CODEN (USA): IJCRGG, ISSN: 0974-4290, ISSN(Online):2455-9555 Vol.11 No.09, pp 298-307, 2018
Abstract : Tin doped Indium Oxide (ITO) thin films with different concentration of tin oxide (5%, 10%,70%) were prepared on glass substrate using dip coating sol gel method. The films were single phase with bixbyte structure. Micro structural parameters were determined from the XRD data. The crystallite size decreased with increase of tin concentration. From EDXA spectrum SnO2 content were presented in the films with different percentage. Surface roughness increased from 0.20 nm to 1.25 nm as the tin concentration decreases. The films showed 85% transmission and transport properties of ITO were studied and these values are reported. The room temperature PL spectra of the samples formed at different concentration of SnO2. From Raman spectrum the vibration mode appeared at 144,175 and 584 cm-1 in the ITO spectrum. These properties are suitable as transparent electrode for DSSC and sensors application. Keywords : SnO2 doped Indium oxide, thin films, sol gel, Transport properties, PL spectrum.
Transparent Conducting Oxide (TCO) films have been widely used for a variety of optoelectronic devices [1-2], energy efficient windows [3], Light Emitting Diodes (LEDs) [4-6] Flat Panel Displays (FPD) [7], and storage-type cathode ray tubes [7], solar cells [8], gas sensors [9], photo catalysts [10], and surface layers in electroluminescent applications [11]. Among the existing TCOs, Sn doped In2O3 (ITO) is one of the most frequently used material because of its unique characteristics such as low resistivity, high optical transmittance over the visible wavelength region, excellent adhesion to substrates and chemical stability. ITO is an n type semiconductor with a band gap between 3.5 and 4.3 eV at room temperature with bixbyte structure [12]. In order to obtain optimal characteristics, i.e. high transparency in visible region and high reflectance in near-IR region, parameters such as film thickness, substrate temperature, doping type and its amount, annealing temperature and the other deposition conditions have to be optimized. Surface coatings can be produced by variety of techniques such as evaporation [13], sol gel technique [14], pulsed laser deposition [15] and electron beam evaporation [16].Although several synthesis and processing methods have been employed for making thin-films of ITO, In this Paper ITO films deposited by the dip coating sol gel technique, and the results are reported.
G. Ramanathan et al /International Journal of ChemTech Research, 2018,11(09): 298-307.
G. Ramanathan et al /International Journal of ChemTech Research, 2018,11(09): 298-307.
Tin doped Indium Oxide (ITO) thin films preparation was given in our earlier paper [17]. After preparation the films formed on the surface was dried in air oven for 15 min followed by different annealing temperatures (350 -450 oC). The films thickness was measure by profilometer in the range of 450 nm to 950 nm with increasing temperature. Investigations of the microstructure were carried out an X-ray diffractometer. The surface morphology of the ITO films was examined by AFM. Optical transmission spectra, Photoluminescence studies and transport properties were studied by four probe method.
Fig.1. shows the XRD patterns of the ITO films of different composition(5%,10%,70%) formed at different temperatures. The samples have the same cubic, bixbyite structure, of In2O3, (JCPDS 6-0416). Also, none of the spectra corresponding to higher annealing temperatures indicate any characteristic peaks of Sn, SnO, SnO2, which indicates that the Sn atoms are probably incorporated substitution alloy into the In2O3 lattice [18,19]. Fig.2. shows the variation crystallite size verse % of SnO2 concentration, the mean crystallite size (38 nm) can be calculated by Debye’s Scherer formula from the line broadening of the (2 2 2) reflection. It is good agreement with AFM observations, a progressive reduction of grains size has been found.
The lattice constant is observed to increase with the content of Sn4+ under the solubility limit. It is likely that the
4+3+ 3+
solute Snions substitute at Insites in the In2O3-SnO2 solid solutions. Because the ionic radius of Inis
4+ 4+
0.080 nm and that of Snis 0.069 nm [20], a slight increase in lattice constant by Snsubstitution appears to
4+ 3+
be somewhat counter-intuitive. However, it should be noted that Snhas different valences than the host Inand charge compensation must be fulfilled during the Sn4+ substitution. The insertion of O2-in the vacant sites is one possible mechanism to compensate this charge unbalance. Insertion of O2-in the vacant sites will increase the lattice constant of the bixbyite structure because of the mutual repulsion between O2-ions, and thus the total lattice constant will be affected by the both Sn4+ substitution (contraction) and the O2-insertion (expansion) effects. The slight increase in lattice constant by Sn4+ addition in In2O3-SnO2 solid solution supports the charge
-4+ 3+
compensation by the O2 insertion into O vacancy sites during the Snsubstitution to the Insites. The crystals defect parameters like microstrain and dislocation density show a decreasing trend with decrease in tin concentration. At low tin concentration, both the microstrain and dislocation density are minimum, which reveals the reduction in the concentration of lattice imperfections leading to preferred orientations [13]. The microstrain and dislocation density were calculated using the following relations.
