Electrical and optical properties of magnetron sputtered Cd2SnO4 transparent conducting oxide thin films for use in CdTe solar devices

Abstract

In this study cadmium stannate Cd2SnO4 (CTO), was sputtered onto borosilicate glass under substrate temperatures of ~25°C (room temp), 200°C, 300°C and 400°C. The CTO films were then annealed in contact with cadmium sulfide (CdS). This annealing process consisted of placing the CTO samples face up in a thermal furnace with a separate piece of CdS film on a borosilicate substrate face down on top of the CTO sample. This arrangement was then heated to a temperature of 600°C and annealed for one hour. The goal was to fabricate a CTO film with the highest transparency without sacrificing good electrical conductivity. The quality of CTO films, i.e. transparency and electrical conductivity can be greatly affected by processing conditions. In this paper the optical and electrical properties of CTO are investigated before and after thermal annealing to determine the affect of varying process conditions on the quality of the film. Spectroscopic ellipsometry analysis was performed on both as-deposited and annealed CTO to determine the nature of the optical response of the films over the spectral range of 0.73 to 3.34eV. The electrical properties of the films were examined using both Hall effect data and optical Drude modeling of free carrier absorption. Structural properties of the films were determined from X-ray diffraction data. The result of this investigation is a high quality CTO film with a sheet resistance of ~8Ω/□.