Academic Open Internet Journal

ISSN 1311-4360

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Volume 17, 2006

 

 

 

Influence of Thermal Annealing on the Optical Properties of Tin Oxide Thin Films Prepared by Chemical Bath Deposition Technique.

 

 

D. D. O. Eya*, A. J. Ekpunobi and C. E. Okeke+

Department of Industrial Physics, Nnamdi Azikiwe University, Awka, Nigeria.

 

*Department of physics, Federal University of Technology, Owerri,Nigeria. E-mail eyadom2003@yahoo.com

 

+Department of physics & Astronomy, University of Nigeria, Nsukka, Nigeria.

 

 

Abstract

Tin oxide thin films have been prepared by chemical bath deposition technique on glass substrates using SnCl2 and NaOH solution with triethanolamine (TEA) as the complexing agent.  The films were subjected to post deposition annealing under various temperatures, 373, 423, 453 and 523 K. The thermal treatment streamlined the properties of the oxide films.  The films are transparent in the entire regions of the electromagnetic spectrum, firmly adhered to the substrate and resistant to chemicals. The transmittance is between 57% and 95% while the reflectance is between 4% and 19%. The band gaps obtained under various thermal treatments are between 2.70eV and 3.0eV. The refractive index is between 1.52 and 2.55. The thickness achieved is in the range of 0.010-0.148mm.

These properties of the oxide film make it suitable for application in solar cell fabrication, gas sensor devices, transparent electrodes for panel displays, etc.

 

Keywords: Tin Oxide, Chemical Bath Deposition, Thin Film, Thermal Annealing, Optical Properties.

 

1. Introduction.

 

Tin oxide (Sn02) is one of the transparent conductive oxides (TCOs). The oxide films are stable, strongly adherent to the substrate, mechanically hard and resistant to moisture and acids [1,2].  In recent times, techniques like atmospheric chemical vapour deposition (APCVD) system [1] Femtosecond pulsed laser deposition [3], chemical vapour deposition [4] etc, have been used in depositing tin oxide films. Band gap range of 3.5 – 4.2 eV has been reported using these techniques. Transparent conductive oxides (TCOs) are unusual materials that are both electrically conductive and visually transparent  [5,6]. Tin oxide films have large transmittance in the visible region of the electromagnetic spectrum as a consequence of the large band gap [4].

Owing to its outstanding electrical, optical and electrochemical properties, Sn02 is extensively used in many applications such as catalytic support materials, transparent electrodes for flat panel displays and solar cells and gas sensors.  In particular, Sn02 thin films have drawn much interest because of their potential application in microsensor devices [3,6-8] They are widely used in high and low technical applications such as antistatic coatings instrument panels, on heaters, electrical contacts in liquid crystals, electrochronic and electroluminescent displays and optical coatings [2,5].

In this paper, we report the successful deposition of thin Tin oxide films using chemical bath deposition technique and the influence of post deposition thermal annealing on the optical properties of the oxide film.

 

2. Experimental Details.

The tin oxide (Sn02) thin films were prepared using chemical bath deposition technique.  The chemical bath system was made up of stannous chloride (SnCl2) sodium hydroxide (NaOH), triethanolamine (TEA) or Ammonia (NH3) as complexing agent, 50ml beakers and 76 mm x 26mm x 1mm glass microscope slides which were used as substrates.  The substrates were degreased in Aqua Regia (3:1 of conc. HCl: HN03) solution for 48 hours, washed clean in detergent solution rinsed out in distilled water and allowed to air dry.  Solution of various concentration ratios ranging from 0.05M to 0.40M prepared and used in arriving at the optimum combination and depositing film of various thickness. Uniform films were obtained in the process. In each case the substrate was suspended vertically in the reaction bath after stirring the solution properly for homogeneity.  The reaction process is of the form

SnCl2 + TEA         ↔    [Sn(TEA)]2++2Cl-

 [Sn(TEA)]2+        ↔    Sn2++TEA

2NaOH  +  40H-      ↔    2Na+ + 302-+ 3H20

Sn4+ + 202-     →    Sn02.

 

The films were removed and washed after various periods of deposition and allowed to dry in air.

Even though the thin film samples were deposited at room temperature, some of them were subjected to post deposition annealing between the temperatures of 373K and 523K.  The optical absorbance/transmittance of the samples were investigated in the spectral range of 340 – 1000nm (UV-VIS-NIR regions) using Unican Helios Gamma UV- Visible spectrophotometer

 

3. Results and Discussion

Tin oxide (Sn02) thin films were successfully deposited on glass substrate using chemical bath deposition technique.  The films are very transparent, firmly adhered to the substrates and resistant to both trioxonitrate (v) acid and hydrochloric acid.  Thermal annealing does not affect the physical nature of the films.  The range of thickness of the films deposited is 0.010 - 0.148mm. Figures 1 and 2 show plots of transmittance and reflectance as functions of wavelength.  The graphs show that the properties of the films become more defined with increase in the annealing temperature.  Generally, the films show very high degree of transmittance and very low degree of reflectance in the entire spectral regions.

