Gareth J. Beirne

Towards deterministically controlled InGaAs/GaAs lateral quantum dot molecules

We report on the fabrication, detailed characterization and modeling of lateral InGaAs quantum dot molecules (QDMs) embedded in a GaAs matrix and we discuss strategies to fully control their spatial configuration and electronic properties. The three-dimensional morphology of encapsulated QDMs was revealed by selective wet chemical etching of the GaAs top capping layer and subsequent imaging by atomic force microscopy (AFM). The AFM investigation showed that different overgrowth procedures have a profound consequence on the QDM height and shape. QDMs partially capped and annealed in situ for micro-photoluminescence spectroscopy consist of shallow but well-defined quantum dots (QDs) in contrast to misleading results usually provided by surface morphology measurements when they are buried by a thin GaAs layer. This uncapping approach is crucial for determining the QDM structural parameters, which are required for modeling the system. A single-band effective-mass approximation is employed to calculate the confined electron and heavy-hole energy levels, taking the geometry and structural information extracted from the uncapping experiments as inputs. The calculated transition

Electrically pumped single-photon emission in the visible spectral range up to 80 K

We present an electrically pumped single-photon emitter in the visible spectral range, working up to 80 K, realized using a self-assembled single InP quantum dot. We confirm that the electroluminescense is emitted from a single quantum dot by performing second-order autocorrelation measurements and show that the deviation from perfect single-photon emission is entirely related to detector limitations and background signal. Emission from both neutral and charged exciton complexes was observed with their relative intensites depending on the injection current and temperature.

Time- and locally resolved photoluminescence of semipolar GaInN∕GaN facet light emitting diodes

The authors investigate the carrier lifetime and photoluminescence (PL) intensity of a semipolar GaInN∕GaN sample which was realized by growing five GaInN∕GaN quantum wells on the {11¯01} side facets of selectively grown n-GaN stripes that have a triangular shape running along the ⟨112¯0⟩ direction. Time- and locally resolved PL measurements show drastically reduced lifetimes for the semipolar sample of only 650ps at 4K whereas lifetimes exceeding 50ns were found for a polar reference sample. Furthermore, more than a doubling of the luminescence intensity and a significantly reduced blueshift of the PL peak wavelength with increasing excitation power density provide further evidence for the presence of reduced piezoelectric fields in the semipolar sample.

Probing the switching mechanism in ZnO nanoparticle memristors

We investigate the resistance switching mechanism in memristors based on colloidal ZnO nanoparticles using electroabsorption (EA) spectroscopy. In this EA experiment, we incorporate a small amount of low-bandgap polymer, poly(9,9-dioctylfluorene-co-benzothiadiazole), as a probe molecule in ZnO-nanoparticle memristors. By characterizing this polymer, we can study the change of built-in potential (VBI) in the device during the resistance switching process without disturbing the resistance state by the EA probe light. Our results show that VBI increases when the device is switched to the high resistance state, suggesting a shift of effective workfunction of the electrode. Thus, we attribute the resistance switching to the field-dependent migration of oxygen vacancies associated with the adsorption and desorption of oxygen molecules at the Al/ZnO interface. This process results in the modulation of the interfacial injection barrier, which governs the resistance state of the device.

Red single-photon emission from an InP∕GaInP quantum dot embedded in a planar monolithic microcavity

Using micro-photoluminescence, we demonstrate single-photon emission in the visible (red) spectral range using self-assembled InP quantum dots embedded in a planar microcavity realized by monolithically grown high reflectivity AlGaAs distributed Bragg reflectors. A full width at half maximum of 130μeV at 5K was observed from a single quantum dot coupled to the fundamental cavity resonance. Photon correlation measurements performed under continuous wave excitation show a clear antibunching behavior [g(2)(0)=0.13] as expected for a single-photon emitter. Saturation count rates up to 1.5MHz (8.1MHz into the first lens, with an extraction efficiency of 4.1%) were observed.

Single-photon emission from a type-B InP∕GaInP quantum dot

Type-B InP∕GaInP quantum dots are expected to exhibit a type-II electronic structure. Evidence for this is provided by the variation in decay time of the ensemble as a function of excitation power density. Photon correlation measurements were subsequently performed on a single type-B InP∕GaInP quantum dot using a Hanbury-Brown and Twiss setup [Nature 178, 1447 (1956)]. Autocorrelation measurements were carried out under both continuous-wave and pulsed excitation conditions with single-photon emission observed in each case. The continuous-wave measurements display a pronounced antibunching dip at zero time delay while pulsed measurements enable the triggered generation of single photons on demand at a wavelength of approximately 750 nm.

Polarization fine structure and enhanced single-photon emission of self-assembled lateral InGaAs quantum dot molecules embedded in a planar microcavity

Single lateral InGaAs quantum dot molecules have been embedded in a planar micro-cavity in order to increase the luminescence extraction efficiency. Using a combination of metal-organic vapor phase and molecular beam epitaxy samples could be produced that exhibit a 30 times enhanced single-photon emission rate. We also show that the single-photon emission is fully switchable between two different molecular excitonic recombination energies by applying a lateral electric field. Furthermore, the presence of a polarization fine-structure splitting of the molecular neutral excitonic states is reported which leads to two polarization-split classically correlated biexciton exciton cascades. The fine-structure splitting is found to be on the order of 10 micro-eV.

Influence of the charge carrier tunneling processes on the recombination dynamics in single lateral quantum dot molecules

We report on the charge carrier dynamics in single lateral quantum dot molecules and the effect of an applied electric field on the molecular states. Controllable electron tunneling manifests itself in a deviation from the typical excitonic decay behavior in dot molecules. It results in a faster population decay and can be strongly influenced by the tuning electric field and intermolecular Coulomb energies. A rate equation model is developed and compared to the experimental data to gain more insight into the charge transfer and tunneling mechanisms. Nonresonant (phonon-mediated) electron tunneling which changes the molecular exciton character from direct to indirect, and vice versa, is found to be the dominant tunable decay mechanism of excitons besides radiative recombination.

Observation of oxygen vacancy migration in memory devices based on ZnO nanoparticles

We investigate the mechanism of resistive switching in non-volatile memory devices based on an ITO/ZnO nanoparticles/Al structure using electroabsorption (EA) spectroscopy and X-ray photoelectron spectroscopy (XPS). By incorporating a small amount of low-bandgap organic semiconductor, poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT), as a probe molecule for EA characterization, we study the change in the built-in potential during the switching process under different ambient conditions. We compare the concentrations of oxygen vacancies between the Al/ZnO interface and the bulk of the ZnO nanoparticle film by XPS. We also investigate the effect of an external electrical field on the concentration of oxygen vacancies at the Al/ZnO interface. We find that the resistive switching can be attributed to the migration of oxygen vacancies driven by the electrical field, accompanied by adsorption/desorption of oxygen molecules at the Al/ZnO interface. This process gives rise to the formation of a dipole layer, ...

Polarization anisotropic luminescence of tunable single lateral quantum dot molecules

We investigate the photoluminescence polarization anisotropy of self-assembled individual lateral InGaAs/GaAs quantum dot molecules. In contrast to similarly grown single quantum dots, the dot molecules exhibit a remarkable degree of linear polarization, which remains almost unchanged when a lateral electric field is applied to tune the exciton wave function and, thus, the luminescence spectral properties. We discuss the nature of this polarization anisotropy and suggest possible causes based on the system’s symmetry and heterostructure alloy composition.