Group Leader:
Univ. Prof. Dr. Armando Rastelli

Post-Docs:
Dr. Ievgen Brytavskyi

PhD Students:
DI Maximilian Aigner
DI Tobias Maria Krieger
M.Sc. Eugenio Maggiolini
M.Sc. Naser Tajik
DI Thomas Oberleitner
DI Gabriel Undeutsch

Undergraduate Students:
Ailton José Garcia Junior, B.Sc.
Andreas Kitzmüller, B.Sc.
Raffael Leithenmayr, B.Sc.
Melina Peter, B.Sc.
Johannes Reindl, B.Sc.
Tobias Steindl, B.Sc.
Christian Weidinger, B.Sc.

We mostly focus on GaAs QDs in AlGaAs matrix, which we grow by Molecular Beam Epitaxy (MBE), opens an external URL in a new window in the cleanroom of our institute. After having tried out different strategies over the past years (hierarchical self-assembly, opens an external URL in a new window, Ga-droplet etching, opens an external URL in a new window), we have selected Al-based droplet etching, opens an external URL in a new window as the most promising route to produce high-quality QDs. The  main reasons are: 1) the whole fabrication process takes place at high temperature, minimizing the occurrence of crystal defects; 2) the QDs can be seamlessly embedded in photonic and optoelectronic structures similar to conventional InGaAs QDs in GaAs matrix, a property which have recently used to enhance light extraction, opens an external URL in a new window and to achieve electrical injection, opens an external URL in a new window; 3) the size of the QDs, opens an external URL in a new window can be changed to achieve any desired wavelength in the range 680-805 nm; 4) QDs with high in-plane symmetry, opens an external URL in a new window can be obtained, facilitating the generation of entangled photons; 5) the radiative recombination times can be substantially shorter than conventional QDs, which is beneficial for fast operation and generation of indistinguishable, opens an external URL in a new window and entangled photons, opens an external URL in a new window.

To control the properties of the QDs after growth we have been exploring different strategies ranging from permanent tuning via laser annealing, opens an external URL in a new window to reversible tuning via electric fields, opens an external URL in a new window and piezoelectric-induced strains, opens an external URL in a new window. Post-growth strain engineering is a powerful method to control the emission wavelength of QDs, for instance in light-emitting diodes, opens an external URL in a new window, and to obtain highly-entangled photons, opens an external URL in a new window from QDs by removing the effect of residual asymmetries. Besides using commercially available piezoelectric substrates, we have patented microstructured piezoelectric actuators, opens an external URL in a new window, which allow the in-plane stress tensor of a semiconductor membrane, opens an external URL in a new window to be fully controlled. This has allowed the demonstration of a QD-based wavelength-tunable source of entangled photons, opens an external URL in a new window. With such a method we envision the possibility of using separate sources to build-up quantum-relays, opens an external URL in a new window. As a first step in this direction, we have performed two-photon interference, opens an external URL in a new window between photons emitted by two separate QDs.

GaAs QDs are also ideally suited for fundamental studies, opens an external URL in a new window, as their structural properties can be conveniently obtained from scanning probe microscopy and they are practically unstrained. Combined with post-growth strain engineering, the optical selection rules, opens an external URL in a new window for excitons confined in these QDs can be widely tuned to match specific applications.