COMPOSITION AND STRUCTURE OF SILICON NITRIDE

The major composition of silicon nitride is Si3N4. Other compositions such as Si2N3, SiN, and Si3N have been reported but remain unconfirmed. 

The exact relationship between the two crystallographic forms is still a subject of debate. The a form may be a metastable modification of the p form or a high-temperature stable phase with a slightly different stoichiometry and chemical composition. Oxygen may stabilize foe a form but it does not appear to be necessary for its formation. The transformation fi om a-Si3N4 to P-Si3N4 has never been observed experimentally, putting in doubt foe polymorph nature of foe forms. 

The bonding in silicon nitride is mostly covalent. These covalent bonds are strong since both atoms are small and the bond length is short (0.179 nm). As discussed in Sec. 3.3 above, the bonding also maintains a large degree of ionicity. The calculated covalent bond energy is 10.87 eV 

---