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Thin Solid Films 468

Highly stable hydrogenated gallium-doped zinc oxide thin films grown

by DC magnetron sputtering using H2/Ar gas

Satoshi Takeda*, Makoto Fukawa

Research Center, Asahi Glass Co., Ltd., 1150 Hazawa-cho, Kanagawa-ku, Yokohama 221-8755, Japan

Received 24 November 2003; received in revised form 25 April 2004; accepted 31 May 2004

Available online 12 August 2004

Abstract

The effects of water partial pressure (PH2O) on electrical and optical properties of Ga-doped ZnO films grown by DC magnetron

sputtering were investigated. With increasing PH2O, the resistivity (q) of the films grown in pure Ar gas (Ar-films) significantly increased due

to the decrease in both free carrier density and Hall mobility. The transmittance in the wavelength region of 300–400 nm for the films also

increased with increasing PH2O. However, no significant PH2O

dependence of the electrical and optical properties was observed for the films

grown in H2/Ar gas mixture (H2/Ar-films). Secondary ion mass spectrometry (SIMS) and X-ray diffraction (XRD) analysis revealed that

hydrogen concentration in the Ar-films increased with increasing PH2Oand grain size of the films decreases with increasing the hydrogen

concentration. These results indicate that the origin of the incorporated hydrogen is attributed to the residual water vapor in the coating

chamber, and that the variation of q and transmittance along with PH2Oof the films resulted from the change in the grain size. On the

contrary, the hydrogen concentration in H2/Ar-films was almost constant irrespective of PH2Oand the degree of change in the grain size of the

films versus PH2Owas much smaller than that of Ar-films. These facts indicate that the hydrogen primarily comes from H2 gas and the

adsorption species due to H2 gas preferentially adsorb to the growing film surface over residual water vapor. Consequently, the effects of

PH2Oon the crystal growth are reduced.

D 2004 Elsevier B.V. All rights reserved.

Keywords: Hydrogen; SIMS; Sputtering; Zinc oxide

1. Introduction

Transparent and conductive oxide (TCO) films, which

are degenerate wide band-gap semiconductors with low

resistance and high transparency in the visible wavelength

range, have been used extensively in optoelectronic devices

such as transparent electrodes in flat panel displays and solar

cells. The majority of TCO films are n-type conductors such

as indium tin oxide (ITO), tin dioxide (SnO2) or zinc oxide

(ZnO) film. Among these materials, ITO film is the most

widely used in these fields because of its low resistivity,

high transparency and excellent etching performance.

Recently, ZnO film has gained much attention for the

0040-6090/$ - see front matter D 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.tsf.2004.05.137

* Corresponding author. Tel.: +81 45 374 8755; fax: +81 45 374 8863.

E-mail address: satoshi-takeda@agc.co.jp (S. Takeda).

TCO applications due to their low cost, non-toxicity and

excellent stability under the exposure to hydrogen plasma

[1]. The conductivity of the ZnO films can be changed by

several orders of magnitude with Al, Ga, or In doping or

upon creation of oxygen vacancies. The films have typically

been prepared by magnetron sputtering [2], chemical vapor

deposition [3] and spraypyrolysis [4]. Among these meth-

ods, the sputtering method is widely used in industrial

products because high quality films; high density, strong

adhesion, high hardness, etc., can be obtained at low

substrate temperature with good uniformity of the film

thickness over a large area. The microstructure and proper-

ties of sputtered films are strongly dependent upon the

process parameters, such as partial pressure of oxygen and

water, sputtering power, substrate temperature [5], etc.

Therefore, precise control of each parameter should be

necessary for obtaining high quality sputtered films.

(2004) 234–239

S. Takeda, M. Fukawa / Thin Solid Films 468 (2004) 234–239 235

In the previous study [6], we have reported that the

electrical properties of Ga-doped ZnO films (GZO) depos-

ited in pure Ar gas by DC magnetron sputtering were

strongly influenced by water partial pressure (PH2O), and

that the resistivity (q) significantly increased with increasingPH2O

. These results indicate that the water vapor in residual

gas is considered to degrade the electrical properties and

repeatability of the film properties. Therefore, it is essential

to elucidate the degradation mechanism for obtaining the

high-quality films.

