Effect of niobium doping on opto-electronic properties of sol–gel based nanostructured indium tin oxide thin films

Optically transparent conducting niobium-doped indium tin oxide (ITNO) thin films were deposited on soda lime glass via sol–gel spin coating technique under various Nb-doping contents. The effect of different Nb-dopant contents on the opto-electronic as well as microstructural properties of the films were characterized by means of UV–vis spectroscopy, four point probe, X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), and atomic force microscopy (AFM). For this purpose, addition of different Nb contents with tin at fixed 6 at% is considered as the first approach. For the second and third approaches, different Nb-doping contents were added at fixed atomic ratios of In:Sn=94:6 and In:Sn=95:5, respectively. Optimum opto-electronic properties, including minimum resistivity of 37.2×10−3 Ω cm with average transmittance of about 85% in the visible region and band gap of 3.48 eV was achieved for ITNO thin films prepared by the first approach at Nb and Sn doping content of 1 and 5 at%, respectively. XRD analysis revealed patterns of cubic bixbyite structure of In2O3 and rhombohedral structure of ITO for Nb-doped films with small shift in major peak position to lower diffraction angles with the addition of Nb. FESEM micrographs show that addition of Nb causes clustering of the grains at higher doping content. However, AFM studies show that addition of Nb dopant leads to the formation of films with compact surface and less average roughness compare to the undoped ITO films.