Amorphous silicon
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Amorphous silicon (a-Si) is the non-crystalline allotropic form of silicon. Silicon is a four-fold coordinated atom that is normally tetrahedrally bonded to four neighboring silicon atoms. In crystaline silicon this tetrahedral structure is continued over a large range, forming a well-ordered lattice (crystal). In amorphous silicon this long range order is not present and the atoms form a continuous random network. Not all the atoms within amorphous silicion are four-fold coordinated. Due to the dissordered nature of the material some atoms have a dangling bond. These dangling bonds are defects in the continuous random network, which cause undesired (electrical) behaviour. The material can be passivated by hydrogen, which bonds to the dangling bonds and neutralises this defect. Hydrogen passivated amorphous silicon has a sufficiently low amount of defects to be used within devices.
Applications
One of the main advantages of amorphous silicon over crystalline silicon is that it is much more uniform over large areas. Since amorphous silicon is full of defects naturally, any other defects, such as impurities, do not affect the overall characteristics of the material too drastically. Also, just the fact that it can be deposited over large areas using PECVD in the first place gives it a huge advantage over crystalline silicon. Amorphous silicon is used as the active layer in thin-film transistors (TFTs) which are most widely used in large-area electronics applications, mainly for liquid-crystal displays (LCDs). It is one of the materials suitable for the microbolometer sensors. It is also considered a photovoltaic material, which can be 1/70th of the thickness of human hair. Large-area solar cells are a new area for amorphous silicon; however, the small solar cells used in pocket calculators have been made with a-Si for many years. a-Si can also be deposited at very low temperatures, as low as 75 degrees Celsius, which allows for deposition on not only glass, but plastic as well. Amorphous silicon is receiving much more attention at the present time because of the potential for roll-to-roll processing, whereby circuits are literally printed onto long sheets of plastic or metal foils. This processing technique is expected to be much cheaper than modern crystalline semiconductor manufacturing.Crystalline silicon generally has better electrical properties than amorphous silicon, but in recent years researchers in the field have been able to close the gap somewhat.
See also
External links
- [National (U.S.) Renewable Energy Laboratory Information on Amorphous Silicon]
- [How electronic inks will work]
- [Xsunx - A company which has developed a manufacturing technique for transparent solar cells]
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