In the present study, we investigate the relationship

between the amount of hydrogen, which may come from

the residual water vapor, and structure of the films by

secondary ion mass spectrometry (SIMS), X-ray diffraction

(XRD), scanning electron microscopy (SEM), X-ray

photoelectron spectroscopy (XPS), UV–Vis optical spec-

troscopy and Hall-effect measurements. The effects of

PH2Oon the optical properties of the films are also

examined. Furthermore, we report that H2 gas introduc-

tion is an effective way to improve the stability of

both electrical and optical properties of the GZO films

versus PH2O.

Fig. 1. The resistivity (q), carrier density (n) and Hall mobility (l) for GZOfilms deposited in Ar 100% or H2/Ar gas mixture as a function of PH2O

.

2. Experimental details

The GZO films were deposited onto glass (Corning

7059) with a thickness of 150–180 nm by DC magnetron

sputtering at room temperature using a ceramics GZO target

(5.7 wt.% Ga2O3, Asahi Glass) under various residual water

pressures (PH2O). Quadrupole mass spectrometry analysis

revealed that base pressure was primarily due to PH2O. Thus,

PH2Owere controlled by the base pressure [7] in this study.

The distance between the target and the substrate was set at

30 mm. The sputtering was carried out under a total gas

pressure of 1.3 Pa of Ar 100%, H2/Ar=1:5 and deuterium

(D2)/Ar=1:5.

Observation of surface morphology of the films was

performed using a SEM (Hitachi S-900). Crystalline

phases of the films were identified by glancing-angle

XRD (Rigaku Rint-2000) using Cu-Ka radiation operated

at 50 kV–200 mA with the incident angle of 0.58.Hydrogen, deuterium and argon in the films were

measured by SIMS (PHI Adept1010). The secondary

ions of 1H� (2D�) and 133Cs40Ar+ were detected using a

5-keV Cs+ primary ion beam with a beam current of 200

and 500 nA, respectively. The angle of incidence was 608to the normal of the sample surface. The chemical

compositions of the films were determined by XPS

(PHI 5500). XPS measurements were carried out with a

monochromatized AlKa source. The detection angle of

the X-ray photoelectrons was 758 to the normal of the

sample surface. The optical transmission and reflection

spectra of the films were measured at room temperature

in air using a dual beam spectrometer (Shimazu UV3100).

The resistivity (q), Hall mobility (l) and free carrier

density (n) were estimated by the four-point probe

method and Hall-effect measurement in the van der Pauw

method.

3. Results and discussion

3.1. Electrical and optical properties

Fig. 1 shows the resistivity (q), carrier density (n) and

Hall mobility (l) of GZO films deposited in Ar 100% (Ar-

films) or H2/Ar gas mixture (H2/Ar-films) as a function of

residual water pressure (PH2O). With increasing PH2O

, the qof Ar-films increases due to the decrease in both n and l.On the other hand, no significant change in q is observed for

H2/Ar-films. These results indicate that the introduction of

H2 gas significantly improves stability of electrical proper-

ties of the films against trace water vapor in the coating

chamber. Here, it is known that the water vapor is

decomposed into the constituent molecules or ions including

O2+, O2, O3, H, OH, H2O and H2O+ during sputtering [8].

Considering this, the decrease in both n and l of the Ar-

films with increasing PH2Omay be closely related with the

Fig. 3. The transmission spectra of GZO films deposited in Ar 100 % or

H2/Ar gas mixture under various PH2O.

S. Takeda, M. Fukawa / Thin Solid Films 468 (2004) 234–239236

decrease in oxygen vacancies as a result of adsorbed oxygen

on the grain boundaries.

Fig. 2 shows the optical transmission and reflection

spectra of Ar-film and H2/Ar-film grown at PH2O=1.3�10�3

Pa. The reflectance in the near infrared region increases for

H2/Ar-film compared with Ar-film due to the increase in n.

In addition, the optical absorption edge of H2/Ar-film is

shifted to higher energy side. This blue shift can be

explained by Burstein-Moss shift caused by increase in free

carrier density (n) [9]. The origin of the increase in n will be

discussed later.

