4th International Science, Social Science, Engineering and Energy Conference
11th-14th December, 2012, Golden Beach Cha-Am Hotel, Petchburi, Thailand
I-SEEC 2012
www.iseec2012.com
Optical and surface characterization of indium tin oxide films
prepared by glancing angle deposition
K. Aiempanakita,e1, S. Kalasunga, D. Daengbutdeea , M. Horprathumbb,
P. Eiamchaib, K. Limwicheanb
b
a
Department of Physics, Faculty of Science and Technology, Thammasat University, Prathumthani, 12120 Thailand
Optical Thin- Film Laboratory, National Electronics and Computer Technology Center, Pathumthani, 12120 Thailand
e1
akamon@tu.ac.th,
Abstract
In this work, indium tin oxide (ITO) films were deposited by using e-beam evaporation on silicon wafer
and glass substrates by employing the glancing angle deposition (GLAD) technique. The GLADs of ITO
films were in the range of 45 to 85 degrees. The optical properties and surface morphology of ITO films
were characterized by spectrophotometer and field emission scanning electron microscopy (Fe-SEM),
respectively. The increasing in the angles produces inclined porous columnar nanostructures due to the
atomic shadowing effect. It was found that the optical transmission characteristics for the wavelength
range of 500 – 900 nm were enhanced (T > 90%) by nanostructure of the ITO films. This resulted in the
absence of the oscillation of light. The ITO films deposited at 85 degree demonstrated the best
nanostructure.
Keywords: Indium tin oxide; e-beam evaporation; glancing angle deposition; nanostructure
1. Introduction
Indium tin oxide (ITO) films are transparent conductive oxide, and show a high transmittance in
the visible spectral range. Moreover, a high reflectance in the infrared (IR) range is related to electrical
conductivity, which this property was depending on the effect of oxygen vacancy with annealing film [1].
The optical and electrical properties of ITO films from the fact that they are n-type degenerated
semiconductor with a wide band gap (Eg  4 eV) [2-3]. From these properties, ITO films are widely used
in many applications, such as cathode in a transparent organic light emitting diode (OLED) [4,5], gas
sensors for detection of ethanol vapors [6], electrode in solar cell [7,8] and transparent coating for solar
energy heat mirrors [9]. The ITO films have been deposited by many techniques such as sputtering,
evaporation, chemical vapor deposition, sol-gel and spray pyrolysis. Among the techniques that are
available for fabricating ITO films, magnetron sputtering is one of the more versatile techniques for ITO
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films preparation. This technique is one of the effective methods for good films performance with high
coating rate and good adhesive film. However, the ITO films prepared at room temperature using
conventional magnetron sputtering have a relatively high electrical resistivity (≥110-3 Ω.cm) [10,11].
High-quality ITO films are commonly obtained by annealing at high temperature (>300°C) in vacuum
after film deposited or substrate heating during film preparation. The annealing can lead to material
crystallization, reducing the crystalline structure defect, and increasing oxygen vacancies in ITO films,
resulting in high transparent and conductive films [12-15]. After annealing, the electrical resistivity of
ITO as low as 2-310-4 Ω.cm and optical transmittance in the visible spectrum above 90% have been
reported for thickness around 300 nm [12-14]. On the other hand, improving the properties of ITO films
for flexible optoelectrical applications has been used ITO/metal/ITO (IMI) multilayer structures, which
have a lower resistivity than single-layer ITO films for the same thickness. Silver (Ag) is a first choice
because it has the lowest resistivity of all materials which is below 210-6 Ω.cm at room temperature for
the bulk material [16]. However, the silver film is easier to degrade with moisture. To improve the optical
transmission can be produced with porous structure. One method for fabrication porous structure is the
glancing angle deposition (GLAD) method, also known as oblique angle deposition is a method to grow
nanorod structures. In this work, we want to develop the optical transmission of ITO films by e-beam
evaporation technique with glancing angle deposition.
2. Experimental details
The ITO films were prepared by GLAD technique with the substrate tiled at the angle of 0, 45,
55, 65, 75, and 85 degree using e-beam evaporation on glass substrate at room temperature with ITO
pieces ( size 1/8 × 1/4, 99.99% ITO composite of mixture 90 wt % In2O3: 90 wt % SnO2). The glass
substrates were ultrasonically cleaned in an acetone and deionized water before depositions. The
evaporation processes were performed in O2 gas (99.99% in purity) which was controlled by mass flow
controller. The O2 gas flow rate was fixed at 8 sccm (standard cubic centimeter per minute). The distance
between the ITO pieces and glass substrate was 10 cm, and the power of e-beam evaporation was kept
constant at voltage of 120 V and current of 1 A. A cryogenic pump coupled with a rotary pump was used
to achieve a base pressure below 110-6 Torr before introducing O2 gas and working pressure of about
2×10-6 Torr. The film preparation conditions as presented in Table 1.
The optical transmittance spectra in UV-Vis-NIR region and the electrical properties of ITO
films were investigated by a spectrophotometer in the wavelength of 300 – 1200 nm. The surface
morphology and film thickness were examined by using field-emission scanning electron microscopy
(FE-SEM).
