Silicon oxynitride
# Silicon oxynitride

Silicon oxynitride is a ceramic material with the chemical formula $SiO_xN_y.$ While in amorphous forms its composition can continuously vary between $SiO_2$ (silica) and $Si_3N_4$ (silicon nitride), the only known intermediate crystalline phase is $Si_2N_2O.$ It is found in nature as the rare mineral sinoite in some meteorites and can be synthesized in the laboratory.

## Properties

The crystalline structure of silicon oxynitride is built by $SiN_3O$ tetrahedra connected through oxygen atoms along the c axis and through nitrogen atoms perpendicular to it. The strong covalent bonding of this structure results in high flexural strength and resistance to heating and oxidation up to temperatures of about 1600 °C.

## Synthesis

Polycrystalline silicon oxynitride ceramics are primarily produced by nitridation of a mixture of Si and silicon dioxide at a temperature above melting point of silicon (1414 °C), in the range 1420–1500 °C:

    3 Si + $SiO_2$ + 2 $N_2$ → 2 $Si_2N_2O$

Silicon oxynitride materials with various stoichiometries may also arise as the products of pyrolysis of preceramic polymers, namely polysilanes and polyethoxysilsesquiazane. SiON materials thus obtained are referred to as polymer derived ceramics or PDCs. By using preceramic polymers, dense or porous Si oxynitride ceramics in complex forms can be obtained using shaping techniques more typically applied for polymers.

## Applications

Thin films of silicon oxynitride can be grown on silicon using a variety of plasma deposition techniques and used in microelectronics as a dielectric layer alternative to silicon dioxide and silicon nitride with the advantages of low leakage currents and high thermal stability. These films have an amorphous structure and therefore their chemical composition can widely deviate from $Si_2N_2O.$ By changing the nitrogen/oxygen ratio in these films, their refractive index can be continuously tuned between the value of ~1.45 for silicon dioxide and ~2.0 for silicon nitride. This property is useful for gradient-index optics components such as graded-index fibers.

Silicon oxynitrides can be doped with metal atoms. The most common example is sialon, a family of quaternary SiAlON compound. Quaternary silicon oxynitrides containing a lanthanide element, such as La, Eu or/and Ce are used as phosphors.

