Effect of Substrate Type on the Electrical and Optical Properties of Cold-sputtered Indium Tin Oxide Films as a function
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Effect of Substrate Type on the Electrical and Optical Properties of Cold-sputtered Indium Tin Oxide Films as a function of Post-deposition Heat Treatment Salil M. Joshi1, Gregory W. Book2 and Rosario A. Gerhardt1,* 1 School of Materials Science and Engineering, Georgia Institute of Technology, J. Erskine Love Building, 771 Ferst Drive NE, Atlanta GA 30332-0245 2 Nanotechnology Research Center, Georgia Institute of Technology, 791 Atlantic Drive, Atlanta, Georgia 30332-0001 *[email protected] ABSTRACT Indium Tin Oxide (ITO) films were deposited by RF sputtering onto glass and quartz substrates with no external heating. An ITO target containing 10 wt% SnO2 was used for the deposition in a Kurt Lesker PVD75 system, in an atmosphere of 50% O2 + 50% Ar. Postdeposition heat treatments were done on these coatings at 150°C, 300°C and 450°C in an atmosphere of commercial air or argon. The effects of these heat treatments on the microstructure and the properties of the films were evaluated using atomic force microscopy, resistivity measurements, and UV-visible absorption spectroscopy. The heat treatments were observed to significantly affect the properties such as transmittance in the visible region, optical band gap and the electrical resistivity of the films. The main differences are caused by the differences in thermal expansion coefficient of the substrates as compared to the sputtered ITO films. INTRODUCTION Indium tin oxide (ITO) is widely used in the fabrication of transparent electrodes and circuits for displays and other optoelectronic applications that need transparency in the visible range, as well as reasonable conductivity 1. Crystalline ITO is transparent in the visible range on account of a large optical band gap, while at the same time, a partially filled conduction band gives it nearly metallic conductivity. The properties of a transparent conducting film not only depend on its chemical composition, but also the manner in which the film is made2. Commercial deposition of ITO onto glass substrates is typically carried out using pulsed laser, dc magnetron sputter systems, or by electron beam evaporation3, 4. These techniques require the substrates to be heated to a relatively high temperature during deposition, as well as undergo an additional post annealing step in order to achieve crystallization and conductivity enhancement in the ITO films5, 6. ITO deposition onto non heat-resistant substrates like polymer films is done by roll-toroll sputtering at room temperature, which results in ITO films with inferior electro-optical properties7, 8. This study investigates the effect of post-deposition heat treatment on ITO films that were deposited by RF sputtering at room temperature. EXPERIMENTAL ITO films were fabricated by RF sputtering onto glass substrates (0.5” × 0.5” pieces of borosilicate glass microscope slides from VWR) and quartz substrates (0.5” × 0.5” pieces of quartz microscope slides from Quartz Scientific). The sputtering was done using a Kurt Lesker PVD 75 sputter coater i
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