Shape-Dependent Stark Shift and Emission-Line Broadening of Quantum Dots and Rings

The impact of charge noise on the line width of the optical emission from semiconductor quantum dots (QDs) is studied using simulations. Several basic dot shapes with varied sizes are addressed: sphere, cone, disk, and ring, for two exemplaric dot classes: epitaxial GaAs/AlGaAs QDs and colloidal core-shell CdSe/CdS QDs. A trap with fluctuating population by a point charge and a thus fluctuating electric field causes a time-dependent Stark shift of the emission energy and finally the line broadening. The trap is located either in a crystal defect for epitaxial dots or on the surface of colloidal core- shell dots. The simulations allow a quantitative estimation of the optical line width, which varies from a few mu eV up to more than 10 meV as a function of the trap density and position, and QD shape and size. A comparison with the Stark shift and radiative lifetime prolongation in a uniform electric field, both caused by a wave function displacement, indicates that line broadening is rather related to a wave function deformation and cannot be evaluated from experiments in a uniform field. With respect to a small line width, for epitaxial QDs, a sphere represents the optimum shape, whereas colloidal dots depend on the respective surface plane populated with the trap. A smaller size yields a smaller line width for epitaxial dots, whereas colloidal dots become broader. Rings in a vertical uniform field establish a significant charge-carrier separation, which causes a very strong Stark shift and a lifetime prolongation up to nearly three orders of magnitude. Accordingly, a vertical point charge induces for rings the largest line widths of up to 100 meV. Furthermore, multiple point charges may cause extremely long radiative lifetimes, which represents a further mechanism for the blinking of core-shell QDs.