Variation of the Optical Conductivity, Dielectric Function and the Energy Bandgap of CdO Using Cadmium Acetate Dehydrate

Transcription

1 International Journal of Advances in Electrical and Electronics Engineering 331 Available online at & ISSN: Variation of the Optical Conductivity, Dielectric Function and the Energy Bandgap of CdO Using Cadmium Acetate Dehydrate Matthew David Femi, Adrian Ohwofosirai, Aboritoli Sunday, Ogah Sunday B. A. Ezekoye F. I. Ezema and R. U. Osuji Department of Physics and Astronomy, University of Nigeria Nsukka, Nigeria. Abstract The CdO thin films were deposited using NaOH as the complexing agent by Successive Ionic Layer Adsorption and Reaction (SILAR) method. The UV-VIS-NIR spectroscopy was used for film characterization. The deposited CdO thin film shows high absorbance in the UV region and low in the visible to near IR region of the spectroscopy. The energy band gap of the deposited CdO thin film ranges from 1.40eV 1.50eV, indicating a wide band gap, this band gap decreases with an increase in No. of cycles for all films, this is attributed to the increase in the thickness of films. Studied is the variation of the film optical conductivity with the band gap, which revealed that the film optical conductivity decreases with an increase in band gap. Lastly, the relationship between the film band gap and the dielectric function of the materials was consider, the dielectric constant varies exponentially with the band gap and also increases with the number of cycles (film thickness). Index Terms CdO, Optical bandgap, Dielectric function, Optical Conductivity. 1. INTRODUCTION During the last few years, CdO has revealed itself as a very promising material for use in the photovoltaic industry. Because of its high optical transparency in the spectral region of the solar radiation, electrical conductivity (in the absence of doping), refractive index between those of air and CdTe and a good match for CdTe lattice, it may also be used as a substitute for CdS and SnO 2 in the SnO 2 /CdS/CdTe photovoltaic heterostructures [1]. In the last ten years, the optical and electrical properties of CdO thin films prepared by various techniques such as spray pyrolysis [2], oxidation of cadmium films [3], chemical vapour deposition [4] and many others have being studied [5]. It was experimentally proven that these properties are very sensitive to the film structure and deposition condition [6]. Rusu and Rusu had worked on the electrical and optical characteristics of CdO thin film. In their work, CdO thin film of thickness range of 0.15µm to 0.70µm were deposited by thermal evaporation under vacuum onto glass substrates kept at 300K and 473K respectively. Depending on the substrate temperature, films with polycrystalline or amorphous structured were obtained, the influence of temperature and post deposition heat treatment on the electrical conductivity (σ) and optical transmittance in visible region was investigated. An irreversible temperature dependence of σ during heating/cooling cycles of as grown samples were observed, the optical band gap were determined to be E g = 2.4eV [7]. Hann et al deposited CdO using CBD and three different complexing agents; ammonia, ethanolamine and methylamine. CdSO 4 was used as the Cd precursor and H 2 SO 4 as the oxidizing agent. Grown films were cubic CdO 2 with some Cd(OH) 2 as well as CdO phases being detected. Annealing at 400 o C in air for 1hr transformed films into cubic CdO. The calculated optical band gap of as grown film ranges from 3.37eV to 4.64eV. Annealed films have a