βcosθ/λ = 1/D + ε sinθ/λ ---------------(1)
The plot of βcosθ/λ vssinθ/λ allows us to determine strain (ε) from slope of the graph. The dislocation density (δ) was obtained from the following relation
δ = 15ε /(aD) ----------------------------------(2)
The strain, lattice parameters, dislocation density and grain size are shown Table.1.
G. Ramanathan et al /International Journal of ChemTech Research, 2018,11(09): 298-307.
Fig.1. Shows X-ray diffraction pattern of 5 %, 10%, 70% tin oxide in ITO films formed at different temperatures (a) 350°C (b) 400°C (c) 450°C
G. Ramanathan et al /International Journal of ChemTech Research, 2018,11(09): 298-307.
The EDS spectra taken on samples of different composition are shown in figure.3 it confirms the presence of Indium, Tin, Oxygen elements and films thickness are listed in the table 2
Table – 1. Micro structural parameters of ITO films with different amounts of Tin oxide
Concen.of tin oxide (%) | Lattice parameters a (Å) | grain size (nm) | Strain (x 10 -3 ) | Dislocation density 1013 lines m -1 |
---|---|---|---|---|
5 | 10.120 | 48 | 4.56 | 1.41 |
10 | 10.130 | 38 | 4.69 | 1.83 |
70 | 10.139 | 7 | 4.93 | 10.21 |
Table-2 shows atomic percentages of ITO films with different amounts of Tin oxide
%SnCo ncen. | Indium | Tin | Oxygen | Thickness (nm) |
---|---|---|---|---|
5 | 33.5 | 1.5 | 65 | 500 |
10 | 31.4 | 2.1 | 66.5 | 530 |
70 | 26 | 4.4 | 69.6 | 890 |
Fig.4.exhibits the Atomic Force Micrographs of ITO films formed at 450°C with different concentrations of tin. It is observed that the grain size decreases with increase of tin concentration. This is
G. Ramanathan et al /International Journal of ChemTech Research, 2018,11(09): 298-307.
supported by the XRD data. The surface roughness increases from 0.20 nm to 1.25 nm as the tin concentration decreases. The films are porous in nature. [21]
Fig.4, Shows Atomic Force Micrographs of ITO films with differentconcentrations of Tin formed at450°C. (1μmx 1 μm area). Z direction: 1 div – 5.0 nm
Fig.5.shows transmission spectra of ITO films formed at different Tin concentration. The transmittance of the films exhibits a sharp increase in the visible region. The ripples pattern seems to arise on account of interference of light which might be due to the characteristic of interference between light and nanostructure materials [22]. The transmission was found to be maximum for low SnO2 concentration of In2O3 and decreased with an increase in SnO2 concentration. The decrease in optical transmission is associated with the loss of light due to (i) Oxygen vacancies and (ii) Scattering at grain boundaries. Since all the films were deposited under similar oxygen environment and processing conditions, the loss of transmission due to oxygen defects was assumed to be same for all the deposited films. The increase in scattering centers due to more grain boundaries with an increase in Sn dopant content might be responsible for the loss of transmission. Thus a decrease in grain size with increase Sn concentration results in an increase in grain boundaries in In2O3 films.
The optical absorption coefficient, ‘a’ of a direct band gap semiconductor near the band edge, for photon energy hν greater than the band-gap energy Eg of the semiconductor, is given by the relation [23].αhν = A( hν – Eg)1/2 ------------------(3)
where h is Planck’s constant and ν is the frequency of the incident photon. The Tauc plot of (ahν)2 versus energy hν for the all ITO films were shown in Fig.5 The band gap energy was obtained by extrapolating the linear part of the Tauc plot curves to intercept the energy axis (at αhν = 0). The estimated values of Eg for the ITO film were found to increase from 3.85 to 4.02 eV with an increase in Sn concentration in the In2O3 lattice. The allowed direct transitions were found previously for different ITO thin films and the direct optical band gap, Eg, values ranging from 3.5 to 4.5 eV were also reported earlier [19,24].
G. Ramanathan et al /International Journal of ChemTech Research, 2018,11(09): 298-307.
Fig.5 Shows Transmission spectra of ITO films with different concentrations of SnO2 (a) glass (b) 5% (c) 10 % (f) 70 %
Fig.6. Tauc’s plot of ITO films with different concentrations of tin oxide formed at 450°C (a) 5 % (b) 15
Fig.6 shows the plot of refractive index as a function of wavelength λ for different percentages of SnO2 in ITO films. The refractive index (n) was evaluated from the measured transmittance versus wavelength graph [25]. The refractive index was found to decrease with wavelength. It was observed that the refractive indices of the Sn-doped In2O3 thin films were smaller than for the pure In2O3 film in the measured wavelength range. The decrease in refractive index was attributed to the lowering in the value of grain size and increase in the porosity of the ITO film with the increase in Sn concentration
Fig.7. Variation of Refractive index with wavelength of ITO films formed at 450°C with different concentrations of Sn (a) 5 % (b) 10 % (c) 30 % (d) 50 % (e) 70 %
G. Ramanathan et al /International Journal of ChemTech Research, 2018,11(09): 298-307.
The resistivity of the films decreased from 20 ohm cm (In2O3 films formed at 450°C) to 0.1 ohm cm as the tin concentration increases to 10 %. Further increase of tin oxide content caused increased in resistivity. Beyond 10 % tin the resistivity increases from 0.1 ohm cm and attain 250 ohm cm .at 70 % tin concentration. The carrier density and mobility increase with increase of tin concentration up to 10 at. % beyond which, they
20 -3 20 -3
decrease.The Nd increased from 1.61x 10cm (In2O3 films formed at 450°C) to 1.25 x 10cm up to 10 at.
