Fabrication and characterization of Zinc Oxide based rib waveguide

摘要

In this work we investigate the possibility to use Zinc Oxide (ZnO) thin films, deposited by RF magnetron sputtering, for the realization of integrated optical structures working at 1550 nm. Structural properties of sputtered zinc oxide thin films were studied by means of X-ray Diffraction (XRD) measurements, while optical properties were investigated by spectrophotometry and Spectroscopic Ellipsometry (SE). In particular, ellipsometric measurements allowed to determine the dispersion law of the ZnO complex refractive index n = n -jk through the multilayer modeling using Tauc-Lorentz (TL) dispersion model. We have found a preferential c-axis growth of ZnO films, with slightly variable deposition rates from 2.5 to 3.8 A/s. Conversely, the refractive index exhibits, from UV to near IR, a considerable and almost linear variation when the oxygen flux value in the deposition chamber varies from 0 to 10 sccm. In order to realize a waveguide structure, a 3-μm-thick ZnO film was deposited onto silicon single crystal substrates, where a 0.5-μm-thick thermal SiO_2 buffer layer was previously realized, acting as lower cladding. Dry and wet chemical etching processes have been investigated to achieve controllable etching rate and step etching profile, with the aim to realize an optical rib waveguide. The etched surfaces were inspected using scanning electron microscopy (SEM) and optical microscopy. Moreover, we carried out the experimental measurements of the fringes pattern and Free Spectral Range (FSR) of an integrated Fabry-Perot etalon, obtained by cleaving of a single mode rib waveguide.