2 Variation of the Optical Conductivity, Dielectric Function and the Energy Bandgap of CdO Using Cadmium Acetate Dehydrate 332 band gap of about 2.53eV, a carrier density as high as cm -3 and a resistivity as low as Ωcm -1 was obtained [8]. Mohaboob et al deposited CdO using the SILAR approach, Cadmium acetate was used as a source of CdO and ammonium hydroxide was the complexing agent. The study determined the effect of molarity of solution on the structural, optical and morphological properties of deposited films. XRD and SEM revealed that the crystallite size is increased with molarity of the precursor solution. UV/VIS spectrum of the films showed that the optical band gap energy increases with concentration of cadmium acetate in the precursor solution [9]. Balu et al also, deposited CdO using the SILAR technique, the effect of solution concentration on the structural, optical, electrical, and morphological properties of the deposited sample was analysed. The structural study revealed that the films were polycrystalline with preferred orientation along the 200 plane. The lattice parameter was found to be equal to 4.690A, grain size increased from 15.96nm to 21nm as the solution concentration increased. Optical absorption measurements showed that the films coated with 0.05M had a maximum transmittance of 84%, band gap energy of the coated films decreased with the increase in solution concentration. It was recorded that the sheet resistance increased from Ω/m 2 to Ω/m 2 as the concentration increased from 0.05M to 0.2M. Film coated with 0.1M had low temperature coefficient of resistance /K [10]. 2. EXPERIMENTAL DETAILS 7.72g of cadmium acetate dihydrate [Cd(CH 3 COO) 2.2H 2 O] of atomic weight g was dissolved in 100cm 3 of distilled water in a beaker, the resulting mixture was stirred. 4g of sodium hydroxide (NaOH) of atomic weight 40g was dissolved in 100cm 3 of distilled water and the resulting mixture was also stirred. Four (4) beakers were set up, the first filled with cadmium acetate dihydrate solution, the second filled with distilled water, the third filled with NaOH solution and fourth filled with distilled water. A prepared glass substrate was first dipped in the beaker containing cadmium acetate dehydrate for 10 seconds reaction time and immediately rinsed in the second beaker containing water for 5 seconds and transferred to the third beaker containing NaOH solution for 10 seconds and rinsed in the fourth beaker containing distilled water for 5 seconds. This process formed one cycle. The same process was repeated for 25 cycles with the first substrate 30, 35 and 40 cycles with the second, third and fourth substrates respectively. Whitish films were observed to deposite on the substrates. The substrates were labeled A, B, C and D respectively. These films were heated at 623K (350 O C) for 3hrs, to facilitate decrease in dislocations and stresses. Finally, the whitish films were observed to change to brown, confirming the formation of CdO. 3. RESULTS AND DISCUSSION The optical properties studied in this research include the absorbance, reflectance, optical conductivity, refractive index, extinction coefficient and dielectric function. A. Absorbance of Films Figure-1 is a plot of absorbance of CdO thin film as a function of wavelength. The optical absorption spectra of the CdO films deposited were obtained at room temperature in the wavelength range of 365nm-1100nm for deposited CdO thin film using NaOH as complexing agent. The optical absorbance spectra were obtained for the deposited CdO at different number of cycles. The curves reveal a decrease in absorbance with longer wavelength. From the graph also, the absorbance of deposited CdO films with fewer number of cycles are high in the UV region.

3 IJAEEE,Volume 2, Number 2 Matthew David Femi et al. It also shows a moderate absorbance in the visible region for all the samples. In the near infra-red (NIR) region, the curves reveal a very low absorption of energy. A critical look at the graph shows a progressive increase in absorbance as the number of cycle s increases, from this experiment one can infer that increase in number of cycles in Successive Ionic Layer Adsorption and Reaction method of depositing CdO thin film increases the absorbance of the film. The deposited films with high absorbance in the UV region and low absorbance in the visible region have useful application in the coating of windscreens and driving mirrors to prevent the effect of the dazzling light into driver s eyes. It could also be useful in p-n junction solar cells [11]. B. Reflectance of Film Figure 1. Plot of absorbance of CdO thin film as a function of wavelength. Figure -2 shows the reflectance curves for the deposited CdO thin films for different numbers of cycles using NaOH complexing agent. The plots reveal that the samples with fewer number of cycles (25 and 30) show high reflectance in the IR region of the electromagnetic spectrum. A careful observation of Figure 3 shows that the reflectance of the films increases as wavelength increases with the highest reflectance value of about 60% for NaOH at approximate wavelength of 1000nm for 25 cycles. The high reflectance exhibited by this material makes it useful in the manufacture of highly reflectance mirrors commonly found in desktop scanners, photocopy machines, astronomical telescope, car head lamps and halogen lamps [12]. C. Absorption Coefficients Figure 2: Reflectance curves for the deposited CdO thin films for different numbers of cycles Figure 3 shows the absorption coefficient plots for the NaOH for the various number of cycles. In order to determine the type of optical transition of the material, the dependence of the absorption coefficient, α on the photon energy, hυ has to be understood. The value of the optical band gap can be determined from the fundamental absorption of the material which corresponds to the excitation of electrons from the valence band to the conduction band. The absorption coefficient was calculated from transmittance values using the expression:

4 Variation of the Optical Conductivity, Dielectric Function and the Energy Bandgap of CdO Using Cadmium Acetate Dehydrate 334 α = [ InT ] t Where, α = Absorption coefficient, T = Transmittance values and t = Thickness of the film. The relation between absorption coefficient, α, and the incident photon energy, hυ, can be expressed as: (αhv) = K(hv E g ) n - 2 Where K is a constant and n is a number which characterizes the transition process and is theoretically equal to 1/2 and 2 for direct and indirect transition, Eg is the optical band gap energy of the material. Figure 3. A graph of absorption coefficient against photon energy D. Optical Band Gap Studies The plot for the optical band gap of the deposited CdO thin films for NaOH is show in Figure 4. The direct band gap energy for the deposited CdO thin films were obtained from the plot of (αhυ) 2 versus the photon energy, hυ, for different No. of cycles and extrapolating the curve to (αhυ) 2 = 0 as shown in figure 5 (a &b ). Table 1 also shows the values of these band gaps which ranges from 1.40eV to 1.50eV. A closer observation of the plot reveals that the deposited CdO band gaps decreases with increasing number of cycles. The decrease in band gap energy is attributed to increase in thickness of the films. Figure 4. A plot of (αhv) 2 against photon energy Table I. Nos of cycles and respective band gaps for NaOH

5 IJAEEE,Volume 2, Number 2 Matthew David Femi et al. No. of cycles Band gap (ev) E. Extinction coefficient of deposited CdO Thin Films The extinction coefficient of the films was calculated using the expression: k = αλ/4π The plot of extinction coefficient versus photon energy for the CdO films deposited at different No. of cycles for the NaOH complexing agent is shown in Figure 5. We observe that the extinction coefficient decreases with an increase in photon energy for all the samples. The extinction coefficient obtained for these materials lie within the range of 3.0 to 0.5. Also noticeable is a decrease in the extinction coefficient of the deposited CdO thin film with increasing No. of cycles. Figure 5. the extinction coefficient versus photon energy. F. Refractive Index of Deposited CdO Films Figure 6 shows the plot of the refractive index against the photon energy of the deposited CdO thin film using NaOH as a complexing agent. The refractive index of the material was calculated from reflectance value using the expression: η = (1 + R 1/2 ) / (1- R 1/2 ) - 4 The refractive index of the material is seen to increase exponentially with an increase in photon energy and also increases with No. of cycles. The refractive index obtained lies within the range of 1.06 to 1.25, with the highest 1.25 obtained at 30 and 40 oscillations. Figure 6. Variation of refractive index with photon energy.

