Optical properties of p-type SnO_x thin films deposited by DC reactive sputtering

Refractive index (n), extinction coefficient (k), effective complex dielectric function (ε) and band gap energy (Eg) of p-type SnO_x thin films from 0.75 to 4 eV are studied. 25 nm thick films were deposited by direct current (DC) magnetron sputtering in reactive argon and oxygen atmosphere at different relative oxygen partial pressure (OPP) followed by a post annealing treatment at 250 °C in air atmosphere for 30 min. The relative high Hall effect mobility (μ) of the SnO_x was attributed to the dominant SnO phase in films grown at 15% OPP. Films deposited at 5 and 11% OPP showed incomplete Sn oxidation resulting in a mixture of Sn and SnO phases with lower hole mobility. Optical transmittance (T) and reflectance (R) are described by assuming a model where the p-type SnO_x films are defined by a dispersion formula based on a generalization of the Lorentz oscillator model. The roughness of the films (r) was modeled by a Bruggeman effective medium approximation (BEMA). From the optical analysis, k in the visible spectral region show high values for films with phase mixture, while films with single SnO phase presented negligible values. Films with single SnO phase have low n, this latter result from the lower compact microstructure of these films. Also, energies associated to direct and indirect transitions of the Brillouin zone of the SnO_x films were identified from the evaluated ε. Finally, the increase in the values of Eg energy was related to the increase in the SnO phase.