Fig. 3 shows the transmission spectra of Ar and H2/Ar-

films deposited under various PH2O. The transmittance in the

wavelength region of 300–400 nm increases with increasing

PH2Ofor the Ar-films. This phenomenon is not observed for

the H2/Ar-films. These results indicate that the introduction

of H2 gas is an effective way to improve the stability of both

electrical and optical properties of the films versus PH2O.

Similar tendency was observed by the introduction of

deuterium (D2) gas.

3.2. Origin of hydrogen and film structure

Fig. 4 shows the secondary ion intensity ratio

(1H�/80ZnO�) which represents the hydrogen concentration

in the films as a function of PH2O. Here, we focus on the

origin of hydrogen because information about chemical

states of hydrogen cannot be obtained in SIMS analysis.

With increasing PH2O, the hydrogen concentration in Ar-

films increases, indicating that the origin of the hydrogen is

Fig. 2. The optical transmission and reflection spectra of GZO films

deposited at PH2O= 1.3�10�3 Pa in Ar 100% or H2/Ar gas mixture.

due to residual water in the coating chamber. On the other

hand, the hydrogen concentration in H2/Ar-films is almost

constant irrespective of PH2Oand the concentration is

obviously higher than that of Ar-films even when PH2Ois

increased up to 4.0�10�3 Pa. It was also revealed that 2D�

was clearly incorporated into D2/Ar-films. These results

indicate that the hydrogen in the H2/Ar-films primarily

comes from H2 gas. It was confirmed that the hydrogen was

uniformly distributed into both Ar- and H2/Ar-films from1H� depth profiles. This observation indicates that some

species including hydrogen is continuously supplied to the

growing film surface during the deposition, that is, the film

surface is covered by the adsorption species due to H2O or

H2. Taking these into consideration, it is considered that the

adsorption species from H2 gas preferentially adsorb to the

growing film surface over residual water vapor in H2/Ar

plasma process.

Fig. 5 shows the relationship between the resistivity (q)and the secondary ion intensity ratio (1H�/80ZnO�) which

represents the hydrogen concentration in the films. It is

clearly seen that the q increases with increasing the

hydrogen concentration in Ar-films. This result indicates

that the increase in q is caused by the incorporation of

hydrogen from residual water vapor. In contrast, both q and

the amount of hydrogen from H2 gas are constant for H2/Ar-

Fig. 4. The secondary ion intensity ratio (1H�/80ZnO�) for GZO films

deposited in Ar 100% or H2/Ar gas mixture as a function of PH2O.

Fig. 5. The relationship between the resistivity (q) and secondary ion

intensity ratio (1H�/80ZnO�) for GZO films deposited in Ar 100% or H2/Ar

gas mixture.

S. Takeda, M. Fukawa / Thin Solid Films 468 (2004) 234–239 237

films and the hydrogen concentration is higher than that of

Ar-films. These suggest that the stability difference in qbetween Ar- and Ar/H2-films versus PH2O

is not simply due

to the hydrogen concentration but due to the difference in

the adsorption species including hydrogen.

Fig. 6 shows XRD patterns of the films grown under

various PH2O. ZnO (002) diffraction peak is clearly observed

for both Ar- and H2/Ar-films. The full width at half

maximum (FWHM) of ZnO (002) peak for the Ar-films

increases with increasing PH2O, indicating that grain size of

the films decreases with increasing PH2O. The decrease in l

of the Ar-films with increasing PH2O, as seen in Fig. 1, can

Fig. 6. 2Q X-ray diffraction patterns for GZO films deposited in Ar 100%

or H2/Ar gas mixture under various PH2O.

