Optical And Electrical Properties Of Pd Doped Sno2 Thin Films ...

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SCOUT software (Theiss, W. in: W. Theiss (Ed.), 2001) in the wavelength range 300nm – 2500nm. Electrical resistivity of the films was calculated from two ...

Australian Journal of Basic and Applied Sciences, 7(2): 89-98, 2013 ISSN 1991-8178

 

Optical And Electrical Properties Of Pd Doped Sno2 Thin Films Deposited By Spray Pyrolysis 1

Benjamin.V.Odari, 1Maxwell Mageto, 2Robinson Musembi, 3Henrick Othieno, 1Francis Gaitho and 1 Valentine Muramba

1

Department of Physics, Masinde Muliro University of Science and Technology, P.O Box 190, 50100, Kakamega, Kenya 2 Department of Physics, University of Nairobi, P.O Box 30197-00100, Nairobi, Kenya 3 Department of Physics, Maseno University, P.O Box 333-40105, Maseno, Kenya Abstract: Thin films of SnO2: Pd have been deposited on glass substrate at 4500C using an alcoholic precursor solution consisting of Tin (IV) Chloride (SnCl4.5H2O) and Palladium Chloride (PdCl2). The influence of increasing Pd concentration on the electrical and optical properties has been investigated. The minimum resistivity of Ωcm for 3.68at% Pd film was obtained. The optical properties were studied in the UV/VIS/NIR region. The optical band gap for undoped SnO2 films lies at 3.93 eV and Palladium doped films lay in the range 3.86 – 3.99 eV. Using dispersion analysis with Drude and Kim terms, optical constants were determined from spectro-photometric measurements for films on glass. The film thickness was determined through analysis using the SCOUT software to be in the range 140 – 223 nm. Key wrods: INTRODUCTION

Tin oxide has received a great scientific interest because of its wide range of applications which include the field of sensors, opacities, transparent electrodes in solar panels and other electrochromic devices, overcoat for thin film magnetic recording media overcoat and as material for Li-ion batteries (Young, S.K., et al., 2010; Díaz, R., 2002). The film is highly transparent in the visible region, chemically inert, mechanically hard and can resist high temperatures (Chitra, A., et al., 1991; Mishra, R.L., et al., 2009; Shamala, K.S., et al., 2004) as it is only attacked by hot concentrated alkalis (Díaz, R., 2002). It belongs to a class of materials that combines high electrical conductivity with optical transparency, and therefore, it constitutes an important component for the optoelectronic applications (Young, S.K., et al., 2010). The efficiency in these applications is usually improved by suitably doping the tin oxide for example, doping with Sb and F increases the conductivity of tin oxide (Díaz, R., 2002; Shadia, J.I., N.A.B. Riyad, 2008; Jebbari, N., et al., 2010; Agashe, C., et al., 1988). Tin oxide is a crystalline solid with a tetragonal crystal lattice. It is a wide gap, non-stoichiometric semiconductor and behaves as a degenerate n-type semiconductor with a low resistivity ( Ωcm) (Agashe, C., et al., 1988). It can exist in two structures belonging to an indirect band gap of about 2.6 eV (Mohammad, T.M., 1990) and direct band gap that ranges from 3.6 eV to 4.6 eV at room temperature (Agashe, C., et al., 1988; Rakhshani, E.A., et al., 1988). Some of the most popular methods of depositing Tin oxide thin films include the following: Reactive sputtering (Stanislav, R., et al., 2004), Plasma enhanced Chemical Vapour Deposition (Young, S.K., et al., 2010), Sol-gel (Sandipan, R., et al., 2010), Electronspun (Joong-Ki, C., et al., 2010), Inkjet printing (Wan, Z.S., et al., 2011), Electron beam Evaporation (Shamala, K.S., et al., 2004) and Spray pyrolysis (Chitra, A., et al., 1991; Mishra, R.L., et al., 2009; Shadia, J.I., N.A.B. Riyad, 2008; Boshta, M.,). Of these methods the spray pyrolysis method represents the less expensive alternative since it can produce large area, high-quality and low cost thin films (Shadia, J.I., N.A.B. Riyad, 2008). The purpose of this work is to improve the properties of the spray deposited SnO2: Pd thin films to be suitable for a window pane gas sensor. Experimental: Sample Preparation: The substrates used were ordinary float glass slides which were 2.5 mm thick and measuring 2.5 by 7.6 cm2. The cleaning procedure involved scrubbing the glass slides gently on both sides using a cotton swab soaked in foam made from a mixture of deionized water, liquid detergent and sodium hydroxide in the ratio of 3:2:1. They were then drag wiped using a lens cleaning tissue held at an angle of 45 degrees before being wiped with Isopropyl alcohol and acetone respectively. Lastly, the substrates were ultrasonically cleaned in distilled water for 30 minutes before drying with spray of pressurized air. Spray pyrolysis technique was used to deposit the films at a substrate temperature of 450 100C using compressed air at 1 bar as the atomization gas. The Corresponding Author: Benjamin.V.Odari, Department of Physics, Masinde Muliro University of Science and Technology, P.O Box 190, 50100, Kakamega, Kenya

