Teppo Hakkarainen

Observation of defect complexes containing Ga vacancies in GaAsN

Positron annihilation spectroscopy was used to study GaAsN/GaAs epilayers. GaAsN layers were found to contain Ga vacancies in defect complexes. The density of the vacancy complexes increases rapidly to the order of 1018 cm−3 with increasing N composition and decreases after annealing at 700 °C. The anticorrelation of the vacancy concentration and the integrated photoluminescence intensity suggests that the Ga vacancy complexes act as nonradiative recombination centers.

Catalyst-free growth of In(As)P nanowires on silicon

The catalyst-free metal organic vapor phase epitaxial growth of In(As)P nanowires on silicon substrates is investigated using in situ deposited In droplets as seeds for nanowire growth. The thin substrate native oxide is found to play a crucial role in the nanowire formation. The structure of the nanowires is characterized by photoluminescence and electron microscopy measurements. The crystal structure of the InP nanowires is wurtzite with its c axis perpendicular to the nanowire axis. Adding arsenic precursor to the gas phase during growth results in a bimodal photoluminescence spectrum exhibiting peak at the InAsP and InP band gap energies.

Enhanced luminescence from catalyst-free grown InP nanowires

The surface effects in the optical properties of catalyst-free grown InP nanowires are investigated. Both as-grown nanowires and nanowires treated with hydrofluoric acid are studied using low- and room-temperature continuous-wave and time-resolved photoluminescence measurements and transmission electron microscopy. It is shown that the room-temperature photoluminescence intensity is increased by two orders of magnitude after the surface treatment, and that there is also a significant increase in the double-exponential photoluminescence decay time.

Effect of substrate orientation on the catalyst-free growth of InP nanowires

We report the fabrication of self-catalysed InP nanowires on (111)B, (111)A, (110), and (001) InP substrates. Indium droplets, deposited in situ using metalorganic vapour phase epitaxy, are used as seeds for nanowire growth. The preferential nanowire growth direction is always on (111)B, (111)A, and (110) oriented substrates. On (111)A substrates some initial growth in the [111]A direction is observed before kinking into one of the available directions. The nanowires are optically active at room temperature. On (001) substrates no nanowire growth off the substrate is observed. However, growth still takes place in the two possible equivalent azimuthal directions.

Self-assembled GaIn(N)As quantum dots: Enhanced luminescence at 1.3 μm

Self-assembled GaIn(N)As quantum dots are fabricated on GaAs by atmospheric pressure metalorganic vapor-phase epitaxy using dimethylhydrazine (DMHy) precursor as a nitrogen source. The incorporation of nitrogen into the islands is observed to be negligible. However, the areal density of the islands is increased by up to one order of magnitude compared to that of the respective GaInAs islands. The GaIn(N)As island size can also be controlled by varying the DMHy flow. An enhancement of the room-temperature luminescence at 1.3 μm is observed in the GaIn(N)As samples grown with DMHy.

Feature size reduction of silicon slot waveguides by partial filling using atomic layer deposition

We propose a novel method to realize silicon-on-insulator (SOI)-based air slot waveguides for sensing applications. The method, based on feature size reduction using conformal thin films grown by atomic layer deposition (ALD), enables a guided slot mode in a silicon slot waveguide with a patterned slot width of more than 200 nm. Feature size reduction of slot structures with ALD grown amorphous TiO2 is demonstrated.

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.

Misfit dislocations in GaAsN-GaAs interface

Highly strained GaAsN layers were grown on GaAs by metal-organic vapor phase epitaxy and studied by synchrotron X-ray topography and X-ray diffraction. The critical thickness for misfit dislocation formation of the GaAs0.965N0.035 epitaxial layer on GaAs was found to be between 50 and 80 nm. In layers thicker than the critical thickness a misfit dislocation network was observed. The network was found to be isotropic and uniform. The relaxation of the strained epilayer begins through the misfit-dislocation generation and continues via formation of cracks. The cracks are not distributed as uniformly as misfit dislocations.

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.

Structural and optical properties of GaInNAs/GaAs quantum structures

Structural and optical properties of GaInNAs/GaAs quantum structures grown by metalorganic vapour phase epitaxy (MOVPE) are studied. The growth of arsenide–nitrides by MOVPE is reviewed and compared to the other major growth technique, molecular beam epitaxy. Post-growth thermal annealing and laser irradiation are employed and found to affect GaInNAs quantum wells differently: with laser treatment no undesired blue-shift of the photoluminescence peak is observed. The critical thickness for misfit dislocation formation of GaAsN on GaAs is found to be about twice as large as the theoretical prediction. GaAsN epilayers are also found to contain Ga vacancies in defect complexes. The current status of experimental research on GaInNAs quantum dot structures is reviewed. Self-organized and strain-induced GaInNAs quantum dots are grown and their formation and optical properties are studied. The emission wavelength of InGaAs quantum dots is extended by using InGaAs and GaInNAs barrier layers.