Influence of Thermal Annealing on the Structural and Optical Properties of Lead Oxide Thin Films Prepared by Chemical Bath Deposition Technique

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1 Influence of Thermal Annealing on the Structural and Optical Properties of Lead Oxide Thin Films Prepared by Chemical Bath Deposition Technique D.D.O. Eya, Ph.D. Department of Physics, Federal University of Technology, Owerri, Nigeria ABSTRACT Thin lead oxide (PbO) films were prepared by Chemical Bath Deposition (CBD) technique. The deposition process was carried out at room temperature but post-deposition thermal treatment was carried out on the films. The films as deposited are lead hydroxide but with the thermal treatment, PbO results. The X-ray diffraction studies of the films reveal that the intensity of the distinct diffraction peaks for the films increased with increase in the annealing temperatures but disappear completely at higher annealing temperatures. The grain size varied from 0.097nm to 0.987nm within the range of annealing temperatures. Film thickness obtained is in the range μm. The film reflectance falls from 20% in the UV region to 13% in the NIR region. On the other hand, the transmittance varies from <40% to 77% within the range. The refractive index varies between 2.10 to The range of band gap obtained for the films is eV corresponding to the wavelength of 6.22 x 10-5 m 4.14 x 10-7 m. The high transmittance of the films in the IR region indicates that the films are potentially useful in window glaze for creating warmth in rooms. (Keywords: lead oxide, thin films, post-deposition annealing, optical properties, structural properties, grain size) INTRODUCTION Lead oxide (PbO) thin films have been prepared by chemical bath deposition technique [1], reactive DC magnetron sputtering [2], and spray pyrolysis [3]. Thin film preparation is special in the sense that the deposition technique adopted determine to a large extent the structural, optical and electrical properties of the films [4]. In this regard, researchers working on thin films not only search for appropriate semiconductor materials but also appropriate deposition techniques that will yield properties of the material that will suit the device application desired. The chemical bath deposition technique from aqueous solution, apart from being the simplest and most economical, has the advantage with respect to other methods of the films being able to be deposited on different kinds, shapes, and sizes of substrates [5]. In the deposition of oxide films, the deposition follows a two step process; deposition of the hydrous film and its pyrolytic decomposition into anhydrous film [1, 6]. In this paper, the influence of post-deposition annealing on the structural and optical properties of thin PbO films is reported. EXPERIMENTAL PROCEDURES The chemical bath deposition technique was adopted in the preparation of the lead oxide (PbO) thin films. Lead nitrate [Pb(NO 3 ) 2 ] and sodium hydroxide (NaOH) were used as sources of Pb 2+ and O 2- respectively. Ethylenediamine tetra acetate (EDTA) served as the complexing agent while 76mm x 26mm x 1mm commercial quality glass microscope slides are the substrates. Details of the deposition process are reported in a recent past work (1). Postdeposition annealing was carried out on the films in a temperature range between 300K and 523K. Energy dispersive X-ray fluorescence (EDXRF) studies of the PbO films was carried out to verify the composition of the films. The analysis was The Pacific Journal of Science and Technology 114

