First-principles study on effective doping to improve the optical properties in spinel nitrides

The high-pressure and high-temperature spinel phases (γ) of the Si₃N₄ and Ge₃N₄ have attracted great attention due to the wide, direct electronic band gaps. Their derivatives, spinel SiGe₂N₄, have also raised notice because of its stability and interesting properties. However we noticed that the optical properties of this group are not satisfied comparing to those of well-known semiconductor. The main purpose of present study is to find the possibility of improving the optical properties of IVA group spinel nitrides by first-principles calculation combining the random phase approximation (RPA). The focus of present work is put on raising the peak value of the imaginary part of dielectric function(ε₂) by site substitution as the simulation of doping. We proposed a method to select doping elements based on detailed analysis on ε₂ to identify the largest interband transitions. Various doping effects by different dopants at different doping sites in different doping concentrations are investigated, by taking γ-SiGe₂N₄ as the example, and the trend of re-distribution of energy band by different dopants is revealed. Calculations show that for γ-SiGe₂N₄, substituting Si in tetrahedrally coordinated cation sites in spinel structure with Ga and As significantly raises the peak value of ε₂, which proposed an effective approach to improve optical property of IVA group nitrides.