Observation of an anomalously broadened donor-induced excitonic emission of CdSe/ZnSe submonolayer quantum dots

Epitaxial CdSe/ZnSe submonolayer quantum dots (SMQDs), formed from CdSe coverages below one monolayer (ML), often exhibit a double-peak excitonic structure in their photoluminescence spectra. The narrower, higher-energy peak corresponds to excitons localized within the SMQDs. The lower-energy peak is broader, which is not typical of donor-bound excitons, and has therefore been attributed in previous studies to morphological features of the quantum dots. The identification of the origin of the low-energy peak is technologically relevant for quantum applications of the SMQDs. To clarify the origin of this excitonic structure, we grew several CdSe SMQDs samples with nominal coverages of 0.5 and 0.25 MLs embedded in both non-intentionally doped and Cl-doped ZnSe barriers. The heterostructures were systematically characterized by photoluminescence spectroscopy. A direct comparison of the excitonic properties of the CdSe SMQDs samples grown within ZnSe barriers with varyingn-type impurity doping clearly demonstrates that the relative intensity and line shape of the lower-energy peak depend strongly on donor concentration. These findings overturn the previous morphological interpretation and conclusively demonstrate that the lower-energy component originates from donor impurities. It corresponds to localized excitons bound to single or multiple neutral donors and/or to negative trions formed at low temperature. The intensity in both cases depends on donor doping level.