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

The high-pressure and high-temperature spinel phases (gamma) 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(epsilon(2)) by site substitution as the simulation of doping. We proposed a method to select doping elements based on detailed analysis on epsilon(2) to identify the largest interband transitions. Various doping effects by different dopants at different doping sites in different doping concentrations are investigated, by taking gamma-SiGe2N4 as the example, and the trend of re-distribution of energy band by different dopants is revealed. Calculations show that for gamma-SiGe2N4, substituting Si in tetrahedrally coordinated cation sites in spinel structure with Ga and As significantly raises the peak value of epsilon(2), which proposed an effective approach to improve optical property of IVA group nitrides. (C) 2009 Elsevier B.V. All rights reserved.