The films as grown and those annealed at lower temperature (373K) show transmittance in the range of 90% - <100% and reflectance in the range of 0-10%.  On the other hand, those annealed at higher temperature show depreciation in transmittance and improvement on

reflectance.  The range is now ~60% - 90% and 4% - 10% respectively. The transmittance rises with increasing wavelength while reflectance decreases with increasing wavelength.

Maximum and minimum values of refractive index, 1.52 and 2.55 respectively were obtained for the Sn02 films.  The values decrease with increasing wavelength as shown in figure 3.  The rate of decrease increases with the annealing temperature.

 

In order to determine the optical band gap of the semiconductor, the following dependence of the absorption coefficient, α on the photon energy equation [6,9,10] is used (αhν) µ (hν-Eg)n

Where Eg is the direct transition band gap and n=1/2 for direct allowed transition

Figure 4 shows a plot of (αhν)2 against the photon energy, hν.  The band gap obtain for various annealing temperature are shown in the table below.  The table shows a decreasing band gap with increasing temperature.  Even the decreased values of Eg still remain wide.

 

Various authors in their separate reports had reported band gap ranging from 3.0ev to 4.2ev in agreement with the wide band gap reports.

 

 Band gap of SnO under various annealing temperatures.

Sample

Annealing temp

Band Gap (eV)

S2

300

3.00

S*5

300

3.00

C4

373

2.95

C2

423

2.90

S5

523

2.70

 

The optical conductivity is given by  [11]        s0    =        αnc

                                                            4p

where α is the absorption co-efficient, n the refractive index, c is the velocity of light and s0 the optical conductivity.

Plots of optical conductivity as a function of photon energy are shown in figure 5.

 

 

At 453k, the rate of increase of s0 with increasing photon energy was very steep. However at 523k, the rate of increase was slower.

Minimum and maximum values of 3.0x1012s-1 and 3.3x1013s-1 respectively are shown the figure.

 

 

4. Conclusion.

Tin oxide (Sn02) thin films have been deposited by chemical bath deposition technique using SnCl2 and Na0H solution. Post deposition annealing of the films at temperatures 373,423,453 and 523K sharpened the properties of the films.  Sn02 film is a transparent oxide film.  It has very high transmittance in all the regions of electromagnetic spectrum.  The transmittance increase from UV-NTR regions up to over 90% The reflectance is generally low and decrease within the same region.

Band gap of 2.70 – 3.0 eV were obtained for the oxide film under various annealing temperatures.  The values are in agreement with theoretical values.  Values of the refractive index are within the range 1.52 to 2.55. The outstanding properties of the oxide films show them as good materials for solar cells, gas sensors, transparent electrodes for panel displays, etc.

 

 

 References.

 

1.   V. Matsui, Y. Yamamoto and S. Takeda; Stability in Electrical                 Properties of Ultra Thin Tin Oxide Films; Mat res. Symp Proc. Vol 621 (2000) Q4.9.1 – Q4.9.6

2.   K.L. Chopra and S.R. Das, Thin Film Solar cells, Plenum Press, New York (1983) 321-327

3.   J. E. Dominguez, L. FU, and X.Q Pan, Optical Nanocrystalline Tin Dioxide thin films grown on (0001) Sapplre by femtosecond pulsed laser deposition, Applied physics letters Vol. 79, 5 (2001) 614-616

4.   A.M. B.V. Mol, Chemical Vapour Deposition of Tin Oxide Thin films, Universiteitsdrukkerij Technische Universiteit Eindhoven (2003) 41-46

5.   S.H. Jeong, S.B. Lee and J. H. Boo, The Insert of Zinc Oxide thin film in Indum tin oxide anode for organic electroluminescence devices, Current Applied Physics 4 (2004) 655-658

6.   B. Thangaraju and P. Kaliannan; Spray Pyrolytic deposition and characterization of SnS and SnS2 thin films, phys D. appl. Phys. 33 (2000) 1054 – 1059.

7.   K. Ihokura and J. Watson, The Stannic Oxide Gas Sensor – principles and applications (CRC, Boca Raton, F.L., 1994)

8.   G. Advani and A. Jordan, J. Electrochem. Soc. 123,29(1990)

9.   D. D. O Eya, A.J. Ekpunobi and C. E. Okeke, Structural and optical properties and applications of Zinc oxide thin films prepared by chemical bath deposition technique, Pacific Journal of Science and Technology 6, 1 (2005) 16-22.

10   A.Tanusevski, Optical and photoelectric properties of SnS thin films prepared by chemical bath deposition, Semincond. Sci. Technol 18 (2003) 501-505

11. J.T. Pankove, Optical processes in semiconductors, prentice Hall, New York (1971)

 

 

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