Photonic crystals in biology

Poster Session, Tuesday, June 15
Theme A1 - B702
Indium Tin Oxide (ITO)
Transparent Conductive Thin Films Elaborated by Sol-Gel Routes
M. Tümerkan Kesim, Hakan Yavaş and Caner Durucan*
Department of Metallurgical and Materials Engineering, Middle East Technical University, Ankara 06531, Turkey
Abstract—Commercial grade ITO thin films are widely produced by sputtering. On the other hand, aqueous based coating
techniques such as sol-gel, which is simple and easily applicable in the economic sense, has the potential for developing ITO
thin films. In this study, sols prepared from indium and tin salts are employed to form ITO thin films on soda lime silica float
glass. The effects of processing parameters such as film deposition parameters, the number of coating layers and heat treatment
on the morphological, electrical, and optical properties of ITO coatings are reported.
Indium tin oxide (ITO) thin films have been used in many
optoelectronic applications such as display panels, solar cells,
touch screens and electrochromic devices. Among different
processing methods, sol-gel approaches offer several
advantages for preparation of ITO films. These include
feasibility in coating large area substrates, low equipment cost
and ease in controlling chemical homogeneity and tin-dopant
incorporation [1-3]. Sol-gel derived ITO films can be prepared
by using organic metal alkoxide precursors or inorganic metal
salts. Organic based routes offer higher chemical homogeneity
and are also advantageous in achieving desired film thickness
(in the order of 200-250 nm) by single coating operation. But,
the need for highly expensive starting chemicals and poor
storage stabilities of organic sols limit industrial scale
production.
In this study, sol-gel processing routes for developing ITO
thin films utilizing inorganic precursors has been established.
The sols were prepared from InCl 3 .4H 2 O and SnCl 4 . The
solvents were ethanol (EtOH) and acetylacetone (AcAc)
which may also serve as chelating agent [4]. ITO sol
preparation was achieved by dissolving 8.00 g of InCl 3 4H 2 O
in 45 mL AcAc at 25 C followed by refluxing at 60 C for 3 h.
Meanwhile, 0.33 mL SnCl 4 were mixed with 4.50 mL ethanol
at 25 C. After complete homogenization of In-solution, two
solutions were mixed under stirring. Final coating sols were
aged for at least 2 days prior to coating and then were
deposited on precleaned glass substrates by spinning. In case
of multiple coating operations 10 min 150 C air drying was
performed in between the subsequent operations. Finished
samples were heat treated in air for 1h at 600 C.
The characterization of the films was performed by X-ray
diffraction (XRD) and scanning electron microscopy (SEM)
examinations. The performance assessments were achieved by
UV-Vis spectroscopy for examining the optical properties and
by four-probe conductivity measurements.
XRD analyses revealed that tin-incorporated indium oxide
films were obtained after heat treatment at 600 C. The SEM
micrographs in Figure 1 exhibit the effect of post coating heat
treatments on film formation behavior. The films exposed to
open air cooling were severely cracked as shown in Figure 1a.
However, a firm film build up can be achieved for coatings
formed by furnace cooling. The micrograph in Figure 1b
shows the representative detailed microstructure of the sol-gel
derived ITO films imaged from pristine region of a 4-layered
film heat treated at 600 C followed by furnace cooling. The
ITO crystals vary in size (10-40 nm) with an average size of
around 20±5 nm.
(a)
5 m 250 nm
Figure 1. (a) SEM micrographs of microcracked ITO coating (4-
layered) heat treated at 600 C for 1h followed by air cooling, and (b)
the details of the microstructure of furnace cooled coating with
similar processing history.
Figure 2a shows that as the film thickness increases from a
single layer to 10 layers, the resistance decreases 2 orders of
magnitude, reaching to value of several thousands ohms/sq.
This value is comparable with the resistance reported for
commercial quality sputtered ITO films. However, the film
thickness also greatly changes the transmittance. As shown in
Figure 2b, the transmittance of the films produced by 1-, 4-
and 7-step coating operations are comparable and are in the
range of 90±5%, and there is only slight reduction in visible
light transmittance with increasing film thickness. However,
the transmittance remarkably decreases for the film produced
by 10 coating operation, reaching to a value of 70%.
Figure 2. (a) Variation in sheet resistance and (b) the visible spectral
transmittance of ITO coatings as a function of number of coating
operations.
In summary, ITO films were prepared by sol-gel using lowcost
non-alkoxide precursors. A comparison of resistivity and
transparency has been performed for the films obtained by
multiple spin coating operations. The properties of the films
are promising and potentially applicable for coating of large
area substrates.
This work is supported by METU-BAP-03-08-2010-04.
*corresponding author: cdurucan@metu.edu.tr
[1] R. B. Hadj Tahar et al., J. Appl. Phys. 82, 15 (1997).
[2] Y. Djaoued et al., Thin Solid Films 293, 108 (1997).
[3] S. R. Ramanan et al., Thin Solid Films 389, 207 (2001).
[4] H. Uchihashi et al., J. Ceram. Soc. Jpn. 97, 396 (1989).
(b)
6th Nanoscience and Nanotechnology Conference, zmir, 2010 409