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

Production of He-4 and (4) in Pb-Pb collisions at root(NN)-N-S=2.76 TeV at the LHC

Results on the production of He-4 and (4) nuclei in Pb-Pb collisions at root(NN)-N-S = 2.76 TeV in the rapidity range vertical bar y vertical bar <1, using the ALICE detector, are presented in this paper. The rapidity densities corresponding to 0-10% central events are found to be dN/dy4(He) = (0.8 +/- 0.4 (stat) +/- 0.3 (syst)) x 10(-6) and dN/dy4 = (1.1 +/- 0.4 (stat) +/- 0.2 (syst)) x 10(-6), respectively. This is in agreement with the statistical thermal model expectation assuming the same chemical freeze-out temperature (T-chem = 156 MeV) as for light hadrons. The measured ratio of (4)/He-4 is 1.4 +/- 0.8 (stat) +/- 0.5 (syst). (C) 2018 Published by Elsevier B.V.Peer reviewe

Ten millennia of hepatitis B virus evolution

Hepatitis B virus (HBV) has been infecting humans for millennia and remains a global health problem, but its past diversity and dispersal routes are largely unknown. We generated HBV genomic data from 137 Eurasians and Native Americans dated between ~10,500 and ~400 years ago. We date the most recent common ancestor of all HBV lineages to between ~20,000 and 12,000 years ago, with the virus present in European and South American hunter-gatherers during the early Holocene. After the European Neolithic transition, Mesolithic HBV strains were replaced by a lineage likely disseminated by early farmers that prevailed throughout western Eurasia for ~4000 years, declining around the end of the 2nd millennium BCE. The only remnant of this prehistoric HBV diversity is the rare genotype G, which appears to have reemerged during the HIV pandemic

New approaches of GaAs/AlGaAs quantum wires defined by Cleaved Edge Overgrowth

With the development of solid-state nanostructures based on MBE-grown high quality GaAs/AlGaAs heterostructures, low-dimensional systems play a growing role. Since the development of the cleaved edge overgrowth method (CEO) over thirty years ago, many one-dimensional systems have been achieved using various approaches. Still, the CEO samples exhibit unsurpassed mean free paths and very strong lateral confinements, making them an ideal test-bed for investigations of electron transport and electron-electron interactions in a one-dimensional system. A well-defined, steep confinement is created by cleaving the sample in-situ within the MBE system before a crystalline overgrowth of the atomically flat cleave. Multiple approaches for CEO and their challenges are investigated within this work. We successfully optimized the challenging fabrication process gaining new insights on crucial steps. However, none of the samples with doped overgrowth have shown signatures of ballistic 1D transport without the influence of a sidegate, although all requirements for its observation were met. For both samples with doped or un-doped overgrowth, we could demonstrate that a side-gate can create enough lateral confinement to from a wire. All the evidence suggests that the doping in [110] direction creates non-suficient lateral con nement potential, which might be due to facet-related doping issues or other difficulties of the second MBE overgrowth. A novel technique to fabricate high-quality quantum wires, based on a simplification of the cleaved edge overgrowth (CEO) method, has been developed. The most challenging part of the in-situ cleave and second overgrowth is overcome by introducing a side-gate onto an ex-situ cleaved edge. This novel method is more widely accessible, opening up the eld to non-MBE specialists. In this cleaved edge deposition (CED) technique, the confinement is achieved by the side-gate on the ex-situ cleaved surface. We could prove ballistic transport features through the 1D-wire. Direct current bias measurements showed transconductance transitions forming curved diamond-like structures from which we could extract a subband spacing of 4:64meV and a geometrical capacitance of 35:6 aF. Additionally, features of the "0.7 structure" could be observed. With the simplification of the CEO process, we believe the now widely accessible technique will foster the 1D experiments and allow straightforward technology transfer to other material systems, e.g. InAs

Gate induced quantum wires in GaAs/AlGaAs heterostructures by cleaved edge deposition

Electric conductors with dimensions reduced to the nanometer scale are the prerequisite of the quantum devices upon which the future advanced electronics is expected to be based. In the past, the fabrication of one-dimensional (1D) wires has been a particular challenge because they have to be defect-free over their whole length, which can be several tens µm. Excellent 1D wires have been produced by cleaving semiconductors (GaAs, AlGaAs) in ultra high vacuum and overgrowing the pristine edge surface by molecular beam epitaxy (MBE)1,2. Unfortunately, this cleaved edge overgrowth (CEO) technique did not find wide-spread use because it requires a series of elaborate steps that are difficult to accomplish. In this Letter, we present a greatly simplified variation of this technique where the cleaving takes place in ambient air and the MBE overgrowth is replaced by a standard deposition process. Wires produced by this cleaved edge deposition (CED) technique have properties that are as least as good as the traditional CEO ones. Due to its simplicity, the CED technique offers a generally accessible way to produce 1D devices.ISSN:2045-232