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N. Yamaguchi, T. Kuroyama, Y. Okuhara and H. Matsubara
Japan Fine Ceramics Center (JFCC), 2-4-1 Mutsuno, atsuta-ku, Nagoya
456- 8587, Japan
E-mail:
[email protected]
Abstract. Al doped ZnO (AZO) films were prepared on quartz
substrates by the co- evaporation of ZnO and Al2O3 ingots by
EB-PVD. EB power applied to the Al2O3 source was ranged from 3kW to
10kW, while EB power to the ZnO source was fixed at 3kW, at a
substrate temperature of 400oC. X-ray diffraction measurement
showed that the AZO films were weakly c-axis oriented.
Transmittance of All the AZO films was over 80% in the visible
range. Highest reflectance in the near IR range was obtained at the
EB power on Al2O3 of 5kW. The lowest resistivity of 3.05 x 10-4 cm
was obtained for the film deposited with the EB power on Al2O3 of
5kW with the deposition time of 300s.
1. Introduction Transparent conductive oxides (TCOs) are
transparent in visible range and shows the high reflectance in the
near-infrared (IR) region. Therefore, TCOs are good candidate for
application as the coatings for IR-reflectors on super-insulating
windows, which realize a considerable energy-saving effect in air
conditioning of houses and buildings. Among TCOs, ZnO is a
promising material, due to its low cost, resource availability and
nontoxicity compared to indium tin oxide and SnO2. It is well known
that group III elements such as Al, In and Ga act as donors in ZnO
[1]. As previously reported, Al doping seems to be the most
successful due to its advantages: Al ions have a similar ionic
radius compared to Zn2+, which results in only small lattice
distortions for high Al concentrations. At present, to our
knowledge, there are few reports on the IR-reflective property of
Al doped ZnO (AZO) films prepared by electron beam physical vapor
deposition (EB-PVD) [2,3].
In this work, structural and optical properties of AZO films
prepared by EB-PVD were investigated, addressing the effect of the
EB power applied on the Al2O3 evaporation source.
2. Experimental procedure AZO layers were prepared on quartz
substrates (30mm × 30mm × 1mm) by electron beam physical vapor
deposition. The source material ingot used in this study was a 40mm
diameter × 20mm thickness of ZnO (Hakusui Tech.) and a 63mm
diameter × 50mm thickness of Al2O3 (Kyoto Thin- Film Materials
Institute). The source materials were co-evaporated by a 100kW EB
gun, using electron beam jumping technique. Electron beam power
applied to the ZnOsource (EBZnO) was fixed at 3kW. Electron beam
power applied to the Al2O3 source (EBAl2O3) was varied from 3 to
10kW. Co- deposition using electron beam jumping technique, the
electron beam is scanned over one target (ZnO) for a certain amount
of time and then moved to the next target (Al2O3). EBx (x=ZnO,
Al2O3) was
ICC3: Symposium 6: Advances in Electro Ceramics IOP Publishing IOP
Conf. Series: Materials Science and Engineering 18 (2011) 092025
doi:10.1088/1757-899X/18/9/092025
c© 2011 Ceramic Society of Japan. Published under licence by IOP
Publishing Ltd1
controlled by dividing the dwell time of electron beam among the
evaporation sources with the ratio of EBx to EBtotal. The substrate
was placed parallel to the ingot surface at a distance of 600mm.
Substrate rotation (in-plane) speed was kept at 30rpm. The vacuum
chamber was evacuated to a base pressure below 5 ×10-3Pa. Substrate
temperature was maintained at 400oC. 2.2 Characterization
Microstructures of AZO films were investigated by field emission
scanning electron microscope (FE- SEM, Hitachi S-8000). Phase
composition and texture were analyzed by X-ray diffraction (XRD,
Rigaku 2000V/PC). Optical transmittance and reflectance were
measured in the range of 190-2500nm by UV-VIS-IR spectro-photometer
(Perkin Elmer Lambda 950). The resistivity, Hall mobility and
carrier density were measured using a four-point probe and Hall
effect apparatus at room temperature.
3. Results and discussion Figure 1 shows the XRD patterns of AZO
films deposited with different EBAl2O3. All layers showed poly
crystalline nature, with (002), (101) and (103) peaks of hexagonal
ZnO, except the AZO film deposited at EBAl2O3 of 10kW which showed
no apparent peak, indicating amorphous nature. Compared with the
standard diffraction pattern, the AZO films exhibited weak (002)
preferential orientation, which differs from the previous
literatures. In general, it is considered that a slow deposition
rate would yield well (002)-oriented ZnO layers. Therefore, this
difference seems to be due to the high deposition rate, shown in
Figure 1. The Al doping does not change the structure of ZnO,
because an obvious peak shift was not observed. The peak intensity
tended to decrease slightly with the increase of EBAl2O3.