Table 1 Film preparation conditions I.
Evaporation type
ITO pieces (size 1/8 × 1/4)
Base pressure
Working pressure
Voltage
Current
Oxygen gas flow rate
Coating time
Substrate
Substrate tilted
Evaporation source
99.99% ITO (90 wt % In2O3: 90 wt % SnO2)
 110-6 Torr
2.0×10-6 Torr
120 V
1A
8 sccm
25 min.
silicon wafer
0, 45, 55, 65, 75, and 85 degree
Author name / Procedia Engineering 00 (2011) 000–000
3. Result and discussion
Fig. 1 shows cross-sectional FE-SEM images of ITO films deposited on substrate tilted at angle
(a) 0, (b) 45, (c) 55, (d) 65, (e) 75, and (f) 85 degree. The film structure changed from dense films to
porous columnar nanostructure as a function of increasing angle of substrate tilted. The ITO films
deposited on substrate tilted at the angle of 75 and 85 degree showed dominate porous columnar
nanostructure due to the atomic showing effect [17].
The angle of substrate tilted affected on film thickness when the same coating time, the films
thickness and deposition rate of ITO films were shown in Table. 2.
0 degree
(a)
45 degree
(b)
55 degree
(b)
65 degree
(d)
75 degree
(e)
85 degree
(f)
Fig. 1. Cross-sectional SEM images of ITO films deposited on substrate tilted at angle
(a) 0, (b) 45, (c) 55, (d) 65, (e) 75, and (f) 85 degree.
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Table. 2. Film thickness and deposition rate.
Substrate tilted
(degree)
0
45
55
65
75
85
Film thickness
(nm)
282
402
374
337
285
139
Deposition rate
(nm/min)
11.28
16.08
14.94
13.44
11.40
5.56
In this work, we focused on the film structure between dense film and porous columnar
structure, which were deposited on substrate tilted at the angle of 0 and 85 degree, respectively.
Generally, the optical property is strongly depending on the film thickness. Therefore, the ITO films for
both cases were prepared with the same film thickness of 200, 300, and 500 nm. The film preparation
conditions were shown in Table 3.
Table 3 Film preparation conditions II.
Evaporation type
ITO pieces (size 1/8 × 1/4)
Base pressure
Working pressure
Voltage
Current
Oxygen gas flow rate
Deposition rate
Film thickness
Substrate
Substrate tilted
Evaporation source
99.99% ITO (90 wt % In2O3: 90 wt % SnO2)
 110-6 Torr
2.0×10-6 Torr
120 V
1A
8 sccm
11.28 and 5.56 nm/min.
200, 300, and 500 nm
Glass slide
0 and 85 degree
Fig. 2 shows the optical transmittance spectra in wavelength ranges of 300 – 1200 nm for ITO
films with with the thickness of 200, 300, and 500 nm deposited on glass substrate tilted at the angle (a) 0
and (b) 85 degree. It could be seen that the substrate tilted enhances the optical transmittance of ITO films
within the visible range.
In Fig. 2(a), the ITO films were shown the optical transmittance spectra with peak of oscillating
light by film thickness. The increasing film thickness affected to increase peak oscillating [18]. In Fig.
2(b), the ITO films for all samples showed high %T about 90% in the visible region with absence of the
oscillation of light. Generally, the mechanisms of transmission in the visible region of the ITO films
depend on grain size and stoichiometry which affected the scattering mechanism in polycrystalline ITO
films as grain boundary scattering [19]. However, in this studied the ITO films of both were deposited in
the same condition, these films were might the same stoichiometry. Therefore, the strongly effect of the
ITO films for both cases were the film structure. Improving the optical property of ITO films succeed
with GLAD technique. The structure of films consisted of a void which enhanced the optical
transmittance.
Author name / Procedia Engineering 00 (2011) 000–000
100
90
Transmittance (%T)
80
70
60
50
40
30
200 nm
300 nm
500 nm
20
10
0
400
600
800
1000
1200
Wavelength (nm)
(a)
100
90
Transmittance (%T)
80
70
60
50
40
200 nm
300 nm
500 nm
30
20
10
0
400
600
800
1000
1200
Wavelength (nm)
(b)
Fig. 2. Transmittance spectra of ITO films with the thickness of 200, 300, and 500 nm
deposited on glass substrate tilted at the angle (a) 0 and (b) 85 degree.
4. Conclusions
Indium tin oxide (ITO) films were deposited by using e-beam evaporation on silicon wafer and
glass substrates by employing the GLAD technique. The structure of ITO films tended to porous
columnar nanostructures with increasing the angle of substrate tilted increased due to atomic shadowing
effect. The optical transmittance of ITO films were enhanced (T > 90%) by GLAD technique and these
films were absence of the oscillation of light. The ITO films deposited at 85 degree demonstrated the best
nanostructure.
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Acknowledgements
The authors would like to thank the Nation Electronics and Computer Technology Center
(NECTEC) for providing the experimental facilities and thank the Faculty of Science and Technology,
Thammasat University for financially supporting this research.
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