Porous silicon a useful imperfection

When semiconductor developers noticed in the 1950s that electropolishing of bulk silicon left certain areas rougher than the rest and somewhat porous, they regarded these simply as imperfect areas. It was not until Leigh Canham, a scientist with DERA (the UK’s Defence Evaluation and Research Agency), discovered in 1990 that porous silicon (PSi) emits visible light when activated by external ultraviolet sources1 that this morphological state of the material came to attract significant research interest. Elementary photonic sensors were soon proposed. In 1992 researchers discovered that PSi also emits light when an electric current is applied, a finding that raised prospects for new optronic sensors and other devices coupling light to electronics, including future high-speed computers. Technology extensions have since been found that make the material chemi- and bio-luminescent as well as photo- and electro-luminescent.In sight by the mid- to late-1990s, therefore, was a whole new class of solid-state sensors offering significant advantages over solid-state gas and other sensors. Many of these were based on bulk silicon and semiconducting oxides such as tin or indium oxide and alumina. Compared with them, PSi offers a high surface area-to-volume ratio and hence high reactivity and, as researchers have established, a porous structure whose morphology could be engineered for high selectivity to particular molecules. Along with high sensitivity and selectivity, comes a rapid response time.