Porous Silicon Electrical and Optical Biosensors

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Porous silicon (PSi) is a form of silicon with unique properties, distinct from those of crystalline, microcrystalline, or amorphous silicon. It was first prepared in 1956 [1] and much later it was identified as etched silicon [2]. The most common fabrication technique to produce PSi is electrochemical etching of a crystalline silicon wafer in a hydrofluoric (HF) acid-based solution [3]. The electrochemical process allows for precise control of the structural properties of PSi such as thickness of the porous layer, porosity, and average pore diameter. The morphology of PSi is important for sensing applications because the pore diameter limits the size of the species that can be captured. PSi can be prepared with a wide range of optical and electrical properties which makes it a very flexible material. The internal surface of PSi is very large, ranging from a few to hundreds of square meters per gram [4]. Therefore, the properties of PSi are affected not only by its crystalline core and nanomorphology but also by its surface. In addition, the properties of the crystalline core differ from those of bulk c-Si, when the size of the silicon structures drops below 10 nm, a size regime in which quantum mechanical effects modify its electronic states [5, 6].