Electron-hole crossover in gate-controlled bilayer graphene quantum dots

Electron and hole Bloch states in gapped bilayer graphene exhibit topological orbital magnetic moments with opposite signs near the band edges, which allows for tunable valley-polarization in an out-of-plane magnetic field. This intrinsic property makes electron and hole quantum dots (QDs) in bilayer graphene interesting for valley and spin-valley qubits. Here we show measurements of the electron-hole crossover in a bilayer graphene QD, demonstrating the opposite sign of the orbital magnetic moments associated with the Berry curvature. Using three layers of metallic top gates, we independently control the tunneling barriers of the QD while tuning the occupation from the few-hole regime to the few-electron regime, crossing the displacement-field controlled band gap. The band gap is around 25 meV, while the charging energies of the electron and hole dots are between 3-5 meV. The extracted valley g-factor is around 17 and leads to opposite valley polarization for electron and hole states at moderate B-fields. Our measurements agree well with tight-binding calculations for our device

Integration of selectively grown topological insulator nanoribbons in superconducting quantum circuits

We report on the precise integration of nm-scale topological insulator Josephson junctions into mm-scale superconducting quantum circuits via selective area epitaxy and local stencil lithography. By studying dielectric losses of superconducting microwave resonators fabricated on top of our selective area growth mask, we verify the compatibility of this in situ technique with microwave applications. We probe the microwave response of on-chip microwave cavities coupled to topological insulator-shunted superconducting qubit devices and observe a power dependence that indicates nonlinear qubit behaviour. Our method enables integration of complex networks of topological insulator nanostructures into superconducting circuits, paving the way for both novel voltage-controlled Josephson and topological qubits.Comment: 11 pages, 6 figure

Successive Increases in the Resistance of Drosophila to Viral Infection through a Transposon Insertion Followed by a Duplication

To understand the molecular basis of how hosts evolve resistance to their parasites, we have investigated the genes that cause variation in the susceptibility of Drosophila melanogaster to viral infection. Using a host-specific pathogen of D. melanogaster called the sigma virus (Rhabdoviridae), we mapped a major-effect polymorphism to a region containing two paralogous genes called CHKov1 and CHKov2. In a panel of inbred fly lines, we found that a transposable element insertion in the protein coding sequence of CHKov1 is associated with increased resistance to infection. Previous research has shown that this insertion results in a truncated messenger RNA that encodes a far shorter protein than the susceptible allele. This resistant allele has rapidly increased in frequency under directional selection and is now the commonest form of the gene in natural populations. Using genetic mapping and site-specific recombination, we identified a third genotype with considerably greater resistance that is currently rare in the wild. In these flies there have been two duplications, resulting in three copies of both the truncated allele of CHKov1 and CHKov2 (one of which is also truncated). Remarkably, the truncated allele of CHKov1 has previously been found to confer resistance to organophosphate insecticides. As estimates of the age of this allele predate the use of insecticides, it is likely that this allele initially functioned as a defence against viruses and fortuitously “pre-adapted” flies to insecticides. These results demonstrate that strong selection by parasites for increased host resistance can result in major genetic changes and rapid shifts in allele frequencies; and, contrary to the prevailing view that resistance to pathogens can be a costly trait to evolve, the pleiotropic effects of these changes can have unexpected benefits

