Porous silicon

Porous silicon (abbreviated as 'PS' or 'pSi') is a form of the chemical element silicon that has introduced nanopores in its microstructure, rendering a large surface to volume ratio in the order of 500 m2/cm3.When purely aqueous HF solutions are used for the PS formation, the hydrogen bubbles stick to the surface and induce lateral and in-depth inhomogeneity Porous silicon (abbreviated as 'PS' or 'pSi') is a form of the chemical element silicon that has introduced nanopores in its microstructure, rendering a large surface to volume ratio in the order of 500 m2/cm3. Porous silicon was discovered by accident in 1956 by Arthur Uhlir Jr. and Ingeborg Uhlir at the Bell Labs in the U.S. At the time, the Ulhirs were in the process of developing a technique for polishing and shaping the surfaces of silicon and germanium. However, it was found that under several conditions a crude product in the form of thick black, red or brown film were formed on the surface of the material. At the time, the findings were not taken further and were only mentioned in Bell Lab's technical notes. Despite the discovery of porous silicon in the 1950s, the scientific community was not interested in porous silicon until the late 1980s. At the time, Leigh Canham – while working at the Defence Research Agency in England – reasoned that the porous silicon may display quantum confinement effects. The intuition was followed by successful experimental results published in 1990. In the published experiment, it was revealed that silicon wafers can emit light if subjected to electrochemical and chemical dissolution. The published result stimulated the interest of the scientific community in its non-linear optical and electrical properties. The growing interest was evidenced in the number of published work concerning the properties and potential applications of porous silicon. In an article published in 2000, it was found that the number of published work grew exponentially in between 1991 and 1995. In 2001, a team of scientists at the Technical University of Munich inadvertently discovered that hydrogenated porous silicon reacts explosively with oxygen at cryogenic temperatures, releasing several times as much energy as an equivalent amount of TNT, at a much greater speed. (An abstract of the study can be found below.) Explosion occurs because the oxygen, which is in a liquid state at the necessary temperatures, is able to oxidize through the porous molecular structure of the silicon extremely rapidly, causing a very quick and efficient detonation. Although hydrogenated porous silicon would probably not be effective as a weapon, due to its functioning only at low temperatures, other uses are being explored for its explosive properties, such as providing thrust for satellites. Fabrication of porous silicon may range from initial formation through stain-etching or by forming an anodization cell. Drying, storage of porous silicon, and surface modification are needed afterwards. If anodization in an aqueous solution is used to form microporous silicon, the material is commonly treated in ethanol immediately after fabrication, to avoid damage to the structure that results due to the stresses of the capillary effect of the aqueous solution. One method of introducing pores in silicon is through the use of an anodization cell. A possible anodization cell employs platinum cathode and silicon wafer anode immersed in hydrogen fluoride (HF) electrolyte. Recently, inert diamond cathodes are used to avoid metallic impurities in the electrolyte and inert diamond anodes form an improved electrical back plate contact to the silicon wafers. Corrosion of the anode is produced by running electric current through the cell. It is noted that the running of constant DC is usually implemented to ensure steady tip-concentration of HF resulting in a more homogeneous porosity layer although pulsed current is more appropriate for the formation of thick silicon wafers bigger than 50 µm. It was noted by Halimaoui that hydrogen evolution occurs during the formation of porous silicon. The hydrogen evolution is normally treated with absolute ethanol in concentration exceeding 15%. It was found that the introduction of ethanol eliminates hydrogen and ensures complete infiltration of HF solution within the pores. Subsequently, uniform distribution of porosity and thickness is improved.