Well-width dependence of optical properties of rare-earth ion-doped ${\mathrm{ZnS}}_{0.8}{\mathrm{Se}}_{0.2}/\mathrm{undoped}$ ZnS multiple quantum wells

We compare optical properties of ${\mathrm{Sm}}^{3+}$-doped ${\mathrm{ZnS}}_{0.8}{\mathrm{Se}}_{0.2}/\mathrm{undoped}$ ZnS multiple-quantum wells (MQWs) with different well widths (2, 5, 10 monolayers) and bulk ${\mathrm{ZnS}}_{0.8}{\mathrm{Se}}_{0.2}:{\mathrm{Sm}}^{3+}$ crystal. The excitonic peak in the photoluminescence excitation spectrum of the ${\mathrm{Sm}}^{3+}$ luminescence shifts to the shorter-wavelength side with reducing well width, which shows that the excitation of ${\mathrm{Sm}}^{3+}$ occurs through the energy transfer from the spatially confined excitons. The activation energy of the thermal quenching of the ${\mathrm{Sm}}^{3+}$ luminescence is found to increase with reducing well width. This result is interpreted as due to the difference in the spatial confinement effect on the binding energy of the free exciton. In addition, the quantum efficiency of the ${\mathrm{Sm}}^{3+}$ luminescence at 4 K is found to increase remarkably with reducing well width. At this temperature, the quantum efficiency of the two monolayers MQW sample is more than 16 times as high as that of the bulk sample. Probable causes of this fact are discussed.