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Vacuum 83 (2009) 552–556

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Vacuum

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The properties of transparent conductive In–Ga–Zn oxide films producedby pulsed laser deposition

Kenichi Inoue a, Kikuo Tominaga a, Takashi Tsuduki a, Michio Mikawa b,*, Toshihiro Moriga a

a Faculty of Engineering, The University of Tokushima, Minami-josanjima 2-1, Tokushima 770-8506, Japanb Department of Telecommunications, Takuma National College of Technology, Kouda 551, Takuma, Mitoyo, Kagawa 769-1192, Japan

Keywords:TCO filmsWide band gapPLDAmorphous

* Corresponding author.E-mail address: tominaga@ee.tokushima-u.ac.jp (M

0042-207X/$ – see front matter � 2008 Elsevier Ltd.doi:10.1016/j.vacuum.2008.04.066

a b s t r a c t

To obtain TCO films for wavelengths shorter than the visible range, Ga2O3 was added to the In2O3–ZnOsystem as an impurity. Using pulsed laser deposition (PLD), two kinds of targets, InGaZnO4 and InGaZn3O6,

were deposited. Although the In–Ga–Zn–O films obtained deviated from the stoichiometry of InGaZnO4,they were amorphous at a substrate temperature below 250 �C. We obtained the lowest resistivityof 2.77� 10�3 U cm within the present experiment at a carrier concentration of 1.38� 1020 cm�3 and aHall mobility of 16.6 cm2/Vs. The optical band gap energy shifted to higher energies and the transmittanceat the blue range was improved dramatically as compared with similar amorphous IZO films.

� 2008 Elsevier Ltd. All rights reserved.

1. Introduction

Various transparent conductive oxide films have beendeveloped for use in optoelectronic devices such as solar cells, flatpanels, and window coatings. The materials with two metal ionsinvestigated to date include SnO2:Sb, ZnO:Ga and indium tinoxide films (ITO) [1–5]. Subsequently, ternary oxide compounds ofCdSnO3, Cd2SnO4 etc. were researched by Shannon [6], andMgIn2O4, ZnGa2O4 etc. by Kawazoe et al. [7–10]. These materialswere edge sharing of In3þ and Sn4þ octahedra. Orita et al.researched the materials with YbFe2O4 structure such as InGaZnO4,

InAlZnO4 etc. and showed that these crystals have a wider band gapand a higher carrier mobility than In2O3 [11].

We have previously deposited In–Zn–O (IZO)films with bothsputtering and pulsed laser deposition (PLD) [12–14]. We foundthat amorphous In–Zn–O films can be effective TCOs with lowresistivity and high transparency. The amorphous films obtainedshowed lower resistivities than homologous polycrystallinefilms. The amorphous phase is not constrained tightly by the com-positional ratio of In, Zn and the atom sites compared to the crystals.InGaZnO4 is thought to be presumably resemble to the In2O3–ZnOsystem. Thus we may deposit TCO films of InGaZnO4, even if thecomposition of In–Ga–Zn–O is not stoichiometric.

2. Experimental

The films were deposited by pulsed laser deposition using a KrFexcimer laser (l¼ 248 nm). The details of apparatus was reported

. Mikawa).

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in the previous paper. [15]. We used two kinds of targets, In2O3–Ga2O3–2ZnO and In2O3–Ga2O3–6ZnO, abbreviated as In–Ga–Zn–Oand In–Ga–3Zn–O, respectively. The 3 cm diameter target wascomposed of a mixture of In2O3, Ga2O3 and ZnO powder sintered at1000 �C for 1 h in air. These targets were evaporated by a pulsedlaser of 50 mJ at a repetition rate of 20 Hz and in a total of 30 000shots. The films were deposited on Corning #1737. The distancebetween the target and the substrate was 4 cm.

The film compositions were estimated by X-ray fluorescenceanalysis, and the carrier concentration and the Hall mobility weremeasured by the Van der Pauw method. Film thickness was mea-sured by the optical interference method using a refractive index, n,of 2.0 [16]. Optical transmittance of the film was measured bya Hitachi Model 330 spectrometer.

3. Result and discussions

As starting materials for targets, we selected In–Ga–Zn–O andIn–Ga–3Zn–O. The latter contained much Zn. The substrate tem-peratures, Ts, during the deposition was 110, 250 and 380 �C. TheX-ray diffractions of the as-grown films deposited at each substratetemperatures are shown in Fig. 1(a)–(c). The crystal structure is notstrongly dependent on the composition of the starting target.Below 250 �C, the films showed no clear peaks and the diffractionintensities were very weak. At the higher substrate temperatureof 380 �C, clear peaks were assigned to bixbyite In2O3. These resultsindicate that crystallization occurs at least above 250 �C, andamorphous films are prepared below 250 �C.

Fig. 2 (a) and (b) show the contents of In, Ga and Zn in the filmsdeposited from the (a) In–Ga–Zn–O and (b) In–Ga–3Zn–O targets,respectively, as a function of substrate temperature. We also plotted

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Fig. 1. X-ray diffractions of as-grown films deposited from In–Ga–3Zn–O and In–Ga–Zn–O targets at a substrate temperature of (a)110 �C, (b) 250 �C and (c) 380 �C, respectively.

K. Inoue et al. / Vacuum 83 (2009) 552–556 553

the compositional ratio of In/(InþGa) and Ga/(InþGa) in order toknow the fluctuation of In and Ga contents.

