reprintsolidistatuspssphysica

Phys. Status Solidi C 8, No. 2, 503 – 505 (2011) / DOI 10.1002/pssc.201000528

cwww.pss-c.com

current topics in solid state physics

Effect of substrate temperature on
ZnO thin film fabrication by using an
atmospheric pressure cold plasma generator

Yoshifumi Suzaki*,1, Akiou Kawaguchi1, Takehiko Murase1, Toshifumi Yuji2, Tomokazu Shikama3,Dong-Bum Shin1,4, and Yoon-Kee Kim4
1

Department of Advanced Materials Science, Kagawa University, Hayashi-cho 2217-20, 761-0396 Takamatsu, Japan
Faculty of Education & Culture, University of Miyazaki, Gakuen Kihanadai-nishi 1-1, 889-2192 Miyazaki, Japan
3
Kagawa National College of Technology, Chokushi-cho 355, 761-8058 Takamatsu, Japan
4
Department of Welding & Production Engineering, Hanbat National University, San 16-1, Duckmyoung-dong, Yuseong-gu, 305-719
Daejeon, Korea
2

Received 15 June 2010, revised 18 August 2010, accepted 7 September 2010
Published online 17 November 2010
Keywords zinc oxide, thin film, open-air plasma, barrier discharge* Corresponding author: e-mail suzaki@eng.kagawa-u.ac.jp, Phone: +81 87 864 2415, Fax: +81 87 864 2438

Web: www.eng.kagawa-u.ac.jp/~suzaki/suzaki-e.html

Under atmospheric pressure, homogeneous nonequilibrium cold plasma was generated stably by highvoltage pulsed power (1 kV, 20 kHz) excitation of Heand O2 gases. By feeding Zn-MOPD (C18H30O6Zn) into
this plasma with He carrier gas, transparent flat ZnO
films about 240 nm thick were successfully fabricated on
glass substrates directly under a slit made into the cathode. Transmittance of the film is about 80 % in the wavelength range from 400 to 600 nm. An XRD measurement
revealed that these ZnO films had a c-axis oriented polycrystalline structure. By increasing the substrate temperature from 250 to 400 °C, electrical resistivity of the film

decreased from 0.42 Ωm to 9.4 × 10–4 Ωm. A high temperature of the substrate may change the growth patternof ZnO film and its microstructure.

Photograph of the atmospheric pressure cold plasma© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

1 Introduction Transparent electrically conductive
zinc oxide (ZnO) films have been studied experimentallyfor their application to low-cost transparent electrodes insolar cells such as amorphous silicon (a-Si). In conventional PECVD (Plasma-enhanced Chemical Vapour Deposition) processes, gas pressure is usually regulated in therange of 10-1 Pa to 1 kPa. If such processing plasma can begenerated under atmospheric pressure, it becomes possible
not only to simplify the processing system but also to makeplasma applicable for wider purposes.
Recently, cold plasma (glow discharge) was generated
by an RF excitation of flowing Ar, He or their mixtures at1 atm [1-3]. Previously we reported on the use of an atmospheric pressure cold plasma generator to fabricate ZnO

and Al doped ZnO films by feeding Zn-MOPD(C18H3O6Zn) and/or Al-MOPD (C27H45O9Al) into theplasma, which was generated stably using He and O2 gases[4-7]. In the present study, the effect of substrate temperature on ZnO thin film fabrication was investigated.
2 Experimental procedure The fabrication system of
the atmospheric pressure cold plasma generator is schematically illustrated in Fig. 1. Our plasma generator iscomposed of an Al cathode with a thin film coating ofalumina created by a natural oxidation process, and agrounded anode of Cu plate. A glass substrate was placed
on top of the anode plate, with a heating system. A slit typehole (20 mm × 1 mm) was made into the cathode (30 mm© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

504

Y. Suzaki et al.: Effect of substrate temperature on ZnO thin film fabrication

carrier He gas plasma He gas O2 gas flowmeter
flowmeter
flowmeter
tape heater
Plasma torch
acrylic boardthermocouple
high frequency
pulse
power supply
Tefloninsulation materialHeO2
thermocathode electrode(Al)coupleGlow discharge
plasmaanode electrode(Cu)mantle heater
exhaustofheating system
Zn-MOPD
stageorganic
Glass substrategas
vaporizer

Figure 1 Schematic diagram of the fabrication system.

× 50 mm), in order to let He gas flow into the gap between
cathode and anode. Also, the anode plate moved with aglass substrate back and forth in the direction perpendicular to the slit in order to fabricate flat films in a large area
(sweep distance: 10 mm). Atmospheric pressure cold
plasma was generated by flowing He and O2 gases. The Hecarrier gas was fed through the inner space of the cathodedown to the gap, where it was excited by the high voltage
pulse supply (HVP-20K, Haiden Laboratory Co. Ltd., Japan). Zn-MOPD (C18H30O6Zn, UBE Industries, Ltd., Japan) was vaporized and carried by the He carrier gas flow
into the plasma generated in the gap. Table 1 lists the fabrication conditions for our experiment.
A film about 240 nm thick grew directly under the slit
made into the cathode with a fabrication rate of about 30
nm/min. Spectral transmittance of the film was measured
by a spectrophotometer (UV-3150, Shimadzu Co., Japan).
The microstructures of the film were examined with a fieldemission scanning electron microscope (FE-SEM; Hitachi,Ltd., Japan) and X-ray diffraction (XRD; XRD-6100, Shimadzu Co., Japan) measurements. Resistivity of the film
was tested by the four-probe method.
3 Results and discussion
3.1 Transmittance and reflectance Figure 2
shows the typical spectral transmittance and reflectance ofthe films for the substrate temperature of 250, 300, 350 andTable 1 Fabrication conditions.
Substrate
Substrate temperature
Source material
Source material temperature
Carrier He gas flow rate
He gas total flow rate
O2 gas flow rate
Voltage
Sweep distance
Sweep speed
Anode and cathode gap
Deposition timeGlass
250–400 °C
Zn-MOPD(UBE Industries, Ltd., Japan)
100 °C
150 ccm
1550 ccm
10 ccm
1 kV
10 mm
1 mm/s
0.5 mm60 min© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Transmittance / Reflectance (%)

solidistatusphysica

pssc1009080706050403020
100300Transmittance

250℃
Absorption edge
Reflectance400500600
Wavelength (nm)

300℃
350℃
400℃

700800Figure 2 Transmittance and reflectance spectra of the ZnO films.