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  experimental set up of the in-house-made spray pyrolysis system is as shown in Figure 1. It consisted of a fume chamber, hot plate, spray nozzle of diameter 1mm, input gas valve, gas compressor, gas flow meter, conduit tube, and pressure gauge.

Fig. 1: Spray pyrolysis setup The undoped SnO2 film was produced from a precursor solution consisting of Tin (iv) chloride (99%) prepared by dissolving completely 6g of stannic chloride with 100ml of ethanol (99.9%) (Shamala, K.S., et al., 2004; Shadia, J.I., N.A.B. Riyad, 2008). Pd-doped SnO2 films were prepared by dissolving 0.5g of PdCl2 (5960%Pd) in 50ml of ethanol (99.9%) then added to the spraying solution at varying volumes ranging from 2ml to 8ml (1.88at% - 7.10at%). The spraying parameters were as follows: substrate temperature: 450 100C, Carrier-gas: Compressed air, Carrier-gas pressure: 1 bar, flow rate of solution 6ml/min, nozzle-to-substrate distance: 33cm horizontally and 32+3cm vertically. After spraying, the films were left to cool with the hot plate before removal for transmittance and reflectance measurements. Sample Characterization: The transmittance and reflectance measurements were done at near normal incidence in the solar wavelength range from 300nm to 2500nm on a computerized double beam solid-spec 3700DUV Shimadzu Spectrophotometer equipped with 198851 Barium Sulphate (BaSO4) integrating sphere. Barium Sulphate plate was used as a reference for the calculation of optical properties such as band gap, absorption coefficient and refractive index. The thickness of the as-deposited samples with Pd doping concentrations were estimated by fitting the experimental spectral data to theoretical spectral data based on Drude and Kim analysis using the SCOUT software (Theiss, W. in: W. Theiss (Ed.), 2001) in the wavelength range 300nm – 2500nm. Electrical resistivity of the films was calculated from two adjustable parameters of Drude: plasma frequency Ωp and damping constant γ. RESULTS AND DISCUSSION Deposition Rate: The deposition rate of the undoped tin oxide and palladium doped tin oxide was determined by plotting the graph of thickness against time as depicted in Figure 2 and was found to be 0.247nm/s. Optical Studies: In order to compare the transparency of SnO2 thin films with various Pd-doping levels, the optical spectra in the UV-VIS-NIR region of the samples was measured. The optical transparency of SnO2: Pd thin films for various Pd-doping levels for both experimental and computed spectral are shown in Figure 3. The films exhibited a transparency near 88% (for undoped tin oxide) at wavelength of 695nm. The transparency at 390 – 440nm decreased with increasing Pd doping concentration which is probably due to the increase in fundamental absorption as photon striking increases with increase in carrier concentration (Yousaf, S.A., S. Ali, 2009). The maximum transmittance observed in the undoped tin oxide thin film may be attributed to a decrease in diffuse and multiple reflections caused by the increase in grain size and a reduction in light-scattering effect (Sankara, N.S., et al., 2006). A sharp fall in transmission at about 310 nm is due to the absorption of the glass substrate (Sandipan, R., et al., 2010).

