Electronic structure of indium-tin-oxide films fabricated by reactive electron-beam deposition

Thin films of indium tin oxide [(ITO), In2O3:Sn] have been grown by reactive electron beam deposition. Annealing in ultrahigh vacuum and, subsequently, in situ room temperature scanning tunneling microscopy (STM) and spectroscopy have been carried out. STM images before annealing show the film composed of grains with sizes around 30 nm while images of samples annealed at 423 K and 573 K indicate that these grains coalesce with sizes of 60-80 nm. Scanning tunneling spectroscopy (STS) curves for the samples in the three different conditions indicate the existence of a small band gap with Fermi level position pinned in the center of the band gap. Optical reflectance and transmittance measurements have been performed in order to deduce optical and electronic properties. STS combined with optical data interpreted with Drude theory shows that the samples have a direct band gap of 3.5 (+/- 0.1) eV and an indirect band gap of 2.1 (+/- 0.2) eV.