Optical and electrical properties of sol-gel derived Ba _1-xLa_xSnO₃ transparent conducting films for potential optoelectronic applications

Perovskite-structured Ba_1-xLa_xSnO₃ (BLSO, x = 0-0.10) films have been directly deposited on (0001) sapphire substrates by a sol-gel method. The effects of La substitution on the microstructure, morphology, and carrier density of the BLSO films have been investigated by X-ray diffraction (XRD), atomic force microscopy (AFM), scanning electron microscopy (SEM), and Hall-effect measurement. XRD analysis shows that all of the films are of a single perovskite phase and the calculated lattice constants agree well with the theoretical lattice parameters. The AFM and SEM pictures show that the BLSO films do not contain cracks with the root-mean-square surface roughness of 2 nm. Temperature dependent conductivity behavior suggests that BLSO films do have a metallic-like conduction, but the resistivity in poly samples is dominated by grain boundary scattering and ionized-dopant scattering. The films exhibit high transparency with more than 80% in the visible region, which is due to the prohibition of electron transition from the conduction band minimum to the second conduction band. Dielectric functions in photon energy of 0.5-6.5 eV can be uniquely extracted by fitting transmittance spectra with the double Tauc-Lorentz and Drude model. It was found that there are two interband transitions (about 4 and 6 eV) and the peak energies increase linearly with increasing La concentration. Far-infrared reflectance spectra was measured in the frequency range of 50-700 cm^-1 and three main reflection bands can be observed. The phonon strength of BLSO films decreases with increasing La concentration.