Study of the coupling of ultra-thin CdSe double quantum wells

• Study of the coupling of symmetric and asymmetric double ultra-thin quantum wells.
• The calculations were performed employing the Transfer Matrix Method.
• The thinner the ultra-thin quantum wells, the thicker the barrier to uncouple them.
• 70 ML ZnSe barrier necessary to uncouple a symmetric double 1 ML CdSe quantum well.
• Coupling/uncoupling of double CdSe quantum wells demonstrated by photoluminescence.

The degree of coupling between the electronic states of adjacent quantum wells (QWs) will depend on the sample structure and material properties of the QW and the barrier. The design of heterostructures containing multiple QWs requires the precise knowledge of the minimum barrier thickness to uncouple them. Due to the finite confinement in ultra-thin QWs (UTQWs, thickness of a few monolayers), the electron and hole wave functions present a large penetration within the barriers; the penetration length may exceed many times the thickness of the UTQW. Following the evolution of the electron and hole states as a function of the barrier thickness of a symmetric double quantum well system, we have determined the thickness of the ZnSe barrier required to uncouple CdSe UTQWs. We found that ZnSe barriers with thickness in the range from 70 to 30 monolayers (ML) are necessary to uncouple symmetric double CdSe UTQWs with thickness in the range from 1 to 5 ML, respectively. Therefore, the thinner the QWs the thicker the barrier required to uncouple them. The coupling/uncoupling of asymmetric double UTQWs (2 and 3 ML CdSe) system was experimentally verified by the analysis of the low temperature excitonic spectra. The sample with a 5 nm ZnSe barrier indicates the presence of coupling because we observe only one peak corresponding to the lowest energy transition; for the case of the uncoupled asymmetric UTQWs, with a 10 nm ZnSe barrier, the spectrum presents two peaks, one for each independent (single) UTQW.

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