Time-resolved spectroscopy of biexciton luminescence in ZnxCd1-xSe-ZnSySe1-y multiple quantum wells

The radiative lifetime of biexcitons in ZnxCd1-xSe-ZnSySe1-y multiple quantum wells has been studied by means of time-resolved luminescence spectroscopy under high-density excitation. It is shown that the rise of the biexciton luminescence becomes more rapid with increasing excitation energy density and that the biexciton luminescence decays with a double exponential form. It is found that the decay-time constant of the faster-decay component in the double-exponential decay corresponds to the radiative lifetime of the biexciton. Its value is about 6 ps at 2 K and is about one-seventh of that in bulk ZnSe (∼40 ps)

Confocal microphotoluminescence of InGaN-based light-emitting diodes

Spatially resolved photoluminescence (PL) of InGaN/GaN/AlGaN-based quantum-well-structured light-emitting diodes (LEDs) with a yellow-green light (530 nm) and an amber light (600 nm) was measured by using confocal microscopy. Submicron-scale spatial inhomogeneities of both PL intensities and spectra were found in confocal micro-PL images. We also found clear correlations between PL intensities and peak wavelength for both LEDs. Such correlations for yellow-green and amber LEDs were different from the reported correlations for blue or green LEDs. This discrepancy should be due to different diffusion, localization, and recombination dynamics of electron-hole pairs generated in InGaN active layers, and should be a very important property for influencing the optical properties of LEDs. In order to explain the results, we proposed a possible carrier dynamics model based on the carrier localization and partial reduction of the quantum confinement Stark effect depending on an indium composition in InGaN active layers. By using this model, we also considered the origin of the reduction of the emission efficiencies with a longer emission wavelength of InGaN LEDs with high indium composition

Localized excitons in cubic Zn1-xCdxS lattice matched to GaAs

Excitonic properties have been studied in cubic Zn0.42Cd0.58S lattice matched to GaAs. At 2 K, the time-integrated photoluminescence spectrum was composed of single emission peaking at 2.863 eV and its LO-phonon replica. The linewidth of the main peak was 18.5 meV, which fairly well agreed with the theoretical value based on the disorder-induced broadening of exciton luminescence in alloys. In order to assess the emission mechanism, the transient luminescence decay was measured at various emission energies. At the high energy tail (2.883 eV), the luminescence showed exponential decay with a time constant of about 72 ps. On the other hand, the decay time increased with decreasing the detected emission energy. It was about 660 ps at the emission peak (2.863 eV). We interpret these features by means of the model of exciton localization