Eero Arola

Single-step fabrication of luminescent GaAs nanocrystals by pulsed laser ablation in liquids

Optical absorption and emission properties of gallium arsenide nanocrystals can be tuned across the visible spectrum by tuning their size. The surface of pure GaAs nanocrystals tends to oxidize, which deteriorates their optical properties. In order to prevent the oxidization, surface passivation has been previously demonstrated for GaAs nanocrystals larger than the Bohr exciton radius. In this paper, we study synthesis of small GaAs nanocrystals by pulsed laser ablation in liquids combined with simultaneous chemical surface passivation. The fabricated nanocrystals are smaller than the Bohr exciton radius and exhibit photoluminescence peaked near 530 nm due to quantum confinement. The photoluminescence properties are stable for at least six months, which is attributed to successful surface passivation. The chemical structure of the nanocrystals and changes caused by thermal annealing are elucidated with Raman spectroscopy, transmission electron microscopy and x-ray photoelectron spectroscopy.

Computational study of GaAs1−xNx and GaN1−yAsy alloys and arsenic impurities in GaN

We have studied the structural and electronic properties of As-rich GaAs1−x Nx and N-rich GaN1−yAsy alloys in a large composition range using first-principles methods. We have systematically investigated the effect of the impurity atom configuration near both GaAs and GaN sides of the concentration range on the total energies, lattice constants and bandgaps. The N (As) atoms, replacing substitutionally As (N) atoms in GaAs (GaN), cause the surrounding Ga atoms to relax inwards (outwards), making the Ga–N (Ga– As) bond length about 15% shorter (longer) than the corresponding Ga–As (Ga–N) bond length in GaAs (GaN). The total energies of the relaxed alloy supercells and the bandgaps experience large fluctuations within different configurations and these fluctuations grow stronger if the impurity concentration is increased. Substituting As atoms with N in GaAs induces modifications near the conduction band minimum, while substituting N atoms with As in GaN modifies the states near the valence band maximum. Both lead to bandgap reduction, which is at first rapid but later slows down. The relative size of the fluctuations is much larger in the case of GaAs1−x Nx alloys. We have also looked into the question of which substitutional site (Ga or N) As occupies in GaN. We find that under Ga-rich conditions arsenic prefers the substitutional N site over the Ga site within a large range of Fermi level values.

Effects of (NH4)2S and NH4OH surface treatments prior to SiO2 capping and thermal annealing on 1.3 μm GaInAsN/GaAs quantum well structures

We report the influence of (NH4)2S and NH4OH surface treatments prior to SiO2 capping and subsequent rapid thermal annealing, on optical properties of GaInAsN/GaAs quantum-well (QW) structures. We observed an increase in QW photoluminescence (PL) emission for the (NH4)2S treated samples as compared to the untreated sample. After annealing, also the NH4OH treated sample showed significant improvement in PL. The treatments were also found to decrease the In out-diffusion and reduce the blueshift upon annealing. The PL results are discussed with x-ray diffraction and x-ray photoemission data from SiO2/GaAs, in particular, with changes found in Ga 3d spectra.

Band offset determination of the GaAs/GaAsN interface using the density?functional theory method

The GaAs/GaAsN interface band offset is calculated from first principles. The electrostatic potential at the core regions of the atoms is used to estimate the interface potential and align the band structures obtained from respective bulk calculations. First, it is shown that the present method performs well on the well-known conventional/conventional AlAs/GaAs (001) superlattice system. Then the method is applied to a more challenging nonconventional/conventional GaAsN/GaAs (001) system, and consequently type I band lineup and valence-band offset of about 35 meV is obtained for a nitrogen concentration of about 3%, in agreement with the recent experiments. We also investigate the effect of strain on the band lineup. For the GaAsN layer longitudinally strained to the GaAs lattice constant, the type II lineup with a nearly vanishing band offset is found, suggesting that the anisotropic strain along the interface is the principal cause for the often observed type I lineup.

Suppression of annealing-induced In diffusion in Be-doped GaInAsN/GaAs quantum well

The authors report on an interesting observation regarding thermal annealing of a beryllium-doped Ga0.65In0.35As0.99N0.01/GaAs quantum well (QW) grown by molecular beam epitaxy. A QW doped at 6×1019 cm−3 exhibited superior thermal properties and about six times larger photoluminescence than an undoped QW of the same structure. X-ray diffraction and secondary ion mass spectrometry provided evidence that beryllium suppressed indium diffusion and stabilized (metastable) dilute nitride heterostructure upon annealing.

Feasibility of density functional methods to predict dielectric properties of polymers.

Feasibility of density functional theory (DFT) to predict dielectric properties such as polarizability of saturated polymers is investigated. Small saturated molecules, methane and propane, which is a monomer of polypropylene chain, are used in testing the methods. Results for polarizabilities based on several density functionals together with different basis sets are compared and contrasted with each other, with results by Hartree-Fock and second-order Moller-Plesset perturbation theory, as well as experimental data. The generalized gradient approximation PW91 method together with the 6-311++G(**) basis set is found to be the most suitable method, in terms of sufficient accuracy and computational efficiency, to calculate polarizabilities for large oligomers of polypropylene. The dielectric constant is then determined using the calculated polarizabilities and the Clausius-Mossotti equation. The molecular DFT methods at the PW916-311++G(**) level together with the Clausius-Mossotti equation give dielectric constants for saturated polymers such as polypropylene in good accordance with the experimental values.

Band offset of InGaAs(N)∕GaAs interfaces from first principles

Valence-band offsets of the InGaAs∕GaAs(001) and InGaAsN∕GaAs(001) interfaces are calculated from first principles. For InGaAs, we study the concentrations up to 25% of indium and for InGaAsN up to 12.5% of indium with 3% of nitrogen. Even though the band offset of the InGaAs∕GaAs interface has a nearly linear dependence on the indium concentration, band offset of the InGaAsN∕GaAs interface is strongly influenced by the amount of In–N bonds. Even a type-II band offset is found in the case of all indium located near to nitrogen and low strain of the InGaAsN layer.

Theoretical Studies on Multiphoton Absorption of Ultrashort Laser Pulses in Sapphire

We discuss in detail the multiphoton absorption theory for wide bandgap dielectrics that we have implemented in the framework of the time-dependent Nth order perturbation theory. In particular, we have carried out calculations on the N-photon absorption coefficient K_N and interband transition rate W_N for a perfect sapphire crystal (α-Al₂O₃). Furthermore, we derive an expression on the laser-pulse induced seed electron density n₀. Application of this theory on sapphire shows that n₀, induced by the ultrashort laser pulse with pulse duration of 30 ps, wavelength of 1 μm, and peak intensity of 5 × 10¹¹W/cm² at the focus point, cannot lead to multiphoton-based ablation. Indeed, our calculations show that the laser pulse in the N = 7 order absorption process generates a conduction electron density n₀ ≈ 6 × 10¹³ cm^-3 that is far too small compared with the required critical density n^cr ≈ 10¹⁹ - 10²¹ cm^-3. This is in agreement with the extremely small value of the probability (P_ind ≈ 2 × 10^-9) that we have estimated for a valence electron to be transferred to the conduction band under the influence of the abovementioned laser-pulse conditions. Therefore, we need to seek other mechanisms than multiphoton absorption, to explain ablation in sapphire. However, we show that by using larger photon energies and smaller order N values, it is possible to induce large enough multiphoton absorption excited electron densities, which will lead to ablation in sapphire, even for the abovementioned laser beam intensity. Finally, we discuss the Keldysh adiabatic parameter y and its interpretation in the case of our experimental pulse-laser setup.