Absence of Quantum-Confined Stark Effect in GaN Quantum Disks Embedded in (Al,Ga)N Nanowires Grown by Molecular Beam Epitaxy.

Several of the key issues of planar (Al,Ga)N-based deep-ultraviolet light-emitting diodes could potentially be overcome by utilizing nanowire heterostructures, exhibiting high structural perfection, and improved light extraction. Here, we study the spontaneous emission of GaN/(Al,Ga)N nanowire ensembles grown on Si(111) by plasma-assisted molecular beam epitaxy. The nanowires contain single GaN quantum disks embedded in long (Al,Ga)N nanowire segments essential for efficient light extraction. These quantum disks are found to exhibit intense light emission at unexpectedly high energies, namely, significantly above the GaN bandgap, and almost independent of the disk thickness. An in-depth investigation of the actual structure and composition of the nanowires reveals a spontaneously formed Al gradient both along and across the nanowire, resulting in a complex core/shell structure with an Al-deficient core and an Al-rich shell with continuously varying Al content along the entire length of the (Al,Ga)N segment. This compositional change along the nanowire growth axis induces a polarization doping of the shell that results in a degenerate electron gas in the disk, thus screening the built-in electric fields. The high carrier density not only results in the unexpectedly high transition energies but also in radiative lifetimes depending only weakly on temperature, leading to a comparatively high internal quantum efficiency of the GaN quantum disks up to room temperature.

Atom-by-Atom Analysis of Semiconductor Nanowires with Parts Per Million Sensitivity.

The functionality of semiconductor devices is determined by the incorporation of dopants at concentrations down to the parts per million (ppm) level and below. Optimization of intentional and unintentional impurity doping relies on methods to detect and map the level of impurities. Detecting such low concentrations of impurities in nanostructures is however challenging to date as on the one hand methods used for macroscopic samples cannot be applied due to the inherent small volumes or faceted surfaces and on the other hand conventional microscopic analysis techniques are not sufficiently sensitive. Here, we show that we can detect and map impurities at the ppm level in semiconductor nanowires using atom probe tomography. We develop a method applicable to a wide variety of nanowires relevant for electronic and optical devices. We expect that it will contribute significantly to the further optimization of the synthesis of nanowires, nanostructures and devices based on these structures.

Boosting Hole Mobility in Coherently Strained [110]-Oriented Ge-Si Core-Shell Nanowires.

The ability of core-shell nanowires to overcome existing limitations of heterostructures is one of the key ingredients for the design of next generation devices. This requires a detailed understanding of the mechanism for strain relaxation in these systems in order to eliminate strain-induced defect formation and thus to boost important electronic properties such as carrier mobility. Here we demonstrate how the hole mobility of [110]-oriented Ge-Si core-shell nanowires can be substantially enhanced thanks to the realization of large band offset and coherent strain in the system, reaching values as high as 4200 cm2/(Vs) at 4 K and 1600 cm2/(Vs) at room temperature for high hole densities of 1019 cm-3. We present a direct correlation of (i) mobility, (ii) crystal direction, (iii) diameter, and (iv) coherent strain, all of which are extracted in our work for individual nanowires. Our results imply [110]-oriented Ge-Si core-shell nanowires as a promising candidate for future electronic and quantum transport devices.

Growth and Optical Properties of Direct Band Gap Ge/Ge0.87Sn0.13 Core/Shell Nanowire Arrays.

Group IV semiconductor optoelectronic devices are now possible by using strain-free direct band gap GeSn alloys grown on a Ge/Si virtual substrate with Sn contents above 9%. Here, we demonstrate the growth of Ge/GeSn core/shell nanowire arrays with Sn incorporation up to 13% and without the formation of Sn clusters. The nanowire geometry promotes strain relaxation in the Ge0.87Sn0.13 shell and limits the formation of structural defects. This results in room-temperature photoluminescence centered at 0.465 eV and enhanced absorption above 98%. Therefore, direct band gap GeSn grown in a nanowire geometry holds promise as a low-cost and high-efficiency material for photodetectors operating in the short-wave infrared and thermal imaging devices.

Spin-orbit-induced circulating currents in a semiconductor nanostructure.

Circulating orbital currents produced by the spin-orbit interaction for a single electron spin in a quantum dot are explicitly evaluated at zero magnetic field, along with their effect on the total magnetic moment (spin and orbital) of the electron spin. The currents are dominated by coherent superpositions of the conduction and valence envelope functions of the electronic state, are smoothly varying within the quantum dot, and are peaked roughly halfway between the dot center and edge. Thus the spatial structure of the spin contribution to the magnetic moment (which is peaked at the dot center) differs greatly from the spatial structure of the orbital contribution. Even when the spin and orbital magnetic moments cancel (for g=0) the spin can interact strongly with local magnetic fields, e.g., from other spins, which has implications for spin lifetimes and spin manipulation.

