Optical Properties of Undoped and Indium-doped Tin Oxide Thin Films

Abstract

Thin films of Tin Oxide (SnO₂), having thickness of 200 nm, were formed on to glass substrates by thermal evaporation of high-purity SnO₂ powder in vacuum at various substrate temperatures (T_S), ranging between 25 and 200°C. SnO₂ films with varying thickness were also prepared for a fixed T_S = 100°C. Further, doping of SnO₂ films with Indium (In) was accomplished through solid state diffusion process by successive deposition of SnO₂ and In films and subsequent annealing at 200°C for 10 minutes. Both undoped and doped films were characterized optically by UV-VIS-NIR spectrophotometry in the photon wavelength ranging from 300 to 2500 nm. In the visible photon wavelength range, the average optical transmittance (T%) of the films with varying T_S was found to be 85%. The maximum value of T % was found to be 89 % around the wavelength of 700nm. The variation of absorption coefficient with photon energy in the fundamental absorption region is the steepest for T_S = 100°C. The sub-band gap (SBG) absorption is also minimum for this T_s. A fluctuating behavior of the band gap energy (Eg) with T_s is observed attaining the highest value of 3.59 eV for T_s = 100°C. The band gap energy increases with thickness but T% in the visible range decreases. The T% in the visible range varies inversely with indium doping, being highest for undoped films. The E_g increases upto 2 wt% In doping and gradually decreases for enhanced doping. It seems reasonable to conclude that In doping does not bring favorable optical characteristics. Undoped SnO₂ films having thickness of 200 nm and formed at substrate temperature of 100°C yield essential acceptable properties for photovoltaic applications.

DOI: http://dx.doi.org/10.3329/jbas.v35i1.7975

Journal of Bangladesh Academy of Sciences, Vol.35, No.1, 99-111, 2011