Abstract
This research project explored the potential of forming an integrated optics technology based on silicon core waveguides suitable for application in sensors and communications in the wavelength range 1.2 to 1.6 mum. Integrated optics has evolved around the use of compounds such as lithium niobate and III-V semiconductors due to their available electro-optic properties. By contrast silicon has received relatively little attention as its indirect band gap has prevented the fabrication of light sources in the material and its centrosymmetric crystal structure means that it has no useful linear electro-optic effect. The lack of a demonstrated low loss integrated optical waveguide compatible with single mode optical fibres has been a further limitation. However, these major drawbacks in silicon waveguide technology may be more than offset by the potential advantages of forming silicon integrated optical devices using well established silicon microelectronics fabrication methods. The project focused research on waveguiding in silicon-on-insulator (SOI) structures with the aim of developing a practical low loss waveguide in these structures and understanding the various loss mechanisms. In principle the optical absorption of pure crystalline silicon over the wavelength range of interest allows waveguides with losses less than 0.1 dB/cm to be formed. SOI material formed by ion implantation has been developed for microelectronic applications and provides a commercial source of a silicon planar waveguide structure with high quality interfaces and low defect density. The project studied waveguides based on this material. Initially planar waveguides with silicon thickness from 0.57 to 7.3 microns and buried oxide thickness of 0.07 to 0.4 microns were studied. Fabrication methods and structures were identified which allowed multi-microns planar SOI waveguides to be formed with losses less than the benchmark of 1 dB/cm. For these structures a buried oxide thickness of 0.4 microns was found to be sufficient to prevent substrate leakage loss. It has been concluded that the predominate loss mechanism is scattering of light at the silicon to buried oxide interface. Rib waveguides were formed in SOI following the insight into loss mechanisms gained in the planar waveguide studies. Optical rib waveguides with widths from 2.73 to 7.73 microns were formed in SIMOX (Separation by IMplantation of OXygen) based SOI structures consisting of a 4.32 micron thick surface silicon layer and a 0.398 micron buried oxide layer. The effect of waveguide width, bend radius, Y-junction splitting and interface roughness on loss and mode characteristics were studied at wavelengths of 1.15 and 1.523 microns. The experimental results support the hypothesis that certain rib dimensions can lead to single mode waveguides even though planar SOI waveguides of similar multi-micron dimension are multimode. The propagation losses of waveguides 3.72 microns wide were found to be 0.0 dB/cm and 0.4 dB/cm for the TE and TM modes respectively when measured at 1.523 microns. The measurement uncertainty was estimated to be +/-0.5 dB/cm. These results are thought to be the lowest loss measurements for silicon integrated optical waveguides reported to date. During the course of the project other researchers have demonstrated useful electro-optic properties in silicon semiconductor junctions based on the free carrier plasma dispersion effect and room temperature electroluminescence in silicon based junctions. The combination of these developments with the practical waveguide structure demonstrated in this project now makes the possibility of developing a practical silicon based integrated optics technology a reality.