High index contrast optical platform using gallium phosphide on sapphire: an alternative to SOI?

Confinement of light at submicron wavelengths is of great importance for highly specific sensing of bio-molecules and for compact photonic circuits based on waveguiding. Currently this confinement can be achieved through the well established high index contrast silicon on insulator (SOI) platform. However this material combination requires light at wavelengths beyond 1 micron where the component cost of the InP based lasers and photodetectors are very expensive. It is thus of great interest to develop a similar platform that could operate in the range of 850 nm where low cost lasers (e.g. Vertical Cavity Surface Emitting Lasers as used in optical mice) and detectors (e.g., as used in camera phones) are readily available. A possible high index material suited to this application is Gallium Phosphide which has a bandgap of 2.26 eV and refractive index of ~ 3.2 at this wavelength. For the highest index contrast, GaP should be grown on a substrate with low index of refraction such as quartz (n=1.5) or sapphire (1.7). We report on the design and characteristics of GaP waveguides grown on c-plane (0001) sapphire substrates using metalorganic vapour phase epitaxy. Growth parameters such as substrate temperature and, in particular, the V:III ratio are reviewed with respect to their effect on the nucleation, surface roughness and uniformity of the films. Modal analysis and the design of a grating coupler at wavelengths around 850 nm have been designed for GaP on sapphire using vectorial finite element method in order to validate the feasibility of GaP waveguides.