In Fig. 2 (a), for the amorphous films deposited at 110 and 250 �Cfrom In–Ga–Zn–O target, we see that the Zn content (z0.18) wasfairly smaller than the value of stoichiometry (0.33). The deviationfrom the stoichiometry is shown by an arrow in Fig. 2 (a) and (b).The compositional ratio of Ga/(InþGa) was 0.42. The changesof In/(InþGa) and Ga/(InþGa) at 110 and 250 �C was relativelysmall.

With increasing Zn composition to In–Ga–3Zn–O in the startingtarget, the Zn content in the amorphous films deposited at 110 and250 �C was increased to 0.40, as shown in Fig. 2 (b). The amorphousfilms deposited at 110 and 250 �C in Fig. 2 (b) did not have stoi-chiometric composition of In–Ga–Zn–O, but the InþGa and Zncontents were closer to their stoichiometric values of 66.7 and 33.3 %than those in Fig.2 (a), as shown by arrows.

In Fig. 2 (a), for the crystalline film at Ts¼ 380 �C , the Zn com-position decreased dramatically. This trend was improved in Fig. 2(b) with using In–Ga–3Zn–O target, but the Zn content in the filmwas still deficient to stoichiometry. This indicates that Zn showslow adhesion on the depositing crystalline film, which will be dueto reevaporation of Zn.

Fig. 3(a)–(c) show the optical transmittance data for films de-posited from the In–Ga–Zn–O and In–Ga–3Zn–O targets at Ts¼ 110,

250 and 380 �C, respectively. The data for the Corning #1737substrate and In–Zn–O crystallization film are also indicated forcomparison. The absorption edge for the films deposited from theIn–Ga–Zn–O and In–Ga–3Zn–O targets shifted to shorter wave-lengths than that of In2Zn3O6 film. [15] The fact that the absorptionedge of film for In–Ga–Zn–O target shifted to shorter wavelengththan that of In–Ga–3Zn–O target corresponds to the results thatGa/(InþGa) for In–Ga–Zn–O target is larger than that for In–Ga–3Zn–O target in Fig. 2 (a) and (b).

For the films deposited at Ts¼ 110 �C in Fig. 3 (a), the opticaltransmittance in the wavelength below 500 nm is decreased, ascompared with the films at Ts¼ 250 and 380 �C in Fig. 3 (b) and (c).This suggests the incomplete oxidization of metallic In, Ga or Zn atthe low substrate temperature.

Fig. 4 (a) and (b) show electrical properties of films pre-pared from the targets (a) In–Ga–Zn–O and (b) In–Ga–3Zn–O,respectively, as a function of substrate temperature. In Fig. 4 (a)and (b), the carrier concentration showed a maximum at 250 �C,but the Hall mobility was a minimum for both targets. Whenthe Hall mobility of the amorphous phase films (Ts¼ 110 and250 �C) decreased, then the carrier concentration increased. Sincethe carrier is generated from the oxygen defects in this system,the largest carrier concentration at 250 �C means that theoxygen defects are likely to be generated at this temperature. Thus

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K. Inoue et al. / Vacuum 83 (2009) 552–556554

the Hall mobility will be related with scattering with the oxygendefects.

Compared with the Hall mobility for In–Ga–Zn–O target in Fig. 4(a), the Hall mobility of amorphous films at 110 and 250 �C forIn–Ga–3Zn–O target increased in Fig. 4 (b). The composition ofInþGa and Zn for In–Ga–3Zn–O target was close to stoichiometryas mentioned above in Fig. 2. In this case, the increase of Hallmobility is supposed to be the improvement of stoichiometry. Thedrastic decrease of Hall mobility for the crystalline film deposited at380 �C in Fig. 4 (b) will be due to the destruction of the In–O net-work with incorporation of Ga and Zn. On the other hand, the Hallmobility for the crystalline film deposited at 110 �C in Fig. 4 (a) isstill high. This film contains less Zn, and then the main structure isbixbyite In2O3, so electrical conduction in In–O network seems tobe maintained.

From the above discussion, we may say that the Hall mobility inthe present system is dominated by the scatterings by native donorof oxygen defects and defects due to the compositional stoichio-metric deviation. Despite the large deviation from stoichiometry inthe crystallographic levels, a high carrier concentration was

obtained and fairly high Hall mobility was attained in amorphousIn–Ga–Zn–O films. This indicates that In–O network is constructedfairly well even in amorphous film, and the carriers can conductfreely enough. This network seems to be maintained under largeoxygen a defects in amorphous film.

4. Conclusions

In this experiment, we used equal amounts of Ga2O3 and In2O3

as starting materials in the form of In–Ga–Zn–O and In–Ga–3Zn–Otargets, and deposited films by the PLD method. Amorphoustransparent conductive oxide films were obtained in the In–Ga–Zn–O system, although the films showed a fairly large deviationfrom stoichiometric InGaZnO4 in the crystallographic levels. Thelowest resistivity was 2.77�10�3 U cm at a carrier concentrationof 1.38� 1020 cm�3 and a Hall mobility of 16.6 cm2/Vs. Opticaltransmittance spectra at the absorption edge were improved at asubstrate temperature of Ts¼ 250 �C , but at Ts¼ 110 �C , thereremained some kind of absorption centers at wavelengths shorterthan 500 nm.

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Fig. 4. Electrical properties of films obtained from the starting materials of (a) In–Ga–Zn–O and (b) In–Ga–3Zn–O targets as a function of substrate temperature.

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