Materials Transactions, Vol. 49, No. 7 (2008) pp. 1694 to 1697
#2008 The Japan Institute of Metals
EXPRESS REGULAR ARTICLE
Effects of Annealing Temperature on the Properties
of Strontium Copper Oxide Prepared by Radio
Frequency Reactive Magnetron Sputtering Method
Jui-wen Liu*1 , Shih-chin Lee*2 and Chih-hao Yang*1
Department of Material Science Engineering, National Cheng Kung University, Tainan, Taiwan, R.O.China
Electrical, structural and optical properties of annealed transparent strontium copper oxide (SCO) films were studied in this paper. These
SCO films were first deposited by reactive radio frequency magnetron sputtering technique on glass substrates at room temperature with ultra
highly pure oxygen and a 100 W sputtering power, and then annealed at different temperatures ranging from 373 to 723 K in an atmosphere of
oxygen controlled at 101 Pa. Results showed that the resistivity increased first as the annealing temperature raised from 373 to 473 K, and then
decreased when annealed at 623 K. Carrier density of an annealed film increased from 7:09 1020 cm3 to 1:21 1021 cm3 as the annealing
temperature increased. Results also showed that the resistivity of a SCO film was highly correlated to the carrier mobility. The highest carrier
mobility observed by annealing a SCO film at 623 K was 4:0 105 m2 V1 S1 . The optical transmittance of an annealed SCO film in the
visible range at 550 nm fell between 52.6% and 58.2%. The Hall coefficient measured at room temperature indicated the nature of the annealed
films was p-type. [doi:10.2320/matertrans.MER2008099]
(Received March 26, 2008; Accepted April 30, 2008; Published June 25, 2008)
Keywords: transparent conductive oxide, thin film, electrical property, p-type semi-conductor, annealing temperature
1.
Introduction
Transparent conductive oxides, generally referred as
TCOs, are semi-conducting materials that have extensive
applications in optoelectronics such as panel displays, solar
cells and coatings for anti-static electricity shielding. Common n-type TCOs have good optical and electrical properties,
such as zinc oxide (ZnO), indium tin oxide (ITO) and tin
oxide doped (SnO2 :F)1) which have been extensively studied.
However, there have only been a few studies2–4) done on
transparent p-type TCO semi-conductor devices due to the
lack of available materials.
The difficulties in designing a material with positive holes
that are not localized on oxygen atoms have contributed to
the lack of p-type transparent oxides.5) This localization
phenomenon affects and limits the movement of carriers, i.e.
positive holes in p-type TCOs, and also leads to low carrier
mobility and high resistivity. Nevertheless, some oxides,
such as CuAlO2 ,6–9) CuInO2 ,10) CuGaO2 11) and CuScO2 ,12)
have been developed by so-called ‘‘chemical modulation of
valence bound’’(CMVB).5) The oxides mentioned above
carry the same chemical formula of AMO2 and a preferred
delafossite structure. With the CMVB strategy, an extended
valence band can be formed by introducing covalence into
the metal-oxygen bonding which hence improves the carrier
mobility. These oxides also exhibit p-type conductivity and
good optoelectrical properties.
Another potential candidate for making multi-layer devices is strontium copper oxide (SCO) due to its low
deposition temperature. Bobeico et al. has demonstrated the
SCO films can be deposited at 623 K by the pulse laser
deposition technique (PLD).13) In the previous article,14) the
feasibility of deposition of SCO films with radio frequency
reactive magnetron sputtering at room temperature was
*1Graduate
Student, National Cheng Kung University
author, E-mail: [email protected]
*2Corresponding
proven valid. However, the carrier mobility of a p-type
TCO film is much inferior to that of a n-type TCO film.15)
Undergoing a post-annealing treatment for a sputtered SCO
film might lower the grain boundary density and carrier
scattering and hence further improves the carrier mobility
and electrical property of the sputtered SCO film. In this
paper, the structural, electrical and optical properties of SCO
films annealed at different temperatures were studied.
2.
Experiment Details
SCO films were first deposited by radio frequency (RF)
magnetron sputtering onto microscopic carrier glasses with
an optical refractive index of 1.5. Big pieces of glass were cut
into 1 1 cm2 substrates for follow-up deposition processes.
