Indium Tin Oxide Thin Films Deposited at Low Temperature Using Dual Ion Beam Sputtering
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Indium Tin Oxide Thin Films Deposited at Low Temperature Using Dual Ion Beam Sputtering Wilhelmus J. Geerts, Nelson A. Simpson, Alan D. Woodall, Maclyn Stuart Compton Department of Physics, Texas State University, San Marcos, TX 78666, USA
ABSTRACT ITO samples were sputtered at room temperature by ion assisted dual ion beam sputtering using atomic or molecular oxygen. The electrical properties appear to depend on the oxygen flow rate during deposition and the resistivity decreases for samples sputtered at a higher oxygen flow rate (1-5 sccm). The resistivity is lowest at an oxygen flow rate of 4 sccm. The average absorption in the visible part of the spectrum also decreases as a function of the oxygen flow rate and is lower for samples sputtered with atomic oxygen. The figure of merit, i.e. the ratio of the conductivity versus the average absorption in the visible range, increases for higher oxygen flow rates and is typically 20-60% higher for samples sputtered using an atomic oxygen assist beam. INTRODUCTION Sn-doped In2O3, or ITO is used as a transparent electrode in various devices including flat panel displays, organic light emitting devices, solar cells, and optical detectors. It is a degenerate semiconductor that is doped by Sn substitutions and doubly charged oxygen vacancies. Although other transparent conducting materials have been proposed recently, ITO is still market leader with a market share of 93% in 2012. The material has a reported transmittance as high as 95% and a conductivity up to 1x104 Ω-1 cm-1. To reach those properties, ITO must be deposited at temperatures higher than 300◦C. It is therefore difficult to deposit high quality ITO on inexpensive and light plastic substrates such as optical grade polymer, as these materials require a deposition temperature below 150◦C. At such low temperatures un-activated tin decreases the carrier density, resulting in a typical conductivity no better than 1-1.6x103 Ω-1 cm-1. High quality ITO films on plastic substrates could reduce the price per watt of solar cells and result in lighter and cheaper flat-panel displays for mobile devices that are less susceptible to breaking. We therefore investigated whether or not it is possible to deposit high quality ITO at room temperature by dual ion beam sputtering (DIBS), while exposing the sample to an atomic oxygen beam during deposition. Although there is a substantial body of literature regarding the creation of ITO films using ion beam deposition and its properties [1-18], few studies have investigated the efficacy of dual ion beam sputtering (DIBS), and those that do, mostly investigated the effect of momentum transfer of an inert atom beam incident on the substrate during deposition. In this paper we investigated the effect of an oxygen assist beam, and compare samples made with atomic oxygen as opposed to samples made with molecular oxygen. Because the plasma used during DIBS is not directly in contact with the substrate but contained in the ion gun’s research tube, the rise in substrate temperature is minimize
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