Jaakko Sormunen

Comparison of epitaxial thin layer GaN and InP passivations on InGaAs∕GaAs near-surface quantum wells

The optical properties of the in situ epitaxial GaN and InP passivated InGaAs∕GaAs near-surface quantum wells, which were fabricated by metal organic vapor phase epitaxy, are investigated. Low-temperature photoluminescence (PL), time-resolved photoluminescence, and photoreflectance are used to study the passivation effect. Both GaN and InP passivations are observed to significantly enhance the PL intensity and carrier lifetime and to reduce the surface electrical fields. Comparison of the methods shows that the epitaxial InP passivation is more effective. However, epitaxial GaN and nitridation methods are comparable with InP passivation.

Genetic algorithm using independent component analysis in x-ray reflectivity curve fitting of periodic layer structures

A novel genetic algorithm (GA) utilizing independent component analysis (ICA) was developed for x-ray reflectivity (XRR) curve fitting. EFICA was used to reduce mutual information, or interparameter dependences, during the combinatorial phase. The performance of the new algorithm was studied by fitting trial XRR curves to target curves which were computed using realistic multilayer models. The median convergence properties of conventional GA, GA using principal component analysis and the novel GA were compared. GA using ICA was found to outperform the other methods with problems having 41 parameters or more to be fitted without additional XRR curve calculations. The computational complexity of the conventional methods was linear but the novel method had a quadratic computational complexity due to the applied ICA method which sets a practical limit for the dimensionality of the problem to be solved. However, the novel algorithm had the best capability to extend the fitting analysis based on Parratts formalism to multiperiodic layer structures.

Fitness function and nonunique solutions in x-ray reflectivity curve fitting: crosserror between surface roughness and mass density

Nonunique solutions of the x-ray reflectivity (XRR) curve fitting problem were studied by modelling layer structures with neural networks and designing a fitness function to handle the nonidealities of measurements. Modelled atomic-layer-deposited aluminium oxide film structures were used in the simulations to calculate XRR curves based on Parratts formalism. This approach reduced the dimensionality of the parameter space and allowed the use of fitness landscapes in the study of nonunique solutions. Fitness landscapes, where the height in a map represents the fitness value as a function of the process parameters, revealed tracks where the local fitness optima lie. The tracks were projected on the physical parameter space thus allowing the construction of the crosserror equation between weakly determined parameters, i.e. between the mass density and the surface roughness of a layer. The equation gives the minimum error for the other parameters which is a consequence of the nonuniqueness of the solution if noise is present. Furthermore, the existence of a possible unique solution in a certain parameter range was found to be dependent on the layer thickness and the signal-to-noise ratio.

Evolution of Self-Assembled InAs/InP Islands into Quantum Rings

Fabrication of quantum rings (QRs) by transforming self-assembled InAs islands on InP is studied. The islands are annealed in-situ in a tertiarybutylphosphine ambient at varying temperatures. The duration of the annealing step is also varied. A partial capping of the InAs islands is not required to achieve the morphological change into rings. The evolution of the transformation is observed ex-situ by atomic force microscopy and photoluminescence measurements. The morphological transformation is accompanied by a compositional change of InAs into InAsP. The results support our conclusions that the QR formation is mostly due to strain-driven kinetic effects.

Effect of surface states on carrier dynamics in InGaAsP/InP stressor quantum dots

The carrier dynamics in strain-induced InGaAsP/InP quantum dots (QDs) is investigated by time-resolved photoluminescence and continuous-wave photoluminescence. The stressor QDs are fabricated by depositing self-assembled InAs islands (or stressor islands) on top of a near-surface InGaAsP/InP quantum well (QW). The temporal behaviour of the QD photoluminescence transients are observed to exhibit a two-phase decay. Rate equation analyses reveal that by increasing the distance of the QW from the surface the surface capture time constant is increased considerably while the capture time constant of an electron from the QW to the QD is decreased.

Highly Tunable Emission from Strain-Induced InGaAsP/InP Quantum Dots

Low-temperature photoluminescence (PL), tunable between 1.3 and 1.7 µm, is obtained from strain-induced quantum dots (SIQDs). The quantum dots are fabricated by growing self-assembled InAs stressor islands on top of a near-surface InGaAs(P)/InP quantum well (QW). The structure is tuned by varying the composition of the QW and its distance from the surface. Time-resolved PL measurements reveal that QD relaxation and recombination time constants (0.55 and 0.50 ns) are virtually independent of the composition of the nearly lattice-matched InGaAsP layer. Also, decreasing the QW distance from the surface is shown to shift the energy states and significantly diminish the PL intensity of the SIQD.

InGaAs/InP Quantum Dots Induced by Self-Organized InAs Stressor-Islands

A material system utilizing InGaAs/InP quantum wells (QWs) and InAs islands to create strain-induced quantum dots (SIQDs) is introduced. The SIQDs are fabricated in situ by growing self-organized stressor-islands on top of a near-surface QW. The confinement of carriers in the SIQDs is studied by photoluminescence. Emission peak from the SIQD ground state transition is redshifted by 64 meV from the quantum well peak. Low-temperature luminescence from the SIQD is observed around 0.8 eV (1.55 ?m). Altogether, four SIQD states are identified having a level splitting of 15 meV.