6 Variation of the Optical Conductivity, Dielectric Function and the Energy Bandgap of CdO Using Cadmium Acetate Dehydrate 336 G. Optical Conductivity of CdO Films The optical conductivity of the material was calculated using the equation below; αnc σ = π Variation of optical conductivity of the material versus photon energy deposited at different No. of cycles using NaOH as complexing agent is shown in figure 7. The optical conductivity decreases exponentially with photon energy and the number of cycles. Figure 7. Optical conductivity as a function of photon energy for CdO thin films at different number of cycles. H. Dielectric Function of the Deposited CdO Films The dielectric function is a complex quantity and a fundamental intrinsic property of the material which consists of both the real and imaginary parts. The real part indicates how the speed of light in the material can be slowed down while the imaginary part deals with the absorption of energy by a dielectric from electric field due to dipole motion. The dielectric function of the deposited CdO thin films grown in this research was calculated from the expression: E T = E r + E i Where, we have used E r to represent the real part and E i to represent the imaginary part and they are given by the relation: E r = η 2 k 2-7 E i = 2ηk, - 8 where η and k represents the refractive index and extinction coefficient of the films respectively. The dielectric function varies exponentially with the bandgap and also increases with the number of cycles as shown in figure-8.

7 IJAEEE,Volume 2, Number 2 Matthew David Femi et al. Figure 8: Dielectric constant as a function of photon energy for CdO thin films at different number of cycles. 4. Conclusions The deposited CdO thin films were prepared using NaOH and NH 3 as complexing agents by Successive Ionic Layer Adsorption and Reaction techniques. The UV-VIS-NIR spectroscopy was used for film characterization. The deposited CdO thin film has high absorbance in the UV region and low in the visible to near infra-red region, conversely low and high transmittance in the UV and visible region respectively. The band gap of the deposited CdO decreases with an increase in No. of cycles for all films in both cases, this was attributed to the increase in the thickness of films. Also studied is the variation of the film optical conductivity with the band gap, which was observed to decrease with an increase in band gap. Lastly, the relationship between the film band gap and the dielectric function of the materials was consider, the dielectric constant varies exponentially with the band gap and also increases with the number of cycles (film thickness). REFERENCE [1] Ferekides, C. S. Marinskiy, D. Viswanathan. V. Tetali, B. Palekis, V. and Morel. D. L. Proceedings of the European Material Conference, E-MRS 1999 Spring Meeting Stransbourg, France,1999. [2] Ramakrishna, K. T. Shanthini, G. M. Johnson, D. and Miles, R. W. Thin Solid Films. 447 (2003) 397. [3] Zhao, Z. Morel, D. L. and Ferekides, C. S. Thin Solid Films. 413 (2002) 203. [4] Carballeda-Galicia, D. M. Castanedo-Perez, R. Jimenez- Sandoval, O. Tores-Delgado, G. Zuniga-Romero, C. I. Thin Solid Films. 371 (2000) 105. [5] Lockande, B. J. and Uplane, M. D. Mater. Res. Bull. 36 (2001) 439. [6] Dakhel, A. A. and Henari, F. Z. Cryst. Res. Technol. 38 (2003) 979. [7] Rusu, R. S. and Rusu G. I. The electrical and optical characteristics of CdO thin films,journal of optoelectronics and Advanced materials. 7 (2005) 823., [8] Khallaf, H. Chia-Tachen, Chanq, L. Lupan, O. Dutta, A. and Heinrich H. H.. Investigation of chemical bath deposition of CdO Thin Films using three different complexing agents. Applied surface science 257 (2011) 9237 [9] Beevi, M. Anusuyab, M. and Saravana, V. IACSIT Member characterization of CdO thin film prepared by SILAR deposition technique, international journal Of chemical Engineering and applications 1 (2010) 10 [10] Balu, A. R. Nagarathinam, V. S. Suganya, M. Arunkumar, N. and Selvan. G. Effect of the solution concentration on the structural, optical and electrical properties of SILAR deposited CdO thin films, Journal of Electron Devices, 12 (2012) 739. [11] Ezenwa, A. I. Effect of Deposition time on the absorbance of chemical bath deposition CuS thin film. Res. J. Engineering Sci. 2(1) (2013) 1. [12] Jeroh M. D. and Okoh, D. N. The influence of dip time on the optical properties of antimony tri-sulphide thin films, IJRPAS 12(3) (2012) 431