Physics:

Below are my (former) research areas - and still areas of interest.

Semiconductors in Extreme Magnetic Fields

Magnetic fields have always been a powerful tool in semiconductor physics. Nevertheless most people use it as a perturbance to lift degeneracies and to probe symmetries. Thus fields beyond 20 Tesla are rarely employed. But the range from 20 Tesla up to several thousand Tesla is a largely unknown area in physics.

Recent experiments, that employed huge capacitor banks discharged within few milliseconds over a single coil copper loop, make it possible to generate pulsed magnetic fields up to 300 Tesla. If a conventional implosion further compresses the magnetic flux, just when it has reached its maximum, field of up to 1000 Tesla can be reached for a few microseconds. While transport experiments are very difficult due to the large induction currents, optical absorption measurements can be performed without interference and from a safe distance.

Up to now there have been very few theoretical studies of semiconductors in those extreme fields and nearly all of them used a perturbative approach whose accuracy is limited to few Tesla.

We developed a nonperturbative approach to calculate the electronic structure on an atomistic level within the tight-binding framework and furthermore expanded the theory of linear optical response to include the magnetic field nonperturbatively.
 

The results are for example that GaSb undergoes the transition from a direct to an indirect semiconductor at 130 T, while GaAs remains direct beyond 1000 T. This is due to the small separation of the L- and the G-valley in GaSb combined with a strongly differing effective mass.

The energy spectrum of the GaAs conduction band becomes a fractal above 1000 Tesla with close resemblance to the Hofstadter butterfly. Suprisingly this fractal spectrum should be visible in optical absorption experiments.

Publications:

C. Strahberger, Vertikaler Transport und extreme Magnetfelder in Halbleitern, Dissertation,
Verein zur Förderung des Walter Schottky Instituts der Technischen Universität München e.V., 2002 ,ISBN 3932749456, Techn. Univ. München (Download)
P. Vogl, C. Strahberger, Self-similar optical absorption spectra in high magnetic fields, in Physics of Semiconductors (Eds. A. R. Long and J. H. Davies, Institute of Physics Publ., Bristol, 2003), E1.3 (2002)

Vertical Transport of Hot Electrons in Metal/Insulator and Insulator/Insulator Heterostructures on Si Substrate


Resonant tunneling diodes based on metal/insulator heterostructures have the potential to become ultra high speed electronic devices. Recent experiments have demonstrated the capability of these materials as room temperature quantum effect switching devices that are compatible with Si technology. The structure in question consists of three sub-nm layers of CaF2 grown pseudomorphically on a n-Si(111) substrate. These three layers enclose two metallic few-nm CoSi2-quantum wells, which are necessary for quantum resonance. The overall vertical extent of this structure is typically below 8nm. Thus coherent quantum transport dominates even at room temperature. I calculated current-voltage characteristics of several types of these devices and thereby studied systematically the influence of well- and barrier thicknesses as well as possible alternatives for the component materials.

These devices show a clear-cut resonance at room temperature with current densities around 103 A/cm2. As it turned out the position of the resonance depends only weakly on the width of the quantum wells but rather sensitively of the width of the barrier. The reason for this surprising result is that the dominant part of the resonant current flows through flat, predominantly Co d-bands of the wells. These are well localized and therefore only weakly affected by confinement.

Publications:

C. Strahberger, und P. Vogl, Model of room temperature resonant tunneling current in metal/insulator and insulator/insulator heterostructures, Physical Review B 62, 7289 (2000) (Download)
C. Strahberger, und P. Vogl, Theoretical insights into CoSi2/CaF2 tunneling diodes, Physica B Condensed Matter 272, 160 (1999) (Download)
C. Strahberger, Theorie der resonanten Metall / Isolator / Halbleiter-Tunneldioden, Diploma Thesis, TU-München / Univ. Regensburg (Download)

k||-Filtering Effects in BEEM-Experiments

In this work, sub-surface and buried GaAs-AlGaAs double barrier resonant diodes (RTDs) structures are investigated by Ballistic Electron Emission Microscopy (BEEM). RTDs grown directly below the sample surface exhibit characteristic steplike features in the BEEM spectrum, whereas for buried RTDs, a linear spectral behavior is observed. In magnetic fields, a Shubnikov de-Haas like oscillatory behavior is observed in the BEEM current of the sub-surface RTDs, but not for the buried structures. To investigate the origin of these effects, the BEEM spectra were simulated using a scattering formalism within the framework of a semiempirical tight binding method. This method enables the incorporation of the position of every single atom and in this way, a realistic description of the electronic structure is obtained. Within this model the spectra are semiquantitatively explained. As main result it is found, that independent of the applied bias, only electrons within a narrow k||-distribution of constant width are transferred through the RTD. In this way, a k||-filter is established and the width of this filter can be derived from the magnetic field data. To interpret these results in a more descriptive way, the commonly accepted Bell-Kaiser model is applied together with an extended Transfer Matrix Method (TMM). Also in the framework of this simple model, the k||-filtering effect can be explained. As result we find that at interfaces between regions of different effective mass, the transmission coefficient for ballistic electrons becomes a function both of E and k|| and that the effective mass changes gradually at the Au-GaAs interface. This demonstrates, that the combination of a locally different effective mass and low dimensional states can lead to a new and unexpected physical behavior.

Publications:

C. Strahberger, J. Smoliner, R. Heer und G. Strasser, Enhanced k||-filtering effects in ballistic electron emission experiments, Physical Review B, 63, 205306 (2001) (Download).
J. Smoliner, R. Heer, G. Strasser und C. Strahberger, Magnetic field effects and k||-filtering in BEEM on GaAs/AlGaAs resonant tunneling structures, Applied Physics A, 72, 223 (2001) (Download).
D. Rakoczy, G. Strasser, C. Strahberger, J. Smoliner,  L-valley electron transport in GaAs-AlAs double-barrier resonant tunneling structures studied by ballistic electron emission microscopy, Phys. Rev. B 66, 033309 (2002) (Download)