Electronic structure of Ge/Si x Sn y Ge 1−x−y quantum dots

The electronic band structures and optical gains of Ge/Si x Sn y Ge 1-x-y truncated pyramid-shaped quantum dots (QDs) are calculated using the 8-band k.p model. The large bowing factors in the calculation of band gaps of both A-conduction and L-conduction valley in the barrier are considered so that the band gaps of the barrier are small. The strains are calculated by constant strain method and valence force field (VFF) method. By constant strain method, strains are uniformly distributed in the QD, and there is no strain in the barrier. Due to the small conduction band offset and strong quantum confinement effect, it requires greater tensile strain to make Ge QD a direct-band material by setting Si and Sn compositions at 11% and 26% respectively. By VFF method which considers the four nearest-neighbour interactions for each atom, the calculated strains are not uniform and the strains exist in the barrier. These barrier strains have raised the valence band edge of the barrier and changed the potential profile of the structure, so that the holes may not be confined in the QD. The VFF method should be more accurate to reflect the real situation.