Gate capacitance of back-gated nanowire field-effect transistors

Gate capacitances of back-gated nanowire field-effect transistors (NW-FETs) are calculated by means of finite element methods and the results are compared with analytical results of the ``metallic cylinder on an infinite metal plate model''. Completely embedded and non-embedded NW-FETs are considered. It is shown that the use of the analytical expressions also for non-embedded NW-FETs gives carrier mobilities that are nearly two times too small. Furthermore, the electric field amplification of non-embedded NW-FETs and the influence of the cross-section shape of the nanowires are discussed.Comment: 4 pages, 5 figures, to be published in Appl. Phys. Let

Ballistic Spin Injection and Detection in Fe/Semiconductor/Fe Junctions

We present {\it ab initio} calculations of the spin-dependent electronic transport in Fe/GaAs/Fe and Fe/ZnSe/Fe (001) junctions simulating the situation of a spin-injection experiment. We follow a ballistic Landauer-B\"uttiker approach for the calculation of the spin-dependent dc conductance in the linear-responce regime, in the limit of zero temperature. We show that the bulk band structure of the leads and of the semiconductor, and even more the electronic structure of a clean and abrupt interface, are responsible for a current polarisation and a magnetoresistance ratio of almost the ideal 100%, if the transport is ballistic. In particular we study the significance of the transmission resonances caused by the presence of two interfaces.Comment: 13 pages, 9 figure

Single quantum dot nanowire LEDs

We report reproducible fabrication of InP-InAsP nanowire light emitting diodes in which electron-hole recombination is restricted to a quantum-dot-sized InAsP section. The nanowire geometry naturally self-aligns the quantum dot with the n-InP and p-InP ends of the wire, making these devices promising candidates for electrically-driven quantum optics experiments. We have investigated the operation of these nano-LEDs with a consistent series of experiments at room temperature and at 10 K, demonstrating the potential of this system for single photon applications

Ferroelectric Thin-Film Capacitors and Piezoelectric Switches for Mobile Communication Applications

Thin-film ferroelectric capacitors have been integrated with resistors and active functions such as ESD protection into small, miniaturized modules, which enable a board space saving of up to 80%. With the optimum materials and processes, integrated capacitors with capacitance densities of up to 100 nF/mm2 for stacked capacitors combined with breakdown voltages of 90 V have been achieved. The integration of these high-density capacitors with extremely high breakdown voltage is a major accomplishment in the world of passive components and has not yet been reported for any other passive integration technology. Furthermore, thin-film tunable capacitors based on barium strontium titanate with high tuning range and high quality factor at 1 GHz have been demonstrated. Finally, piezoelectric thin films for piezoelectric switches with high switching speed have been realized

Untersuchungen zum spinabhängigen ballistischen Transport

In this thesis two phenomena of the spin-dependent ballistic transport are investigated theoretically by means of ab initio Korringa-Kohn-Rostocker calculations and an analytical model. The ballistic conductance is evaluated in linear response theory (Landauer formula) by a Green-function formalism. In the first part the hot spots found in ab initio calculations of the kll resolved conductance of tunneling magneto resistance junctions are investigated. Based on an analytical model the full transmission of the electrons in the hot spots is attributed to a resonance effect of interface states on both sides of the barrier. Detailed ab initio studies show that the total conductance can be dominated by the hot spots up to a critical barrier thickness. In the second part one of the fundamental needs for spinelectronics is investigated: the injection of a spin-polarized current into a non-magnetic semiconductor. Here the ballistic spin injection from the ferromagnet Fe into the semiconductors ZnSe and GaAs is calculated for different growth orientations and injection processes. It will be shown that for (001) oriented ideal heterojunctions a nearly 100% spin polarized current is injected into the semiconductor. This can be explained by the symmetry mismatch of the Fe majority and minority states at the Fermi energy