Thin Solid Films 506 – 507 (2006) 155 – 158
www.elsevier.com/locate/tsf

Deposition of ZnO film using an open-air cold plasma generator
Yoshifumi Suzaki a,*, Seiki Ejima a, Tomokazu Shikama b, Shunryo Azuma c, Osamu Tanaka c,
Takahiro Kajitani c, Hideomi Koinuma da

Faculty of Engineering, Kagawa University, 2217-20, Hayashi-cho, Takamatsu, Kagawa 761-0396, Japanb
Takamatsu National College of Technology, Takamatsu, Kagawa 761-8058, Japanc
Research Laboratory, Okura Industry Co. Ltd., Marugame, Kagawa 763-8508, JapandCeramics Materials and Structures Laboratory, Tokyo Institute of Technology, Yokohama, Kanagawa 226-8503, Japan
Available online 8 September 2005

Abstract
Under open-air atmosphere, homogeneous non-equilibrium cold plasma was generated stably by high voltage pulsed power (0.8 kV, 20Hz) excitation of He and O2 gases. By feeding bis(dipivaloylmethanato)zinc (DPM2Zn) into this plasma, transparent flat ZnO films about 200
nm thick were successfully deposited on glass substrates directly under the slit made into the cathode. An XRD measurement revealed thatZnO films had a polycrystalline structure oriented c-axis. By increasing the O2 gas flow rate, the grain size of the polycrystalline ZnO became
larger and its crystallinity was improved.
D 2005 Elsevier B.V. All rights reserved.
Keywords: Plasma deposition; Zinc oxide; Atmospheric pressure cold plasma; Helium gas

1. Introduction
Transparent electrically conductive zinc oxide (ZnO)
films have been studied experimentally for application to
low-cost transparent electrodes in solar cells such as
amorphous silicon (a-Si) or copper indium diselenide(CuInSe2) [1]. ZnO films are usually prepared by methods
such as high temperature chemical vapor deposition (CVD),
plasma-enhanced chemical vapor deposition (PECVD) [2,3]and sputtering [4,5]. Compared to high temperature chemical vapor deposition (CVD), plasma-enhanced chemicalvapor deposition (PECVD) is an attractive process for film
growth due to its lower substrate temperature requirement,
which is sometimes indispensable for preparing multilayered structures and for deposition on thermally unstablematerials. In conventional PECVD processes, gas pressureis usually regulated in the range of 1 mTorr to several Torr.
If such a processing plasma can be generated underatmospheric pressure, it would become possible not only

* Corresponding author.
E-mail address: suzaki@eng.kagawa-u.ac.jp (Y. Suzaki).
0040-6090/$ - see front matter D 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.tsf.2005.08.166

to simplify the processing system but also to make plasma
applicable for wider purposes.
Recently, low temperature plasma (glow discharge) wasgenerated by an RF excitation of flowing Ar, He, O2 or theirmixtures at 1 atm [6– 8]. In this paper, we will report on theuse of the open-air cold plasma generator to deposit ZnO
films by feeding bis(dipivaloylmethanato)zinc (DPM2Zn)
into the plasma, which was generated stably by using Heand O2 gases.

2. Experimental details
The deposition system in the open-air cold plasma
generator is schematically illustrated in Fig. 1. Our plasma
generator is composed of an Al cathode with a thin film
coating of alumina created by a natural oxidation process,and a grounded anode of Al plate. A glass substrate wasplaced on top of the anode plate. A slit (1 mm  20 mm) was
prepared on the cathode (30 mm  50 mm), in order to let
He gas flow into the gap between cathode and anode. Also,
the anode plate moves with a glass substrate back and forthin the direction perpendicular to the slit in order to fabricate

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Y. Suzaki et al. / Thin Solid Films 506 – 507 (2006) 155 – 158

plasma He gas

O2He

plasma O2 gas

pulse power suply

carrier He gas

plasma torchheatercathode (Al)cold plasmastageanode (Al)

Zoil bath
DPM2ZncomputerXglass substrate

Fig. 1. Schematic diagram of the deposition system with open-air cold plasma.

flat films in a large area (sweep distance: 10 mm). A
schematic diagram of its movement is shown in Fig. 2. Cold
plasma was generated by flowing He and O2 gases. The He
carrier 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.).
Bis(dipivaloylmethanato)zinc (DPM2Zn, Tri Chemical Laboratories Inc.) was vaporized and carried by the He carriergas flow into the plasma generated at the gap. Table 1 lists
the deposition conditions for our experiment. Under theseconditions, a transparent flat film about 200 nm thick was
successfully deposited. The film grows only directly under
the slit made into the cathode with a deposition rate of about
0.6 nm/s. The transmittance of all films deposited was morethan 90% in the wavelength range from 400 nm to 600 nm.
The microstructures of the films were studied by a fieldemission scanning electron microscope (FE-SEM; Hitachi,Ltd.) and X-ray diffraction (XRD; XRD-6100, Shimadzu
Co.) measurement. Resistivity of the film was tested by thefour-probe method with a multimeter (E2373A, Hewlett
Packard Co., Ltd.).

