Abstract
Total internal reflection (TIR) based optical switches offer wavelength insensitivity, thermal stability, a short device length, the ability for over-driving without pre-emphasis and polarisation independence. When implemented in silicon the plasma dispersion effect is normally employed, using a PIN diode to inject carriers into the active region of the device, and allowing electrical control of the switching operation. The free diffusion of these injected carriers inhibits the formation of a large and abrupt spatial variation in free carrier concentration (and therefore refractive index) as required for an efficient switching operation. In this work the use of two types of carrier restrictive barrier have been investigated as feasible methods of improving the injected free carrier profile. The first barrier consists of an insulating silicon dioxide layer which completely isolates the PIN diode from the surrounding silicon. A 2mm thick layer has been shown to be thin enough such that the propagating light is not significantly perturbed, whilst being effective at blocking free carrier diffusion. Full device modelling has demonstrated an improved performance over the only other carrier injection based TIR switch in silicon-on-insulator (SOI) from the literature. The second barrier consists of a region of ion implantation induced defective silicon which is positioned along the opposite half of the switching region to the PIN diode. Defective silicon has a decreased free carrier lifetime and therefore any free carriers which diffuse into such material experience an enhanced recombination rate, resulting in reduced diffusion lengths. Experimentally it has been shown that the required electrical and optical properties can be produced by fully amorphising the SOI overlayer and then thermally regrowing it into polycrystalline silicon. Results obtained from fabricated devices with defective silicon barriers have shown an improvement in switching performance over those without barriers, hence successfully demonstrating the principle of barrier enhanced switching.