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  240

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Fig. 2: Thickness versus deposition time for undoped tin oxide and Pd-doped tin oxide at temp of 450 100C 100

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Fig. 3: Experimental and Computed Spectral Transmittance and Reflectance of undoped tin oxide and Pd-doped tin oxide films prepared at 450 100C Dispersion analysis using a model for the dielectric susceptibility of the film consisting of Drude (Ashcroft, N.W., N.D. Mermin, 1976) and Kim terms (Kim, C.C., et al., 1992) was used to model the measured reflectance and transmittance. For doped semiconductors, the charge carriers set free by the donors or acceptors can be accelerated by very little energies and hence do respond to applied electric fields with frequencies in the infrared region (Theiss, W. in: W. Theiss (Ed.), 2001). Drude is a free electron contribution which describes the intraband contributions to the optical properties. This model has two adjustable parameters: plasma frequency,

Ωp

and damping constant, γ . The plasma frequency is proportional to the square root of the carrier density and

the damping constant is proportional to the inverse of the mobility. The Drude dielectric susceptibility, χ Drude , expressed as a function of frequency ω, is given as (Theiss, W. in: W. Theiss (Ed.), 2001; Ashcroft, N.W., N.D. Mermin, 1976)

χ Drude ω 

Ωp

2

ω 2  i 

The one oscillator contribution developed by Kim contains four adjustable parameters: Ω TO resonance frequency,

Ωp

oscillator strength, Ω τ damping constant and Gauss–Lorentz-switch constant σ . σ may vary

between zero and infinity. For σ = 0, a Gaussian line shape is achieved. A large value of σ (larger than 5) leads to a Lorentzian line shape. The Kim oscillator models the weak broad interband absorption in the measured wavelength range. The interband dielectric susceptibility described by Kim is given by (Kim, C.C., et al., 1992);

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χ Kim, Oscillator 

Ωp

2

ΩTO  ω2  i ω τ ω 2

 1 τω  Ω τ exp  2  1  σ

where

 ω  Ω TO     Ωτ 

2

  

Model parameters were determined from the best fit between computed and experimental data, using Scout software (Theiss, W. in: W. Theiss (Ed.), 2001). The best fit gives us directly the optical constants of the film under study. Figure 4 shows spectral absorption coefficient (α) for SnO2 and SnO2:Pd with four doping levels. For all the films, the absorption edge lies in the UV region and increases with increase in palladium concentration. The samples show a high absorption coefficient for α > 104 cm-1 for λ < 500 nm as shown in Figure 4(i). The dependence on α on hν near the band edge is shown in Figure 4(ii). It is clear that the value of α increases with increasing photon energy in the range 2.5 eV – 3.7 eV. This may be due to increase in palladium concentration.

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Fig. 4: Absorption coefficient (α) against wavelength (i) and Absorption coefficient (α) against Energy for undoped tin oxide and Pd-doped tin oxide films prepared at 450 100C Absorption coefficient, α, also depends on the thickness of the film at the band edge as shown in Figure 5. At shorter wavelengths (high energies), it was seen that the thinner the film, the greater the absorption coefficient. 12.0 140000

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Fig. 5: Effect of thickness on Absorption coefficient (α) of undoped tin oxide and Pd-doped tin oxide films prepared at 450 100C The optical band gap, Eg, was determined using the standard formula (Maghanga, C.M., et al., 2010; Mageto, J.M. and M. Mwamburi,): αhν (hν – Eg)n ,