The deposition rate was 20.5 nm/h, with a 100 W sputtering
power, in an atmosphere of pure oxygen at 4 101 Pa. The
oxygen gas used in this experiment was of 99.999% ultra
high purity. All substrates were ultrasonically cleansed in
acetone and alcohol solution. The organic contaminants were
removed by de-ionized water before the deposition process
began. The SCO target of 99.9% purity was purchased from
GUV Team International Co., Ltd. The X-ray diffraction
patterns indicated that the target was poly-crystallized
SrCuO2 . Prior to the sputtering process, the background
pressure of the vacuum chamber was reduced to below
1:5 104 Pa. There were three sputtering targets in our
sputtering system with target diameter of 76.2 mm. The
substrate rotated at a speed of 10 rpm. The actual time for the
film’s growth was only a quarter of the sputtering duration.
The substrate was kept at room temperature throughout the
sputtering process. Film thickness was measured using an
alpha-step surface profiler (TENCOR MA-1450). The films
were typically 100 nm-thick. Deposited samples were taken
from the same sputtering batch and then annealed at different
temperatures ranging from 373 to 723 K. Annealing treatments were performed in the atmosphere whose oxygen
65
64
Rsistivity, σ / Ω m
63
62
61
60
400
500
600
700
Annealing Temperature, T/K
Fig. 1
2
4
3
2
1
0
300
400
500
600
700
Carrier mobility of SCO films at various annealing temperatures.
Resistivity of SCO films at various annealing temperatures.
-3
14
20
3.1 Electrical and structural properties
Figure 1 shows the film resistivity against annealing
temperature plotted for as-deposited and annealed SCO
films. Resistivities of all samples were measured by a
standard four-point probe technique. The resistivity increased
first as a film was annealed at 373 and 473 K, and then started
to decrease as the annealing temperature reached 623 K. The
changes in carrier mobility of as-deposited and annealed
films are shown in Fig. 2. The mobility of a film decreased
with annealing temperature at 373 and 473 K, and then
started to increase at 623 K. This trend matched the changes
occurring in resistivity between 373–623 K. Figure 3 depicts
the carrier density vs. annealing temperature. It is clear that
the carrier density increased with annealing temperature.
According to previous research work, the increase in the
number of carriers would decrease the carrier mobility due to
the scattering of carriers.14) It is agreed widely that the film
resistivity increases as the carrier mobility decreases in TCO
films. As discussed above, the major difficulty in developing
a p-type TCO is to increase the mobility of carriers in the
film, so one distinctive factor that affects film resistivity is
the variations in the mobility of carriers. However, it was
300
5
-5
Fig. 2
Results and Discussions
5
4
3
2
1
0
1695
Annealing Temperature, T/K
Carrier density, p/10 cm
3.
Carrier mobility, µp / 10 m V s
purity was as in the sputtering process, under a pressure of
101 Pa. All the SCO films were annealed for 3 hours. The
changes in structure with corresponding annealing treatment
variations were measured by a grazing incident angle X-ray
diffractormeter (GIAXRD). A four-point probe (Model RT70, NAPSON, Japan) was employed to measure the sheet
resistivity for all films. The carrier density and mobility of the
films were measured using the Hall effect in Van der Pauw
method. All film thickness values and electrical characteristics were the averages calculated from measurements at
different positions on the samples. Optical transmittance
and absorbance were measured by an ultraviolet/visible/
near-infrared spectrophotometer (UV/VIS/NIR spectrophotometer, Hitachi U4001) within a wavelength range between
200–1100 nm. The transmission percentages were then
calculated from the percentage transmission data of the
corresponding bare substrates.
-1 -1
Effects of Annealing Temperature on the Properties of Strontium Copper Oxide
12
10
8
6
4
2
0
300
400
500
600
700
Annealing Temperature, T/K
Fig. 3 Carrier density of SCO films at various annealing temperatures.
observed that when the annealing temperature reached 623 K
there were larger SrCu2 O2 crystals formed in the film being
annealed.
