Research

This section reviews my scientific work during my employment at the Walter Schottky Institute (WSI) of the Technical University Munich (TUM). In particular, I joined the Experimental Semiconductor Physics I (E24) division lead by Prof. Gerhard Abstreiter as a PhD student.

My research activities focused on the electronic magneto-transport properties of semiconductor devices possessing low-dimensional electron systems. Investigated samples were grown using highquality molecular beam epitaxy and fabricated by means of the sophisticated cleaved edge overgrowth technique. Abstracts of the research projects are compiled in the following subsections, results are listed in the publications and presentations section.

Conducted research projects were funded by the Deutsche Forschungsgemein- schaft (SFB348, SPP1092) and the Bundesministerium für Bildung und Forschung (01BM912).

molecular beam epitaxy

Molecular Beam Epitaxy (MBE) is employed to grow atomically precise hetero- structures in the lattice-matched InGaAlAs material-system with n-type Si doping. This MBE facility is optimized for fabrication of highmobility samples on (001) and (110) crystal surfaces. Therefore great care is taken to provide a ultra-clean environment and extremely-pure source materials during the growth. Peak values in the mobility of a modulation doped single interface hetero-structure grown on these different surfaces are summarized in the following table:

surface	mobility	density	temperature
(001)	11.5 x 10⁶ cm²/Vs	2.05 x 10¹¹ 1/cm²	1.3 K
(110)	4.6 x 10⁶ cm²/Vs	1.25 x 10¹¹ 1/cm²	1.3 K

Research projects including highmobility electron systems were dedicated to the anisotropic behavior in the quantum Hall regime, see the highmobility electron systems section for further information.

cleaved edge overgrowth

The power of the Cleaved Edge Overgrowth (CEO) technique is to realize special low-dimensional systems of nearly arbitrary geometry grown on GaAs single crystals. This technique relies on two important facts: First, epitaxially overgrown (001)-GaAs single crystals naturally cleave in (110)-directions, leaving large areas of atomically flat regions; Second, epitaxial growth on the (110)-surface of GaAs is also established with highest quality. Exploiting these properties the CEO method consists of following steps: First, growth of a hetero-structure in the (001)-direction; Then, in-situ cleaving the crystal; Finally, growing a layer-sequence on the freshly exposed (110)-plane (the last two steps could be repeated another time). By combining layer-sequences with a confining potential in two or more spatially orthogonal directions, a low-dimensional electron system can be created this way.

Research projects on cleaved edge overgrowth devices covered the following areas: highquality quantum wires, shortchannel transistors, tunneling transistors and superlattice transistors.