Control of biexciton localization and optical functionality in ZnMgCdS-based semiconductor quantum structures

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We have studied the effects of quantum confinement and localization on biexcitons in Cd_xZn_<1-x>S/(ZnS) quantum-well structures and Cd_xZn_<1-x>S/(ZnS)/(Mg_yZn_<1-y>S) separate-confinement heterostructures for application to highly efficient biexcitonic ultraviolet laser diodes. The samples used in the present study were grown by low-pressure metalorganic chemical vapor deposition. We first optimized growth conditions of Mg_yZn_<1-y>S ternary alloys and fabricated Cd_xZn_<1-x>S/(ZnS)/(Mg_yZn_<1-y>S) separate-confinement heterostructures. We confirmed that the insertion of Mg_yZn_<1-y>S ternary alloys as a cladding layer was effective for carrier confinement into Cd_xZn_<1-x>S quantum wells and resulted in the improvement of luminescence efficiency at RT. We also succeeded in the reduction of inhomogeneous broadening of both exciton and biexciton density of states due to alloy disorder by refining structural design and growth conditions of quantum wells. The binding energy of biexciton … More s was found to be larger than 40 meV in the quantum well with optimized quantum confinement.The degree of biexciton localization due to alloy disorder was evaluated quantitatively for the first time on the basis of experimental observations by photoluminescence excitation spectroscopy of biexcitons. The value was defined as a Stokes shift of biexcitons. We observed that the Stokes shift of biexcitons (S_<xx>) was smaller than that of excitons (S_x) and the value of S_<xx>/S_x was about 0.54. This resulted from the larger spatial extent of biexcitons compared with that of excitons. Finally, we studied stimulated emission properties of a variety of quantum-well samples with different structural parameters. We observed that the threshold excitation density for stimulated emission due to radiative recombination of biexcitons was concerned with the degree of biexciton localization as well as the binding energy of biexcitons. In particular, we clarified that the stability enhanced by the localization of biexcitons resulted in the suppression of temperature-induced increase in the threshold excitation density for stimulated emission. Less