We report on an experimental study of the temperature dependent band gap shift and the homogeneous line broadening of the excitonic luminescence in ZnO. This data is derived from photoluminescence (PL) measurements at temperatures from T = 10 -290 K and fitting the PL line shape with a theoretical model for the exciton line and its LO-phonon replica. Additionally, transmission measurements are utilized to extend the data of the temperature dependent band gap into the temperature range from 290 K up to 800 K. The experimentally determined shift is compared with one that is obtained numerically from the phonon density of states, the pressure dependent band gap, the thermal expansion coefficient and the bulk modulus with only a single fit parameter. The interaction of excitons with phonons leads to a homogeneous broadening of the excitonic emission which reaches values of 40 meV at room temperature. This mechanism results in a substantially increased threshold for stimulated emission from every process involving excitons.
We have studied the temperature dependence of the photoluminescence ͑PL͒ spectra of molecular beam epitaxy grown ultrathin Zn 1Ϫx Cd x Se/ZnSe quantum wells with random and inhomogeneous Cd distributions over cation sublattice within the temperature interval 2-300 K. Depending on the Cd concentration, the PL band maximum position E max PL (T) follows either a ''normal'' or an ''anomalous'' ͑known as ''S-shaped''͒ temperature dependence. We have analyzed both dependences in detail for a model of an island ensemble which can be characterized by a single-mode distribution of the most important parameters governing the optical properties of the quantum well. We demonstrate that the anomalous behavior arises due to the strong temperature dependence of the lifetimes of a family of metastable states participating in formation of the PL band at low temperatures. The metastablility of some island states is ascribed to a complex topological structure of the islands. The mechanism of the exciton-phonon interaction responsible for the fast decrease of the lifetime of these states with the increase of temperature has the same origin as the mechanism leading to the vanishing of narrow lines in -PL. We also present results of time-resolved experiments which yield the shift of the PL band for hot excitons cooling in a cold lattice.
By means of time resolved spectroscopy we compare two samples of ZnO nanorods with respect to their suitability as stimulated emitters. In the case of narrow nanorods their wave guiding quality causes a suppression of exciton -exciton scattering whereas no laser emission is detectable. Unlike their narrow counterparts, wide nanorods not only benefit from a larger overlap of the guided mode with the gain medium but a variation in VLS growth results in gold nanoparticles being present at the bottom of nanorods. Consequently, laser emission from single wide rods is evidenced up to 150 K. In addition to experimental studies we carry out 3D numerical simulations of the electric field distribution to evaluate the influence of gold nanoparticles at the nanorod/substrate interface. This finite element analysis confirms that gold leads to an enhancement of confinement within the resonator.
We investigate ZnO epitaxial layers grown by MBE (Molecular Beam Epitaxy) and MOVPE (Metal Organic Vapor Phase Epitaxy) techniques. The samples show similar optical behavior in temperature dependent photoluminescence measurements, reflection and photoluminescence excitation spectroscopy in the low density regime. High excitation measurements show different behavior. While the MBE sample leads to stimulated emission from the exciton-exciton-scattering, an electron hole plasma is formed in the MOVPE sample which leads to stimulated emission at higher excitation intensities. The gain value measured by the variable stripe length method is much higher for the MBE grown sample.
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