be explained by the decrease in the grain size. Similar

tendency is clearly observed in SEM images as shown in

Fig. 7. However, no significant PH2Odependence of surface

morphology was observed for H2/Ar-films. Also, the degree

of change in FWHM for the H2/Ar-films is much smaller

than that of Ar-films although the values of FWHM are

larger than those of Ar-films. These observations clearly

indicate that the grain size of the H2/Ar-films is smaller

than that of Ar-films, but that the degree of change in

the grain size of the H2/Ar-films along with PH2Ois

much smaller than that of the Ar-films. The decrease in lof the H2/Ar-films compared with that of the Ar-films,

as seen in Fig. 1, can be ascribed to the decrease in the

grain size. Furthermore, the chemical compositions (ob-

tained from XPS depth profile) of H2/Ar-film deposited

at PH2O=1.3�10�3 Pa is (at.%): 53.8-Zn, 39.0-O, 7.2-Ga,

which was almost same as that of Ar-film. On the basis

of these experimental facts, it is concluded that the variation

of electrical and optical properties of the films results from

the change in microstructure of the films along with PH2O.

Fig. 7. SEM images for GZO films deposited in Ar 100% at (a) PH2O=

0.8�10�3 Pa and (b) 4.0 � 10�3 Pa.

Table 1

Sputtering voltage at certain sputtering power for Ar and H2/Ar plasma

processes

Sputtering power, W Ar plasma, V H2/Ar plasma, V

130 �338 �320

200 �361 �344

250 �370 �360

S. Takeda, M. Fukawa / Thin Solid Films 468 (2004) 234–239238

3.3. Origin of change in microstructure

As mentioned above, instability of electrical and optical

properties versus PH2Ois induced as a result of change in

microstructure along with PH2O. In order to clarify the origin

of the changes, let us consider two effects which are one of

the major factors affecting the crystal growth in sputtering

process; One is bombardment of recoiled high-energy argon

neutrals (Ar0) and negative oxygen ions (O�) on the

growing film surface [10,11] and the other is the adsorption

species from the atmosphere to the growing film surface.

3.3.1. Bombardment of Ar 0 and O�

Fig. 8 shows SIMS depth profile of 133Cs40Ar+ which

represents incorporated Ar concentration into the film. It can

be clearly seen that the Ar concentration in H2/Ar-film is

lower than that of Ar-film, indicating that the damage due to

the recoiled Ar0 is reduced by the introduction of H2 gas.

Table 1 shows sputtering voltage at a certain sputtering

power. It is found that the sputtering voltage of H2/Ar

plasma process is lower than that of Ar plasma process

when the sputtering power is kept at certain value.

According to the research of Ishibashi et al. [12], the

resistivity of ITO films reduced by using a lower sputtering

voltage because of the reduction of the bombardment by

high energetic O�. These suggest that the defect density

induced by high-energetic Ar0 and O� is considered to be

reduced in H2/Ar plasma process.

The increase in free carrier density of the H2/Ar-films

compared with that of Ar-films, as shown in Fig. 1, may be

closely related with the decrease in the defect density of the

films because ineffective dopants trapped at the crystalline

defects are decreased. However, further investigation should

be necessary to elucidate the effects of hydrogen on the

Fig. 8. SIMS depth profiles of 133Cs40Ar+ in GZO films deposited at

PH2O= 1.3 � 10�3 Pa in Ar 100% or H2/Ar gas mixture.

electrical properties because it is known that the hydrogen in

ZnO (single crystals and films grown by metal-organic

chemical vapor deposition) induces a donor state and

thereby increase the free electron concentration [13–15].

Here, if the damage is reduced, the crystallinity of the

film should be improved [11]. However, as shown in Figs. 1

and 6, no significant improvement of the crystallinity is

observed for the H2/Ar-films. Also, the Hall mobility of the

films does not increase but decrease. We also confirmed that

the amount of incorporated Ar and the sputtering voltage of

the Ar-films was not dependent upon PH2O. Based on these

analyses, it is concluded that the bombardment of Ar0 and

O� is not responsible for the change in microstructure along

with PH2O.

3.3.2. Adsorption species due to H2O or H2

Next, let us consider the effects of the adsorption species

on the crystal growth. Aforementioned, it was found that the

origin of the hydrogen of Ar and H2/Ar-film is due to H2O

and H2, respectively. Additionally, SIMS depth profile of1H� revealed that the hydrogen was uniformly distributed in

both films. These results indicate that the adsorption species

from H2O or H2 are continuously supplied to the growing

film surface during the deposition. Considering that the

incorporated hydrogen concentration in the H2/Ar-films was

almost constant irrespective of PH2Oin H2/Ar plasma

process, it is considered that the adsorption species from

H2 gas preferentially adsorb to the growing film surface

over residual water vapor. That is, the film surface is

covered by the adsorption species due to H2 during the

deposition, so that the effects of PH2Oon the crystal growth

are reduced. Consequently, stability of the microstructure

versus PH2Omay be significantly improved for H2/Ar-films.

