Luminescence investigations of the GaP-based dilute nitride Ga(NAsP) material system

Multi-quantum well heterostructures (MQWHs) of the novel Ga(NAsP)/GaP material system have been grown, pseudomorphically strained to GaP-substrate. The crystalline perfection is verified by transmission electron microscopy (TEM). For As-concentrations in excess of about 70%, a direct band structure and adequate luminescence efficiency for laser device application is observed. Temperature-dependent photoluminescence (PL) investigations show the influence of carrier localisation and non-radiative recombination processes typical for dilute nitride materials. With rising N content in the active material, the emission wavelength shifts towards longer wavelength, leading to Ga(NAs)/GaP MQW structures with photon energies below the indirect band gap of silicon (Si). At the same time the luminescence intensity drops due to an increase in non-radiative carrier traps and/or structural degradation.     

Spectral Theory for Slowly Oscillating Potentials II. Schrödinger Operators

This paper continues the investigation about the singularity theory in dual rich quasi–Banach spaces given in T. Runst [Ru 2]. The abstract results are applied to the study of the solution structure of semilinear elliptic boundary value problems in spaces of Besov – Triebel – Lizorkin type.     

Asymptotic and boundary integral formulae for the computational reconstruction of small conductivity imperfections

In this paper we investigate the electrostatic problem of determining conductivity profiles from the knowledge of boundary currents and voltages. We obtain an improved estimate for the voltage potential of a two-dimensional conductor having finitely many circular inclusions and piecewise constant conductivity profile. We derive an asymptotic expansion for the voltage potential in terms of the reference voltage potential and the location, size, and conductivity of the inhomogeneities. This representation is used to formulate the nonlinear least squares problem for estimating the location and size of the inhomogeneities. Required boundary data for the voltage potential are generated numerically by solving a system of integral equations. Computational experiments are presented to demonstrate the effectiveness of our identification procedure.     

Trace norm estimates for products of integral operators and diffusion semigroups

We give trace norm estimates for products of integral operators and for diffusion semigroups. These are applied to differences of heat semigroups. A natural example of an integral operator with finite trace which is not trace class is given.     

Properties of Josephson junctions in the inhomogeneous magnetic field of a system of ferromagnetic particles

The effect of an array of ferromagnetic nanoparticles on the field-dependent critical current of the short overlap Josephson junction is experimentally studied. Large reversible variations of the maximum critical current are observed depending on the magnetic state of the particles. The pronounced commensurability effects are detected which are proved by the additional peaks of magnetic field induced diffraction pattern.     

Production of slow muonic hydrogen isotopes in vacuum

Muonic hydrogen isotopes (μ⁻ p, μ⁻ d, and μ⁻t) are simple quantum mechanical systems ideally suited for studies of numerous fundamental phenomena in electroweak and strong interactions as well as in applied areas such as muon chemistry or muon catalyzed fusion. Emission of muonic hydrogen isotopes into vacuum helps to overcome the limitations which are normally imposed on conventional investigations with gaseous and liquid targets. A proof of principle experiment for this new technique was performed at TRIUMF last year. Negative muons with 30 MeV/c momentum were stopped in a thin film of solid hydrogen and produced very low energy μ⁻d in vacuum. The distribution center of the normal velocity components of emitted μ⁻d atoms was measured to be ∼1 cm/μs. The yield of μ⁻d in vacuum is an increasing function of H₂ film thickness δ up to a value of δ≥1 mm.     

Atomic scale properties of interior interfaces of semiconductor heterostructures as determined by quasi-digital highly selective etching and atomic force microscopy

We report on first experiments combining quasi-digital highly selective etching and atomic force microscopy (AFM) to examine the interior interfaces of semiconductor heterostructures. Lattice matched (GaIn)As/InP heterostructures grown by metalorganic vapour-phase epitaxy (MOVPE) are taken as a model system to check the capabilities of this new method. Standard selective etchants for different material systems have been optimized in selectivity and etch rate to achieve a quasi-digital etching behaviour. In this way, the real structure of interior interfaces can be determined by AFM. We find a significant difference between the surface of the heterostructure and the interior interfaces.