Photoluminescence properties of epitaxial asymmetric triple CdSe quantum wells

The authors present the results of the photoluminescence characterization of three asymmetric triple quantum well (ATQW) systems. Each one contains three ultrathin CdSe quantum wells of 1, 2, and 3 monolayers (ML) thickness grown by atomic layer epitaxy between ZnSe separating barriers. Two systems have coupled QWs due to thin 5 nm thick ZnSe separating barriers, and the uncoupled system has 100 nm ZnSe separating barriers. Two of them (5 and 100 nm ZnSe barriers) were grown in the sequence 3–2–1, with the 3 ML QW closer to the substrate, while the third ATQW (5 nm ZnSe barrier) was grown in the reverse order. The photoluminescence spectrum of the uncoupled ATQW (3–2–1 sequence) presents three excitonic peaks, one peak for each QW, with the same emission energies of corresponding isolated single QWs. In the case of two coupled ATQW systems (3–2–1 and 1–2–3 growth sequence), the authors observed only one peak, corresponding to the lowest transition energy in the system formed by the three coupled QWs. The calculation of the wavefunctions shows no overlap for 100 nm ZnSe separating barriers, as expected for this thick barrier. For the systems with 5 nm ZnSe barriers, the calculations indicate that the overlap occurs mainly between the wavefunctions of the central 2 ML thick CdSe QW and the neighboring 1 ML and 3 ML QW wavefunctions; the overlap between the 1 and 3 ML QWs is small as a consequence of their 10 nm separation. The authors found that the effect of the coupling between the QWs is more easily identified by the reduction (or absence) of the intensity of the higher energy transitions of the multiple QW system than by the change of the ATQW energy levels with barrier thickness reduction.