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Masked droplet etching for site-controlled quantum structures

Applicant Dr. Christian Heyn
Subject Area Experimental Condensed Matter Physics
Term from 2019 to 2024
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 414041112
 
Final Report Year 2025

Final Report Abstract

This project studies Gallium Arsenide quantum dots (QDs) that are fabricated using the local droplet etching (LDE) method in our molecular beam epitaxy system. For this, Aluminium is deposited on an AlGaAs surface where it forms droplets in a self-assembled fashion. Diffusion of Arsenic from the AlGaAs into the droplets yields the local etching of nanoholes, whose depth can be precisely controlled via the process parameters. Finally, the nanoholes are filled with GaAs for the creation of various types of quantum structures. The initial goal of the project was devoted to a study of the creation of site-controlled LDE quantum structures. However, the Corona pandemic induced a supply bottleneck which causes significant delays of essential installations in our labs and the project goal was modified to an experimental and theoretical study of the optical properties of self-assembled LDE QDs. The fabrication of LDE QDs was first demonstrated by us in 2008 and they established in the following to be very attractive for applications in the field of quantum cryptography. Important milestones were for instance their ultra-small fine-structure splitting and optical linewidths, as well as the entanglement of photons being emitted from two different QDs. We have studied here in particular larger LDE-QDs which have a unique so-called cone-shell shape. Due to this shape, LDE- QDs exhibit very interesting optical properties when subjected to external electric or magnetic fields. Single LDE-QDs are investigated using low-temperature photoluminescence spectroscopy, where in addition to the energy of the emitted radiation also its time and polarization dependence is measured. The experimental data are interpreted in combination with simulation results. As a first central outcome the combination of measurements in vertical electric fields with simulations allows a quantitative determination of the shape and size of LDE-QDs. Furthermore, we noticed that electric fields enable a wide manipulation of the charge-carrier probability densities and for instance a controlled transformation from dot-like to ring shaped probability densities with tuneable radius is possible. Quantum rings are highly interesting objects of the quantum world both for fundamental science as well as for applications. The comparison of measured and simulated radiative lifetimes of excitonic states in vertical electric fields demonstrate a novel hybrid state with an electron in a strong and a hole in a weak confinement, which to our knowledge has not been observed so far. Furthermore, our simulations predict that an additional magnetic field allows the switching between optical bright and dark states. This would allow the creation of a QD-based storage for photo-excited charge carriers, which can be used in integrated photonic circuits for quantum technologies. Finally, a lateral electric field tunes the electron and hole inside a LDE-QD to form a lateral dipole with orientation controlled by the field.

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