EFFECT OF THERMAL ANNEALING ON OPTICAL PROPERTIES OF SILVER ALUMINIUM SELENIDE THIN FILM DEPOSITED BY ELECTRO-DEPOSITION METHOD.

EFFECT OF THERMAL ANNEALING ON OPTICAL PROPERTIES OF SILVER ALUMINIUM SELENIDE THIN FILM DEPOSITED BY ELECTRO-DEPOSITION METHOD. Muomeliri B.C1., Okereke N. A2. and Chibuogwu I.U1 Email: [email protected] ngozigoddy[email protected] 1. Department Physics and Industrial Physics, Nnamdi Azikiwe University, Awka,. Anambra State, Nigeria. 2. Department of Industrial Physics, Chukwemeka Odumegwu Ojukwu University Uli. Anambra State, Nigeria. ABSTRACT Semiconductor thin films of AgAlSe were synthesized by electro-deposition method unto ITO glass substrate and subsequent heat treatment process was carried out. The deposited films have been characterized for optical analysis from absorption spectra within the wavelength range of 300nm-1000nm which was recorded by uv-visible spectrophotometer. The optical parameters such as optical band gap, refractive index, extinction coefficient, dielectric constant have been calculated from absorption spectra. The results of the optical analysis showed that heat treatment on the deposited thin films has some effect on the optical properties studied. The films show high absorbance in the UV region but decrease in the visible and infrared regions of the electromagnetic spectrum, the absorbance values increased further when annealed. The transmittance on the other hand has a reduction in values as a result of annealing but increases as wavelength increases. The extinction coefficient of the deposited films increases as a result of thermal annealing but both annealed and un-annealed decrease as wavelength increases. The refractive index values decreased on annealing. The direct band gap energy of as-deposited films ranges from 3.7 eV to 4.0 eV while on thermal annealing the films revealed the energy band gap of 3.3 eV to 3.8 eV, hence there is decrease in the band gap energy as a result of thermal annealing. The thin films that exhibit these properties are desirable for optical and optoelectronic applications. Keywords: AgAlSe, ternary thin film, electrochemical deposition method, optical properties. 1. INTRODUCTION Thin film technology is the basis of astounding development in the now world dominated solid state electronics. The usefulness of the optical properties of metal films and scientific agitation about the behaviour of two dimensional solids has been responsible for the immense interest in the study of science and technology of the thin films. Thin film studies have directly or indirectly advanced many new areas of research in solid state physics and chemistry which are based on phenomena uniquely characteristic of the thickness, geometry and structure of the film (West, 2003). The phenomenal rise in thin film researches is no doubt due to their extensive applications in the diverse fields of electronics, optics, space science, aircrafts, defense, solar energy research and many other industries. These investigations have led a numerous inventions in the forms of active devices and passive components, piezo-electric devices, micro-miniaturization of power supply, rectification and amplification, sensor elements, storage of solar energy and its conversion to other form, magnetic memories, super conduction films, interference filters, reflecting and antireflection coatings and many others (Kuanr et al, 2008) and (Rao and Shekhawat, 2013). Compound semiconductor thin films have been found to be of importance for many of these applications. These groups of semiconductors are now taking the place of the elemental semiconductors in many applications as their cost of production is far less than the elemental semiconductor counterparts. Most of the compound semiconductors are group II-IV, II-V and II-VI of the periodic table and many others. The combination of these elements to form compound will either be a binary, ternary or quaternary compound thin film depending on the number of elements that come together. These combinations in thin film form have been found to be of importance for solar energy development and many electronics device fabrications. Electro-deposition is a liquid phase thin film deposition method that is based on electrochemical reactions (reductions or oxidations) carried out using an external power supply. In addition to the power supply, at least two electrodes are needed, between which the current flows in the deposition solution. One of the electrodes is a working electrode, or substrate on which the film grows, and the other one is a counter electrode. According to Gamburg and Zangari, (2011) electro-deposition is a film growth process that consists in the formation of metallic or semiconducting coatings on conductive substrates, starting from metal ion precursors in a suitable solvent and occurring via a charge transfer process. This method has been used to deposit several thin films for many applications (Eom et al, 2010) and Kadirgan, et al, (2000). To the best of our knowledge there is no report in the literature on any properties combining the hetrostructure of silver aluminum selenide thin films. Subsequently, there are few reports on the depositions of aluminum selenide thin films, but it is of interest due to various uses of aluminum element which is among the group III elements in the periodic table. Silver which is a transition metal is used for many applications such as in electrical equipment, mirrors, medical and dental equipment, and jewelry. It is often used to make alloys with gold for some of these applications. With these attributes of silver and aluminum in mind, it is of interest to form chalcogenide films with them for important applications. Thus in this work, we employed electrochemical deposition method to deposit thin films of silver aluminum selenide so as to determine its optical properties for applications. 