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Vacuum 74 (2004) 607–611

*Correspondin

88-655-7025.

E-mail addre

(Y. Hayashi).

0042-207X/$ - see

doi:10.1016/j.vac

ZnO–SnO2 transparent conductive films deposited by opposedtarget sputtering system of ZnO and SnO2 targets

Y. Hayashia,*, K. Kondoa, K. Muraia, T. Morigaa, I. Nakabayashia,H. Fukumotob, K. Tominagab

aDepartment of Chemical Science and Technology, University of Tokushima, Minami-josanjima 2-1, Tokushima 770-8506, JapanbDepartment of Electrical and Electronic Engineering, Faculty of Engineering, The University of Tokushima,

Minami-josanjima 2-1, Tokushima 770-8506, Japan

Abstract

Thin ZnO–SnO2 films were deposited on glass substrates by opposed planar magnetron sputtering, in which ZnO and

SnO2:Sb (Sb2O5 3wt% doped) targets face each other. Glass substrate temperatures (Ts) were held at 150C and 250C.

As an experimental parameter, current ratio d; ZnO target current divided by the sum of ZnO and SnO2 target currents,

was used. Compositions of as-deposited film were changed with the current ratio d: Amorphous transparent films

appeared over the range of 0.47pdp0.80 (Zn/(Zn+Sn)=0.28–0.76) at Ts=150C, over the range of 0.33pdp0.73

(Zn/(Zn+Sn)=0.32–0.66) at Ts=250C. Crystalline ZnSnO3 and crystalline Zn2SnO4 was not obtained in any of the

as-deposited films, even at d=0.62 (Zn/(Zn+Sn)=1/2) or d=0.73 (Zn/(Zn+Sn)=2/3). Amorphous films exist as form

of (ZnSnO3)1x(SnO2)x over the range of 0.50pdp0.62 (0pxp0.5) and (ZnSnO3)1y(ZnO)y over the range of

0.62pdp0.73 (0pyp0.5). The minimum resistivity of the deposited amorphous films was 3.6 102O cm at d=0.50

(Zn/(Zn+Sn) =0.33), Ts=250C.

r 2004 Elsevier Ltd. All rights reserved.

Keywords: Transparent conductive film; ZnO; SnO2; Sputtering; Amorphous; ZnSnO3; Zn2SnO4

1. Introduction

Transparent Conductive oxide (TCO) films arenecessary in applications such as transparentelectrodes for solar cells, flat panel displays andwindow coatings [1,2]. At present, indium tin oxide(ITO) is mainly used in TCO materials, because ofthe low resistivity and low optical absorptance in

g author. Tel.: +81-88-656-7435; fax: +81-

ss: yuka@chem.tokushima-u.ac.jp

front matter r 2004 Elsevier Ltd. All rights reserv

uum.2004.01.033

the visible spectrum. Previously, ZnO–In2O3 thinfilms have been deposited by simultaneous DCsputtering at Ts=150C [3–6]. Amorphous phasefilms exhibit a minimum resistivity of 2.3104O cm and have potential applications inTCO films. To explore other compound films,experiments were conducted with In2O3 replacedby SnO2. In the ZnO–SnO2 system, two com-pounds have been reported in TCO films, theilumenite ZnSnO3 and the spinel Zn2SnO4 [2,7,8].In this study, films of all compositional ratios ofZn/(Sn+Zn) were deposited. Film structures werefirst examined by XRD, in particular the presence

ed.

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Y. Hayashi et al. / Vacuum 74 (2004) 607–611608

of amorphous phase in the ZnO–SnO2 system.Electrical and optical properties were examinednext. The effects of annealing the amorphous filmson structural property were also investigated inAr gas.

2. Experimental

An Opposed target sputtering system was usedto deposit ZnO–SnO2 films. Fig. 1 shows aschematic diagram of the system. A ZnO targetand an SnO2:Sb target doped with 3wt% Sb2O5

face each other by a distance of 10 cm. Thesetargets were held on target holders, which containpermanent ferrite magnet arrays to producestrongly unbalanced planar magnetron sputtering.Sputtering was performed at 0.133 Pa (1mTorr) inpure Ar for 2 h. Corning 7059 glass substratetemperatures ðTsÞ were held at 150C and 250C.Discharge currents for ZnO and SnO2:Sb aredenoted as IZn and ISn, respectively. The currentratio d ¼ IZn=ðISn þ IZnÞ was varied as a parameterof deposition. For 0pdp0:5; ISn was fixed at80mA and IZn was varied from 0 to 80mA,

Fig. 1. Schematic drawing of the DC-magnetron sputtering

system.

whereas at 0.5pdp1, IZn was fixed at 80mA andISn was varied from 80mA to 0.