On the contrary, the amount of the adsorption species from

residual water vapor changes along with PH2Oin Ar plasma

process. As a result, the microstructure of the Ar-films

changes with varying PH2O.

4. Conclusions

In this paper, we have reported the effects of residual

water pressure (PH2O) on electrical and optical properties of

GZO films prepared by DC magnetron sputtering. With

increasing PH2O, the resistivity (q) and the optical trans-

mittance in the wavelength region of 300–400 nm of the

films grown in pure Ar gas (Ar-films) significantly

increased. However, no significant PH2Odependence of

S. Takeda, M. Fukawa / Thin Solid Films 468 (2004) 234–239 239

the electrical and optical properties was observed for the

films grown in H2/Ar gas mixtures (H2/Ar-films). It was

found that grain size of Ar-films decreases with increasing

incorporated hydrogen due to residual water vapor, and that

the variation of q and transmittance along with PH2Oof the

films resulted from change in the grain size. However, the

incorporated hydrogen of H2/Ar-films was almost constant

irrespective of PH2Oand the degree of change in grain size

of the films was much smaller than that of the Ar-films.

These indicate that the hydrogen primarily comes from H2

gas and the adsorption species due to H2 gas adsorbs to the

growing film surface in preference to the residual water

vapor. That is, the film surface is covered by the adsorption

species from H2 gas during the deposition in H2/Ar plasma

process, so that the stability of electrical and optical

properties versus PH2Ois significantly improved. These

results may serve as clues on how to modify film properties

using hydrogen gas.

Acknowledgements

The authors are grateful to Dr. S. Suzuki for a critical

reading of the manuscript.

References

[1] S. Major, S. Kumar, M. Bhatnagar, K.L. Chopra, Appl. Phys. Lett. 49

(1986) 394.

[2] W. Wendt, K. Ellmer, K. Wiesemann, J. Appl. Phys. 82 (1997) 2115.

[3] J. Hu, R.G. Gordon, J. Appl. Phys. 71 (1992) 880.

[4] J. Aranovich, A. Ortiz, R.H. Bube, J. Vac. Sci. Technol. 16 (1979)

994.

[5] Y. Igasaki, H. Saito, J. Appl. Phys. 69 (1991) 2190.

[6] J. Ebisawa, K. Sato, A. Mitsui, S. Takeda, Proceedings of 3rd

International Conference on Coating on Glass, 2000, p. 751,

Maastricht, Netherlands.

[7] Y. Shigesato, Y. Hayashi, A. Masui, T. Haranoh, Jpn. J. Appl. Phys.

30 (1993) 814.

[8] T. Nakada, Y. Ohkubo, A. Kunioka, Jpn. J. Appl. Phys. 30 (1991)

3344.

[9] I. Hamberg, C.G. Granqvist, K.-F. Berggren, B.E. Sernelius, L.

Engstrfm, Phys. Rev., B 30 (1984) 3240.

[10] J.A. Thornton, J. Tabock, D.W. Hoffman, Thin Solid Films 171

(1989) 5.

[11] M. Kon, P.K. Song, A. Mitsui, Y. Shigesato, Jpn. J. Appl. Phys. 41

(2002) 6174.

[12] K. Ishibashi, K. Hiratam, N. Hosokawa, J. Vac. Sci. Technol., A 8

(1990) 1403.

[13] C.G. Van de Walle, Phys. Rev. Lett. 85 (2000) 1012.

[14] B. Theys, V. Sallet, F. Jomard, A. Lusson, J.-F. Rommeluere, Z.

Teukam, J. Appl. Phys. 91 (2002) 3922.

[15] S.Y. Myong, K.S. Lim, Appl. Phys. Lett. 82 (2003) 3026.