2 done using ANNULAR 25mCi109Cd excitation source that emits Ag-K X-rays (22.1kev). The optical Absorbance/Transmittance were measured by a Unicam Helios Gamma UVvisible having spectral range of nm. The structure and lattice parameters of the oxide films were analysed by a PW1800 X-ray diffractometer with Cu anode. RESULTS AND DISCUSSION The deposition process of PbO films was carried out at room temperatures (about 300K). The films as deposited are Pb(OH) 2 and white in colour. Various samples of the films were subjected to post-deposition annealing temperatures in the range of K. With increasing annealing temperatures, the film gradually assumes the yellow colour of lead oxide [7]. The EDXRF of the films showed energy of KeV for lead corresponding to peak intensity of c/s and hence confirm Pb as a major component of the film. The optical method was used in determining the thickness of the films. The range of thickness achieved for the hydroxide and oxide films as shown in Table 1 are µm. The thickness increases with increasing annealing temperature. Figure 1 shows the X-ray diffraction (XRD) patterns of the PbO films under various thermal treatments. The analysis of the patterns show (100), (110), and (111) distinct diffraction peaks for the films as grown as shown in Figure 1a. Figure 1b shows more distinct and dense diffraction peaks for films annealed at 403K. These are at the (110), (011), (200), (020), and (002) planes. However, it is remarkable that when the annealing temperature of 463K was applied, the structure of the films resulted in only one broad peak at (111) diffraction plane as shown in Figure 1c. When the annealing temperature was further increased to 503K the distinct peaks disappeared completely and the resulting structure being similar to that of as grown film as shown in Figure 1d. The grain sizes D for each of the annealed films were estimated by the well known Scherrer s formula (8-10): D = Kλ x βcosθ (1) Where K is a dimensionless constant (0.9), 2θ is the diffraction angle, λ x is the X-ray wavelength and β is the full width at half maximum (FWHM) of the diffraction peak. The grain size corresponding to the various diffraction peak are shown in Table 2. Figures 2a and 2b show the transmittance and reflectance of PbO as functions of wavelength under various thermal treatments. The erratic fluctuation in Figure 2a is partly as a result of molecules of water contained in the films which disappeared to a great extent in Figure 2b where the modification in the structure by the thermal treatment contributed to the definite trend of the transmittance and reflectance. For instance, when the annealing temperature is 523K the transmittance is between 45-77% and reflection between 13-20%. The high transmittance in the IR region indicates that PbO films are good materials for warming applications in homes in temperate regions and in agriculture (1). Table 1: Deposition Parameters and Thickness of PbO Thin Films. Sample Molar Ratio: Volume Period of Annealing Average Pb(NO 3 ) 2 :EDTA:NaOH Ratio Deposition (hrs) Temp.(K) Thickness (µm) P :0.1 : : 5 : P : 0.1 : : 4.5 : P : 0.1 : : 4.5 : P : 0.1 : : 2.5 : P : 0.1 : : 2.5 : P : 0.1 : : 4.5 : P : 0.1 : : 5 : The Pacific Journal of Science and Technology 115

3 This is an indication that the rate at which light is slowed down in the oxide film is very high in the UV-region and decays sharply to relatively low rate in the IR region. Table 2: Grain Size of PbO Thin Films under various Thermal Treatments and Diffraction Peaks. Diffraction Peaks D values for P01TR (nm) D values for POP (nm) D values for P03T190 (nm) Using the same sample, the real dielectric constant of the film varies between and the values for the imaginary between within the regions respectively. From the figures, one can infer that the overall effect of the post-deposition annealing on the PbO films is to stabilize the properties of the films. The direct transition dependence of photon energy is given by the relation (1,8,11-12): (αhν) (hν - E g ) 1/2 (2) Figure 1: X-Ray Diffraction Pattern for PbO Films. Figures 3a and 3b show the variation of refractive index of the films with wavelength. The sample annealed at 523K indicates high values of the refractive index (maximum value = 2.64) in the UV-visible regions but lower values (minimum value = 2.10) in the IR region. where α is the absorption coefficient, ν is photon frequency, E g is the band gap of the material and h, the Planck s constant. When (αhν) 2 is plotted as a function of hν and the linear portion of the curve is extrapolated to (αhν) 2 = 0, the band gap of the oxide film results as shown in Figure 4. The Pacific Journal of Science and Technology 116

4 K 423K T, R K 423K 300K 423K n Fi 3 R f ti i d ( ) f ti f l th Figure 2a: Transmittance (T) and Reflectance (R) as functions of Wavelength under Various Thermal Treatments for PbO. Figure 3a: Reflective Index (n) as a Function of Wavelength under Various Thermal Conditions K 523K T, R K 523K 453K 523K n Figure 2b: Transmittance (T) and Reflectance (R) as Functions of Wavelength under Various Thermal Conditions. 2 Figure 3a: Reflective Index (n) as a Function of Wavelength under Various Thermal Conditions. The values obtained for various samples under various post-deposition annealing temperatures are shown in Table 3. The table shows a band gap range of eV corresponding to wavelength range of 6.22x10-5 m x10-7 m. The Pacific Journal of Science and Technology 117

