P.C.M. Christianen

Ellipsoidal InAs quantum dots observed by cross-sectional scanning tunneling microscopy

We report a detailed analysis of the shape, size, and composition of self-assembled InAs quantum dots based on cross-sectional scanning tunneling microscopy (X-STM) experiments. X-STM measurements on 13 individual quantum dots reveal an ellipsoidal dot shape with an average height of 8 nm and a diameter of 26 nm. Analysis of the outward relaxation and lattice constant profiles shows that the dots consist of an InGaAs alloy with a profound gradient in the indium concentration in both horizontal and vertical directions. These results are important to obtain a deeper understanding of the relationship between the structural and electronic properties of semiconductor quantum dots.

Electric field dependent EL2 capture coefficient in semi‐insulating GaAs obtained from propagating high field domains

We have determined the electric field dependence of the carrier capture coefficient of the EL2 traps in semi‐insulating GaAs up to 7 kV/cm by means of the quantitative analysis of propagating high electric field domains. The experimental data show a trapping coefficient which is proportional to the electron drift velocity, supporting the microscopic model of electric field enhanced trapping of carriers over a configurational barrier.

Suppression of convection using gradient magnetic fields during crystal growth of NiSO4-6H2O

A magnetic field was successfully used to suppress buoyancy driven convection during solution growth of a NiSO4∙6H2O crystal. The disappearance of the convection plume and the expansion of the depletion zones, typical for crystal growth in the absence of gravity, were observed with schlieren microscopy when the product of magnetic field and field gradient corresponds to the condition that for all relevant concentrations buoyancy is compensated by paramagnetic counterforces. We show both theoretically and experimentally, that levitation of the growth solution is not the correct condition to suppress convection.

MAGNETIC TRAP FOR EXCITONS

Abstract High in-plane magnetic field gradients are used to control the lateral motion of excitons in an InGaAs quantum well. The gradients, as large as 6.10 4 xa0T/m, are produced by positioning a thin magnetized stripe of dysprosium on top of the quantum well. By measuring the exciton photoluminescence energy and intensity, spatially resolved with μm resolution, it is shown that the diamagnetic excitons are forced to regions of low magnetic field, because of the interaction of their magnetic moment with the inhomogeneous field. These results show the possibility to magnetically confine excitons to a limited region of space, which opens the way for the realization of the first magnetic trap in solids.

Optical investigation of shallow acceptor states in GaN grown by hydride vapor-phase epitaxy

The evolution of low-temperature photoluminescence (PL) spectra with the thickness of the layer (3–400 μm) is investigated on high-quality GaN grown by hydride vapor-phase epitaxy. With increasing layer thickness, three acceptor bound exciton peaks are found to reduce in intensity, although the impurity concentrations, measured by secondary ion mass spectrometry, do not depend on the sample thickness. The observed acceptor transitions are attributed to intrinsic defects, originating from the substrate/layer interface and decreasing in density with the thickness of the layer. The optical properties, studied by reflectance, temperature and excitation power dependent PL, are compared to those of homoepitaxial GaN films grown by metalorganic chemical vapor deposition.

Optically excited multi-band conduction in LaAlO3/SrTiO3 heterostructures

The low-temperature resistance of a conducting LaAlO3/SrTiO3 interface with a 10u2009nm LaAlO3 film decreases by more than 50% after illumination with light of energy higher than the SrTiO3 band-gap. We explain our observations by optical excitation of an additional high mobility electron channel, which is spatially separated from the photo-excited holes. After illumination, we measure a strongly non-linear Hall resistance which is governed by the concentration and mobility of the photo-excited carriers. This can be explained within a two-carrier model where illumination creates a high mobility electron channel in addition to a low mobility electron channel which exists before illumination.

Spatially indirect excitons in GaAs/Al0.35Ga0.65As double quantum wells in the presence of in-plane magnetic fields

We studied the optical properties of GaAs/Al0.35Ga0.65As asymmetric double quantum wells at T = 4.2 K and in the presence of in-plane magnetic fields up to 20 T. In an electric field F ≳ 45 kV/cm, electrons and holes are respectively confined in the wide and narrow well and form spatially indirect excitons with a binding energy of∼3.5 meV. The photoluminescence (PL) peak shifts diamagnetically by 0.02 meV/T2 and is quenched by fields of ∼5 T, the suppression being stronger at higher biases. The experimental results are well explained by numerical calculations which show that the magnetic field makes the e–h recombination indirect also in k-space. The suppression of the PL cannot be described in the picture of free e–h pairs, but rather scales as the ratio between the magnetic length and the e–h distance; we therefore believe that it is a peculiar manifestation of the exciton binding.

Investigation of optical and structural properties of GaN grown by hydride vapor-phase epitaxy

Photoluminescence and reflectance experiments of high quality GaN layers grown by hydride vapor-phase epitaxy reveal narrow (2-3 meV) free and bound exciton transitions. Quenching of three acceptor-bound exciton peaks is observed with increasing film thickness (3.5-400 μm). Changes in the PL features appear to be independent of the impurity concentrations, as measured by secondary ion mass spectrometry, and the dislocation densities, obtained by photo-enhanced chemical etching. Therefore, the observed acceptor levels are assigned to intrinsic defects originating from the substrate/layer interface and decreasing in density with the thickness of the film.

Magneto-optical study of ZnSe-based quantum wells

Abstract Heterostructures with ZnSe quantum wells and different barrier materials (Zn 1xa0−xa0 x Mg x S 1xa0−xa0 y Se y and Zn 1xa0−xa0 x Be x Se) are investigated by means of photoluminescence and reflectivity in external magnetic fields up to 20 T. Up to four exciton excited states are well-resolved in the photoluminescence excitation spectra of undoped structures. Model calculations of the magneto-exciton fan chart are performed. In n-type modulation doped quantum wells negatively charged excitons associated with heavy-hole and light-hole excitonic states are found.

Laser emission on a cavity-polariton line in a II–VI microcavity

Abstract The occurrence of a stimulated emission line exactly superimposed on the lower branch of a II–VI microcavity polariton shows that laser emission can be obtained from a strongly coupled exciton—photon system. We present spectral, time and power data as experimental evidence for this.