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.