Compositional ratios Zn/(Zn+Sn) of the de-posited films were estimated by X-ray fluorescencemeasurements. Compositional ratio increased withincreasing d but was slightly smaller than the valueof d: Values of Zn/(Zn+Sn)=1/2 and 2/3 corre-sponded to d=0.62 and 0.73, respectively.

3. Results and discussion

Fig. 2 shows compositional ratios of Zn/(Zn+Sn) as a function of d for films deposited at250C. The deposition of ZnO was less than thatof SnO2. Values of d corresponding to thecomposition of ilumenite-like ZnSnO3 and spinel-like Zn2SnO4 were 0.62 and 0.73, respectively.

X-ray diffraction patterns for films deposited atTs=250C are shown in Fig. 3 as a function of d:Diffraction patterns for 0pdp0.47 indicate thatthe films consist mainly of rutile SnO2 phase bycomparison with JCPDS cards. At 0.38pdp0.47the signals are weaker and broader. This indicatesthat amorphous phase of some kind is included inrutile SnO2 as ZnO content increases. Diffractionpatterns for 0.50pdp0.73 exhibit no definitepeaks, indicating that the films are amorphous instructure. Neither ZnSnO3 nor Zn2SnO4 crystal-line phase was observed at d=0.62 and 0.72, where

Fig. 2. The composition ratio for ZnO–SnO2 films deposited at

Ts=250C as a function of d:

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20 30 40 50 60 70 80

Inte

nsit

y(A

rb.U

nits

)

2 (CuK )/deg.

=0.00

=0.11

=0.20

=0.27

=0.33

=0.38

=0.43

=0.47

=0.50

=0.53

=0.57

=0.62

=0.67

=0.73

=0.80

=0.89

=1.00

SnO ZnO(0

02)

(200

)

(211

)

(301

)

(101

)

(110

)

Am

orph

ous

phas

e

SnO

2

+ A

mor

phou

s ph

ase

20 30 40 50 60 70 80

Inte

nsit

y(A

rb.U

nits

)

2 (CuK )/deg.

=0.00

=0.11

=0.20

=0.27

=0.33

=0.38

=0.43

=0.47

=0.50

=0.53

=0.57

=0.62

=0.67

=0.73

=0.80

=0.89

=1.00

SnO ZnO(0

02)

(200

)

(211

)

(301

)

(101

)

(110

)

× 1/3

× 1/2

× 1/10

× 1/16

20 30 40 50 60 70 80

Inte

nsit

y(A

rb.U

nits

)

2 (CuK )/deg.

=0.00

=0.11

=0.20

=0.27

=0.33

=0.38

=0.43

=0.47

=0.50

=0.53

=0.57

=0.62

=0.67

=0.73

=0.80

=0.89

=1.00

SnO ZnO(0

02)

(200

)

(211

)

(301

)

(101

)

(110

)

20 30 40 50 60 70 80

Inte

nsit

y(A

rb.U

nits

)

2 (CuK )/deg.

=0.00

=0.11

=0.20

=0.27

=0.33

=0.38

=0.43

=0.47

=0.50

=0.53

=0.57

=0.62

=0.67

=0.73

=0.80

=0.89

=1.00

SnO ZnO(0

02)

(200

)

(211

)

(301

)

(101

)

(110

)

Am

orph

ous

phas

e

SnO

2

+ A

mor

phou

s ph

ase

20 30 40 50 60 70 80

Inte

nsit

y(A

rb.U

nits

)

2 (CuK αθ )/deg.