Bistable charge configuration of donor systems near the GaAs(110) surfaces.

In gated semiconductor devices, the space charge layer that is located under the gate electrode acts as the functional element. With increasing gate voltage, the microscopic process forming this space charge layer involves the subsequent ionization or electron capture of individual dopants within the semiconductor. In this Letter, a scanning tunneling microscope tip is used as a movable gate above the (110) surface of n-doped GaAs. We study the build-up process of the space charge region considering donors and visualize the charge states of individual and multi donor systems. The charge configuration of single donors is determined by the position of the tip and the applied gate voltage. In contrast, a two donor system with interdonor distances smaller than 10 nm shows a more complex behavior. The electrostatic interaction between the donors in combination with the modification of their electronic properties close to the surface results in ionization gaps and bistable charge switching behavior.

Shape and size control of InAs/InP (113)B quantum dots by Sb deposition during the capping procedure.

The role of Sb atoms present on the growth front during capping of InAs/InP (113)B quantum dots (QDs) is investigated by cross-sectional scanning tunnelling microscopy, atomic force microscopy, and photoluminescence spectroscopy. Direct capping of InAs QDs by InP results in partial disassembly of InAs QDs due to the As/P exchange occurring at the surface. However, when Sb atoms are supplied to the growth surface before InP capping layer overgrowth, the QDs preserve their uncapped shape, indicating that QD decomposition is suppressed. When GaAs(0.51)Sb(0.49) layers are deposited on the QDs, conformal growth is observed, despite the strain inhomogeneity existing at the growth front. This indicates that kinetics rather than the strain plays the major role during QD capping with Sb compounds. Thus Sb opens up a new way to control the shape of InAs QDs.

Nanoscale potential fluctuations in (GaMn)As/GaAs heterostructures: from individual ions to charge clusters and electrostatic quantum dots.

During growth of the dilute p-type ferromagnetic semiconductor Ga1-xMnxAs, interstitial manganese, Mni(2+), is formed when x exceeds 2%. The double donor Mni(2+) compensates the free holes that mediate ferromagnetism. Annealing causes out-diffusion of these interstitials, thereby increasing the Curie temperature. Here, we use cross sectional scanning tunneling microscopy and spectroscopy to visualize the potential landscape which arises due to the clustering of Mni(2+) in annealed p-i-n (GaMn)As-GaAs double barrier heterostructures. We map the local minima in the potential landscape, link them to clusters of individual Mni(2+) ions, and show that the ions are doubly charged.

Surface induced asymmetry of acceptor wave functions.

Measurements of the local density of states of individual acceptors in III-V semiconductors show that the symmetry of the acceptor states strongly depends on the depth of the atom below a (110) surface. Tight-binding calculations performed for a uniformly strained bulk material demonstrate that strain induced by the surface relaxation is responsible for the observed depth-dependent symmetry breaking of acceptor wave functions. As this effect is strongest for weakly bound acceptors, it explains within a unified approach the commonly observed triangular shapes of shallow acceptors and the crosslike shapes of deeply bound acceptor states in III-V materials.

An atomically resolved study of InGaAs quantum dot layers grown with an indium flush step.

In this cross-sectional scanning tunnelling microscopy study we investigate the indium flush method as a means to control the height of self-assembled InGaAs quantum dots and wetting layers. The results show that application of an indium flush step during growth results in flattened dots and a reduced wetting layer of which the height can be precisely controlled by varying the height of the first capping layer.

An atomic scale study of surface termination and digital alloy growth in InGaAs/AlAsSb multi-quantum wells.

An atomic scale study has been performed to understand the influence of the (As,Sb) shutter sequences during interface formation on the optical properties of InGaAs/AlAsSb quantum wells. Our cross-sectional scanning tunneling microscopy results show that the onset of the Sb profile is steep in the Sb-containing layers whereas an appreciable segregation of Sb in the subsequently grown Sb free layers is observed. The steep rise of the Sb profile is due to extra Sb that is supplied to the surface prior to the growth of the Sb-containing layers. No relation is found between the (As,Sb) termination conditions of the Sb-containing layers and the resulting Sb profiles in the capping layers. Correspondingly we see that the optical properties of these quantum wells are also nearly independent on the (As,Sb) shutter sequences at the interface. Digital alloy growth in comparison to conventional molecular beam epitaxy growth was also explored. X-ray results suggest that the structural properties of the quantum well structures grown by conventional molecular beam epitaxy techniques are slightly better than those formed by digital alloy growth. However photoluminescence studies indicate that the digital alloy samples give rise to a more intense and broader photoluminescence emission. Cross-sectional scanning tunneling microscopy measurements reveal that lateral composition modulations present in the digital alloys are responsible for the enhancement of the photoluminescence intensity and inhomogeneous broadening.

