Control of the electronic properties of CdSe submonolayer superlattices via vertical correlation of quantum dots

Structural and optical properties of submonolayer CdSe/ZnSe superlattices grown with varying thickness of the ZnSe spacer layer are studied. High-resolution electron microscopy images demonstrate that submonolayer CdSe depositions result in two-dimensional nanoscale CdSe islands which are anticorrelated for spacer layer thicknesses exceeding 3 nm, while predominantly vertically correlated growth occurs for thinner spacers, in agreement with most recent theoretical predictions. Vertical ordering of the CdSe islands leads to two lines in photoluminescence (PL) and optical reflectance spectra originating from excitons localized at vertically coupled and uncoupled CdSe quantum islands, respectively. In edge PL, these lines exhibit different polarizations: predominantly TM and predominantly TE for coupled and uncoupled states, respectively. Stimulated emission in edge geometry and resonant waveguiding effects are observed for both states. The TE and TM components of the stimulated emission of the same state show an energy splitting. At the highest excitation densities we observe saturation of the stimulated emission in the edge geometry, and the development of a peak in surface emission that is strongly increasing with excitation intensity. This peak is attributed to stimulated emission in surface geometry, which is made possible by the ultrahigh material gain in quantum dots and the self-adjustment of the gain spectrum and the cavity mode.