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Optical properties of strained II/VI semiconductor quantum structures

The exciton binding energies of bulk crystals of most of the wide gap II-VI semiconductors are comparable to the thermal energy at room temperature. Typically, the confinement potentials in QWs and QDs are considerably larger than bulk exciton binding energies.

The excitonic states in strained II-VI semiconductor quantum structures with shallow confinement potentials are suitable for quantum devices which require small inhomogeneous broadening of the excitonic transitions. We studied the optical properties of ZnMgSe/ZnSe quantum structures with various spectroscopic methods, in which the binding energy of quasi-two-dimensional excitons in the QWs can reach 40-60 meV. Furthermore the strain in the barriers and the QWs offers a further degree of freedom that modifies the band structure and the optical properties of such QWs and leads to interesting excitonic features. Recently we developed a sophisticated strain-compensation technique to overcome the formation of Stranski-Krastanov quantum dots (QDs) and/or plastic lattice relaxation of highly mismatched ZnSe/CdSe quantum structures (6.7% lattice mismatch). This enabled us to perform molecular beam epitaxial growth of ZnMgSe/ZnSe/CdSe QWs with high internal quantum efficiency at room and low temperatures and spectral tunability over the whole visible range by variation of the CdSe QW width between 1-6 monolayers.

PL NanostructuresLeft: Room temperature photoluminescence spectra of various ZnMgSe/ZnSe/CdSe QWs with different CdSe well width between 1-6 ML. Right: Webcam shots of the surface luminescence spots indicating the high quantum efficiency and color tunability.

Selected Publications in this topic:

“Exciton states in shallow ZnSe/(Zn,Mg)Se quantum wells: Interaction of confined and continuum electron and hole states”, A. Pawlis, T. Berstermann, C. Brüggemann, M. Bombeck, D. Dunker, D.R. Yakovlev, N.A. Gippius, K. Lischka, M. Bayer, Phys. Rev. B 83, 115302 (2011).

“Extending the spectral range of CdSe/ZnSe quantum wells by strain engineering”, A. Finke, M. Ruth, S. Scholz, A. Ludwig, A.D. Wieck, D. Reuter, A. Pawlis, Phys. Rev. B 91, 035409 (2015).