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Aluminum nitride (AlN) is an important group III-V semiconductor material that has attracted great interest because of its wide direct bandgap, high thermal conductivity and excellent matching with both silicon carbide (SiC) and gallium nitride (GaN). In addition to the above, it has unique intrinsic properties such as spontaneous polarization. The origin of this phenomenon is the strong ionic character of chemical bonds in the wurtzite AlN crystal structure. The non-centrosymmetric nature of the atomic structure also contributes to this property.
AlN is synthesized by the heating of boric oxide with ammonia gas at a temperature of about 1800 degC. The resulting product is a platy powder that contains sheets of hexagonal rings that slide easily past each other similar to the carbon mineral graphite. It has an electrical resistivity that is about 1000 times lower than SiC and GaN, and its thermal conductivity is about 2.5 times that of copper at room temperature. This makes it a very useful ceramic material, especially when it is alloyed with copper, magnesium, manganese or silicon for various purposes.
Ionic compounds are composed of a large number of ions that have the same charge. They can be classified as either organic or inorganic, based on whether the elements have organic carbon or not. Chemical formulas for ionic compounds are written using the crisscross method: The symbol of each cation is placed in front of the symbol of each anion, and the charges are arranged according to their relative magnitude. The lowest whole-number ratio of cations and anions is chosen to write the formula. For example, calcium nitrate is written as CaN because there are equal numbers of calcium and nitrate ions in the crystal.
