What is polysilicon?
Polycrystalline silicon (Rod and Nugget form)
Silicon (Si) exists usually as an oxide (silica stone), being an element among about 100 different elements. Silicon is found near the earth's surface, in an abundance second only to oxygen, and is considered to be limitless in supply.
To change silica stone into polycrystalline silicon, metallic silicon is first made with a purity of 99% by reduction with carbon. Trichlorosilane is produced from metallic silicon and purified by distillation refining. Reduction is performed with hydrogen at temperatures near 1,000oC, depositing a 99.999…% (in eleven 9s) pure polycrystalline silicon in rod form.
[ Raw material for semiconductors ]
Polycrystalline silicon (polysilicon) is changed into singlecrystalline silicon and, in turn, singlecrystalline wafers, and further, integrated circuits that are indispensable components of electronics, such as TVs, radios, and computers.
Why is polycrystalline silicon processed into singlecrystalline silicon?
Polycrystalline silicon is an aggregation of small silicon crystals. As a result, silicon molecules are irregularly aligned within the crystals, and do not allow a regular flow of current even if electricity is charged.
Singlecrystalline silicon features a uniform alignment of silicon molecules in three dimensions, resulting in a set level of resistance to electrical current in integrated circuits. Integrated circuits control electrical current, so it is desirable to have a constant electrical resistance. For this reason, polysilicon is melted once (melting point:1,414oC) and recrystallized to singlecrystalline silicon.
Why is high purity polycrystalline silicon necessary?
When polycrystalline silicon is turned into singlecrystalline silicon, impurities cause an ununiform alignment of molecules in singlecrystalline silicon. For this reason, a high level of purity is required for making singlecrystalline silicon.
Singlecrystalline silicon itself does not allow passage of electric currents. Boron or phosphorus is added as additive in a trace of amount to provide the singlecrystalline silicon with the property for transmission of electrons. However, if boron or phosphorus exists as impurities, in situ, even before addition of an additive, it is difficult to make integrated circuits of intended characteristics. For this reason, polycrystalline silicon also must be of a very high purity.
[ Raw material for solar cells ]
Polycrystalline silicon (polysilicon) is used in the production of solar cells that use monocrystalline or multicrystalline silicon for substrates. These crystalline type silicon solar cells account for over 90% of the worldwide production of solar cells. Excelling in power generation efficiency, demand for such solar cells is increasing mainly for housing use. Their widespread use is expected as a renewable energy source.
Diagram of production from silica stone to polycrystalline silicon and finished product