%, beyond this value of SnO2 content, the carrier density decreases. The value of mobility decreased from 49.93
2 -1 -1 2 -1 -1
cm Vs to 3.73 cmVs up to 10 at. % SnO2, beyond which the mobility decreases. Fig.8. shows the
variation of resistivity, mobility and carrier concentration with concentration of tin. The data are presented in table-3.The increase in resistivity beyond 10 % Sn can be explained as follows, this evolution could be described in terms of solubility of Sn atoms in the In2O3 lattice. As given in the literature [26], the maximum solubility of Sn-atoms in the In2O3 lattice is reached at approximately 10 at. %. So for an atomic concentration
4+3+ 4+3+
of Sn lower than 10 %, the Snions substitute Inions in the cationsublattice. As Snreplace InSn atoms act as n-type donors, and the resistivity decreases with the Sn concentration. For higher tin content, no more Snatoms can be embedded in the In2O3-lattice, and the resistivity increases. This increase may be due to interstitial Sn-atoms in the lattice, which act as charged trapping centers for the electrons [27].
Fig.8. Shows the Variation of room temperature Resistivity, mobility and carrier concentration of ITO films with different concentration of SnO2
G. Ramanathan et al /International Journal of ChemTech Research, 2018,11(09): 298-307.
Table -3 Transport parameters of ITO films with different Concentrations of Tin
Sn ( at. %) | Resistivity (ohm cm) | Mobility (cm2V -1 s -1) | Carrier concentration (cm -3) |
---|---|---|---|
5 | 0.01 | 49.93 | 1.25 Χ 1020 |
10 | 0.10 | 38.75 | 1.61 Χ 1018 |
70 | 250.00 | 3.73 | 6.69 Χ 1015 |
phase separation of SnO2 in the crystalline ITO-material does not occur since no Bragg reflections belonging to a crystalline SnO2 phase were detected for films with SnO2 concentration up to 70 % in XRD measurements. Raman spectrum of undoped In2O3 shows vibrational modes at 109, 135, 307, 366, 495, 517 and 630 cm-1 which is an unambiguous signature of the cubic In2O3 structure [28]. These same features are readily observed in the ITO spectra, because the main contribution to the vibrational modes comes from the cubic In2O3 host lattice
The high frequency line in the ITO spectrum at 631 cm-1 is due to the superposition of the contribution of the In–O vibrational modes (630 cm-1) and the Sn–O vibrational modes (633 cm-1) in this work, vibration modes appeared at 144, 175 and 584 cm-1 in the ITO spectrum ( Fig.9). These result matches with previous report [29]. The height of the peaks decreased as the tin oxide concentration increased. In fact the observation of these modes in the doped structure can be related to the Sn incorporation in the lattice. In this way, these modes can be indexed as Sn–O vibrations which make ITO distinguishable from the undoped material. Moreover, a narrow line width of Raman in all the spectra indicates that the films have a very good crystal quality.
The photoluminescence (PL) spectra (Fig. 10) of the films excited at the band gap energy (Eg = 4.0 eV,
i.e.310 nm, λex) of the excitons. Three PL bands appeared at 423, 486, 529 nm with a small broad peak at 439 nm. It is earlier reported [30, 31] that in the nanoclusters of semiconductors associated with defect centers,
G. Ramanathan et al /International Journal of ChemTech Research, 2018,11(09): 298-307.
emissions for both bound and free excitons wouldappear when kex is equivalent to Eg and emissions for only free excitons in the case λex is >Eg or <Eg. Hence, we may predict that the PL bands at 423, 486 and 529 nm are for bound excitons trapped in the defect centers of ITO. The 439 nm band is possibly due to the defect of In– O vacancy [32].
Fig.10. Room temperature photoluminescence emission spectra of ITO films containing different concentrations of Sn (a) 70 % (b) 60 % (c) 30 % (d) 20 %(e) 5 %
The results of this work illustrate that single phase cubic, bixbyite structure of In2O3 films, grain size in the range of 7-48 nm can be deposited by dip coating sol gel method. Films with transmission in the range of 80-90% can be obtained. Thickness of the films varying from 500 nm to 890 nm. Low resistivity films can be obtained its band gap in the range of 3.85-4.02eV and with carrier density 1020 cm-3 can be deposited. Also this investigation clearly points to the possibility of using them solar cells and gas sensor applications.
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