Manus Hayne

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.

Electron localization by self-assembled GaSb/GaAs quantum dots

We have studied the photoluminescence from type-II GaSb/GaAs self-assembled quantum dots in magnetic fields up to 50 T. Our results show that at low laser power, electrons are more weakly bound to the dots than to the wetting layer, but that at high laser power, the situation is reversed. We attribute this effect to an enhanced Coulomb interaction between a single electron and dots that are multiply charged with holes.

GaSb/GaAs quantum dot formation and demolition studied with cross-sectional scanning tunneling microscopy

We present a cross-sectional scanning tunneling microscopy study of GaSb/GaAs quantum dots grown by molecular beam epitaxy. Various nanostructures are observed as a function of the growth parameters. During growth, relaxation of the high local strain fields of the nanostructures plays an important role in their formation. Pyramidal dots with a high Sb content are often accompanied by threading dislocations above them. GaSb ring formation is favored by the use of a thin GaAs first cap layer and a high growth temperature of the second cap layer. At these capping conditions, strain-driven Sb diffusion combined with As/Sb exchange and Sb segregation remove the center of a nanostructure, creating a ring. Clusters of GaSb without a well defined morphology also appear regularly, often with a highly inhomogeneous structure which is sometimes divided up in fragments.

High-field magnetoexcitons in unstrained GaAs ∕ Al x Ga 1 − x As quantum dots

The magnetic field dependence of the excitonic states in unstrained GaAs/AlxGa1-xAs quantum dots is investigated theoretically and experimentally. The diamagnetic shift for the ground and the excited states are studied in magnetic fields of varying orientation. In the theoretical study, calculations are performed within the single band effective mass approximation, including band nonparabolicity, the full experimental three-dimensional dot shape and the electron-hole Coulomb interaction. These calculations are compared with the experimental results for both the ground and the excited states in fields up to 50 Tesla. Good agreement is found between theory and experiment.

Optical observation of single-carrier charging in type-II quantum ring ensembles

A high-purity GaSb/GaAs quantum ring system is introduced that provides both strong hole-confinement in the GaSb ring and electron confinement in its GaAs core. The latter is responsible for a reduced inhomogeous linewidth measured in photoluminescence, in comparison to the previous measurements made on nanostructures with differing morphology in this material system. This allows the resolution of multiple peaks in the photoluminescence due to discrete charging with holes, revealing the mechanism responsible for the excitation-power-induced blueshift.

The structural, electronic and optical properties of GaSb/GaAs nanostructures for charge-based memory

The potential for GaSb nanostructures embedded in GaAs to operate as charge-based memory elements at room temperature is introduced and explored. Cross-sectional scanning-tunnelling microscopy is employed to directly probe and optimize the growth of nanostructures by molecular beam epitaxy. The results of structural analysis are combined with electrical measurements made with deep-level transient spectroscopy, showing excellent agreement with theoretical calculations which model the electronic structure of the nanostructures using 8-band kp theory. Hole-localization energies exceeding 600 meV in quantum dots and near-100% material contrast between GaSb-rich quantum rings (QRs) and the surrounding GaAs matrix are revealed (no intermixing). Optical measurements confirm the depth of the hole localization, and demonstrate substantially lower inhomogeneous broadening than has previously been reported. Multiple peaks are partially resolved in ensemble photoluminescence of GaSb/GaAs QRs, and are attributed to charge states from discrete numbers of confined holes.

GaSb quantum dot morphology for different growth temperatures and the dissolution effect of the GaAs capping layer

We compare the characteristics of GaSb quantum dots (QDs) grown by molecular beam epitaxy on GaAs at temperatures from 400°C to 490°C. The dot morphology, in terms of size, shape and density, as determined by atomic force microscopy on uncapped QDs, was found to be highly sensitive to the growth temperature. Photoluminescence spectra of capped QDs are also strongly dependent on growth temperature, but for samples with the highest dot density, where the QD luminescence would be expected to be the most intense, it is absent. We attribute this to dissolution of the dots by the capping layer. This explanation is confirmed by atomic force microscopy of a sample that is thinly capped at 490°C. Deposition of the capping layer at low temperature resolves this problem, resulting in strong QD photoluminescence from a sample with a high dot-density.

MAGNETOPHOTOLUMINESCENCE OF STACKED SELF-ASSEMBLED INP QUANTUM DOTS

We report magnetophotoluminescence measurements of stacked layers of self-assembled InP quantum dots. With a magnetic field applied in the growth direction we have determined the exciton reduced mass from the field dependence of the photoluminescence energy. By applying a magnetic field perpendicular to the growth direction, we have analyzed the spatial confinement of the dots in the growth direction. We observe a large increase in the shift of the exciton energy between 0 and 50 T when the thickness of the GaInP spacer layer between the dots is reduced from 8 to 4 nm. This indicates a decrease in spatial confinement in the growth direction which we attribute to strong electronic coupling between vertically stacked dots.

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.

Impact of the capping layers on lateral confinement in InAs∕InP quantum dots for 1.55μm laser applications studied by magnetophotoluminescence

We have used magnetophotoluminescence to study the impact of different capping layer material combinations (InP, GaInAsP quaternary alloy, or both InP and quaternary alloy) on lateral confinement in InAs∕InP quantum dots (QDs) grown on (311)B orientated substrates. Exciton effective masses, Bohr radii, and binding energies are measured for these samples. Conclusions regarding the strength of the lateral confinement in the different samples are supported by photoluminescence at high excitation power. Contrary to theoretical predictions, InAs QDs in quaternary alloy are found to have better confinement properties than InAs∕InP QDs. This is attributed to a lack of lateral intermixing with the quaternary alloy, which is present when InP is used to (partially) cap the dots. The implications of the results for reducing the temperature sensitivity of QD lasers are discussed.