Figure 3 Resistivity of the ZnO films.

400 °C. The average transmittance of the films was about
80 %, in the wavelength range from 400 to 800 nm. These
spectra have absorption edges. When substrate temperature
increased, the absorption edge shifts to a short wavelength.
When the substrate temperature is 250 °C, the optical band
gap calculated from the absorption edge is 3.25 eV, which
is close to the value of ZnO (3.26 eV) reported by B. G.
Bylander [8]. When the substrate is between 250 to 400 °C,
the band gap of the film increases from 3.25 to 3. 35 eV.
3.2 Electrical resistivity Figure 3 shows the electrical resistivity of the ZnO films. The resistivity of the film
is 0.40 Ωm at a substrate temperature of 250 °C. By increasing the substrate temperature, resistivity of the film
decreased to 9.4 × 10–4 Ωm at a substrate temperature of400 °C. This finding suggests the need for further experiments in order to produce lower resistivity of the ZnOfilms.
3.3 Analysis of the film structure Typical XRD
profiles of the films fabricated at substrate temperatures of
250 and 400 °C are shown in Fig. 4. The profile shows apeak only for the (002) plane of ZnO. This result reveals
that ZnO films have a c-axis oriented polycrystalline
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ContributedArticle
Phys. Status Solidi C 8, No. 2 (2011)

505structure. A high substrate temperature may cause thegrowth pattern of ZnO film and its microstructure to become smoothZnO (002)

Intensity (a.u.)▼Substrate temperature
400 ℃

250 ℃

2030405060702θ (°)

Figure 4 Typical XRD profiles of the ZnO films.

(a) 250 °C

300 nm4 Conclusions Atmospheric pressure cold plasma
was generated stably by high voltage pulsed power (1 kV,20 kHz) excitation of a mixture of He and O2 gases. We
fabricated ZnO films on glass substrates by feeding ZnMOPD into the plasma. Results are summarized as follows.
(1) Transparent flat films about 240 nm thick are successfully fabricated directly under the slit made into thecathode. The deposition rate of the film is 30 nm/min.
(2) The average transmittance is about 80 % in therange from 400 to 800 nm. The optical band gap calculated
from the absorption edge is 3.35 eV at a substrate temperature of 400 °C.
(3) ZnO films have a c-axis oriented polycrystalline
structure. When the substrate temperature is 400 °C, themicrostructure of the ZnO film becomes smooth and its
crystallinity is improved.
(4) Electrical resistivity of the ZnO film is 9.4 × 10–4
Ωm at a substrate temperature of 400 °C.
Further studies and experiments on the effects of thegas flow rate in the gap and other conditions are in progress to enable ZnO of better quality and resistivity to beprepared in the near future. Our goal is to develop a simple
depositing system using low cost materials such as N2 gas
or air at 1 atm.
Acknowledgements We thank Mr. Ian Willey for his
careful proofreading of our manuscript.

References(b) 400 °C

300 nmFigure 5 Typical FE-SEM observations of the ZnO film for thesubstrate temperature of (a) 250 and (b) 400 °C.

structure. By increasing the substrate temperature, the peak
becomes larger and sharper, which means that the grain
size of the polycrystalline ZnO becomes larger and its
crystallinity is improved.
Typical FE-SEM observations of the ZnO film at substrate temperatures of 250 and 400 °C are shown in Fig. 5(a) and (b), respectively. Flat and continuous films wereobtained. The cross-sectional image in Fig. 5(a) shows acolumnar structure. The grain columns were tightly packed
and grew through the entire film. However, the crosssectional image in Fig. 5(b) shows a uniform and smoothwww.pss-c.com[1] H. K. Ha, K. Inomata, and H. Koinuma, J. Electrochem. Soc.
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Conference on Production Engineering, Osaka, Japan, 1999
(the Japan Society for Precision Engineering, Tokyo, 1999),
pp. 596-600.
[3] T. Shikama, Y. Suzaki, S. Ejima, Y. Ide, S. Azuma, O. Tanaka, T. Kajitani, and H. Koinuma, J. Soc. Mater. Sci. Jpn.
54, 279 (2005).
[4] Y. Suzaki, S. Ejima, T. Shikama, S. Azuma, O. Tanaka, T.
Kajitani, and H. Koinuma, Thin Solid Films 506/507, 155(2006).
[5] Y. Suzaki, A. Obika, T. Shikama, and S. Ejima, J. Jpn. Soc.
Prec. Eng. 74, 1097 (2008) (in Japanese).
[6] Y. Suzaki, H. Miyagawa, A. Obika, A. Kawaguchi, T. Shikama, and T. Yuji, Front. Appl. Plasma Technol. 3, 23(2010).
[7] Y. Suzaki, H. Miyagawa, A. Obika, A. Kawaguchi, T. Shikama, and Toshifumi Yuji, Adv. Appl. Plasma Sci. 7, 123(2009).
[8] B. G. Bylander, J. Appl. Phys. 49, 1188 (1987).

© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim