Effects of multiple heat treatment cycles on structure, optical and electrical properties of indium-tin-oxide thin films

Post-annealed optimized RF-sputtered indium-tin-oxide (ITO) films were further treated at high temperature environments to investigate the effects of multiple heat treatment cycles on their structure, optical and electrical properties. Identification of intrinsic changes to the ITO characteristics is crucial because ITO films are eventually used in further applications that involve more treatment processes, which in turn, could alter the initial optimized properties. After further heating the post-annealed optimized films in atmospheric air, a red-shift occurred in the optical transmittance with a narrowed bandgap energy from 4.00 eV to 3.94 eV due to decrease in the free electron charge density caused by the growth of ITO crystal grains. Additional heating at temperatures more than 300 °C was also found to increase the electrical sheet resistance of the ITO rapidly from 13.4 Ω per square at 300 °C up to 54.1 Ω per square at 600 °C. The results indicate that the ITO electrical conductivity is more greatly influenced by the decrease of the electrically active tin dopant concentration and oxygen vacancies than by improvement of its crystallinity and stoichiometry. This work provides fundamental insights regarding multiple heat treatments of ITO thin films.