The SEM images of AZO films, as given in Figure 2 (a) to (e),
revealed a poly-crystalline structure
with grain size of 100 - 200nm. Thickness of the AZO films
increased with increasing from 230nm (EBAl2O3 of 3kW) to 300nm
(EBAl2O3 of 6kW). However, at 10kW, thickness of the AZO film
decreased.
In co-deposition of ZnO and Al2O3 with medium EBAl2O3, the
deposition rate of AZO was dominated by the evaporation rate of
ZnO, because ZnO sublimes and its vapor pressure is much higher
than that of Al2O3, which was in semi-melt state and the vapor
pressure is very low at EBAl2O3 below 6kW. Increasing EBAl2O3
decreases the dwell time on the ZnO source and the amount of ZnO
vapor generated in certain time and the effective deposition rate
dropped insignificantly. In addition, at EBAl2O3 of 10kW, Al2O3
changed to molten state, and the vapor pressure of Al2O3 rose
suddenly, and the rate of Al2O3 vapor in the whole vapor increased
and the AZO film approached to Al2O3. The substrate temperature of
400oC is too low for Al2O3 to crystallize. As a result, amorphous
phase was formed and the film thickness decreased.
Figure 1. XRD patterns of AZO films deposited with different
EBAl2O3. 20 30 40 50 60 70 80
2θ (degree)
(0 02
ICC3: Symposium 6: Advances in Electro Ceramics IOP Publishing IOP
Conf. Series: Materials Science and Engineering 18 (2011) 092025
doi:10.1088/1757-899X/18/9/092025
2
Optical properties of AZO films were shown in Figure 3 (a), (b).
All the AZO films show high
transmittance (> 80%) in visible range (Figure 3 (a)). However,
in the near-IR region, significant difference was observed. The
drop of transmittance in near-IR region mainly comes from the
increase of reflectance which is due to the plasma resonance of
electron gas in the conduction band. IR- reflectance was observed
in all the AZO films, except for the film deposited at EBAl2O3 of
10kW. Highest reflectance in near-IR region was obtained EBAl2O3 of
5kW (Figure 3 (b)). At higher EBAl2O3, IR-reflectance decreased
with increasing EBAl2O3. Therefore, the effect of deposition time
on optical property of AZO films deposited at 5kW of EBAl2O3 was
surveyed. Increase in deposition time resulted in increase in
IR-reflectance up to 68% at 2500 nm at 300s, which is much higher
than that of reported films by EB-PVD [2]. As the deposition time
increases, the AZO film thickens and thus the number of carriers
(electrons) in the AZO film increase, resulting in increase in
IR-reflectance which originates from the plasma arising from the
high electron concentration.
Electrical property of the most IR-reflective AZO films (EBAl2O3 of
5kW with deposition time of
300s) was measured by Hall measurement using van der Pauw’s
technique. The AZO film achieved electrical resistivity of
3.05×10-4 cm, carrier concentration of 0.86×1021 cm-3 and Hall
mobility of 23.8 cm2/Vs. These values are comparable to that of the
reported IR-reflective AZO films prepared by
500nm
(d) EBAl2O3=6kW
Figure 2. Cross-sectional SEM images of the AZO films deposited at
EBAl2O3 of: (a)3kW, (b)4kW, (c)5kW, (d)6kW and (e)10kW.
Figure 3. Optical transmittance (a) and reflectance (b) spectra of
AZO films deposited at different EBAl2O3 and deposition time as a
function of wavelength.
500 1000 1500 2000 25000
20
40
60
80
100
500 1000 1500 2000 2500 Wavelength (nm)
R ef
le ct
an ce
(a) (b)
ICC3: Symposium 6: Advances in Electro Ceramics IOP Publishing IOP
Conf. Series: Materials Science and Engineering 18 (2011) 092025
doi:10.1088/1757-899X/18/9/092025
3
EB-PVD [3] and by sputtering [4, 5]. Further improvement in optical
and electrical properties of AZO films is expected by optimizing
the deposition parameters.
4. Conclusions Al doped ZnO (AZO) films were prepared on quartz
substrates by the co-deposition of ZnO and Al2O3 evaporation
sources by EB-PVD. The effect of EB power applied to the Al2O3
(EBAl2O3) source on the structure, morphology and optical
properties of AZO film was investigated. X-ray diffraction
measurement showed that the AZO films were weakly c-axis oriented.
Transmittance of All the AZO films was over 80% in the visible
range. Highest reflectance in the near IR range was obtained at
EBAl2O3 of 5kW. The lowest resistivity of 3.05 × 10-4 cm was
obtained for the film deposited with the EBAl2O3 of 5kW with the
deposition time of 300s.
Acknowledgement This work was entrusted by NEDO as ‘the Project of
Development of Multiceramic Film for New Thermal Insulators’.
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ICC3: Symposium 6: Advances in Electro Ceramics IOP Publishing IOP
Conf. Series: Materials Science and Engineering 18 (2011) 092025
doi:10.1088/1757-899X/18/9/092025
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