=0.00δ

δ

δ

δ

δ

δ

δ

δ

δ

δ

δ

δ

δ

δ

δ

δ

δ

δ

=0.11

=0.20

=0.27

=0.33

=0.38

=0.43

=0.47

=0.50

=0.53

=0.57

=0.62

=0.67

=0.73

=0.80

=0.89

=1.00

SnO2 ZnO(0

02)

(200

)

(211

)

(301

)

(101

)

(110

)

× 1/3

× 1/2

× 1/10

× 1/16

: :

Fig. 3. X-ray diffraction patterns for films deposited at

Ts=250C as a function of d: Magnification of diffraction

intensity for d=0.27, 0.80, 0.89 and 1.00 is shown at the right

margin of the figure.

SnO2 +amorphous

Amorphousphase

SnO2 ZnSnO3

0.5

SnO2 +amorphous

Amorphousphase

SnO2 ZnSnO3

0.50 1

10

10

102

=IZn/(IZn+ISn)

SnO2 +amorphous

Amorphousphase

SnO2 ZnSnO3

0.5

ZnSnO3

0

-2 SnO2 +amorphous

Amorphousphase

SnO2 ZnSnO3

0.5

SnO2 +amorphous

Amorphousphase

SnO2 ZnSnO3

0.50 1

10

10

102

=Iδρ/

Ω c

m

Zn/(IZn+ISn)

SnO2 +amorphous

Amorphousphase

SnO2 ZnSnO3

0.5

ZnSnO3

0

-2

.

Fig. 4. Resistivities of the films deposited at Ts=250C as a

function of d:

Y. Hayashi et al. / Vacuum 74 (2004) 607–611 609

the appearance of stoichiometric films was ex-pected. At d=0.80, a strong peak appeared nearly2yE33. This peak could not be identified fromthe JCPDS cards of ZnO and SnO2, althoughidentification is not definite at the present stage.XRD pattern for films of 0.83pdp1.00 indicatewurtzite ZnO. XRD patterns of films deposited atTs=150C are almost the same as those depositedat Ts=250C. An amorphous phase appeared inthe range of 0.47pdp0.80 at 150C.

Fig. 4 shows the resistivities of the filmsdeposited at Ts=250C as a function d: Theminimum resistivity was 5.4 103O cm atd=0.11. This film was thought to be rutile SnO2

doped with ZnO. Further ZnO doping of SnO2 ledto a rapid increase in film resistivity. As amor-phous phase becomes mixed with rutile SnO2 at0.38pdp0.50 film resistivity decreases with in-creasing Zn content. At d ¼ 0:50 where thediffraction peaks corresponding to SnO2 disap-peared completely, film resistivity reached aminimum of 3.6 102O cm. The trend in resis-

tivity at 0.38pdp0.50 would be a consequence ofchanging the ratio of the rutile SnO2 phase withhigh resistivity to the amorphous phase seen atd ¼ 0:50: Resistivity then increased again withincreasing Zn content up to d ¼ 0:73 where thecomposition ratio Zn/(Zn+Sn) corresponds to thespinel Zn2SnO4.

The presence of a minimum resistivity ascomposition changed has also been reported byMinami et al. [7], where ZnSnO3 was formed at theminimum resistivity. In our films, the compositionof the film at d ¼ 0:50 having the minimumresistivity deviated from that of ZnSnO3. Thecomposition ratio of Zn/(Zn+Sn) is nearly equalto 1/3. This corresponds to ZnSn2O5. The filmwith a composition ratio corresponding toZnSnO3 was deposited at d=0.62, much higherthan minimum resistivity. Therefore, amor-phous ZnSnO3 and SnO2 can be presumed to bemixed. Amorphous films exist in the forms of(ZnSnO3)1x(SnO2)x over the range of 0.50pdp0.62 (0pxp0.5) and in the form of (ZnSnO3)1y

(ZnO)y over the range of 0.62pdp0.73(0pyp0.5). Amorphous SnO2 or ZnO were

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Inte

nsit

y(A

rb.U

nits

)

10 20 30 40 50 60 70 80

Inte

nsit

y(A

rb.U

nits

)

after annealing

=0.80

=0.73

=0.62

=0.50

:Zn2SnO4

:ZnO

=0.80

=0.73

=0.62

=0.50

× 1/4

× 1/5

× 1/8

10 20 30 40 50 60 70 80

Inte

nsit

y(A

rb.U

nits

)

10 20 30 40 50 60 70 80

Inte

nsit

y(A

rb.U

nits

)

after annealing

=0.80

=0.73

=0.62

=0.50

:Zn2SnO4

:ZnO

=0.80

=0.73

=0.62

=0.50

10 20 30 40 50 60 70 80

Inte

nsit

y(A

rb.U

nits

)

10 20 30 40 50 60 70 80

Inte

nsit

y(A

rb.U

nits

)

2 (CuK )/deg. 2 (CuK )/deg.

