Amorphous silicon

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 crystalline 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 silicon are four-fold coordinated. Due to the disordered 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 can reduce the dangling bond density by several orders of magnitude. Hydrogenated amorphous silicon (a-Si:H) has a sufficiently low amount of defects to be used within devices. However, the hydrogen is unfortunately associated with light induced degradation of the material, termed the Staebler-Wronski Effect.

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Applications

One of the main advantages of amorphous, over crystalline silicon relies in its production technique, as thin films of it can be deposited over large areas by PECVD, as opposite to crystalline silicon (c-Si) wafers, which are sliced from bulk monocristalline boules. It can be doped in a fashion similar to c-Si, to form p- or n-type layers and ultimately to form electronic devices. For this reason, amorphous silicon has become the material of choice for 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 also used to produce large-area photovoltaic solar cells. This is a relatively new application, although the small solar cells used in some 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, making it a candidate for a roll-to-roll processing technique. The relatively lower electronic performance of low-temperature a-Si devices could be compensated by the cheaper production, for future, ultra-low-cost, high-volume applications (e.g. RFID tags).

Amorphous alloys of silicon and carbon (amorphous silicon carbide, also hydrogenated, a-Si_1-xC_x:H) are an interesting variant to this material. Introduction of carbon adds extra freedom to controlling the properties of the material, as increasing concentrations of carbon in the alloy (x) widen the electronic gap between conduction and valence bands (see semiconductors), also called "optical gap", in order to potentially increase the light efficiency of solar cells made with amorphous silicon carbide layers. The film could also be made transparent to visible light. On the other hand, the electronic properties as a semiconductor (mainly electron mobility), are badly affected by the increasing content of carbon in the alloy, due to the increased disorder in the atomic network. Bringing x to the opposite extreme (100%) we have amorphous carbon, or synthetic diamond-like films. Several studies are found in the scientific literature, mainly investigating the effects of deposition parameters on electronic quality, but practical applications of amorphous silicon carbide in commercial devices are still lacking.

See also

• Nanocrystalline silicon
• Polycrystalline silicon