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  is the absorption coefficient, hν, the photon energy, and n=1/2 accounts for the fact that Where the directly allowed transitions across the bandgap are expected to dominate. Figure 6 shows plots of versus photo energy, hν, in the high absorption region. Extrapolation of the curve to hν = 0 gave the direct band gap of SnO2:Pd films in the range 3.86 eV – 3.99 eV for the Pd doped tin oxide films and 3.93 eV for the undoped tin oxide film; which is comparable with the values already reported (Shadia, J.I., N.A.B. Riyad, 2008; Boshta, M., F.A. Mahmud, M.H. Sayed; Sankara, N.S., et al., 2006; Brajesh, N., et al., 2010). The band gap for palladium doped films is found to be wider than the undoped tin oxide film for 1.88at% Pd and 3.68at% Pd. The increase in the energy gap can be correlated with the Moss-Burstein effect since the absorption edge of the films shifts to shorter wavelength. This indicates an optimum level palladium doping on tin oxide causes a widening effect on the band gap and this may be attributed to the gradual increase in carrier concentration and mobility (Sankara, N.S., et al., 2006). 11

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Fig. 6: Energy bandgap for undoped tin oxide and Pd-doped tin oxide films prepared at 450 100C, (i) Undoped SnO2 (ii) 1.88at% SnO2:Pd (iii) 3.68at% SnO2:Pd (iv) 5.42at% SnO2:Pd (v) 7.10at% SnO2:Pd 93

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  Figure 7 shows spectral refractive index (η) and extinction coefficient k for SnO2 and SnO2:Pd. The refractive index (η), was estimated from the transmission and reflectance data and it was found to be around 1.95 at 500 nm for the undoped SnO2 film of thickness 167nm, deposited at 450 100C. It was observed that the refractive index of all the films decreases with wavelength and then attains almost a constant value towards higher wavelengths (Shamala, K.S., et al., 2004). 2.25

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Fig. 7: Spectral refractive index and extinction coefficient for SnO2 and SnO2:Pd thin films. 4.00 3.98

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3.96 3.94 3.92 3.90 3.88 3.86 3.84 -1

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Fig. 8: Energy gap versus Pd concentration Figure 8 shows the band gap energy of the undoped and Pd doped SnO2 thin films with different percentage of Pd. It is observed that the band gap energy increases with Pd percentage up to 1.88at% then it decreases when Pd percentage is more than 1.88at% which is in accordance with the findings of (Fatema et.al., 2011). Electrical Properties: In most cases it is the electric field of the probing light wave that interacts with the sample. Hence excitations can be observed in optical experiments that are going along with a polarization. The polarization P induced by an externally applied electric field E in a homogeneous material is given by the electric susceptibility χ;

The dielectric function ε which connects the dielectric displacement and the electric field vector is closely related to the susceptibility χ;

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The frequency dependence of the susceptibility is very characteristic for a material since it incorporates vibrations of the electronic system and the atomic cores as well as contributions from free charge carriers. The doping of semiconductors leads to free charge carriers which can be investigated by IR spectroscopy. The response of the free carriers to oscillating electric fields can be described to a good approximation by the Drude model. The parameters of that model relate the concentration of the charge carriers and their mobility to properties of the dielectric function. After a model parameter fit of the simulated spectrum to measured data the carrier concentration and the mobility or resistivity can be computed (Theiss, W. in: W. Theiss (Ed.), 2001). The Drude model relates the macroscopic susceptibility to the microscopic quantities carrier concentration n and mobility μ;

and where e is the elementary charge , is the permittivity of the free space and m the effective mass of the charge carriers ( ) (Roy, G.G., 2000) and is the mass of an electron. Resistivity can also be computed using the formula (Theiss, W. in: W. Theiss (Ed.), 2001);

