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					                             IPASJ International Journal of Electrical Engineering (IIJEE)
                                                                         Web Site: http://www.ipasj.org/IIJEE/IIJEE.htm
A Publisher for Research Motivation........                                        Email: editoriijee@ipasj.org
Volume 1, Issue 6, December 2013                                                                 ISSN 2321-600X


    Preparation of ITO thin film by Sol-Gel method
                                         Harith Ibrahem1, Mariam Moghdad2
                                                   1,2
                                                     University of Baghdad




                                                         ABSTRACT
Highly transparent and conductive thin films ITO have been prepared on glass substrate using spray pyrolysis method. The
ITO deposited at substrate temperature 400⁰ C their thickness was approximately (150 nm), and at different doping
ratios(5,10,15,20) of Sn Wt.% annealed at ( 500-550 )⁰C for one hour . The structural, optical and electrical properties for all
films were studied. X-ray diffraction data showed that all films have polycrystalline nature cubic for ITOwith major reflex
along (222) plane .The absorbance and transmittance spectra data for the films were recorded at room temperature in the
wavelength range (200-1100nm), which were used to determine the optical properties of the films. All films are highly
transparent (greater than 91%) in visible region of electromagnetic spectrum, the direct band gap was in the range (3.6-3.88 )
eV.
Keywords: ITO, Sol-Gel, Thin film

    1. INTRODUCTION
TIN-DOPED indium oxide films, commonly referred to as ITO films, are n-type, degenerate semiconductors with wide
band-gap, which have high transmittance, high infrared reflectance, good electrical conductivity, excellent substrate
adherence and chemical inertness hence [1],[2] they have been extensively used as coating electrodes for
optoelectronics such as organic light-emitting diodes (OLEDs), liquid crystal displays (LCDs), solar cells, etc. in recent
years, ITO films are used extensively and play the leading role in the field of display ,ITO films need additional fine
patterning when they are used in the concrete integrated apparatus so as to obtain specified fine patterns It is difficult to
obtain a fine pattern at a sub-nanometer scale through conventional chemical etching methods, such as wet etching, dry
etching, etc. [3],[4].ITO particles have been obtained by various techniques, such as thermal hydrolysis, thermal
decomposition, spray pyrolysis, and coprecipitation However, these methods have not proved very successful in
obtaining nanometric particles due to the high degree of agglomeration presented by the oxide. Other methods that
have been used include direct precipitation, co-precipitation, and hydrothermal methods, but these do not allow for
control of the particle size distribution new techniques for obtaining metal oxides with controlled morphological
characteristics have recently been developed Among these techniques, the sol-gel method stands out for its advantages,
such as the possibility of obtaining high purity materials using simple equipment Another advantage of the sol-gel
method is the ability to process the material in the form of thin films, which, in the particular case of ITO, is the only
technique that allows for deposition on relatively large substrates at a feasible cost,the sol-gel methodology comprises
wet and dry routes one of the dry routes is the sol-gel method for nonhydrolytic gels (NHG), developed in the 1990s as
an alternative to the hydrolytic sol-gel route one of the advantages of the NHG method is the greater homogeneity
achieved for heterometallic oxides, not easily achieved by the hydrolytic sol-gel route due to the different reactivities of
their alkoxidesPromising results have been obtained through the NGH method, which not only allows for greater
reproducibility but also involves the use of less expensive precursors, thus representing lower costs and better quality
[5],[6].
 Although the sol-gel process is one of the candidates for the fabrication method of ITO films, a subsequent heat-
treatment at a relatively high temperature is needed for crystallization and removal of residual organic and hydroxyl
groups recently, various irradiation techniques using energetic ions and ultraviolet (UV) light were applied to
modification of sol-gel films as an alternative to conventional heat-treatments. These studies revealed that collisional
atomic displacements and electronic excitations could densify sol-gel derived films at low temperatures [7].
 some previous works showed that the best performances, can only be obtained for an annealing temperature around
550 °C. The lowering of this high sintering temperature still remains a challenge as regards the use of the sol–gel
process for the coatings on plastic substrates or smart textile the aim of our study is to investigate the electrical
properties, optical properties of the sol–gel derived ITO films as a function of metal concentration of the sol [8],[9].



Volume 1, Issue 6, December 2013                                                                                     Page 18
                           IPASJ International Journal of Electrical Engineering (IIJEE)
                                                                     Web Site: http://www.ipasj.org/IIJEE/IIJEE.htm
A Publisher for Research Motivation........                                    Email: editoriijee@ipasj.org
Volume 1, Issue 6, December 2013                                                             ISSN 2321-600X

 ITO is a wide-gap, degenerated semiconductor where electrons are the major charge carriers the Sn atoms are
considered to substitute for In, without ordering the difference of valence between In3 and Sn4 results in the donation of
a free electron to the lattice for most applications, the ITO films must be as conducting as possible, which requires a
high carrier density (n c) and a high carrier mobility (µ) [10].

