Curie temperature and carrier concentration gradients in MBE grown GaMnAs layers

We report on detailed investigations of the electronic and magnetic properties of ferromagnetic GaMnAs layers, which have been fabricated by low-temperature molecular-beam epitaxy. Superconducting quantum interference device measurements reveal a decrease of the Curie temperature from the surface to the GaMnAs/GaAs interface. While high resolution x-ray diffraction clearly shows a homogeneous Mn distribution, a pronounced decrease of the carrier concentration from the surface towards the GaMnAs/GaAs interface has been found by Raman spectroscopy as well as electrochemical capacitance-voltage profiling. The gradient in Curie temperature seems to be a general feature of GaMnAs layers grown at low-temperature. Possible explanations are discussed.Comment: 3 pages, 4 figures, submitted to AP

Influence of micro-patterning of the growth template on defect reduction and optical properties of non-polar (11-20) GaN

We investigate the influence of different types of template micro-patterning on defect reduction and optical properties of non-polar GaN using detailed luminescence studies. Non-polar (11-20) (or a-plane) GaN exhibits a range of different extended defects compared with its more commonly used c-plane counterpart. In order to reduce the number of defects and investigate their impact on luminescence uniformity, non-polar GaN was overgrown on four different GaN microstructures. The micro-patterned structures consist of a regular microrod array; a microrod array where the -c-side of the microrods has been etched to suppress defect generation; etched periodic stripes and finally a subsequent combination of etched stripes and etched microrods (double overgrowth). Overall the presence of extended defects, namely threading dislocations and stacking faults (SFs) is greatly reduced for the two samples containing stripes compared with the two microrod samples. This is evidenced by more uniform emission and reduction in dark regions of non-radiative recombination in room temperature cathodoluminescence imaging as well as a reduction of the SF emission line in low temperature photoluminescence. The observed energy shifts of the GaN near band edge emission are related to anisotropic strain relaxation occurring during the overgrowth on these microstructures. A combination of stripes and microrods is a promising approach for defect reduction and emission uniformity in non-polar GaN for applications in light-emitting devices as well as power electronics

Luminescence behavior of semipolar (101¯1) InGaN/GaN “bow-tie” structures on patterned Si substrates

In this work, we report on the innovative growth of semipolar “bow-tie”-shaped GaN structures containing InGaN/GaN multiple quantum wells (MQWs) and their structural and luminescence characterization. We investigate the impact of growth on patterned (113) Si substrates, which results in the bow-tie cross section with upper surfaces having the (101¯1) orientation. Room temperature cathodoluminescence (CL) hyperspectral imaging reveals two types of extended defects: black spots appearing in intensity images of the GaN near band edge emission and dark lines running parallel in the direction of the Si stripes in MQW intensity images. Electron channeling contrast imaging (ECCI) identifies the black spots as threading dislocations propagating to the inclined (101¯1) surfaces. Line defects in ECCI, propagating in the [12¯10] direction parallel to the Si stripes, are attributed to misfit dislocations (MDs) introduced by glide in the basal (0001) planes at the interfaces of the MQW structure. Identification of these line defects as MDs within the MQWs is only possible because they are revealed as dark lines in the MQW CL intensity images, but not in the GaN intensity images. Low temperature CL spectra exhibit additional emission lines at energies below the GaN bound exciton emission line. These emission lines only appear at the edge or the center of the structures where two (0001) growth fronts meet and coalesce (join of the bow-tie). They are most likely related to basal-plane or prismatic stacking faults or partial dislocations at the GaN/Si interface and the coalescence region

Structure and dynamics of central European amphibian populations: A comparison between Triturus dobrogicus (Amphibia, Urodela) and Pelobates fuscus (Amphibia, Anura)

During a long-term study of the amphibian fauna on an artificial island near Vienna (Austria), one isolated site was completely encircled with a permanent drift fence and pitfall traps. Eleven amphibian species occurred at the study site. For the Common Spadefoot Toad (Pelobates fuscus) and for the Danube Crested Newt (Triturus dobrogicus). individuals could be recognized by photographing the highly variable dorsal/ventral patterns. Daily patrols of the drift fence, for 6 years since 1986, enabled us to monitor the demography and dynamics of these two species. For T. dobrogicus, the adult part of the censused population decreased from 207 to 87 individuals during the first 2 years of investigation and then remained stable. Pelobates fuscus showed a constant decrease over 6 years, from 626 to 62 individuals. Juveniles were produced annually; a massive increase in this age class was observed for both species during the period of investigation. Triturus dobrogicus showed higher adult survival than P. fuscus. The constancy of several population parameters of both species may reflect the stability of the cultured parkland habitat in which the study site is located

Semiconductor nanostructures defined with self-organizing polymers

We describe a technique to create very small semiconductor nanostructures, with sizes far beyond the limit of conventional optical lithography processes, by the use self-assembling diblock copolymers as nanolithographic masks. Quantum structures with very high aspect ratio of 1:10 were fabricated by dry etching. In a first step, so-called diblock copolymer micelles were generated in a toluene solution. These micelles were loaded by a noble-metal salt. After dipping a substrate into this solution, a monolayer of ordered micelles is generated, covering almost the complete surface. After treatment in a hydrogen plasma all of the organic components are removed and only crystalline metal clusters of ≈12 nm size remain. This metal cluster mask can be used directly in a chlorine dry etching process to etch cylinders in GaAs and its alloys of In and Al. It is also possible to etch through a quantum well layer underneath the surface in order to produce quantum dots. The resulting nanostructures were investigated by scanning force microscopy, by high resolution transmission electron microscopy, and also by low temperature photoluminescence spectroscopy