H. Welsch

Lithographically defined metal-semiconductor-hybrid nanoscrolls

We report on two-layer metal-semiconductor-hybrid scrolls fabricated from rolled-up strained metal∕InGaAs-layers. As the central approach, the metallic layer itself acts as a stressor in contact with the semiconductor. Position and length of the scrolls can be precisely tuned by patterning the e-beam-evaporated metallic stressor with conventional lithographic techniques. The thickness of the metallization determines the radius of the resulting scrolls. This fabrication technique significantly improves the reliability and simplifies the fabrication of metal∕semiconductor three-dimensional objects which employ bending up layers. Even more important, using this technique the bending radius of such three-dimensional objects can easily be downsized to very small radii in the nanometer scale, e.g. in order to build nano-electro-mechanical systems.

Faceting during GaAs quantum dot self-assembly by droplet epitaxy

Strain-free GaAs quantum dots (QDs) are grown in a self-assembled fashion by applying Ga droplet epitaxy. The QDs are studied using electron diffraction and atomic force microscopy. Two distinct regimes are observed for the QD shape. QDs whose volume exceeds approximately 3×105 Ga atoms are shaped like truncated pyramids with side facets having an angle of about 55°. Smaller QDs are pyramidlike with 25° facets.

Three-Dimensionally Confined Optical Modes in Quantum Well Microtube Ring Resonators

We report on microtube ring resonators with quantum wells embedded as an optically active material. Optical modes are observed over a broad energy range. Their properties strongly depend on the exact geometry of the microtube along its axis. In particular, we observe (i) preferential emission of light on the inside edge of the microtube and (ii) confinement of light also in the direction of the tube axis by an axially varying geometry, which is explained in an expanded waveguide model.

Evenly curved two-dimensional electron systems in rolled-up Hall bars

We present evenly curved two-dimensional electron systems in Hall-bar geometry. Expanding the method of self-rolling strained layers to fragile thickness modulated layer systems enables us to roll-up complete Hall-bar geometries into microscrolls. These structures represent model systems for the investigation of transport phenomena in sinusoidally modulated magnetic fields. The sinusoidal shape of the modulation and the tunability of the modulation phase are illustrated by comparing finite element calculations with transport data in the classical regime. Furthermore, we demonstrate the tunability of the carrier density in our structures using rolled-in metal gates.

Interlocking mechanism for the fabrication of closed single-walled semiconductor microtubes

We introduce a method to realize closed single-walled semiconductor microtubes. By applying a novel interlocking mechanism to the method of self-rolling strained bilayers we are able to fabricate closed InGaAs/GaAs-microtubes with precisely tuneable parameters. Such 3D objects are promising candidates for the realization of different kinds of free-standing optical resonators.

Spatially and energetically resolved optical mapping of self-aligned InAs quantum dots

We perform photoluminescence measurements on self-assembled InAs quantum dots which are self-aligned by a deliberately induced underlying misfit dislocation network. Scanning micro photoluminescence measurements allow for a spatial mapping of the structures and reveal different emission energies for quantum dots aligned along [1 1 0] and [−1 1 0] crystal directions.

Optical Microtube Ring Resonators

In this work, we demonstrate that semiconductor microtubes fabricated by utilizing the self‐rolling mechanism of epitaxially grown strained bilayers can act as optical ring resonators. The mode structure is probed by the photoluminescence of an optically active material, i.e. self‐assembled quantum dots, embedded inside the microtube wall. We find a spectrum of sharp modes, which is in very good agreement with the result of a theoretical modelling of the microtube as a closed thin dielectric waveguide. This novel kind of microcavity, in which the optically active material is intrinsically located close to the optical field maximum, is a good candidate for both, new optoelectronic devices and cavity quantum electrodynamic experiments.

Magnetotransport On Evenly Curved Hall‐Bars In InGaAs/GaAs‐Microtubes

We present transport measurements on evenly curved two dimensional electron systems in Hall‐bar geometry. By combining the method of self‐rolling strained double‐layers with a special lithographic procedure we are able to roll‐in and contact AlGaAs/GaAs/AlGaAs quantum well structures into tubes or curved lamellas. Applying a magnetic field to such structures results in a sinusoidal modulation of the magnetic field components perpendicular to the curved 2 DES plane. Transport measurements with current direction parallel to the rolling axis exhibit weak oscillations reflecting the superposition of magneto oscillations caused by different perpendicular magnetic field vectors. In contrast to this, magneto transport along the curvature of the tubes shows pronounced Shubnikov‐de Haas oscillations which can be associated with the maximum perpendicular magnetic field component between the contacts. Furthermore, a breakdown of magnetic field inversion symmetry depending on the perpendicular field component distrib...