Optical transparency and electrical conductivity of nonstoichiometric ultrathin InxOy films

The effect of thickness and composition on the electrical conductivity and optical transparency, mainly in the infrared, of ultrathin InxOy films was studied. InxOy films 35–470 Å thick with oxygen atomic fractions of ∼0.3 and ∼0.5 were prepared via dc magnetron sputtering. All films were polycrystalline, consisting of only the cubic bixbiyte phase of In2O3. The average grain size of the films increased from 30 to 95 nm as the film thickness increased. The weak dependence of the electrical conductivity on the frequency and the low activation energies for conduction, a few hundredths of an eV, provided an indication that free band conduction was the primary electrical conduction mechanism in the case of all ultrathin InxOy films. It was found that introducing a high degree of nonstoichiometry in the form of oxygen deficiency did not help improve the electrical conductivity, since not all vacancies contributed two free electrons for conduction and due to impurity scattering. The optical nature of these films, studied mainly by ellipsometry, was found to be dependent on the film’s composition and thickness. In the infrared, the dielectric function of all InxOy films was consistent with the Drude model, inferring that the transparency loss in this region was a result of free charge carriers. In the visible however, InxOy films under 170 Å, which had an oxygen atomic fraction of ∼0.5, were modeled by extending the Drude model to the shorter wavelengths. Films over 170 Å, with the same composition, were modeled using the Cauchy dispersion model, meaning that no absorption was measured. These results indicate that, optically, under specific compositions, ultrathin InxOy films undergo a transition from metalliclike behavior to dielectric behavior with increasing film thickness. Using a figure of merit approach, it was determined that a nonstoichiometric 230 Å thick InxOy film, with an oxygen atomic fraction of ∼0.3, had the best combination of conductivity and transparency, namely, absorption of less than 20% in the infrared, about 10% in the visible, and electrical resistance of only 230 Ω at 20 kHz. Such a film may be classified as a highly conductive transparent oxide even in the infrared.