High-quality transparent conductive indium oxide film deposition by reactive pulsed magnetron sputtering: Determining the limits of substrate heating

Abstract

A transparent conductive oxide (TCO) should have a combination of high electrical conductivity and optical transmission property to fulfill the challenging demands of industrial applications. So far, doped TCOs have been mostly considered to fulfill the technological challenges. In this study, we demonstrate that indium oxide (In2O3) thin films without intentional doping can be deposited with high crystallinity under specific film growth conditions leading to thin films with high mobility, high electrical conductivity, and high transmittance. In2O3 thin films have been deposited by reactive pulsed direct current magnetron sputtering (pulsed DCMS) from an indium target on substrates in a temperature range from room temperature (RT) to 600 °C. Detailed investigations on In2O3 thin films are performed and the film properties such as crystallinity, microstructure, chemical bonding states, electrical and optical properties are revealed. Enhanced plasma density and ionization degree provided by the employed reactive pulsed DCMS and the intentional substrate heating during the thin film deposition give possibility to deposit highly crystalline thin films which are preferentially oriented in (222) direction. The substrate heating enhances the crystallinity of the grown films up to a certain optimum temperature: 400 °C. When the substrate temperature is above 400 °C, the carrier concentration and mobility decrease due to the grain refinement effect caused by growth competition of grains in different orientations. The films grown at 400 °C indicate the presence of high oxygen vacancy concentrations, which can directly be associated with a high charge carrier concentration despite the lack of intentional doping. The undoped In2O3 films in this study grown at 400 °C show highly promising and competitive electrical properties such as low resistivity of 1.28 × 10-4 Ω⋅cm and the high carrier mobility of 69 cm2/Vs. The average transmittance of the In2O3 films with the highest conductivity is found to be greater than 80% in the visible to the near-infrared spectral region owing to an enhancement in the carrier mobility and an optical bandgap in the range from 3.61 eV to 3.77 eV.