The improved inverted AlGaAs/GaAs interface: its relevance for high-mobility quantum wells and hybrid systems

Two dimensional electron gases (2DEGs) realized at GaAs/AlGaAs single interfaces by molecular-beam epitaxy (MBE) reach mobilities of about 15 million cm^2/Vs if the AlGaAs alloy is grown after the GaAs. Surprisingly, the mobilities may drop to a few millions for the identical but inverted AlGaAs/GaAs interface, i.e. reversed layering. Here we report on a series of inverted heterostructures with varying growth parameters including temperature, doping, and composition. Minimizing the segregation of both dopants and background impurities leads to mobilities of 13 million cm^2/Vs for inverted structures. The dependence of the mobility on electron density tunes by a gate or by illumination is found to be the identical if no doping layers exist between the 2DEG and the respective gate. Otherwise, it differs significantly compared to normal interface structures. Reducing the distance of the 2DEG to the surface down to 50nm requires an additional doping layer between 2DEG and surface in order to compensate for the surface-Schottky barrier. The suitability of such shallow inverted structures for future semiconductor-superconductor hybrid systems is discussed. Lastly, our understanding of the improved inverted interface enables us to produce optimized double-sided doped quantum wells exhibiting an electron mobility of 40 million cm^2/Vs at 1K.Comment: 19 pages, 9 figure

Electronic g-factor and Magneto-transport in InSb Quantum Wells

High mobility InSb quantum wells with tunable carrier densities are investigated by transport experiments in magnetic fields tilted with respect to the sample normal. We employ the coincidence method and the temperature dependence of the Shubnikov-de Haas oscillations and find a value for the effective g-factor of $\mid g^{\ast}\mid$ =35$\pm$4 and a value for the effective mass of $m^*\approx0.017 m_0$, where $m_0$ is the electron mass in vacuum. Our measurements are performed in a magnetic field and a density range where the enhancement mechanism of the effective g-factor can be neglected. Accordingly, the obtained effective g-factor and the effective mass can be quantitatively explained in a single particle picture. Additionally, we explore the magneto-transport up to magnetic fields of 35 T and do not find features related to the fractional quantum Hall effect.Comment: 18 Pages, 5 Figure

Ion implanted Back-gates developed for high-mobility Two-dimensional Electron systems

Since discovering the quantum Hall effect, two-dimensional electron systems (2DEGs) in (perpendicular) magnetic fields are a field of intense research. With the continuous improvement of sample quality, multiple new exotic quantum phases have been discovered, some of which have potentially topological protected states with quantum computing applications. Almost all of the correlated phases are stabilized by electron-electron interaction, which can be controlled using sophisticated gating approaches. While the application of metallic top-gates is straightforward, back-gates are a technological challenge. The presented new development of planar structured back-gates based on ion implantation overcomes traditional designs' common problems, allowing the combination of multiple back-gates with very high-quality 2DEGs. Two techniques to fabricate patterned back-gates are investigated. One applies either oxygen or gallium ion implantation to passivate doped regions. Another approach utilizes the implantation of donors, tested with silicon, selenium, and tellurium, to form conducting areas in semi-insulating gallium arsenide. While both techniques allow the fabrication of conducting, patterned back-gates, passivation using oxygen implantation results in both the most reliable and least detrimental method, crucial for obtaining high mobility 2DEGs. The high-quality MBE-growth on patterned back-gated substrates has been demonstrated on a single 2DEG, equipped with ion-implanted back- and metallic top-gate. By tuning the gates to exactly center the wave-function of the 2DEG in the quantum well, a peak mobility of \SI{40E6}{\centi\meter\squared\per\volt\per\second} could be achieved. Additionally, the new gate design has overcome typical problems in magneto-transport measurements of gate-enriched 2DEGs. Bilayer systems, consisting of two closely-spaced 2DEGs, have shown an even richer phase diagram due to the additional degree of freedom. Preparing a bilayer system with both top- and back-gates allows to control their interaction energy and set individual 2DEG densities. Bilayer systems, with and without separated contacts to the individual layers, are tuned across a large parameter space. While in the investigated parameter range no novel quantum states were found, we believe the planar back-gate design holds great promise to produce controllable bilayers suitable to investigate the exotic (potentially non-Abelian) properties of correlated states

Die Schweizer Migros: ein Einzelhandelsriese zwischen Genossenschaft und Manager-Konzern

Welskopp T. Die Schweizer Migros: ein Einzelhandelsriese zwischen Genossenschaft und Manager-Konzern. In: Hesse J-O, Scharnetzky T, Scholten J, eds. Das Unternehmen als gesellschaftliches Reformprojekt: Strukturen und Entwicklungen von Unternehmen der "moralischen Ökonomie" nach 1945. Bochumer Schriften zur Unternehmens- und Industriegeschichte. Vol 12. Essen: Klartext-Verlag; 2004: 127-146