Porous silicon thin films for photonic sensor technologies

[Truncated abstract] Semiconductor based sensing technologies are now ubiquitous in all areas of modern life, in such diverse applications as air-bag systems, pollution monitoring, chemical residue testing and proximity sensors in mobile phones. The proliferation of semiconductor based sensors is due to both the simple integration of such sensors into modern electronic devices and the rapid development of semiconductor research outside the realm of pure microelectronics. Semiconductors such as the group-III nitrides, diamond and porous silicon (PS) have recently created much interest due to their biocompatibility and stability, compared to the far more technologically mature materials such as Si and GaAs. The study of PS for sensor based devices has stemmed from discovery of its room temperature luminescence, large surface area and tunable porosity. The majority of PS based sensors are optical due to the relative ease at which multilayer optical devices such as Bragg mirrors and microcavities can be fabricated. PS is formed via the partial electrochemical dissolution of crystalline silicon in a HF based solution, hence the silicon starting wafer has a major effect on the resultant PS thin film. This thesis is focused on developing techniques that allow the fabrication of high quality, stable PS thin films from lightly doped starting wafers. Fabricating PS from lightly doped silicon starting wafers would enable transmissive devices to be fabricated, which have a number of benefits compared to reflectance devices. These benefits include the improvement of sensitivity due to the increased extinction ratio of transmissive over reflectance devices, and the ease of measuring transmissive devices compared to reflectance devices. There are a number of challenges associated with PS anodised from lightly doped starting wafers. The first is large unwanted porosity gradients that cause significant reduction in optical device performance...