Tin Oxide (SnO2) thin film is one of the important transparent conducting oxides (TCOs) and applied in various fields such as in solar cells, optoelectronic devices, heat mirror, gas sensors, etc due to its electrical and optical transparency in visible light spectrum. In this paper, we presented the optical properties of tin oxide thin film at four different annealing temperatures (373 K, 437 K, 573 K and 673 K) prepared by radio frequency sputtering technique. The optical characteristic of these films was investigated using the UV-VIS 3101-PC Spectrophotometer. From this study, all samples exhibit high transmittance more than 70% in the visible light spectrum. Sample annealed at 473 K shows the maximum transmittance which is 87%. Refractive index, n were in the range of 2.33 &ndash; 2.80 at  = 550 nm and enhanced with the annealing temperature. However the extinction of coefficient, k was found to be very small. The optical band-edge absorption coefficients were found in the range of 104 &ndash; 105cm-1. The energy gap value was decreased with increasing annealing temperature and the type of photon transition was allowed direct transition

Baco, Saturi

Chik, Abdullah

Md. Yassin, Fouziah

English

Tin Oxide (SnO2) thin film is one of the important transparent conducting oxides (TCOs) and applied in various fields such as in solar cells, optoelectronic devices, heat mirror, gas sensors, etc due to its electrical and optical transparency in visible light spectrum. In this paper, we presented the optical properties of tin oxide thin film at four different annealing temperatures (373 K, 437 K, 573 K and 673 K) prepared by radio frequency sputtering technique. The optical characteristic of these films was investigated using the UV-VIS 3101-PC Spectrophotometer. From this study, all samples exhibit high transmittance more than 70% in the visible light spectrum. Sample annealed at 473 K shows the maximum transmittance which is 87%. Refractive index, n were in the range of 2.33 &ndash; 2.80 at ï¬ = 550 nm and enhanced with the annealing temperature. However the extinction of coefficient, k was found to be very small. The optical band-edge absorption coefficients were found in the range of 104 &ndash; 105cm-1. The energy gap value was decreased with increasing annealing temperature and the type of photon transition was allowed direct transition

Journals of Universiti Tun Hussein Onn Malaysia (UTHM)

