Annealing Effect on Structural and Optical Properties of SnO2 Thin Films

Abstract

Thin solid films of SnO2 were deposited by chemical vapor deposition (CVD) method on glass substrate at 450C and 2.5L/min.air flow rate. Deposited films were annealed at different temperature (200,300,400,500) C in air for 60 minutes. Structural properties have been studied for annealed and unannealed films. It was found that the films have polycrystalline structure with grain diameter(190-230)A depend on annealing temperature. And the crystal structure is tetragonal with lattice constants (a=b=4.782 ,c=3.233)A. Optical properties also studied for annealed and unannealed films using UV-visible spectroscopy. The transmission spectra showed higher transparency over the visible wave length region with maximum value (85-95)% depend on annealing temperature. Absorption edge energy is increased by annealing temperature from (3.93 to 4.1)eV while Optical energy gap is not affected by annealing and it is found to be (4.17)eV. Annealing Effect on Structural.... 100 Introduction Transparent, electrically conductive films have been prepared from a wide variety of materials. These include semiconducting oxides of tin, indium, zinc and cadmium; and metals such as silver, gold and titanium nitride .They have been used for many years as electrodes or resistive elements and applied to semiconductor devices as transparent gates on charge injection devices and charge coupled devices and as transparent barrier layers on solar cells[1]. The materials mostly used for such applications are tin oxide (SnO2), antimony-doped tin oxide and tindoped indium oxide. These films have been deposited by chemical vapour deposition, sputtering, evaporation and spraying [2]. Tin oxide films is the most used material because of its cheap raw material and highly existing in nature. It is characterized by a wide band gap and high electrical conductivity through the existing of O2 vacancies and interestial tin atoms in the crystal structure of the films and that is depend on method of deposition. Also it is a good transparent antireflecting material. Temperature stability and reproducibility of the film properties are important for device reliability. The films are exposed to heat during the preparation and operation of the device. This may cause structural changes which tend to either influence or degradation in most of the physical properties of the films. The effects of annealing are complex because of the variety of phenomenon that may observe. Many studies have been done on different transparent conducting oxides to find the effect of annealing on certain property of the films such as the refractive index (n), extinction index (k) and energy gap (Eg) for ZnO-SnO2 [3], resistivity and factor of merit for antimony-doped tin oxide[4], grain size, porosity and conductivity of tin-doped indium oxide[5] and many others [6,7,8,9]. In this study structural and optical properties of tin oxide films prepared by chemical vapour deposition are described. Also, the effect of annealing at different temperature in air on structural and optical properties of SnO2 thin films are studied including: grain size, lattice constant, transmission, optical energy gap and absorption edge. Experimental Detail The SnO2 thin films were prepared by CVD technique. Although this technique is well known and used by many workers[10], it still need some modification. We have modified and built this system in physics department, Education college, Mosul university, using the stannous chloride oxidation principle. The diagram of the experimental set-up used for SnO2 deposition is shown in Figure (1). Material used was SnCl2.2H2O (purity 99.99% supplied by Fluka Company). Films were Sanaa M. Al -Delaimy & Rajaa Abdulah Basheer 101 deposited on glass type (Gloim 1003/Italy) at 450°C substrate temperature and 2.5 L/min. gas flow rate. Fig.(1)Schematic Diagram of CVD System for SnO2 Films Deposition When the desired deposition factors were attained and stabilized the tin chloride was melted (melting point 247°C) using coil heater (350°C). The tin chloride vapour was driven through the reactor by maintaining an air flow. Air carries the SnCl2 vapour to form SnO2 film deposited on the hot substrate and the reaction takes place as follows: SnCl2 + O2  SnO2 + Cl2 After finishing the deposition the substrates were brought to room temperature inside the reactor. The deposition systems as well as the method have been described briefly in reference [11]. Some of the films were annealed in air by controllable electric oven type (Heraeus) at temperature (200,300,400,500)°C for 60 minutes, while the others were left without annealing. Structural properties were studied for annealed and unannealed films by X-ray diffraction (XRD) using a Philips X-ray diffractometer model (PW 1130). Source of radiation was Cu K (=1.5405 A°) and the scanning range of 2 was restricted to range (10-60)°. Perpendicular distance between adjacent crystal layers(dhkl) for each grain, Lattice constants (a=b, c) and grain diameter (D) of SnO2 film were calculated using the XRD pattern data and the following equations [12] : 2d hkl sin = n (1) 1 h k l ــ = ــ + ــ + ــ (2)