Integration of Redox-based Resistive Switching Memory Devices

The steadily growing market for consumer electronics and the rapid proliferation of mobile devices such as tablet computers, MP3 players and smart phones make high demands for the nonvolatile memory. Present FLASH memory technology approaches to the end due to physical scalability limits. Therefore, an alternative technology must be developed. For memory technology, not only the storage density and cost are important factors but the power consumption and the writing/reading speed must also be taken in account. Redox-based resistive memory (ReRAM) offers a potential alternative to the FLASH technology and presently is in the focus of research activities. The operating principle of the ReRAM is based on the non-volatile reversible change in resistance by electrical stimuli in a simple metal-insulator-metal(MIM) device architecture. This simple structure enables the integration of ReRAM in passive crossbar arrays, in which each crosspoint consumes only 4F$^{2}$ (F- feature size) device area. This leads to an ultra-high storage density at reduced cost. Research on the ReRAM memory elements requires a technology platform that ensures a cost-effective fabrication of the crossbar devices with nanometer feature size. In this thesis, the fabrication processes have been developed based on the nanoimprint lithography, which facilitates both the high resolution (<50nm) and the high throughput at low cost. The stamp for the UV-nanoimprinting is developed with plasma etching and electron-beam lithography. This process facilitates the fabrication of the ReRAM devices sizes ranging from 40x40 nm$^{2}$ to 100x100 nm$^{2}$. The fabricated nano-crosspoint ReRAM of different switching layer thickness and different device areas are electrically characterized. In order to toggle the resistance state in the ReRAM device, an electroforming step is generally required. In this work, a systematic analysis of the electroforming process is carried out on TiO$_{2}$ and WO$_{3}$-based ReRAM cells and the respective switching characteristics are investigated. The switching mechanism is explained by the filamentary conduction model. The forming voltage decreases with decreasing oxide layer thickness whereas it increases for the smaller device size. Due to overshoot phenomena during the electroforming process, these devices show a significant increased switching current, lower non-linearity, and lower endurance. The ReRAM device performance is improved by integration in the backend of a 65nm CMOS process. In the integrated 1T-1R stack, the electroforming is performed by controlling the current flow with the gate electrode. By employing this approach, the switching current in the ReRAM devices is reduced to 1 $\mu$A. In order to lower the sneak path current in the passive crossbar arrays, a high degree of nonlinearity is required. This nonlinearity parameter has been investigated with 100ns transient pulses in the nano-crossbar devices and in the 1T-1R structures. This parameter depends on the switching current and switching material properties. The lower switching current in the TiO$_{2}$ ReRAM leads to the higher nonlinearity. Furthermore, the ReRAM nanodevices inherently exhibit open clamp voltage in the switching characteristics. This phenomenon is explained by the electromotive force(EMF). The amplitude of the generated EMF voltage depends on the nature of the switching materials and can be several hundred mV. This degrades the conducting filament and thereby limits the ON state retention properties of the ReRAM devices. Additionally, the non-zero crossing of the I-V characteristics, caused by the EMF voltage demands the refinement of the memristor theory

Light Management in Silicon Heterojunction Solar Cells via Nanocrystalline Silicon Oxide Films and Nano-Imprint Textures

Excellent light management is essential to increase the amount of light being captured in the absorber of silicon heterojunction solar cells in order to obtain a high photoelectric current. Three possible ways to achieve this are improving the cell anti-reflectance, increasing the light path through the absorber material, and minimizing the parasitic losses in the other layers. The former two goals can be realized via surface texturing and the latter by using highly transparent materials. In this study, we focus on implementing hydrogenated nanocrystalline silicon oxide (nc‑SiOx:H) in combination with front side nano-imprint textures in silicon heterojuction solar cells. Nc‑SiOx:H offering a unique combination of high conductivity and high transparency is perfectly suited as an alternative wide-gap doped layer to minimize parasitic absorption. At the same time, nano-imprint technology provides a way to realize various textures on “flat” silicon solar cells without inevitably promoting recombination at the absorber interface by enlarging the surface area and increasing the number of defect states. We show by a systematic investigation how the interplay between the imprinted layer and the underlying thin films of the silicon heterojunction based solar cell affects the generated current. Ultimately, we demonstrate very high current densities and efficiencies beyond 20% without wet-chemically texturing the Si-wafer by combining the benefits of the highly transparent nanocrystalline silicon oxide layers and the favourable properties of the nano-imprint technology

Light management in planar silicon heterojunction solar cells via nanocrystalline silicon oxide films and nano-imprint textures

In order to increase the efficiency of high performance silicon heterojunction solar cells even further, it is paramount to increase the photoelectric current by enhancing the amount of light being captured within the absorber. Therefore, to reduce the parasitic absorption in the other layers, optoelectronically favorable hydrogenated nanocrystalline silicon oxide films can substitute the commonly used hydrogenated amorphous silicon layers. In this work, we systematically investigate the combination of hydrogenated nanocrystalline silicon oxide and front side nano-imprint textures as anti-reflection layers in silicon heterojunction solar cells. Ultimately, we were able to tune the parasitic absorption via variation of the front surface field layer and enhance the short-circuit current of the planar solar cells by about 2 mA cm−2 due to a random silicon pyramid textured imprint layer. A maximum active area efficiency of 20.4% was achieved with a short-circuit current of 37.7 mA cm−2

Design of deterministic light-trapping structures for thin silicon heterojunction solar cells

We optically designed and investigated two deterministic light-trapping concepts named “Hutong” (wafer thickness dependent, patch-like arrangement of “V” grooves with alternating orientations) and “VOSTBAT” (one directional “V” grooves at the front and saw-tooth like structures at the back) for the application in emerging thin silicon heterojunction (SHJ) solar cells. Calculated photocurrent density (Jph) (by weighting the spectrally resolved absorptance with AM1.5g spectrum and integrating over the wavelength) showed that both the Hutong and VOSTBAT structures exceed the Lambertian reference and achieved Jph of 41.72 mA/cm2 and 41.86 mA/cm2, respectively, on 60 μm thin wafers in the case of directional, normal incidence