Już 12.12.2023 o godz. 14:15 w Audytorium Aleksandra Jabłońskiego prof. Andreas D. Wieck (Lehrstuhl für Angewandte Festkörperphysik, Ruhr-Universität Bochum, Germany) wygłosi wykład pt: "Molecular-Beam-Epitaxy Growth of GaAs-based Quantum Dots and In-Plane-Gate Transistors".

Streszczenie:
Semiconductor quantum dots (QDs) belong to the most promising realisations of quantum bits. Among them, self-organized InGaAs – QDs, grown by molecular beam epitaxy, have been developed.  
This talk addresses issues of QD-optimization concerning purity, apparative details, growth and layer design. In particular, the purity of commercially available evaporation material, its purification in the MBE, ultrahigh-vacuum issues like sample holders and shutter design, growth sequences and epitaxial junction gates for better optical access and stability are discussed. To localise the QDs, we apply the non-perpendicular impact of the evaporated MBE-particles to obtain a thickness gradient over the wafer surface which yields nearly circular, concentric stripes of flatter and rougher regions. The QDs prefer nucleation on the atomically rough stripes which yield higher QD densities compared with the flat stripes in between. In this manner, low-density QDs (for single QD spectroscopy) and high-density QDs (for ensemble QD spectroscopy) can be analysed on the same wafer.
For transport experiments, we developed lateral gates: By writing of linear insulation paths in 2-dimensional electron gases (2DEGs) in AlGaAs heterostructures by focused ion beam implantation, etched trenches or other techniques, we define In-Plane-Gates (IPGs) adjacent to narrow conductive paths (channels) of the 2DEG. Biasing these IPGs, the channel conductivity can be tuned, resulting in a field-effect transistor. Here, the gates are not sandwiched over the channel, but are coplanar BESIDE it, consisting of the same material 2DEG as the channel itself. The in-plane electric fields which confine the channel lead to strong width variations, depending linearly of the IPG voltage between 2 half-planes rather than the conventional depletion width between two half-spaces which varies with the square root of the negative bias.