Modeling of transitions in Mn2+ doped ZnS nanocrystals and predicting reduced lasing threshold current density and enhanced electro-optic effects in ZnCdSe–ZnMgSSe and InGaN–AlGaN pseudomorphic quantum dots

A three-dimensional coupled-well excitonic model is presented to explain the observed shift in the photoluminescence excitation spectrum, emission peak from Mn2+ ions, and five-to-six orders reduction of radiative lifetime in ZnS:Mn2+(35 Å) doped nanocrystals. For pseudomorphic cladded nanocrystals such as ZnCdSe–ZnMgSSe, a modified excitonic model predicts enhancements in the absorption coefficient (α∼160 000 cm−1) and electric field-dependent index of refraction change (Δn/n∼0.1–0.2), and a significant reduction of radiative lifetime τr∼14.5 fs. Optical gain and threshold current density (Jth) are computed for ZnCdSe–ZnMgSSe and InGaN–AlGaN quantum dot lasers. In the case of InGaN–AlGaN quantum dot lasers, the effect of dislocation-induced traps, enhancement of optical gain due to excitonic transitions, and dot size are considered in the computation of Jth. It is shown that InGaN dots with larger cross sections (∼200×250 Å, such as self-organized dots) and consisting of a trap density of 2.9×1017 cm−3 yield Jth of 4500 A/cm2 in comparison to 136 A/cm2 for dots of size 35×35×35 Å. This explains the observation of higher current density in InGaN quantum well lasers having self-organized dots.