Nonlinear thickness and oxidation-dependent transparency and conductance of sputtered titanium suboxide nanofilms

Optically transparent and electrically conductive semiconducting titanium suboxide nanometer thick films, which are technologically important in a wide variety of applications, show a nonlinear dependency of optical and electrical properties on film thickness and oxidation time. The optical, electrical, chemical, and structural properties of the sputter-deposited titanium suboxide nanofilms on fused quartz glass at room temperature are investigated by varying the film thickness, and the additional oxidation is controlled by the duration of air exposure. The optical properties of the nanofilms are simulated by considering them as both a homogeneous film as well as an inhomogeneous film with the combination of the Lorentz Drude model and the Maxwell Garnett effective medium theory (MG-EMT). Their electrical properties are simulated with the MG-EMT. The optical transmittance, electrical conductivity, secondary ion mass spectroscopy, x-ray photoemission spectroscopy analysis, and simulation of titanium suboxide nanofilms indicate that inhomogeneous film growth and oxidation are responsible for the nonlinear dependency. The oxygen atomic ratios to titanium for the as-deposited films depend on the deposition time and vary in 1.60-1.73, while those exposed to air for seven days increase to 1.79-1.99.