Optical properties and growth mechanism of multiple type-II Zn Te ∕ Zn Se quantum dots grown by migration-enhanced epitaxy

The properties of multiple type-II $\mathrm{Zn}\mathrm{Te}∕\mathrm{Zn}\mathrm{Se}$ quantum dots (QDs), which are coexistent with isoelectronic centers formed by Te, grown by migration-enhanced epitaxy, are studied. The samples with a single deposition cycle of Zn-Te-Zn sandwiched between ZnSe barriers are investigated via temperature- and excitation-dependent photoluminescence (PL) as well as magneto-PL measurements. It is found that the PL consists of two broad bands: a ``blue'' band, which is dominant at low temperatures, and a ``green'' band, which is observed at $T\ensuremath{\ge}60\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. Upon increasing the excitation intensity by about 4 orders of magnitude, the peak energy position of the blue band remains nearly the same, whereas the green band exhibits a large blueshift of $\ensuremath{\sim}50\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$, which suggests that the green band is due to, at least partially, the recombination of excitons bound to type-II QDs. The existence of type-II $\mathrm{Zn}\mathrm{Te}∕\mathrm{Zn}\mathrm{Se}$ QDs is further supported by the results of magneto-PL, for which the oscillation in the PL intensity as a function of magnetic field is observed. The properties of $\mathrm{Zn}\mathrm{Te}∕\mathrm{Zn}\mathrm{Se}$ QDs grown under the same $\mathrm{Zn}∕\mathrm{Te}$ flux ratio but with one and three contiguous deposition cycles of Zn-Te-Zn are compared. It is concluded that type-II QDs are formed in both types of samples; however, the density, size, and chemical composition of QDs strongly depend on the deposition of the submonolayer quantities of ZnTe.