Excitonic carrier dynamics taking place in InxGa1-xN/GaN multi-quantum-well systems have been studied by low temperature picosecond time resolved photoluminescence (LT-TRPL), HR-TEM, XPS, Dynamic TOF-SIMS, and quantum mechanical simulation methods. Both time-integrated and time-resolved photoluminescence spectra of InxGa1-xN/GaN multi-quantum-wells with different well thickness and Indium composition were measured at 10 K. We assigned the natural radiative lifetime of each sample from the time resolved PL. We observed that the natural radiative lifetime of In InxGa1-xN/GaN multi-quantum-wells depends strongly on the well thickness and Indium composition. To support the measured natural radiative lifetimes, excitonic oscillator strengths of the InxGa1-xN/GaN multi-quantumwells were calculated by using a 2-D particle-in-a-box model as functions of well thickness and Indium composition. Values of the well thickness and Indium compositions from the HR-TEM and XPS compositional depth profiling were used to achieve more realistic computational results and to corroborate the measured natural radiative lifetimes of InxGa1-xN/GaN multi-quantum wells.
We report on photoluminescent properties of ultrafine ZnO nanorods and ZnO/Zn0.8Mg0.2O nanorod quantum-well structures. The catalyst-free metalorganic chemical vapor deposition (MOCVD) technique enables control of ZnO nanorod diameters in the range of 5 to 150 nm. From the PL spectra of ultrafine ZnO nanorods with a mean diameter smaller than 10 nm, a systematic blue-shift in their PL peak position was observed by decreasing their diameter, presumably due to the quantum confinement effect along the radial direction in ZnO nanorods. In addition, we obtained time-integrated and time-resolved PL spectra of ZnO/Zn0.8Mg0.2O nanorod single-quantum-well structures (SQWs) in the temperature range of 10 K to 300 K. The nanorod SQWs also showed a PL blue-shift and the energy shift was dependent on ZnO well layer width. The PL peak position shift originates from the quantum confinement effect of carriers in nanorod quantum structures. Furthermore, we investigated spatially-resolved PL spectra of individual nanorod SQWs using scanning near-field optical microscopy.
The dynamics of the bound and free excitons and exiton polaritons of the ZnO nanorods have been investigated by time resolved photoluminescence in the temperature range from 10 K to 300 K. The samples have been fabricated by catalyst-free metal organic chemical vapor deposition (MOCVD), and have a diameter 35 nm and lengths in the range of 150 nm to 1.1 μm. In the temperature range of 10 K to 50 K, the photoluminescence lifetime of the bound exciton increases as the temperature increases. Photoluminescence lifetime of the free excitons, however, decreases with the temperature. The low temperature (10 K) time resolved photoluminescence spectra reconstructed from the time profiles measured at different frequencies clearly show that the bound exciton decay faster than the free A exciton. This result may be due to the transition from the bound exciton to free exciton because of the local temperature increase. Free B exciton is dominant above 50 K, and forms exciton polariton at high temperatures. At low temperature, photoluminescence lifetimes of the free A and B excitons do not show a clear correlation with the length of the nanorods. At room temperature, however, the photoluminescence lifetime increases monotonically as the length of the nanorods increase in the range of 150 nm to 600 nm. Decrease of the radiative decay rate of the exciton polariton has been invoked to account for the results.
We report on photoluminescence (PL) properties of ZnO epitaxial films and single-crystal nanorods grown by low pressure metalorganic vapor phase epitaxy. Time-integrated PL spectra of the films at 10 K clearly exhibited free A and B excitons at 3.376 and 3.382 eV and bound exciton peaks at 3.360, 3.364, and 3.367 eV. With increasing temperature, intensities of the bound exciton peaks drastically decreased and a free exciton peak was dominant above 40 K. Similarly, vertically well-aligned ZnO nanorod arrays also exhibited free exciton peaks at 3.374 and 3.381 eV, which indicates that ZnO nanorods prepared by the catalyst-fee method are of high optical quality. Furthermore, time-resolved PL measurements at a free exciton peak were carried out at room temperature. The decay profiles were of double-exponential form, and the decay time constants of 180 ps and 1.0 ns were obtained using a least-square fit of the data. Excitation power-dependent PL of ZnO epilayers is also discussed.