Microstructure effects on light propagation in zinc-sulfide thin film waveguides.

Abstract

The optical propagation losses resulting from the internal microstructure of ZnS thin films were investigated using a wavelength technique. Waveguide losses were determined by measuring the scattered light as a function of propagation distance along the film. Accurate measurements were obtained by using a technique we developed that employees a coherent fiber bundle to transfer the scattered light streak to a remote image plane that was scanned with an apertured photomultiplier tube. Microstructure effects on losses were found to dominate effects caused by substrate surface finish. The magnitude of the loss was found to depend upon two independent parameters: the average grain size of the polycrystalline films and the refractive index difference between ZnS and the interstitial material. Increasing the H₂O partial pressure led to lower losses as a result of reduced crystallite size, and a change in preferential crystallite orientation. A similar change in orientation was observed for films deposited onto heated substrates. Increasing the O₂ partial pressure during deposition also resulted in slightly lower waveguide losses, possibly as a result of void filling with ZnO. The modal dependence of the losses for ZnS films deposited at ambient temperature suggests that volume losses dominate surface losses for the lowest order mode while the ratio of surface to volume losses increases for higher order modes. By depositing ZnS onto substrates cooled with liquid nitrogen, adatom surface mobility was reduced which resulted in amorphous films. Losses were minimized (≤0.5 dB/cm at λ = 633 nm) for a substrate temperature of -50°C. These losses are lower than any previously reported for ZnS. However, further reduction of the substrate temperature resulted in an increase in tensile stress which eventually led to higher waveguide losses and crazing. The films deposited onto cooled substrates exhibited a low refractive index which indicates a low packing density and increased porosity. Differential water desorption, which is further evidence of increased porosity, was most noticeable in films with lower refractive indices when nonlinear prism coupling was attempted.