Effect of annealing on refractive indices of radio-frequency magnetron sputtered waveguiding zinc oxide films on glass

Abstract

The effects of annealing and gas composition on the refractive indices of zinc oxide films were studied in light of the structural properties. ZnO films (1μm) were deposited by rf magnetron sputtering in different oxygen:argon mixtures on glass and annealed at 380°C in air, at different times. Waveguide modes were excited in the films by prism coupling using a He-Ne laser. The estimated values of the extraordinary and ordinary refractive indices of the films, which were close to the corresponding bulk values (ne=2.006, no=1.990), initially decreased with annealing time and later increased before becoming constant with further annealing. The variation in refractive indices was explained on the basis of contribution from both packing density p and lattice constant c of the films. The initial decrease in refractive indices was attributed to the observed lattice contraction, and the latter increase was explained in terms of the increase in packing density (p) of the films on annealing. A relation is proposed to estimate the refractive indices of films, which have the lattice constant c different from the bulk value.