Effects of bias pulse frequencies on reactively sputter deposited NbOx films

Within the current work, niobium oxide thin films have been deposited by reactive d.c. magnetron sputtering from cold gas sprayed niobium targets, applying a systematic variation of oxygen partial pressure, bias pulse frequency and substrate temperature. The films grown without additional substrate heating are amorphous. X-ray photoelectron spectroscopy shows the Nb5+ oxidation state, characteristic for the Nb2O5 phase, for films grown at the highest oxygen partial pressure applied, while the Nb4+ state of NbO2 is additionally present at lower oxygen partial pressures. With increasing oxygen partial pressure, an optical shift from non-transparent to nearly fully transparent was observed, whereby the higher bias pulse frequency leads to a transition already at lower oxygen partial pressures. The transparent films are characterized by a maximum transparency of 80% and a refractive index of 2.5. Increasing the substrate temperature up to 600 °C results in the formation of well crystalline coatings consisting of the orthorhombic Nb2O5 phase, which are whitish and semi-transparent.