Bistable Si dopants in the GaAs (1 1 0) surface.

In this review, recent work is discussed on bistable Si dopants in the GaAs (1 1 0) surface, studied by scanning tunneling microscopy (STM). The bistability arises because the dopant atom can switch between a positive and a negative charge state, which are associated with two different lattice configurations. Manipulation of the Si atom charge configuration is achieved by tuning the local band bending with the STM tip. Furthermore, illuminating the sample with a laser also influences the charge state, allowing the operation of the dopant atom as an optical switch. The switching dynamics without illumination is investigated in detail as a function of temperature, lateral tip position, and applied tunneling conditions. A physical model is presented that independently describes the thermal and quantum tunneling contributions to the switching frequency and charge state occupation of a single Si atom. The basic functionality of a memory cell is demonstrated employing a single bistable Si dopant as the active element, using the STM tip as a gate to write and read the information.

Apparent antifungal activity of several lactic acid bacteria against Penicillium discolor is due to acetic acid in the medium.

Fifty-six dairy bacteria belonging to the genera Lactococcus, Lactobacillus, Pediococcus, Propionibacterium, Streptococcus, Enterococcus, Leuconostoc, and Brevibacterium were screened for antifungal activity against four species of fungi relevant to the cheese industry (Penicillium discolor, Penicillium commune, Penicillium roqueforti, and Aspergillus vesicolor). Most of the active strains belonged to the genus Lactobacillus, whereas Penicillium discolor was found to be the most sensitive of the four fungi investigated. Further studies on P. discolor showed antifungal activity only below pH 5. This effect of pH suggests that organic acids present in the culture could be involved in the detected activity. Determination of acid composition revealed lactic acid production for active dairy strains and the presence of acetic acid in active as well as inactive strains. It was demonstrated that the undissociated acetic acid originates from the bacterial growth medium. The synergistic effect of the acetic acid present and the lactic acid produced was likely the main factor responsible for the antifungal properties of the selected bacteria. These results could explain some discrepancies in reports of the antifungal properties of lactic acid bacteria, since the role of acetic acid has not been considered in previous studies.

In vitro susceptibility of Campylobacter and Salmonella isolates from broilers to quinolones, ampicillin, tetracycline, and erythromycin.

Recently, an increased resistance of Campylobacter to fluoroquinolones, a newer class of antimicrobial agents in both human and veterinary medicine, has been reported. Campylobacter isolates (617) from 150 broiler flocks were tested for their susceptibility to cephalothin (control), ampicillin, tetracycline, erythromycin, and the quinolones nalidixic acid, flumequine, enrofloxacin, and ciprofloxacin by a disc diffusion method. Almost complete cross-resistance was found between the quinolones tested. Campylobacter isolates (181, 29%), originating from 55 flocks (37%), were quinolone resistant. Salmonella isolates (94) from 40 flocks were also tested for their antimicrobial susceptibility. Eight isolates (8.5%), from three broiler flocks (7.5%), showed resistance to nalidixic acid and flumequine (and tetracycline), but not to ciprofloxacin or enrofloxacin.

A Versatile and High-Yield Route to Active and Well-Defined Catalysts [Ru(bisphosphane)(H)(solvent)3 ](BF4 ).

Simple and efficient: Protonation of [Ru(1,2:5,6-η-cod)(η6 -cot)] (cod=1,5-cyclooctadiene, cot=1,3,5-cyclooctatriene) with HBF4 ⋅Et2 O and then reaction with chiral bisphosphane ligands ($_{\rm PP}^{\frown }$=Me-DuPHOS, BINAP, Tol-BINAP, JOSIPHOS) affords the corresponding [Ru($_{\rm PP}^{\frown }$)(H)(η6 -cot)]+ or [Ru($_{\rm PP}^{\frown }$)(1,2,3,4,5-η-C8 H11 ')]+ (C8 H11 '=2,4-cyclooctadienyl; see scheme). Exposure of these cations to H2 in solvents (sol) such as acetone, methanol, and THF affords [Ru($_{\rm PP}^{\frown }$)(H)(sol)3 ]+ , which are catalysts for (amongst other things) enantioselective hydrogenations of alkenes.

Simple and efficient scanning tunneling luminescence detection at low-temperature.

We have designed and built an optical system to collect light that is generated in the tunneling region of a low-temperature scanning tunneling microscope. The optical system consists of an in situ lens placed approximately 1.5 cm from the tunneling region and an ex situ optical lens system to analyze the emitted light, for instance, by directing the light into a spectrometer. As a demonstration, we measured tip induced photoluminescence spectra of a gold surface. Furthermore, we demonstrate that we can simultaneously record scanning tunneling microscope induced luminescence and topography of the surface both with atomic resolution.