2.0 EXPERIMENTAL DETAILS Thin films of silver aluminum selenide were deposited on ITO Glass substrates using electrodeposition method. 0.1M of AgNO3 and 0.1M of AlCl3.6H2O solutions were used as the source of silver and aluminum ions respectively while selenium metal powder was the source of selenide ions. Variousf molar concentration of selenide source which are of the order 0.1 M, 0.2 M, 0.4 M and 0.5 M were optimized to determine the optimal combination. The thin films were deposited at room temperature by passing a constant voltage of 10 volts for 5 minutes over electrochemical deposition apparatus containing 12 ml solutions of each of Ag, Al and Se ions sources, pure carbon (graphite) electrode and working electrode (ITO/Glass substrates). Four different thin film samples via different selenide concentration were obtained. The whole process were repeated to obtain two sets of sample thin films in which one set is annealed at 150oC for 5 minutes. The samples annealed are coded as B1, B2, B4 and B5 while unannealed are coded as A1, A2, A4 and A5. The summary of the deposition are depicted in tables 2.1 and 2.2 below. The deposited films were characterized for the optical analysis using UV-VIS Spectrophotometers in the wavelength range of 300 nm to 1000 nm. Table 2.1: Variation of molar concentration of selenide source for annealed samples S/N Sample Conc. Selenium (Mole) Working Voltage, WV (V) Working Current, WC (A) Weight, Wt (g) 1 B1 0.1M 1.26 0.5 0.99 2 B2 0.2M 1.04 0.4 0.96 4 B4 0.4M 1.62 0.9 1.06 5 B5 0.5M 1.42 1.10 0.96 Table 2.2: Variation of molar concentration of selenide source for un-annealed samples S/N Sample Conc. Selenium (Mole) Working Voltage, WV (V) Working Current, WC (A) Weight, Wt (g) 1 A1 0.1M 1.35 0.5 1.21 2 A2 0.2M 1.03 0.4 1.41 4 A4 0.4M 1.14 0.9 0.83 5 A5 0.5M 1.41 1.2 0.82 3.0 RESULTS AND DISCUSSIONS Figure 3.1: Plot of absorbance against wavelength for un-annealed samples. Figure 3.2: Plot of absorbance against wavelength for annealed samples. 0 20 40 60 80 100 120 140 160 180 300 400 500 600 700 800 900 1000 1100 Absorbance A Wavelength λ (nm) A1 A2 A4 A5 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200 210 220 230 300 500 700 900 1100 Absorbance A Wavelength λ (nm) B1 B2 B4 B5 The plots of percentage absorbance against wavelength for deposited the films for un-annealed and annealed samples are displayed in figure 3.1 and 3.2 respectively. The figures showed that the absorbance of the films is very high in the UV regions for both sample but decreases towards the visible and infrared regions of electromagnetic spectrum. The effect of annealing on the as deposited thin films rather increases their absorbance values as shown in figure 3.2. Figure 3.3: plot of Percentage transmittance against wavelength for un-annealed samples. 0 10 20 30 40 50 60 70 300 450 600 750 900 1050 1200 % Transmittance Wavelength λ (nm) A1 A2 A4 A5 Figure 3.4: plot of Percentage transmittance against wavelength for annealed samples. Figures 3.3 and 3.4 are plots of percentage transmittance against wavelength for both unannealed and annealed samples respectively. The films transmit high in infrared regions, but thermal annealing of the films decreases the transmittance value in the region to the range of 10% to 50%. 0 10 20 30 40 50 60 300 400 500 600 700 800 900 1000 1100 % Transmittance Wavelength λ (nm) B1 B2 B4 B5 Figure 3.5: plot of reflectance against wavelength for un-annealed samples. Figure 3.6: plot of reflectance against wavelength for annealed samples -80 -70 -60 -50 -40 -30 -20 -10 0 10 20 30 300 400 500 600 700 800 900 1000 1100 % Reflectance Wavelength λ (nm) A1 A2 A4 A5 -130 -120 -110 -100 -90 -80 -70 -60 -50 -40 -30 -20 -10 0 10 20 30 300 400 500 600 700 800 900 1000 1100 % Reflectance R Wavelength λ (nm) B1 B2 B4 B5 The plots of reflectance against wavelength for the two sets (un-annealed and annealed) of the thin film samples are displayed in figures 3.5 and 3.6 respectively. The plots reveal that the reflectance is generally low in the range of 15% to 20% for un-annealed and 0% to 10% for annealed samples. Figure 3.7: plot of refractive index against wavelength for un-annealed samples 0 0.5 1 1.5 2 2.5 3 300 400 500 600 700 800 900 1000 1100 Refractive index (n) Wavelength λ (nm) A1 A2 A4 A5 Figure 3.8: plot of refractive index against wavelength for annealed samples The plots of refractive index against wavelength of the as deposited thin films as shown in figures 3.8 and 3.9 for both un-annealed and annealed samples indicate that the refractive index is in the range of 2.0 to 2.5 for un-annealed samples but the values decrease as a result of thermal annealing. 1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8 300 400 500 600 700 800 900 1000 1100 Refractive index (n) Wavelength λ (nm) B1 B2 B4 B5 properties of the deposited AgAlSe films. Based on the various optical behavours exhibited by the films of this nature, they are suitable for many applications such as absorber layer for solar cell and high temperature devices due to wide band gap value, poutry houses due high transmittance in infrared range and anti-reflection coating due to high refractive index and low reflectance values. REFERENCE West A.R (2003). Solid State Chemistry, John willey & Sons, Singapore. Page 50. Kuanr, B.K, Maat, S Chandrashekariaih, S,. Veerakumar, V., Camley R.E., and Z. Celinski Z., (2008). Journal of Appl. Phys. page 103, 07C107. Gamburg, Y.D and Zangari, G. (2011). Theory and Practice of Metal Electrodeposition; Springer: New York, NY, USA. Rao M. C and Shekhawat M. S. (2013). Basic Properties of Thin Films for Device Application International Journal of Modern Physics: Conference Series Vol. 22, page 576–582. Eom H, Jeon B, Kim D and Yoo B (2010). Electrodeposition of Silver-Nickel Thin Films with a Galvanostatic. 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