Theoretical study of the effect of different n-doping elements on band structure and optical gain of GeSn alloys.

N-Doping is an effective approach for improving the lighting efficiency of GeSn alloys. As each doping element has an atomic radius and electronegativity value different from those of the host atoms, the shape of the GeSn band is affected. However, no recent studies considering this phenomenon have been reported. For this reason, first-principles calculations combined with the GGA+U method and supercell models have been employed to precisely investigate the structural properties, band structures, and optical gains of Ge0.9375Sn0.0625 when doped with different V-group elements (including P, As, Sb, and Bi). With regard to the structural properties, the results indicate that they all exhibit a positive deviation from Vegard's law; Ge0.9375-mSn0.0625Pm has the largest bowing coefficient. The bandgap results indicate that doping with P and As does not assist in converting GeSn into a direct bandgap material, while doping with Sb and Bi has positive effects on the transition of GeSn; the corresponding crossover values are 1.89 and 1.58%, respectively. The calculated optical gain indicates that the net gain of Ge0.9375-mSn0.0625 will reach a maximum when the injected carrier density is similar to 1 x 10(19) cm(-3), and it will increase as the doping concentration increases. The effects of the doping elements on the optical gain of GeSn can be ranked as Bi > Sb > As > P.