Extended excitons and compact heliumlike biexcitons in type-II quantum dots.

We have used magneto-photoluminescence measurements to establish that InP/GaAs quantum dots have a type-II band (staggered) alignment. The average excitonic Bohr radius and the binding energy are estimated to be 15 nm and 1.5 meV respectively. When compared to bulk InP, the excitonic binding is weaker due to the repulsive (type-II) potential at the hetero-interface. The measurements are extended to over almost six orders of magnitude of laser excitation powers and to magnetic fields of up to 50 tesla. It is shown that the excitation power can be used to tune the average hole occupancy of the quantum dots, and hence the strength of the electron-hole binding. The diamagnetic shift coe±cient is observed to drastically reduce as the quantum dot ensemble makes a gradual transition from a regime where the emission is from (hydrogen-like) two-particle excitonic states to a regime where the emission from (helium-like) four-particle biexcitonic states also become significant

Self-assembled InAs quantum dot formation on GaAs ring-like nanostructure templates

The evolution of InAs quantum dot (QD) formation is studied on GaAs ring-like nanostructures fabricated by droplet homo-epitaxy. This growth mode, exclusively performed by a hybrid approach of droplet homo-epitaxy and Stransky-Krastanor (S-K) based QD self-assembly, enables one to form new QD morphologies that may find use in optoelectronic applications. Increased deposition of InAs on the GaAs ring first produced a QD in the hole followed by QDs around the GaAs ring and on the GaAs (100) surface. This behavior indicates that the QDs prefer to nucleate at locations of high monolayer (ML) step density

Optical characterisation of silicon nanocrystals embedded in SiO2/Si3N4 hybrid matrix for third generation photovoltaics

Silicon nanocrystals with an average size of approximately 4 nm dispersed in SiO2/Si3N4 hybrid matrix have been synthesised by magnetron sputtering followed by a high-temperature anneal. To gain understanding of the photon absorption and emission mechanisms of this material, several samples are characterised optically via spectroscopy and photoluminescence measurements. The values of optical band gap are extracted from interference-minimised absorption and luminescence spectra. Measurement results suggest that these nanocrystals exhibit transitions of both direct and indirect types. Possible mechanisms of absorption and emission as well as an estimation of exciton binding energy are also discussed

Magnetophotoluminescence study of the influence of substrate orientation and growth interruption on the electronic properties of InAs/GaAs quantum dots

We have used photoluminescence in pulsed (less than or equal to50 T) and dc (less than or equal to12 T) magnetic fields to investigate the influence of substrate orientation and growth interruption (GI) on the electronic properties of InAs/GaAs quantum dots, grown by molecular beam epitaxy at 480 degreesC. Dot formation is very efficient on the (100) substrate: electronic confinement is already strong without GI and no significant change in confinement is observed with GI. On the contrary, for the (311)B substrate strong confinement of the charges only occurs after a GI is introduced. When longer GIs are applied the dots become higher. (C) 2004 American Institute of Physics

Classification and control of the origin of photoluminescence from Si nanocrystals.

Silicon dominates the electronics industry, but its poor optical properties mean that III–V compound semiconductors are preferred for photonics applications. Photoluminescence at visible wavelengths was observed from porous Si at room temperature in 1990, but the origin of these photons (do they arise from highly localized defect states or quantum confinement effects?) has been the subject of intense debate ever since. Attention has subsequently shifted from porous Si to Si nanocrystals, but the same fundamental question about the origin of the photoluminescence has remained. Here we show, based on measurements in high magnetic fields, that defects are the dominant source of light from Si nanocrystals. Moreover, we show that it is possible to control the origin of the photoluminescence in a single sample: passivation with hydrogen removes the defects, resulting in photoluminescence from quantum-confined states, but subsequent ultraviolet illumination reintroduces the defects, making them the origin of the light again