Figure 4 displays the GIAXRD patterns (2) as a function
of annealing temperature. There were no identical peaks
found among the patterns of the films that had been annealed
at 373 and 473 K. However, the patterns of a SCO film
annealed at 623 K revealed a peak of (211) plane (JCPDS
relative card #38-1178) and indicated that larger conductive
SrCu2 O2 crystals had been created. Crystals of larger sizes
lead to less grain boundaries and reduce the scattering of
grain boundaries. Therefore, the carrier mobility and film
resistivity were improved, similar to what had been reported
from Lee et al.16) The resistivity of the SCO film annealed at
623 K in an atmosphere of pure oxygen was hence further
improved. It was also found that the peak of SrCu2 O2 had
disappeared after annealing at 723 K. When the conductive
SrCu2 O2 phase in the film vanished due to the annealing
process, it resulted in an increase of film resistivity. Similar
results were found in previous research.13) Due to the high
resistivity which typically leads to less practicality, films
annealed at 723 K will be excluded in discussions on optical
properties.
1696
J. Liu, S. Lee and C. Yang
0.02
(e) 723K
As deposited
373K
473K
623K
2
(211)
-1
α hν (cm eV)
Intensity (arb. units)
(d) 623K
0.01
2
(c) 473K
(b) 373K
0.00
(a) As deposited
o
o
20
o
30
o
40
o
50
60
Diffraction Angle, 2θ
2
3
4
Photon energy, E/eV
o
70
Fig. 6
Band gap energy of SCO films at various annealing temperatures.
Fig. 4 GIAXRD patterns of SCO films at various annealing temperatures:
(a) as-deposited; (b) 373 K; (c) 473 K; (d) 623 K; (e) 723 K.
The Eg evaluated slightly shifted from 3.6 to 3.65 eV as the
annealing temperature increased. This increase could have
been caused by the increase of carrier density in annealed
SCO films. During the annealing process, excess oxygen
could have been introduced into SCO films and provided
additional carriers.20) This shift in the band gap was as
expected.
100
Transmittance (%)
90
80
70
60
50
As deposited
373K
473K
623K
Blank glass
40
30
20
10
0
200
400
600
800
1000
1200
Wavelength, λ /nm
Fig. 5
Transmittances of SCO films at various annealing temperatures.
3.2 Optical properties
Figure 5 shows the transmission spectra of films annealed
at different temperatures with oxygen. The transmission
spectra of the films were measured for wavelengths between
200–1100 nm. The transmittance of an annealed film was
improved as the annealing temperature increased. The
transmittance in the visible range at 550 nm was slightly
higher from 52.6% to 58.2%. The defects formed during the
sputtering deposition process could have hindered the transmission of light. The annealing process could provide
sufficient energy for atoms to migrate and by which less
defects could be obtained. Thus the annealing process could
help reduce defect density and improve transmittance.17–19)
It is a general rule that the absorption coefficient, , in
relation to the photon energy, h, is given by
2
ðhÞ ¼ Aðh Eg Þ
Where A is a constant and Eg is the optical energy gap.
Figure 6 is a plot of ðhÞ2 vs. photon energy (h) for asdeposited and annealed SCO films. The optical band gap (Eg )
was obtained from the linear portion where h was zero.18)
3.3 Hall measurements
To identify the major conducting type of the annealed
films, Hall measurements were taken at 298 K. The measured
Hall coefficients of these films were all positive values. The
Hall coefficients of annealed SCO films fell in the range of
9:28 102 to +5.46. The conducting type of the films
was affirmatively p-type.
4.
Conclusion
Annealed SCO films exhibited that the resistivity of a SCO
film could be improved with a proper annealing temperature.
The demerits of low carrier mobility strongly affect the
resistivity of a SCO film but which can be reduced by a
common annealing technique. Analysis of GIAXRD results
showed that larger crystals of SrCu2 O2 were generated on the
SCO film when annealed at 623 K, and lowest resistivity of
1.63 m was observed due to the improved carrier mobility.
However, an annealing temperature above 723 K could
damage the conductive SrCu2 O2 phase and the conductivity
of a SCO film. The carrier density and optical transmittance
were both improved as the annealing temperature increased.
Hall measurements verified all the SCO films to be p-type.
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