δ

δ

δ

δ

δ

δ

δ

δ

10 20 30 40 50 60 70 80

Inte

nsit

y(A

rb.U

nits

)

10 20 30 40 50 60 70 80

Inte

nsit

y(A

rb.U

nits

)

as deposition at 150˚C after annealing

=0.80

=0.73

=0.62

=0.50

:Zn2SnO4

:ZnO

=0.80

=0.73

=0.62

=0.50

× 1/4

× 1/5

× 1/8

Fig. 5. X-ray diffraction patterns as -deposited and after annealing of amorphous phase films at Ts=150C for 1 h in Ar gas at 650C.

Magnification of diffraction intensity is shown at the right margin of each figure.

200 300 400 500 600 700 800 9000

20

40

60

80

100

Wavelength/nm

δ =0.50δ =0.62δ =0.73

δ=0.80

Tra

nsm

itta

nce

/%

Fig. 6. Optical transmission spectra for films deposited at

Ts=250C as a function of d:

Y. Hayashi et al. / Vacuum 74 (2004) 607–611610

highly dispersed throughout the amorphousZnSnO3.

For films containing amorphous phase, at0.50pdp0.73, the carrier concentration of thissystem is thought to be generated from oxygenvacancies in the zinc–stannate lattice, as suggestedby Minami et al. [7] Their data shows that moreoxygen vacancies in amorphous ZnSnO3 aregenerated by excess Sn. In our data in Fig. 4,excess SnO2 in ZnSnO3 is thus expected to attractmore oxygen in the ZnSnO3. It is also found thatexcess ZnO in ZnSnO3 attracts less oxygen in theZnSnO3 from the data in Fig. 4. These will beascribed to the affinity of excess SnO2 or ZnO tooxygen in ZnSnO3. The same trends in resistivitycould be observed for the films deposited atTs=150C.

Fig. 5 shows X-ray diffraction patterns afterannealing of amorphous phase films deposited atTs=150C in Ar gas at 650C Peaks correspond-ing to Zn2SnO4 appeared in the films of0.62pdp0.80. This seems to be due to decom-position of ZnSnO3 into Zn2SnO4 and SnO2

during annealing [9]. This confirms the presenceof ZnSnO3 in the amorphous-phase film. Thoughresistivity of film deposited at d=0.50 decreasedslightly, amorphous films deposited at larger dvalues in Fig. 5 showed increasing resistivity afterannealing.

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Optical transmission spectra of films depositedat Ts=250C in the amorphous region are shownin Fig. 6 as a function of d: Optical transparenciesof the amorphous films were 70–90% in the visibleregion, higher than those for Ts=150C films.Estimate of the optical bandgap energies of thefilms from these optical transmission spectraranged from 3.3 to 3.6 eV. The amorphousZnSnO3 at d ¼ 0:63 and Zn2SnO4 at d=0.72 gavethe largest value of 3.6 eV. Optical bandgap energyof the amorphous film at minimum resistivity was3.47 eV. Further addition of SnO2 to ZnSnO3

increased the band tailing.

4. Conclusion

Amorphous Phase in the ZnO–SnO2 systemwere observed in films deposited by simultan-eous sputtering at Ts=150C and 250C Amor-phous transparent films appeared for Zn/(Zn+Sn)=0.28–0.76 at Ts=150C, in Zn/(Zn+Sn)=0.32–0.66 at Ts=250C. The presenceof both amorphous SnO2 and ZnSnO3 could havean important role in the reduction of resistivity.The presence of both amorphous ZnO andZnSnO3 induced an increase in resistivity. Theseamorphous films exhibited minimum resistivity atZn/(Zn+Sn)=0.33 (d=0.50), Ts¼ 250C which

indicates that the film is composed of a mixture ofamorphous ZnSnO3 and SnO2.

Acknowledgements

This work was supported by a Grant-in-Aid forScientific Research from the Ministry of Educa-tion, Culture, Sports, Science and Technology ofJapan, Nos.14550301 and 14750555.

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