Influence of deposition pressure on the optical properties of DC-Magnetron sputtered Indium Tin Oxide (ITO) thin films

Abstract

Indium tin oxide (ITO) has continued to attract great interest in the research and technological world because it bears a valuable compromise of both high optical transparency and high electrical conductivity. These twin properties make it very useful in architectural glass applications and in numerous optoelectronic devices such as front electrodes for solar cells, liquid crystal and gas discharge displays, photodiodes, image sensors and organic light-emitting devices. Magnetron sputtering, especially its reactive version, is the preferred method of preparing thin films of ITO. However, `target poisoning' remains a major drawback in preparing films with optimal characteristics. In this work, ITO films have been prepared by DC magnetron sputtering of a ceramic ITO (90% In2O3, 10% SnO2) target, without the use of oxygen gas. By varying the deposition pressure (0.5-3.0 Pa) due to argon gas, films of thickness 300-375 nm have been obtained, and their optical behaviour studied by optical spectroscopy. The optical analysis showed that the refractive index decreases while the optical band gap increases (3.24-3.33 eV) with increasing deposition pressure. This variation in the refractive index has been relaxed to the increase in film porosity, and the change in porosity calculated using Bruggeman's and Maxwell Garnett's effective medium theories. The shift in the optical band gap can be explained by the Moss rule. Films deposited at an argon pressure of 2.5-3.0 Pa have shown an optical transmittance of 85 % averaged over the visible region.