Journal of Science and Technology  61 Study on Optical Properties of Tin Oxide Thin Film at Different Annealing Temperature  Saturi Baco1,*, Abdullah Chik2, Fouziah Md. Yassin1  1Physics with Electronic Programme, School of Science & Technology, Universiti Malaysia Sabah, Jalan UMS, 88400 Kota Kinabalu, Sabah, Malaysia 2School of Material Engineering, Universiti Malaysia Perlis, Kampus Kubang Gajah, 02600 Arau Perlis, Malaysia.  *Corresponding email: saturi@ums.edu.my   Abstract Tin Oxide (SnO2) thin film is one of the important transparent conducting oxides (TCOs) and applied in various fields such as in solar cells, optoelectronic devices, heat mirror, gas sensors, etc due to its electrical and optical transparency in visible light spectrum. In this paper, we presented the optical properties of tin oxide thin film at four different annealing temperatures (373 K, 437 K, 573 K and 673 K) prepared by radio frequency sputtering technique. The optical characteristic of these films was investigated using the UV-VIS 3101-PC Spectrophotometer. From this study, all samples exhibit high transmittance more than 70% in the visible light spectrum. Sample annealed at 473 K shows the maximum transmittance which is 87%. Refractive index, n were in the range of 2.33 – 2.80 at  = 550 nm and enhanced with the annealing temperature. However the extinction of coefficient, k was found to be very small. The optical band-edge absorption coefficients were found in the range of 104 – 105cm-1. The energy gap value was decreased with increasing annealing temperature and the type of photon transition was allowed direct transition.  Keywords: tin oxide; radio frequency sputtering; energy band gap; annealing        Journal of Science and Technology  62  1.   INTRODUCTION  Tin Oxide (SnO2) is a n-type semiconductor with wide energy band gap (3.7 eV). The undoped SnO2 [1] has low electrical resistance and high optical transparency in the visible range of the electromagnetic spectrum. These properties make tin oxide suitable for many applications, particularly as an electrode material in solar cell, light emitting diodes, transparent electromagnetic shielding materials, etc [2].   Numerous works have been reported concerning doped and undoped  tin oxide thin films using various preparation techniques such as chemical vapor deposition [3], thermal beam evaporation [4], spray pyrolysis [5], pulse laser deposition [6], and sputtering techniques [7]. Sputtering technique has advantages among other methods, since film deposition can be carried out at low temperature, whilst yielding the preferred orientation and uniform properties [8]. Furthermore, sputtering technique is known to reproduce thin films of various types of materials especially the oxides [9]. Studies on effect of substrate type and temperature, deposition rate, oxygen partial pressure and annealing temperature are widely performed by many researchers to improve the structural, electrical and optical properties of the sputtered thin film. The annealing processes, for instance, are usually performed to reduce the intrinsic stress, to improve the lattice mismatch and create longer mean paths for the free electrons in getting better electrical conductivity [10].   In this work, tin oxide thin films were deposited by using radio frequency sputtering technique and later annealed at four different temperatures.  The optical properties of SnO2 films at different annealing temperatures were analyzed to determine their four optical parameters which are refractive index, extinction coefficient, absorption coefficient, and energy gap values.   2.   MATERIALS AND METHODS  In this study, the SnO2 films were deposited onto soda lime glass substrate (7.6 cm x 2.6 cm x 0.1 cm) via radio frequency sputtering technique. First, before the deposition of thin films, the substrates were cleaned using an Ultrasonic Branson 3200 Cleaner by immersing the substrate in chromic acid and distilled water solution for 30 minutes at temperature of 45 oC. Next, the substrates were washed using distilled water and soaked in the beaker that also contained the distilled water for 30 minutes. The working pressure and RF power used in this study were 10 mTorr and 50 Watt respectively and the substrate temperature was kept at room temperature as listed in Table 1. After deposition of SnO2, the films were annealed at four different temperatures which are 373 K, 473 K, 573 K and 673 K for two hours in air. The optical properties of films afterward were characterized using UV-VIS 3101-PC Spectrophotometer. The transmission spectrum in the visible light (300nm – 800nm) Journal of Science and Technology  63 was analyzed to determine the refractive index, extinction coefficient, optical band-edge absorption coefficient and energy gap value for each sample.  