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Surface & Coatings Technolog

Effects of substrate temperature on crystallinity and electrical properties ofGa-doped ZnO films prepared on glass substrate by ion-plating

method using DC arc discharge

Takahiro Yamada ⁎, Aki Miyake, Seiichi Kishimoto, Hisao Makino,Naoki Yamamoto, Tetsuya Yamamoto

Kochi University of Technology, 185, Miyanokuchi, Tosayamada-cho, Kami-shi, Kochi, 782-8502, Japan

Available online 24 May 2007

Abstract

The effects of substrate temperature, Ts, on the crystallinity and the electrical properties of Ga-doped ZnO films (GZO) with a thickness of200 nm were investigated. GZO films were prepared on glass substrates at various Ts in the range from 150 to 400 °C by ion-plating method withDC arc discharge. X-ray diffraction analysis reveals that GZO film prepared at 250 °C shows the preferential orientation of c-axis and the highestcrystallinity. Williamson–Hall analysis indicates that the crystallite size of GZO films remains nearly constant with increasing Ts up to 300 °C, andthen, with further increasing Ts, decreases gradually. The Ga concentration in the films, estimated by secondary ion mass spectroscopy and X-rayfluorescence analyses, increases monotonically with increasing Ts above 250 °C. Hall effect measurements show that resistivity decreases slightlywith increasing Ts up to 250 °C, leading to the lowest resistivity of 2.1×10−4 Ω cm at 250 °C, and then exhibits a gradual increase with furtherincreasing Ts.© 2007 Elsevier B.V. All rights reserved.

Keywords: ZnO; GZO; Transparent conductive film; Ion plating; Substrate temperature; Low resistivity

1. Introduction

Polycrystalline ZnO films on glass and plastic substrates havebeen extensively developed for many applications such astransparent electrodes, sensors, thin film transistors, and micro-wave shielding. In particular, low resistivity ZnO films with highvisible transmittance and small surface roughness allow for theapplication to transparent electrodes of this material. At present,indium tin oxide (ITO) is mainly used as the electrodes in variousoptoelectronics devices. However, it has the problem of limitedresource of its principal component, indium. Thus, ZnO filmscomposed of low cost and abundant material are expected at themoment as an alternative to ITO electrodes in large size devicessuch as flat panel displays and solar cells.

We have deposited Ga-doped ZnO (GZO) films on glasssubstrate at 200 °C by an ion-plating method with a DC arcdischarge. This deposition system works very well from anindustrial view point, because it enables the deposition of GZO

⁎ Corresponding author. Tel.: +81 887 57 2734; fax: +81 887 57 2181.E-mail address: yamada.takahiro@kochi-tech.ac.jp (T. Yamada).

0257-8972/$ - see front matter © 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.surfcoat.2007.05.051

films: 1) with low plasma damage, 2) at high rate of e.g.170 nm/min, and 3) on large-area substrate [1,2]. In ourprevious works [3,4], the effects of O2 gas flow rate during thedeposition, Ga2O3 content in target source and film thickness onthe electrical properties of GZO films have been investigated.Consequently, we have succeeded in obtaining a resistivity aslow as 2.2×10−4 Ω cm in GZO film with a thickness of 200 nmon glass substrate at a temperature of 200 °C.

The aim of this work is to investigate the effects of substratetemperature, Ts, on the crystallinity and the electrical propertiesof GZO films. The variation in the electrical properties of GZOfilms with Ts is investigated in terms of its crystallinity,crystallite size and Ga concentration.

2. Experimental

GZO films were deposited on alkali-free glass substrate byion-plating system with DC arc discharge using a pressure-gradient-type plasma gun [5]. In this deposition system, thesubstrate traveled through the deposition room for the filmgrowth. A series of GZO films with a thickness of 200±5 nm

Fig. 2. Typical XRD θ–2θ scan profile of GZO film deposited at 250 °C. Thediffraction intensity on the vertical axis is expressed in a logarithmic scale.

974 T. Yamada et al. / Surface & Coatings Technology 202 (2007) 973–976

was prepared at various Ts in the range from 150 to 400 °C.Deposition rate of GZO films strongly depended on Ts. As willbe shown in Fig. 1, the difference in the deposition ratesbetween GZO films prepared at Ts of 150 °C and 400 °C wasabout 85 nm/min. Therefore, the traveling speed of the substratewas adjusted in the range of 3 to 6 mm/s to realize the samethickness of 200 nm in all GZO films.