    2. Experimental Section
 To prepare ITO thin film ,firstly we prepared Indium chloride solution(sol I) which prepared by dissolving (0.5g of
InCl3 molecular weight 221.18 g/mole ) in (25ml) of methanol the solution was stirred for half hour on magnetic stirr
.the second solution is the tin chloride solution (sol II) (SnCl2.2H2O molecular weight 225.6 g/mole) which is used as
a dopant solution depending on the weight of the tin chloride in (15ml)of methanol where the doping ratio of the tin
with Indium was token from the following equation [11]                (5,10,15,20)% The last solution was also stirred
for half hour on magnetic stirr , then it add gradually to the Indium chloride solution. The final solution stirred again
at lest for 20 minuet a few drops of Hydrochloric acid were add as a catalyst, this solution called ITO solution.this
solution leaves for one day to be gelation. In this work a microscopic glass and quartz slides of dimension, (25 25 1)
mm are used as a substrate .this substrate must be thoroughly cleaned before deposition, In the present study the
cleaning of the substrates was done by cleaning the substrates with deionized water, then they immersed in the ethanol
and put in an agitated ultrasonic bath at least for half hour , finally the substrates were dried by lens paper and dry
air.Spray pyrolysis, which is a process in which a thin film is deposited by spraying a solution on a preheated substrate
, where the constituents react to form a chemical compound. the substrate temperature reach to (400-450 ) ⁰C in this
temperature the solution was sprayed on the substrate then left to dry for about 20 minute , this process was returned
twice to get the suitable thickness . The produced film from the above process must be annealed to get the completely
oxidization process . so they put into the oven at temperature about (500-550) ⁰C for glass and at (600) ⁰C for quartz
substrates ,for one hour and then left in the oven until it reach to the room temperature . Thefilm crystalline structure
was investigated by a used X-ray instrument is supplied by SHIMADZU 6000 X-ray diffractometer (illustrated in figure
2- 7) using Cu K radiation (1.5406 Å) in reflection geometry. A proportional counter with an operating voltage of 40
kV and a current of 30 mA is used. XRD patterns are recorded at a scanning rate of (0.08333°) s-1 in the (2 ) ranges
from (20° to 60°). The SEM study carried out by (Hitachi FE-SEM model S-4160, Japan) in University of Tehran, The
optical properties of the prepared thin films deposited on quartz slides which includes the transmittance spectra are
studied over the wavelength range (200-1100nm) by using Shimadzu UV/Visible recorder spectrometer model (Centra
5).

    3. RESULT AND DISCUSSION
The X-ray diffractions pattern of the ITO thin films for the four doping ratio (5,10,15,20) wt. % of Sn with In are
shown in figure(1) The analyses evidence at low doping ratio the film has low crystallinaty comparing with other films
as illustrated in figure (3-1-a); also we can observed from experimental data appearing of peaks at (222) ,(400) and
(440) which refers to cubic structure of In2O3 the intensity of this peaks are very low ; this means that the
interaction did not reach the level of saturation which are compensate tin atoms within the structure of In2O3 [12] . By
increasing doping ratio above (5%) the films begin to take a polycrystalline nature [13, [14]. The growing of the peaks
(400),(440) of cubic In2O3 crystal also can be observed but with lower intensity than (222) plane [15].No phases
corresponding to tin or other tin compounds were detect showing that the Sn was in solution in the In2O3. As a result
the crystallinaty increase because substitutional replacement of indium by tin in the lattice took place.




                Figure (1) X-ray diffraction pattern of ITO films prepared at (5,10,15,20) wt. % of Sn.



Volume 1, Issue 6, December 2013                                                                                Page 19
                           IPASJ International Journal of Electrical Engineering (IIJEE)
                                                                     Web Site: http://www.ipasj.org/IIJEE/IIJEE.htm
A Publisher for Research Motivation........                                    Email: editoriijee@ipasj.org
Volume 1, Issue 6, December 2013                                                             ISSN 2321-600X

The SEM pictures of the ITO film which it prepared by chemical spray method on a glass substrate at temperature
(400ºC)and annealed for one hour at(500-550) ºC for the doping ratio of (20) Wt.% of Sn with In are demonstrated in
the regions (a,b,c,d,e,f) where all regions showing the identity of the film except region (e) that explain a patch
resulting from sudden spray of the solution on the hot substrate . The mean average value of the grains was found to be
about (60nm) for regions (a,b,d) , (1µm) for (c) ,(0.2µm) for(e) and (10 µm) for(f),this convert from the nano to the
micro scale associated with the increasing the crystallization of the film so that in the region (d) we can see the cubic
shapes which for the In2O3 crystal [16]. All regions are approximately homogenous so this will more affects on the
optical and electrical properties [17],[18].