Table 1: Process Parameters used in this study Target SnO2 (99.99% purity) Substrate Soda lime glass Substrate Temperature Room Temperature Working Pressure 10 mTorr RF Power  50 Watt Film Thickness 49 nm     3. RESULTS AND DISCUSSION  3.1 Optical Transmittance  SnO2 thin films were successfully deposited onto glass substrate and the films were very transparent. This may be attributed by the formation of the Fermi level in the conduction band [11]. Fig. 1 shows the optical transmittance spectra in the visible light region (300 nm- 800nm) for samples annealed at different temperature. From the graph, the film annealed at 473 K exhibits highest transmittance in the range of 500nm – 700nm which maximum transmittance is 87%. This is in a good agreement with the results obtained by [12] where the SnO2 films annealed at 473 K shows high transmittance above 80%. Conversely at the wavelength between 300 nm- 450 nm, the film at 473 K initially shows the lowest transmittance and gradually increases. On the other hand film annealed at 673 K shows the lowest transmittance. Nevertheless, all samples exhibit more than 70% of transmittance in the visible light region. The transmittance for all annealed samples rises with increasing wavelength and this also approved as in [13]. The ripples as shown in the transmittance spectra may resulted from the interference of light, since they show wave forms that are characteristic of the interference light [14].  Journal of Science and Technology  64  0102030405060708090100300 350 400 450 500 550 600 650 700 750 800Wavelength (nm)Transmittance (%)373 K473 K573 K673 K  Figure 1: Optical Transmittance against wavelength in visible light spectra at different annealing temperature (373 K, 473 K, 573 K and 673 K)  3.2 Refractive Index, n and Extinction Coefficient, k  Optical constants of SnO2 films which are refractive index, n and the extinction coefficient, k were calculated from transmission and reflectance spectra. The refractive index of SnO2 films can be obtained with the following equation [15]: 2/111RRnn s                                                                (1) with ns is the refractive index of substrate and R is the reflectance. Refractive index for soda lime glass substrate used in this study is 1.51. The extinction coefficient, k is given by [9]: 4k                                                                           (2) where  is the absorption coefficient of the film, and  is the wavelength of light. Fig.2 shows the variation of refractive index and the extinction coefficient with wavelength. It can be seen that, the refractive index were decrease with the wavelength according to the behavior of reflectance spectrum. At  = 550 nm, the values of n were in the range of 2.33 – 2.80 and enhanced with the annealing temperature. As reported in [1] the refractive index of SnO2 at 550 nm was 2.00 and the extinction coefficient was 0.03. However the extinction of coefficient in this study was found very small and this similar behavior also found as in [16] that showing the SnO2 is transparent in the visible light spectrum.   Journal of Science and Technology  65  Figure 2: (a) Dispersion of refractive index, n (b) Dispersion of extinction coefficient, k with wavelength for SnO2 films annealed at different temperature.  3.3 Analysis on Optical Band-Edge Absorption Coefficient   If the light with intensity Io incident on the sample with thickness d, the intensity that is transmitted is expressed as the Lambert-Beer-Bouguer Law [17]:  dII o  exp                                                                                 (3) where I is transmitted intensity; Io is incident intensity; is absorption coefficient  (cm-1) and d is a sample thickness. The equation (3) can be simplified and the absorption coefficient can be obtained from the measured transmission spectra, T according to the following equation: TInd11                                                                               (4) The graph of absorption coefficient as a function of photon energy was plotted in Fig.3. The graph shows the absorption coefficient is related to the photon energy, since as the photon energy increases, the absorption edge shifted and broadened to high energy. At very low wavelength (high energy) absorption is observed due to the domination of fundamental band-gap, and for higher wavelength, high reflection is observed due to the released of free electrons in SnO2 [18].  Based on the graph, the absorption coefficient values in this study are in the range of 104-105 cm-1 and similar to previous study [7].   1.82.32.83.33.84.34.8400 450 500 550 600 650 700 750 800Wavelength (nm)Refractive index, n373 K473 K573 K673 K     0.0000.0010.0020.0030.0040.0050.0060.0070.0080.0090.010400 450 500 550 600 650 700 750 800Wavelength (nm)Extinction Coefficient, k373 K473 K573 K673 K  (a)  (b)  Journal of Science and Technology  66  0.00E+001.00E+052.00E+053.00E+054.00E+055.00E+056.00E+057.00E+058.00E+051.50 2.00 2.50 3.00 3.50 4.00 4.50Energy (eV)Absorption (cm-1)373 K473 K573 K673 K  Figure 3: Optical absorption coefficient as a function of photon energy at different annealing temperature.  