Roger R. Lowe-Webb

Determination of the shape of self-organized InAs/GaAs quantum dots by reflection high energy electron diffraction

We report a reflection high energy electron diffraction study of InAs self-organized quantum dots grown on GaAs (001). We observe facet reflections along the [310] and [130] azimuths, which indicate that the quantum dot shape is pyramidal with bounding facets corresponding to a family of four {136} planes. The determined structure, which possesses C2v symmetry, is quite different both from square-base pyramidal geometries which have been assumed in recent electronic structure calculations, and from previously proposed structures which have been based upon incomplete reflection high energy electron diffraction data.

FORMATION OF INAS/GAAS QUANTUM DOTS BY MOLECULAR BEAM EPITAXY : REVERSIBILITY OF THE ISLANDING TRANSITION

We report a study of the dynamics of coherent island formation in InAs/GaAs films grown by molecular beam epitaxy. A comparison of the temperature dependence of the critical layer thickness for islanding between the migration-enhanced and the continuous growth modes confirms that surface adatom diffusion and indium desorption are the controlling processes which determine the variation of the measured critical layer thickness with temperature. We find that under conditions in which indium desorption is significant, the islanding transition is reversible, which provides a new way to study the dynamics of the islanding transition. Applying this technique, we find that the size distribution of the three-dimensional islands evolves into a bimodal distribution during the reverse process.

Photoluminescence study of in situ annealed InAs quantum dots: Double-peak emission associated with bimodal size distribution

We report a photoluminescence study of self-assembled InAs islands subjected to in situ annealing prior to the growth of a capping layer. A distinctive double-peak feature is observed in the photoluminescence spectra of annealed samples. The power dependence of the photoluminescence spectra reveals that the double-peak emission is associated with the ground-state transition of islands in two different size branches. This observation agrees with a previous study, which demonstrated that the InAs island size distribution bifurcates during post-growth annealing. The temperature dependence of the photoluminescence intensities from samples with bimodal island size distributions illustrates that different thermal activation energies for carrier emission are associated with islands in different size branches.

Correlation between photoluminescence and infrared absorption spectra of oxidized nanoscale silicon clusters

We report in situ photoluminescence and ex situ Fourier transform infrared spectra of nanoscale silicon clusters exposed to atomic hydrogen, molecular oxygen, and humidified argon. Comparisons between infrared absorption spectra of fresh and aged samples indicate that photoluminescence efficiency is correlated with a stoichiometric oxide shell and the presence of Si dangling bond passivants at the core/oxide interface. Photoluminescence quenching is demonstrated in efficiently luminescing samples upon exposure to atomic hydrogen with recovery of photoluminescence occurring upon subsequent exposure to air. The photoluminescence quenching and recovery is correlated with a partial quenching and recovery of absorption due to interfacial silane groups. The correlations between photoluminescence and infrared absorption spectra, together with the hydrogen quenching results, provide evidence that radiative recombination in these samples is associated with interfacial oxide-related defects.