Effects of vacancy on the electronic and optical properties of β-Si3N4 by first-principles

We use a first-principles calculation to explore the effects of vacancies on structural, electronic and optical properties of beta-Si3N4 based on density functional theory (DFT). The results show that after optimization, the Si vacancy system of beta-Si3N4 is more difficult to produce than N vacancy system under the same thermodynamic conditions. The band gaps including N vacancy and Si vacancy systems are smaller than that of the perfect crystal. The charge density difference and population analysis show that the bonding near Si vacancy has stronger covalent property, whereas those near nitrogen vacancy have stronger ionic property. For Si vacancy system, the materials have much higher values of the imaginary part of the dielectric constant than those of N vacancies and perfect beta-Si3N4. The maximum value of the Si vacancy system in absorption and reflection spectra is lower than those in different nitrogen vacancy systems.