Table 1 below gives the calculated values of carrier concentration (n), mobility (μ), resistivity (ρ) and conductivity (σ) of the undoped and palladium doped films. Table 1: Calculated values of carrier concentration (n), mobility (μ), resistivity (ρ) and conductivity (σ) of the undoped tin oxide and palladium doped tin oxide films Damping Carrier Plasma frequency constant (Ωτ) cm- concentration (n) Mobility (μ) Resistivity (ρ) Conductivity (σ) 1 Film (Ωp) cm-1 cm-3 cm2/Vs Ωcm Ω-1cm-1 0at% Pd:SnO2 13809.9639 343585.625 6.38024E+22 5.69E-03 0.1080051 9.258821481 1.88at% Pd:SnO2 23835.9805 631534.25 1.90072E+23 3.10E-03 0.0666385 15.00633867 3.68at% Pd:SnO2 23082.4746 456856.8125 1.78245E+23 4.28E-03 0.0514055 19.45316349 5.42at% Pd:SnO2 97.6613 160.2429 3.19078E+18 1.22E+01 1.0072289 0.992822968 7.10at% Pd:SnO2 8.6206 1658.2019 2.48615E+16 1.18E+00 1337.6941 0.000747555

The relatively low resistivity ( Ω cm) observed for the un-doped films is attributed to the deviation from stoichiometry due to oxygen vacancies, which act as electron donors and increase of the free carrier concentration (Mohamed, H.A., 2009) as shown in Figure 9. It can be seen here that initially with doping, electrical resistance decreases due to increase in number of charge carriers. But as we kept on increasing the doping concentration, at a point resistance increases again. This may be attributed to the fact that at higher concentration, Palladium atoms incorporates at the interstitial sites and crystal structure of the films start to deteriorate, hence decreases the mobility of the free electrons and increases the electrical resistivity (Yousaf, S.A., S. Ali, 2009; Chatterjee, K., et al., 2003; Elangovan, E., K. Ramamurthi, 2003). The resistivity of Pd doped SnO2 films are higher compared to fluorine and antimony doped tin oxide reported in other peoples’ work. This is due to the adsorbed oxygen ions on the surface of the Pd doped SnO2 films which inhibit intraband and interband transitions. Therefore, in the presence of a reducing gas, desorption of the oxygen ions is expected to take place setting free more charge carriers in the film hence increasing film conductivity.

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Fig. 9: Resistivity versus Pd Concentration The electrical charge concentration and conductivity as a function of palladium concentration is shown in Figure 10 and 11 respectively. The electrical conductivity and carrier concentration tend to follow a similar behavior. Initially there is a rise in their values up to 3.68at% Pd, beyond which a consistent fall is observed. The changes in carrier mobility are comparatively small as shown in Table 1. Hence the electrical conductivity seems to be governed by the carrier concentration. The rise in charge concentration is because of the rise in palladium incorporation at regular and interstitial sites. Further increase in palladium concentration, the interstitial incorporation of palladium must be large enough so that due to the carrier compensation phenomenon the carrier concentration drops. Such a compensation of donors has been observed in the case of excess tin doped indium oxide films and tin incorporation on the growth mechanism of sprayed SnO2 films (Chitra, A., et al., 1991). 20

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Fig. 10: Carrier concentration versus Pd/Sn concentration

Fig. 11: Conductivity versus Pd/Sn concentration

Conclusion: In the present work, thin films of pure tin oxide and palladium doped tin oxide are prepared by Spray pyrolysis technique from SnCl4 precursor. The resistivity of the undoped films decreases with initial doping of palladium to attain a minimum value and increases for higher level of doping. The resistivity achieved for the films doped with 3.68at % Pd is the lowest ( Ωcm) for these films from SnCl4 precursor. Initial doping of SnO2 with Palladium leads to the widening of the band gap which decreases on further doping which may be due to the change in structure of the films as the dopant concentration increases. Increase in dopant concentration also leads to decrease in transmittance in the visible wavelengths with high transmittance of the undoped SnO2 film found to be 88%.

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