The key of ITO films with high transparency and conductivity: Grain size and surface chemical composition

•Grain size / surface chemical composition are keys for ITO films with high performance.•Oxygen vacancy and surface oxygen desorption depend on the annealing atmosphere.•Low conductivity of ITO film can be improved by annealing in reducing gas.•The resistivity and transmittance of ITO coated glass are 1.75 × 10−4 Ω cm and 88%. In this paper, tin doped indium oxide (ITO) films with high transparency and conductivity were fabricated on quartz by sol-gel method. The effect of four kinds of annealing atmospheres on the microstructure, morphology, surface chemical composition and optical-electrical properties has been investigated. Results show that the microstructure and morphology of the ITO films does not change much after heat treatment under different atmospheres. All annealed samples have the cubic bixbyite crystal structure of In2O3, with fine and uniformly distributed grains. The average grain size of each sample is approximately 10 nm. However, a noticeable dependence on the type of atmosphere is observed for oxygen vacancies and surface oxygen desorption. Different from oxidizing atmosphere, reducing or inert atmosphere is conducive to the formation of oxygen vacancies and desorption of adsorbed oxygen. Oxygen vacancies can effectively increase the carrier concentration of ITO. Oxygen desorption cannot only increase the carrier concentration, but also improve the carrier mobility. The issue for low conductivity of ITO nanocrystalline films can be solved by annealing in reducing atmosphere. This method can effectively control the grain growth and the chemical state of surface oxygen. The resistivity, average transmittance and Haacke FOM of the optimized ITO coated glass are 1.75 × 10−4 Ω cm, 88% and 0.0398 Ω−1, respectively.