3.4 Estimation of SnO2 Optical Energy Band Gap  Based on [19], the excitation of electron from valence band to conduction band by absorption of photon energy can occur in two ways usually either in direct or indirect transitions. The energy band gap of the film can roughly determined by the transmittance spectra. According to the Tauc relation [20], the absorption coefficient is given by:  mgEhvBhv                              (5) where B is constant and has different values for different transitions, Eg is the energy gap, hv is the photon energy and m is an exponent which assumes the values ½, 3/2, 2 and 3 depends on the electronic transition. In order to determine the optical energy band gap of films, the graph of ( hv )n versus hv has been plotted by replacing n = 1/m which are n = 2 for allowed direct transition, n = ½ for allowed indirect, n = 1/3 for forbidden indirect and n = 2/3 for forbidden direct of optical transitions [21]. In this study, ( hv )n versus hv for n=2,1/2,1/3 and 2/3 values respectively have been plotted.  In our case, the value of n was selected to be 2 (for allowed direct transition band gap)  as illustrated in Fig.4 because it was most probable transition and by using n=2,  the graph gave the best line fit [22][23]. By extrapolating the linear portion of the plot to ( hv )2 = 0 as plotted in Figure 5, the optical band gap was obtained. It was found that the energy gap decreased with annealing temperature. Reference [9] reported that apart from eliminating the oxygen vacancies, annealing process also localizes the oxygen atoms at interstitials. The induced oxygen interstitials form separate band defects in the band gap region, causing the reduction in Eg value with increasing annealing temperature [24]. The variation of direct band gap with annealing temperature was presented in Table 2 along with the comparisons Journal of Science and Technology  67 with the published values of energy band gap [24]. The highest optical band gap was obtained from the sample annealed at 473 K while the lowest energy band gap was from the sample annealed at 673 K with 3.39 eV and 2.85 eV respectively.  0.00E+002.00E+124.00E+126.00E+128.00E+121.00E+131.20E+131.40E+131.60E+131.00 1.50 2.00 2.50 3.00 3.50 4.00 4.50Photon Energy (eV)(αhv)2 cm-2 eV 2373 K473 K573 K673 K  Figure 4: Plot of ( hv )2 vs. photon energy for SnO2 films annealed at different temperature.  Table 2: Comparison of energy bad gap obtained with other researcher on tin oxide thin film at various annealing temperature.   Method Annealing Temperature (K) 373 473 573 672 673 Energy Gap (eV) (In present study) RF Sputtering 3.00 3.39 2.95 - 2.85 Energy Gap (eV) [24] Chemical Bath Deposition 3.20 2.60 2.40 1.60 -      Journal of Science and Technology  68  0.00E+005.00E+111.00E+121.50E+122.00E+122.50E+123.00E+123.50E+124.00E+124.50E+125.00E+120.00 1.00 2.00 3.00 4.00 5.00Photon Energy (eV)(αhv)2  cm-2 eV 2        0.00E+002.00E+124.00E+126.00E+128.00E+121.00E+131.20E+131.40E+131.60E+130.00 1.00 2.00 3.00 4.00 5.00Photon Energy (eV)(αhv)2  cm-2 eV 2 0.00E+005.00E+111.00E+121.50E+122.00E+122.50E+123.00E+123.50E+124.00E+120.00 1.00 2.00 3.00 4.00 5.00Photon Energy (eV)(αhv)2  cm-2 eV 2   0.00E+005.00E+111.00E+121.50E+122.00E+122.50E+123.00E+123.50E+120.00 1.00 2.00 3.00 4.00 5.00Photon Energy (eV)(αhv)2 cm-2 eV 2  Figure 5: Graph of extrapolation on ( hv )2 = 0 at photon energy for SnO2 film at (a) 373 K, (b) 473 K, (c) 573 K and (d) 673 K   4.  CONCLUSIONS  Tin Oxide (SnO2) thin films have been successfully deposited using radio frequency sputtering and the four optical parameters of films at different annealing temperature were successfully measured. All samples exhibit high transmittance more than 70% in the visible light spectrum.. Refractive index were in the range of 2.33 – 2.80 at  = 550 nm and the absorption coefficients were in the range of 104 – 105cm-1. Energy gap of SnO2 was found to be decreased with increasing in annealing temperature with the photon transition was allowed direct transition.   Acknowledgement  The authors gratefully acknowledge to Universiti Malaysia Sabah (UMS) for financial support.   a bdc Journal of Science and Technology  69 REFERENCES  [1] J.C. Manifacier, M. De Murcia, J.P. Fillard, E. Vicario. (1977). Optical and electrical properties of SnO2 thin films in relation to their stoichiometric deviation and their crystalline structure. Thin Solid Films.41: 127-144.  [2] H.L. 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Study on Optical Properties of Tin Oxide Thin Film at Different Annealing Temperature

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Study on Optical Properties of Tin Oxide Thin Film at Different Annealing Temperature

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