3. Results and discussion
3.1. Glow discharge range
The range of glow discharge in our experiment isshown in Fig. 3. The horizontal and vertical axes
indicate the O2 gas flow rate and the voltage of thepulse supply, respectively. Triangle (r) and diamond (O)data show maximum and minimum values of the glowdischarge, respectively. When the O2 gas flow rate is 5ccm, stable glow discharge was observed for voltages
ranging from 0.4 to 0.8 kV. But above 0.8 kV, unstablestreamer discharge was observed instead. Since oxygen
has high electronegativity it recombines with electrons in
the plasma easily. So the density of electrons in theplasma is held lower than the limit value for transition
to the streamer discharge. Stable discharge can beobtained at a higher voltage with a higher oxygen flowrate.
3.2. Analysis of crystal structure using XRD
Typical XRD profiles of films deposited at the O2 flow
rates of 5 to 50 ccm are shown in Fig. 4. The profile shows a1 mm

top view
20 mm

slitgas flow

cathode (Al)cold plasma
flat filmgap
Zmovement

glass
substrate
anode plate (Al)XFig. 2. Schematic diagram of movement of the anode plate. The anodemoves back and forth during plasma deposition.

Table 1
Deposition conditions
Substrate

Glass

Substrate temperature
Source material
Source material temperature
He gas total flow rate
He carrier gas flow rate
O2 gas flow rate
Voltage
Sweep distance
Sweep speed
Anode and cathode gap
Deposition time140 -CBis(dipivaloylmethanato)zinc (DPM2Zn)
110 -C
2300 ccm
300 ccm
5¨50 ccm
0.8 kV
10 mm
1 mm/s
0.5 mm60 minY. Suzaki et al. / Thin Solid Films 506 – 507 (2006) 155 – 158

2.0157

glow discharge maximum value

1.8

glow discharge minimum value

1.6

streamer region

Voltage (kV)

1.4
1.2
1.0

glow discharge region

0.8(a) O2 gas flow rate: 10 ccm

0.60.4

no discharge

0.205

1015202530

35404550

O2 gas flow rate (ccm)Fig. 3. Glow discharge region of the open-air cold plasma generator.

(b) O2 gas flow rate: 30 ccm

peak only for the (002) plane of ZnO. This result reveals
that ZnO films have a polycrystalline structure oriented caxis. By increasing the O2 gas flow rate, the peak becomeslarger and sharper, which means that the grain size of thepolycrystalline ZnO becomes larger and its crystallinity isimproved.
3.3. FE-SEM observations
Typical FE-SEM observations of the ZnO film for the O2flow rate of 10 ccm and for 30 ccm are shown in Fig. 5(a)and (b), respectively, whereas panel (c) shows the same
sample as panel (b) viewed from a 30- angle. Fig. 5(a)shows fine sub-grains of about 10 nm, but panel (b) shows
crystal grains of about 50 nm with wide grain boundaries.
Fig. 6(c) reveals that the ZnO crystal grain has a conic shapewhich looks like a bamboo shoot grown on the glass(c) O2 gas flow rate: 30 ccm (sample inclined 30°°)
Fig. 5. Typical FE-SEM observation. (a) O2 gas flow rate: 10 ccm. (b) O2
gas flow rate: 30 ccm. (c) O2 gas flow rate: 30 ccm (sample inclined 30-).

substrate. A high flow rate of O2 gas may change the growth
pattern of ZnO film and its microstructure into one ofcolumnar crystallites.

352 θ (º)

4045Fig. 4. Typical XRD profiles of the films.

50

as

302 g25

O20w

flo

20
10rate(ccm504030)Intensity (a.u.)ZnO (002)

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Y. Suzaki et al. / Thin Solid Films 506 – 507 (2006) 155 – 158

Resistivity (Ωm)42) The transmittance of the films was more than 90% in therange from 400 nm to 600 nm.
3) XRD measurement revealed that ZnO films had apolycrystalline structure oriented c-axis. By increasing
the O2 gas flow rate, the grain size of the polycrystalline
ZnO became larger and its crystallinity was improved.
4) By increasing the O2 gas flow rate, the surface of theZnO film developed a somewhat rough structure.

321010203040
O2 gas flow rate (ccm)

50Fig. 6. Resistivity of the ZnO films.

3.4. Electrical resistivity
Fig. 6 shows the resistivity of the ZnO films deposited atO2 flow rates of 5 to 50 ccm. By increasing the O2 flow rate,
resistivity of the film first decreased and reached a minimum
value of 0.273 V m at an O2 flow rate of 20 ccm, and thenthe resistivity increased. Decrease of the resistivity for therates below 20 ccm is due to the progress of ZnO
crystallinity as revealed by the XRD measurements. However, when the O2 flow rate increases above 20 ccm, thegrain boundary becomes wider, as shown in Fig. 5. This
could be attributed to increasing resistivity of the film. This
research suggests the need for further experiments in orderto produce higher quality ZnO films.

4. Conclusion
Our open-air plasma generator successfully generated
stable glow discharge by high voltage pulsed power (0.8 kV,20 Hz) excitation of He and O2 gases. Using this generator,
we deposited ZnO films on the glass substrates, and theresults are summarized as follows.
1) By feeding bis(dipivaloylmethanato)zinc (DPM2Zn) into
the plasma, transparent flat films about 200 nm thickwere successfully deposited directly under the slit madeinto the cathode. The typical input power was 30 W.

ZnO film has a rough structure and its resistivity appears
to be high. Further studies and experiments on the effects ofthe gas flow rate in the gap or 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 by using low cost materials such as N2
gas or air at 1 atm.

Acknowledgement
We thank Mr. Ian Willey for his careful correction of ourmanuscript. This work was funded by the Kagawa TechnoFoundation.

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