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Amorphous silicon

Amorphous silicon (a-Si) is the non-crystalline form of silicon. Silicon is normally tetrahedrally bonded to four neighboring silicon atoms. This is also the case in amorphous silcon, however, it does not form a continuous crystalline lattice as in crystalline silicon. Some atoms may actually have dangling bonds, which occur when it does not bond to four neighboring atoms. Since not all the atoms are four-fold coordinated, amorphous silicon is said to be under-coordinated. These dangling bonds are defects in the continuous random network, which can be passivated by introducing hydrogen into the silicon. It then becomes Hydrogenated amorphous silicon.

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). 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 deposted 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.

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