Study On Silicon Optical Waveguides Based On Free Carrier Dispersion Effect

Optical communication advances so fast that higher and higher performance optoelectronic devices are demanded. High performance and low cost optoelectronic devices can be effectively implemented by utilizing semiconductor integration technology. The primary optoelectronic materials include III-V compounds and wide band gapâ…¢-â…¤nitride such as AIN, GaN and InN. However, the application areas of these materials are limited due to their incompatibility with silicon material and high fabrication costs. Silicon photonics, which combines the silicon technology with the integrated optoelectronic technology and makes the silicon technology be applied from microelectronics to optoelectronics, is worthy of intensive investigation.It is well known that silicon exhibits no linear electro-optic (Pockels) effect because of its centrosymmetric crystal structure. However, efficient electro-optic modulation can be achieved by employing free carrier dispersion (FCD) effect. Additionnally, device size can greatly shrink due to the large difference of refractive index in silicon-on-insulator (SOI). Minimum line width of 30 or 40 nm can be fabricated by the microelectronic process, which has advanced for decades and can be well applied to the fabrication of silicon photonic devices. In this thesis, silicon optical waveguide device based on the FCD effect is theoreticly and experimentally investigated, to explore the feasibility of fabricating this kind of device by utilizing the CMOS process. This thesis makes innovative works mainly in the following three aspects:(1) The characteristic of SOI material, the electric structure of the pin diode and the requirements of the CMOS process are considered together to design feasible process flows. By using these process flows, silicon optical waveguide devices based on the FCD effect were successfully fabricated. In China, we are the first group to fabricate this kind of device by employing a commercial CMOS process.(2) pin-MZI based silicon optical waveguide devices are theoreticly and experimentally analyzed. Analysis shows that the heavily doping position can influence absorption loss, heat dissipation, extinction ratio (ER) and speed of the device. Moreover, a method to improve the ER is analyzed by adjusting dynamically the splitting coefficient. By employing this method, the ER can be improved to more than 45 dB in theory.(3) The photocurrent effect in silicon waveguides at 1550 nm wavelength is experimentally investigated. Obvious avalanche photocurrent due to the wide avalanche impacting region in the pin diode is measured, with multiplication factor of about 5. On the other hand, the enhancement of the photocurrent in the pn silicon waveguide is mainly attributed to (â…°) the overlap between the optical field and the depletion layer and (â…±) the strong electric field induced tunnelling effect. Finally, the influence of the photocarriers to a pn-MZI silicon optical modulator and methods to alleviate this influence are also discussed.