The substrate was rinsed ultrasonically in deionized waterand isopropyl alcohol, and then jet-dried using high-puritynitrogen gas. A base pressure in the vacuum chamber prior todeposition was about 2.0×10−5 Pa. After evacuation of thechamber, argon and oxygen gases were introduced at the gasflow rate of 140 and 8 sccm, respectively. The depositionpressure was 2.2×10−1 Pa. A sintered ZnO ceramic targetwhich contains a Ga2O3 powder of 4 wt.% was used as a sourcematerial (99.99% purity, Hakusui Tech.). The applied DC arcdischarge current between the target and the plasma gun wasautomatically controlled to be about 142 A. The distance fromthe target to the traveling substrate was fixed at about 0.6 m.

Thickness of the film was measured by using a surfaceprofilometer (Alfa-Step, IQ). Crystallinity and crystallite size ofthe films were characterized by high-resolution X-ray diffrac-tion (XRD: RIGAKU, ATX-G system) analysis using CuKαradiation (λ=1.5406 Å). The electrical properties of the filmswere characterized by Hall effect measurements in the van derPauw configuration (Accent, HL5500PC) at room temperature.Ga concentration in the films was estimated by secondary ionmass spectroscopy (SIMS) and X-ray fluorescence (XRF)spectroscopy analyses.

3. Results and discussion

3.1. Deposition rate

Fig. 1 shows the deposition rate of GZO films as a function ofTs. From Fig. 1, it is obvious that the deposition rate decreasesmonotonously from 200 to 115 nm/min, as the Ts increased from150 to 400 °C. For this result, we have confirmed that there is no

Fig. 1. Deposition rate of GZO films as a function of substrate temperature.

change of deposition parameters other than Ts. During thedeposition, the total and O2 partial pressure remained a constantindependently of Ts, respectively. The plasma conditions such asarc current, discharge voltage and target bias etc. were also thesame among the samples. Thus, it is suggested that the decreaseof the deposition rate is only due to the increase of the Ts. Theincrease of the Ts enhances the migration of species on thesurface. This will possibly lead to the formation of more densefilm. The increase of film density could effectively decrease thedeposition rate. In addition, desorption from the film has beenoften attributed to the decrease of deposition rate with theincrease of Ts.We observed in a thermal desorption spectroscopyanalysis that, by a post-annealing at the temperature above about300 °C, zinc begins to desorb from the film prepared at 200 °C.Accordingly, the decrease of the deposition rate may beexplained by the migration effect and the desorption of zinc.

3.2. Crystalline quality and crystallite size

In order to investigate the crystallinity of GZO films pre-pared at different Ts, out-of-plane XRD measurements wereperformed. Fig. 2 shows a typical XRD θ–2θ scan profile ofGZO film prepared at 250 °C. The diffraction intensity on thevertical axis in Fig. 2 is expressed in a logarithmic scale. InFig. 2, only two diffraction peaks from ZnO (0002) and (0004)planes were observed, where no other phases were detected. Allthe films prepared at different Ts basically showed same dif-fraction pattern. This result indicates that the films arecomposed of a wurtzite type hexagonal structure and have apreferential orientation of c-axis normal to the substrate plane.It is well known that ZnO films consist of columnar grainsgrown with the c-axis orientation [6].

The full width at half maximum of the (0002) diffractionpeak, FWHMθ–2θ scan, is plotted as a function of Ts in Fig. 3.The FWHMθ–2θ scan decreases with increasing Ts from 150 to

Fig. 4. Crystallite size of GZO films as a function of substrate temperature.

Fig. 5. Depth profile of Ga concentration in GZO film prepared at differentsubstrate temperatures.

Fig. 3. Full width at half maximum of (0002) diffraction peak, FWHMθ–2θ scan,as a function of substrate temperature.

975T. Yamada et al. / Surface & Coatings Technology 202 (2007) 973–976

200 °C. It seems to be a plateau in the Ts range from 200 to300 °C, and then increases gradually with further increasing Tsup to 400 °C. The decrease in the FWHMθ–2θ scan represents theimprovement of the film crystallinity. Thus, these experimentalfindings indicate that the GZO film deposited at the Ts around250 °C has the highest crystallinity. The improvement of thecrystallinity can be probably explained by the enhancement ofthe migration effect, as described in Section 3.1. On thecontrary, the crystallinity is deteriorated at the higher Ts morethan 300 °C. This is attributed to the increase of Ga con-centration into the films, as will be shown in Fig. 6. In ourprevious work [3], we reported that the crystallinity of GZOfilms is deteriorated by the increase of the Ga concentration.

