International Journal of Analytical, Experimental and Finite Element Analysis (IJAEFEA), Issue. 2, Vol. 1, April 2014 © 2014 RAME IJAEFEA 1 Research Association of Masters of Engineering www.rame.org.in Santhosh Prabhu 1 prabhusanthoshmsc@gmail.com Dr. Dhananjaya Kekuda 2 kekuda@gmail.com Department of Physics, Manipal University, Manipal, Karnataka, India Blue Shift of Optical Band Gap in ZnO Thin Film Grown by Spin Coating Process Abstract - Optical band gap of ZnO thin films deposited on glass substrate by spin coating process was studied. The optical band gap of as-grown ZnO blue shifted from 3.11eV to 3.69 eV as the annealing temperature decreased from 300 to 100 °C. Also observed that there is a variation in the transmittance of ZnO thin film. X-ray diffraction measurements showed that samples deposited at 100 0 C, 200 0 C and 300 0 C consisted of amorphous phases without any prominent diffraction peaks. The AFM images showed that there is a decrease in the grain size, which is the evidence for the decrease in the transmittance. Index Terms- Spin coating, UV-visible, XRD, AFM, Optical properties I. INTRODUCTION Metal oxides exhibit a wide range of functional properties depending on their crystal structure and bonding between the metal cat ion and oxygen. Indeed, the electrical properties of metal oxides range from insulating to highly conducting like a metal, or even superconducting. Some metal oxides also exhibit magnetic properties such as ferromagnetism or ferrimagnetisms. Because of this diverse functionality, metal oxides have become one of the most fascinating inorganic materials in device applications such as light emitting diodes, field effect transistors, solar cells and spintronic devices. Among the metal oxides, ZnO is attractive because the metals are abundant on earth, inexpensive and non- toxic. Moreover, these oxides have useful optical and electrical properties suitable for a wide variety of electrical devices. As synthesized ZnO is almost always an n-type semiconductor with high mobility (10 ~ 200 cm 2 / V sec) in thin high quality films at room temperature. This is due to the oxygen vacancies in the grown films. ZnO is a wide band gap (3.3 eV) semiconductor and therefore, it has been considered as a potential material for transparent electrode and electronics and widely used in solar cells as window layer and transparent thin film transistors. II. EXPERIMENT The ZnO thin film used in this study are grown on the glass substrate by spin coating process. The glass substrates were cleaned with acid (con.H 2 SO 4 ), then soap solution followed by ultra-sonication in distilled water for 1 hr and subsequently dried. For sol preparation Zinc acetate di-hydrate (Zn (CH 3 COO) 2 ·2H 2 O) was first dissolved in a 2-methoxyethanol ((CH 2 )2CHOH) with mono ethanolamine (MEA: H 2 NCH 2 CH 2 OH) which was used as a stabilizer. The molar ratio of MEA to zinc acetate was kept to 1.0 and concentration of zinc acetate was 0.8 mol/l. The resultant solution was stirred at 60˚C for 1 hr to yield a clear and homogeneous solution ready for coating. The spinning rate was kept at 1000rpm. The wet films were dried at room temperature for 10 min. The process was repeated to obtain the desired thickness of the film. Multilayer films were post annealed at 100 0 C, 200 0 C and 300 0 C for 1hr to study the effect of annealing. The structural properties of the prepared films were studied by X-ray diffraction measurements (Rigaku-miniflex600). The Atomic Force Microscopy (Innova SPM) was used to find the surface morphology of the grown films. III. RESULT AND DISCUSSION A. Optical Properties Fig. (1) Shows the representative transmittance spectra of 0.8 M ZnO thin films, annealed at different temperatures. It is observed that as annealing temperature increases there is increase in the transmittance. Fig:1 Effect of annealing temperature on optical transmittance It is probably due to the decrease in the grain size of the ZnO thin films [1]. In this experiment 0.8M concentration solution is used. The transmittance is observed between 91% to 95%. for all the films. The highest transmittance is obtained for the film which was annealed at 300 0 C and lowest in the case of the film which was annealed at 200 0 c. The absorption coefficient can be calculated from the relation [2] T = A exp (-αd) Where T is the transmittance of thin film, A is a constant, and d is the film thickness. The constant A is approximately unity, as the reflectivity is negligible and insignificant near the absorption edge. The optical band gap of the films is determined by applying the Tauc model [3]