Figure 2 UV-VIS transmittance spectra of the ITO/quartz films with thickness of (150) nm and different doping ratios
                                                  of Sn Wt. %




Figure (3) shows the optical transmittance as a function of wave length in the range (200–1100) nm) with different
doping ratios of (5,10,15,20) Wt. % Sn of thickness (150) nm. This reveals that the transmittance depends on the Sn
doping . The maximum transmittance observed at 5% equal to (96.7%) in the wavelength (600)nm , while for
10%,15% films the maximum transmittance equal (92%) and (83.2%) at 20% doping Sn . The behavior of the
transmittance spectra is opposite completely to that of the absorption spectra. In general, we can observe from this
figure that transmittance decreases with increasing of doping ratios of Sn and this may be due to impurities and
decreases in the hemogensaty of the film which means an increase in the absorption. The films were found to be highly
transmittance in the visible wavelength region with an average transmittance in excess of 91% [19],[20],[21].

The energy gap values depend in general on the films crystal structure, the arrangement and distribution of atoms in
the crystal lattice; also affected by crystal regularity. The optical energy gap (Eg) was derived assuming a direct
transition between the edge of the valence and conduction band. The optical energy gap values (Egopt) for ITO films
have been determined by using Tauc formula by plotting the relations of (αhν)2 vs E (eV) for direct energy gap as
shown in figure (3-6).The energy gap is obtained from intercept of the extrapolated linear part of the curve with the
energy axis, the direct band gap of the ITO films increases from (3.7-3.88)eV when doping ratio increases from 5%
Wt.Sn to 20%Wt.Sn . We can see that at 5% optical energy gap approximately equal to 3.7eV,for 10% equal to
3.6eV,for 15% equal to 3.7eV and for 20% doping Sn equal to 3.88eV. Mean that the lowest optical energy gap in the
doping ratio of 10 % Sn in this ratio the mobility increases because of the increase in the carrier concentration and this
led to narrowing optical band gab because of the modification of structure . So the shift in the optical energy gap to


Volume 1, Issue 6, December 2013                                                                                Page 20
                           IPASJ International Journal of Electrical Engineering (IIJEE)
                                                                     Web Site: http://www.ipasj.org/IIJEE/IIJEE.htm
A Publisher for Research Motivation........                                    Email: editoriijee@ipasj.org
Volume 1, Issue 6, December 2013                                                             ISSN 2321-600X

approximately 3.88eV for 20% Sn attributed to the decreases in the carrier concentration there for in this ratio the
mobility will decreases because of increasing optical energy gap[22].Our results are nearly in agreement with
[23],[24],[25].




    4. Conclusions
The analyses evidence at low doping ratio the film has low crystallinaty comparing with other films .By increasing
doping ratio above (5%) the films begin to take a polycrystalline nature. Appearing of peaks at (222) ,(400) and (440)
which refers to cubic structure of In2O3 the intensity of this peaks are very low at low doping ratio and it increase
with increase doping ratio.No phases corresponding to tin or other tin compounds were detect showing that the Sn was
in solution in the In2O3.The SEM pictures for the doping ratio of (20) Wt.% of Sn with In, all regions are
approximately homogenous so this will more affects on the optical and electrical properties. The transmittance
decreases with increasing of doping ratios of Sn from(96.7%-83.2%) this may be due to impurities and decreases in the
hemogensaty of the film which means an increase in the absorption The direct band gap of the ITO films increases
from( 3.7-3.88) eV when doping ratio increases from 5% Wt.Sn to 20%Wt.Sn , the lowest optical energy gap in the
doping ratio of 10 % Sn(3.6) eV, in this ratio the mobility increases because of the increase in the carrier concentration
and this led to narrowing optical band gab because of the modification of structure of the ITO films.

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Volume 1, Issue 6, December 2013                                                                                Page 21
                           IPASJ International Journal of Electrical Engineering (IIJEE)
                                                                    Web Site: http://www.ipasj.org/IIJEE/IIJEE.htm
A Publisher for Research Motivation........                                   Email: editoriijee@ipasj.org
Volume 1, Issue 6, December 2013                                                            ISSN 2321-600X

[14] M.J. AlamU, D.C. Cameron Optical and electrical properties of transparent conductive ITO thin films deposited
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Volume 1, Issue 6, December 2013                                                                             Page 22

				
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Description: IPASJ International Journal of Electrical Engineering (IIJEE) Web Site: http://www.ipasj.org/IIJEE/IIJEE.htm A Publisher for Research Motivation........ Email: editoriijee@ipasj.org Volume 1, Issue 6, December 2013 ISSN 2321-600X