Next, a crystallite size of a-axis parallel to the substrate planewas estimated from in-plane XRD data analysis on the basis of aWilliamson–Hall method [7]. Fig. 4 shows the crystallite size asa function of Ts. With increasing Ts up to 300 °C, the crystallitesize remained nearly constant at approximately 30 nm, and then,with further increasing Ts, reduced to about 20 nm. Here, thecrystallite size will be related to a nucleation density in theinitial stage of film deposition. As described in Section 3.1, themigration and desorption effects were argued as a factor causingthe decrease of the deposition rate with increasing Ts. Theenhancement of the migration generally limits the density ofnuclei formed in the nucleation stage, and simultaneously, thiseffect contributes the crystallite growth of the a-axis with thelowest surface energy plane. However, the result of Fig. 4differs from this consideration. The desorption of zinc could beremarkable at higher temperature. The desorption may affect thenucleation stage and result in the decrease of crystallite size. Afurther study is necessary to reveal that.

3.3. Ga profile

SIMSmeasurements were performed with Cs+ primary ions toinvestigate the distribution of Ga in GZO films. The Gaconcentration was quantified using the relative sensitivity factor,

which was obtained by measuring the Ga-implanted ZnOreference sample. The reference sample was prepared by Ga ionimplantation to an undoped-ZnO polycrystalline film, where theGa ions are implanted at a dose of 5×1014 ions/cm2, with anacceleration voltage of 200 keV. The error range was estimatedwithin ±10%.

A SIMS profile of GZO film prepared at 250 °C is shown inFig. 5. The artificial peak located at a depth of 200 nm probablycorresponds to the interface between the film and substrate, asindicated by a broken line. From Fig. 5, the distribution of theGa concentration was found to be nearly uniform from thesurface to the interface.

3.4. Electrical properties

Fig. 6 shows the resistivity, carrier concentration and Hallmobility of GZO films as a function of Ts. In addition, the Ga

Fig. 6. Resistivity, carrier concentration and Hall mobility of GZO film as afunction of substrate temperature. In addition, the Ga concentration in the films,estimated by SIMS and XRF analyses, is also shown.

976 T. Yamada et al. / Surface & Coatings Technology 202 (2007) 973–976

concentration in the films, estimated by SIMS and XRFanalyses, is also shown. As seen from Figs. 3 and 6, it should benoted that the variation trend in the resistivity with Ts is verysimilar to that of FWHMθ–2θ scan. This suggests that thecrystallinity of the films can be closely associated with thebehavior of the electrical properties. In Fig. 6, their variationtrends with Ts could be mainly divided into two regions; RegionI up to 250 °C, where the resistivity indicates a slight decrease,and Region II above 300 °C, where the resistivity increasesgradually.

In Region I, the carrier concentration and the Ga concentrationof GZO films are not directly affected by Ts, while the Hallmobility increases, where the FWHMθ–2θ scan decreases withincreasing Ts in this range. The increase of Hall mobility can beexplained by the improvement of the crystallinity. As a result, atTsof 250 °C, the lowest resistivity of 2.1×10−4 Ω cm was obtainedin GZO film with carrier concentration of 1.1×1021 cm−3 andHall mobility of 27.6 cm2/Vs. In addition, as shown in Fig. 4, thecrystallite size remains a constant in Region I.

In Region II, both the carrier concentration and Hall mobilitydecreased. As shown in Figs. 3 and 4, the deterioration of thecrystallinity and the reduction of the crystallite size are observed

with increasing Ts in this range. They correspond to the increaseof scattering and trapping factors for carrier electron in the grainand grain boundary, respectively. In addition, the Ga concen-tration in Region II is higher than that in Region I. This will beinduced by the desorption of zinc from the film, as described inSection 3.1. Excess Ga will give rise to the segregation of Ga-oxides in the films, which acts as a donor killer. However, wehave obtained no direct evidence concerning the structure of thegrain boundary to date.

4. Summary

The effects of substrate temperature, Ts, in the range from 150to 400 °C on the crystallinity and electrical properties of GZOfilms were investigated. Deposition rate of the films decreasesmonotonically from 200 to 115 nm/min with increasing Ts. XRDanalysis shows the improvement of the film crystallinity withincreasing Ts up to 250 °C, while, at higher Ts, they aredeteriorated. Williamson–Hall analysis indicates the crystallitesize decrease from 30 to 20 nm with increasing Ts above 300 °C.SIMS and XRF measurements reveal that Ga concentrationchanges a little with increasing Ts up to 250 °C, while, at higherTs, an increase in Ga concentration is observed. Hall effectmeasurements show that the lowest resistivity of 2.1×10−4 Ωcm is obtained in GZO film at 250 °C.

Acknowledgements

The financial support from the collaboration of RegionalEntities for the Advancement of Technological Excellence ofthe Japan Science and Technology Agency is gratefullyacknowledged.

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