis whendeposition temperatures were increased over 150 �C. The struc-tural properties of grown ZnO thin films varied with O2 plasma

power before ZnO deposition. As the growth temperature was in-creased, the plasma power required for the strongest (0 0 2) peakintensity decreased. At a growth temperature of 250 �C, no plasmatreatment was necessary to obtain a strong (0 0 2) peak.

Acknowledgements

This work was supported by the Korea Science and EngineeringFoundation (KOSEF) grant funded by the Korea government (MO-EST) (No. R01-2007-000-20580-0).

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