CdSe quantum dots grown on ZnSe and Zn0.97Be0.03Se by molecular-beam epitaxy: Optical studies

Abstract

We report detailed studies of the photoluminescence (PL) properties of CdSe quantum dots (QDs) grown on ZnSe and Zn0.97Be0.03Se by molecular-beam epitaxy. We performed steady-state and time-resolved PL measurements and observed that nonradiative processes dominate at room temperature (RT) in the CdSe/Zn0.97Be0.03Se QD structures while these nonradiative processes do not dominate in the CdSe/ZnSe QD structures up to RT. We developed a method to estimate the capped CdSe/Zn0.97Be0.03Se QD size and composition, based on PL and photoluminescence excitation as well as Raman scattering spectroscopy measurements. Assuming spherical QDs, we estimate the QD radii to be in the range of 2.5 nm to 4.0 nm with the Cd concentration in the range of 47%–54%. This size is smaller than the exciton Bohr radius, suggesting a nearly zero-dimensional character. We also performed contactless electroreflectance measurements on the CdSe/ZnSe QDs, and observed the transitions due to the QDs and the wetting layer. In this case, assuming lens-shaped QDs, we extracted the QD radius to be around 9.52 nm, the height about 3.24 nm. This size is larger than the exciton Bohr radius, indicating a quasi-two-dimensional character. Studies based on Raman scattering spectroscopy suggest that the Cd concentration is about 44% for this structure. The temperature dependences of the PL lifetimes are consistent with the results of the QD size and Cd concentration estimated by these two methods.