Excitonic Diffusion in InGaN/GaN Core-Shell Nanowires.

We report on the direct observation of the diffusion of carriers in graded InGaN/GaN quantum wells in a nanowire. By probing the local dynamics at the nanoscale, along the wire for different temperatures between 4 and 250 K, we conclude that this diffusion process is thermally activated. In addition, the analysis of the cathodoluminescence lifetime for different temperatures shows that the carrier motion is isotropic and does not follow the indium gradient.

Biexcitonic molecules survive excitons at the Mott transition.

When the carrier density is increased in a semiconductor, according to the predictions of Sir Nevil Mott, a transition should occur from an insulating state consisting of a gas of excitons to a conductive electron-hole plasma. This crossover, usually referred to as the Mott transition, is driven by the mutual effects of phase-space filling and Coulomb screening because of the presence of other charges nearby. It drastically affects the optical and electrical characteristics of semiconductors and may, for example, drive the transition from a polariton laser to a vertical cavity surface-emitting laser.

Exciton drift in semiconductors under uniform strain gradients: application to bent ZnO microwires.

Optimizing the electronic structures and carrier dynamics in semiconductors at atomic scale is an essential issue for innovative device applications. Besides the traditional chemical doping and the use of homo/heterostructures, elastic strain has been proposed as a promising possibility. Here, we report on the direct observation of the dynamics of exciton transport in a ZnO microwire under pure elastic bending deformation, by using cathodoluminescence with high temporal, spatial, and energy resolutions.

Biexciton emission and crystalline quality of ZnO nano-objects.

The design of cost-effective standards for the quality of nano-objects is currently a key issue toward their massive use for optoelectronic applications. The observation by photoluminescence of narrow excitonic and biexcitonic emission lines in semiconductor nanowires is usually accepted as evidence for high structural quality. Here, we perform time-resolved cathodoluminescence experiments on isolated ZnO nanobelts grown by chemical vapor deposition.

Probing carrier dynamics in nanostructures by picosecond cathodoluminescence.

Picosecond and femtosecond spectroscopy allow the detailed study of carrier dynamics in nanostructured materials. In such experiments, a laser pulse normally excites several nanostructures at once. However, spectroscopic information may also be acquired using pulses from an electron beam in a modern electron microscope, exploiting a phenomenon called cathodoluminescence.

Local quantification of the composition in GaAs/Al(x)Ga(1-x)As structures by thickness fringe analysis.

The increase of the complexity in semiconductor structures raises more and more the need for local evaluation techniques. For example, laser structures with graded-index waveguides are now widespread, but the characterisation of the shape of the gradient is still empirical. In this study, we use transmission electron microscopy (TEM) for the local determination of the chemical composition of GaAs/Al9x)Ga(1-x)As semiconductor laser structures, by quantifying the variation of the thickness fringe profiles with the aluminium content on wedge shaped samples.