Processing and size effects on the optical properties of sputtered oxide thin films

The variation in refractive index, extinction coefficient and optical absorption edge with thickness of magnetron sputtered vanadium and niobium oxide films and ytterbium oxide films deposited by both magnetron sputtering (MS) and ion beam sputtering (IBS) has been investigated. The thickness of the films was varied in the range 50–400 nm. In all the cases it is shown that with increase in thickness the extinction coefficient and the absorption edge decreased, while the refractive index increased. Typically, for the MS ytterbia films the variation was between 1.6 and 1.75 whereas it was between 1.7 and 1.8 for the IBS films. The absorption edge varied from 4.5 to 4.7 eV for ytterbia MS thin films while it was between 4.6 and 4.8 eV for the IBS films. It is demonstrated that independent of the material, below a critical thickness the variations in refractive index, extinction coefficient and absorption edge are interrelated. It is therefore postulated that independent of the processing technique a material dependent critical thickness has to be achieved for the films to exhibit bulk like behaviour and the absorption edge and refractive index to become independent of each other.