A photoluminescence (PL) band, the M band, was observed in photoluminescence spectra for various excitation densities in high-quality ZnO epitaxial thin films. The M band intensity increased superlinearly with an increase in the excitation intensity, suggesting that the observed PL band is due to a biexciton state. In order to prove this, the photoluminescence excitation (PLE) spectrum of the M band and time dependence of the PL intensity were measured. A shoulder that originates from two-photon absorption of the biexciton state appeared in the PLE spectrum. The biexciton binding energy was estimated to be 15 meV. The temporal behavior of the PL intensities of the M and free-exciton bands can be explained by modified rate equations assuming that the M band is caused by radiative annihilation of a biexciton leaving a free exciton and that the rate of creation of biexcitons is proportional to the 1.5th power of the exciton density. This power dependence is consistent with experimental results showing that the M band intensity is proportional to the 1.5th power of the excitation density.
Picosecond time-resolved photoluminescence from biexcitons in CuCl quantum dots (QDs) embedded in a NaCl matrix has been measured using an optical Kerr gate method. Ultrafast pulsed emission from the biexciton states was observed for the first time, only under resonant two-photon excitation of biexcitons. This implies that complete population inversion between the biexciton and exciton states is necessary in order to trigger the pulsed emission. In addition, the nature of the dependence of the time profiles of the pulsed emission on the excitation intensity reveals that the peak intensity is directly proportional to the square of the number of excited QDs. We conclude that this phenomenon is caused by superfluorescence, that is, the cooperative spontaneous radiative decay of many isolated excited states coupled by a resonant electromagnetic wave. Such a phenomenon has been observed for the first time in an ensemble of semiconductor QDs in this study. The results presented in this paper show that it is possible to control the microscopic coherent dynamics of electronic excited states in a QD ensemble.
We have investigated the optical and structural properties of high-quality ZnO films grown on epitaxial GaN (epi-GaN) by plasma-assisted molecular-beam epitaxy employing low-temperature buffer layers. High-resolution x-ray diffraction for both symmetric and asymmetric reflexes shows that crystalline defects in ZnO films have a similarity to epi-GaN used as a substrate. The quality of ZnO epilayers grown on epi-GaN is basically determined by epi-GaN. The photoluminescence (PL) spectrum at 10 K exhibits very sharp exciton emission with a linewidth of 1.5 meV, while deep-level emission is negligible, indicative of small residual strain. At 77 K, PL is dominated by a free-exciton emission line in the low-excitation regime, while it is overtaken by a new emission band due to biexcitons at its low-energy side as the excitation intensity increases. This biexciton emission band emerges even under the intermediate excitation regime of 100 W/cm2, which is 100 times smaller than the previously reported threshold for bulk ZnO. The biexciton binding energy is estimated to be 15 meV, in agreement with previous results. At the higher excitation regime, the emission line due to exciton–exciton scattering dominates the PL spectrum.
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