Porous Silicon Sensors - from Single Layers to Multilayer Structures

1.1 Origin and discovery Silicon, one of the more common elements in nature, is defined as a metalloid, which corresponds to the number 14 in the Mendeleyev Periodic Table. It is heavier than Carbon (element number 6 in the periodic table and a key component in biochemistry), but both have chemical characteristics that are very close. Since human civilization began, volcanic stones containing this metalloid in the form of dioxide were used to create the first tools and weapons. Roman historian Plinio the Elder (23AD-79AD) mentioned the Silex-Silicis (silicon stones) in one of his works as very hard stones. These roman words are the Latin origin of the name Silicon (Tomkeieff, 1941). J.J.Berzelius was credited for the discovery of this element in 1824 in Stockholm, Sweden, but Gay-Lussac and Thenard had already prepared impure amorphous silicon by 1811. After World War II, once the applied mechanical technology was ready to produce very pure silicon wafers (under the form of monolithic crystals) and succeeded to manage the problem of the surface impurities and contamination (Hull, 1999), the electronic industry jumped from the Germanium diodes to the Silicon integrated circuits and metal-oxidesemiconductor (MOS) microprocessors that helped man reach the Moon. In summary, it is safe to say that Silicon’s role along our evolution extends from prehistoric times to the exploration of the Solar System. In 1956 at the U.S. Bell Laboratories, Arthur Uhlir Jr. and Ingeborg Uhlir while trying a new technique for polishing Silicon crystalline wafers observed for the first time a red-green film formed on the wafer surface (Kilian et al, 2009). Since the discovery of its luminescence properties by Leigh Canham in 1990 (Kilian et al, 2009), researchers started to study the nonlinear optical, electric and mechanical properties of this nanostructure. This effort has permitted the fabrication of uniform porous layers with diameters as small as one nanometer, and permitting an enormous inner surface density.