5 25 523K (1) 523K (2) also modified the structure and optical properties of the films resulting in more stable properties of the films. ACKNOWLEDGEMENT (αhν) 2 x ev 2 m hν (ev) Figure 4: A Plot (αhν) 2 of as a function of Photon Energy for PbO. Table3: Band Gaps of PbO under Various Annealing Temperatures. Sample Annealing Band gap Temp. (k) (ev) P P P P P P P The author is very grateful to Prof. C.E. Okeke and Prof. A.J. Ekpunobi for their wonderful assistance and advice in the course of the work. I remain grateful to them. REFERENCES 1. Eya, D.D.O., A.J. Ekpunobi, and C.E. Okeke Optical Properties and Applications of Lead Oxide Thin Film Prepared by Chemical Bath Deposition Technique. Academic Open Internet Journal. 14(5). 2. Venkataraj, S., et al Structural and Optical Properties of Thin Lead Oxide Films Produced by Reactive Direct Current Magnetron Sputtering. Journal of Vacuum Science and Technology A. 19(6): Thangaraju, B. and P. Kaliannam Optical and Structural Studies on Spray Deposited α- PbO Thin Films. Semicond. Sci. Technol. 15: Chopra, K.L. and S.R. Das Thin Film Solar Cells. Plenum Press: New York. 5. Crus-Vazquez, C., F. Rocha-Alonzo, S. E. Burruel Ibarra, and M. Inoue Fabrication and Characterization of Sulphur Doped Zinc Oxide Thin Film. Sociedad Mexicana de Ciencia de Superficies y de Vacío. 13: Eze, F.C. and C.E. Okeke Chemical Bath Deposited Cobalt Sulplide Films: Preparation Effects. Material Chemistry and Physics. 47: Cotton, F.A. and G. Wilkinson Basic Organic Chemistry. John Wiley & Sons Inc.: New York. CONCLUSION Lead oxide (PbO) thin films were prepared from Pb(NO 3 ) 2 and NaOH with EDTA as the complexing agent. Some of the films deposited were annealed under various annealing temperatures up to 523K. The annealing of these films not only produced the oxide films but 8. Eya, D.D.O., A.J. Ekpunobi, and C. E. Okeke Structural and Optical Properties and Applications of Zinc Oxide Thin Film Prepared by Chemical Bath Deposition Technique. Pacific Journal of Science and Technology. 6 (1): Ion, L., V.A. Antole, and S. Antole Defects Induced by Electron Irradiation in CdSe Thin Films. Journal of Optoelectronics and Advanced Material. 7(4): The Pacific Journal of Science and Technology 118

6 10. Gomez M., J. Rodriguez, S. E. Lindquist, and C. G. Grangvist Photoelectrochemical Studies of Dye-Sensitized Polycrystalline Titanium Oxide Thin Films Prepared by Sputtering. Thin Solid Films. 342: Tanusevski, A Optical and Photoelectric Properties of SnS Thin Films Prepared by Chemical Bath Deposition. Semicond. Sci Technol. 18: Thanganaju, B. and P. Kaliannan Spray Pyrolytic Deposition and Characterization of SnS And SnS2 Thin Film. J. Phys. D., Appl. Phys. 33: SUGGESTED CITATION Eya, D.D.O Influence of Thermal Annealing on the Structural and Optical Properties of Lead Oxide Thin Films Prepared by Chemical Bath Deposition Technique. Pacific Journal of Science and Technology. 7(2): Pacific Journal of Science and Technology ABOUT THE AUTHOR D.D.O. Eya, Ph.D. is an academic staff member in the Department of Physics at the Federal University of Technology in Owerri, Nigeria. His research interests are in the areas of photovoltaics, thin film deposition, and characterization. The Pacific Journal of Science and Technology 119