Correlation between structure, stress and optical properties in direct current sputtered molybdenum oxide films

MoO x films were prepared by reactive DC magnetron sputtering in an atmosphere of argon plus oxygen in an effort to determine the influence of the oxygen partial pressure on structural and optical properties of the films, as well as the resulting stress. Sputtering was carried out on glass and Si(100) substrates held at room temperature. X-Ray diffraction revealed the amorphous nature for most of the as-deposited films. Rutherford backscattering spectroscopy showed an almost linear increase in oxygen concentration at low oxygen partial pressure, while a small amount of excess oxygen was found at higher oxygen partial pressure. Optical spectroscopy and spectroscopic ellipsometry revealed that films, which were slightly substoichiometric, still showed polaron absorption of approximately 1.4 eV. For higher oxygen partial pressure, fully transparent films were deposited, which showed a slight increase in optical bandgap with increasing oxygen partial pressure, while the refractive index simultaneously decreased. This is in line with the slight decrease in film density observed in this regime by X-ray reflectometry. Possibly in this regime films possess an increasing amount of voids, since a moderate tensile stress is observed for these samples. On the other hand, a very high compressive stress is observed for low oxygen partial pressure.