Submonolayer epitaxy growth of fractional monolayer CdSe/ZnSe quantum dots

A heterostructure containing several sheets of CdSe fractional monolayer quantum dots with nominal coverages of 0.5 and close to 0.25 of a monolayer (ML) embedded in a ZnSe matrix was successfully grown by a combination of submonolayer epitaxy and molecular beam epitaxy. Due to surface reconstruction properties of cation- and anion-terminated surfaces, one complete Cd–Se alternate source exposure cycle produces a submonolayer saturation coverage of around 0.5 ML of CdSe. To obtain specific coverages lower than 0.5 ML, it is necessary to work with an unsaturated Cd surface, then the control of Cd exposure time becomes crucial. To determine the appropriate Cd exposure times, a careful study of the adsorption process and knowledge of the time to obtain the saturation coverage are required. Four separated sheets of CdSe fractional monolayer quantum dots with coverage Θ  ≃ 0.5 ML were deposited in the first section of the heterostructure to analyze the surface reconstruction streaks in the reflection high energy electron diffraction (RHEED) patterns. The analysis performed during the adsorption process to reach a Cd saturated surface (nominal coverage Θ  = 0.5 ML) suggests that the large Cd atoms produce strain-induced disorder in the underlying layers during the initial stages of the Cd chemisorption process. The results of the RHEED study were employed to estimate the Cd exposure time for the growth of the last sheet of CdSe fractional monolayer quantum dots with Θ  ≃ 0.25 ML. The low temperature photoluminescence spectrum presented two intense and narrow excitonic peaks at energies of 2.750 eV and 2.778 eV, corresponding to the CdSe fractional monolayer quantum dots with nominal coverages of 0.5 ML and ∼0.25 ML, respectively, with narrower linewidth for lower CdSe coverage.