6. Functionalization of Droplet Etching for Quantum Rings

We give an overview on various types of strain-free semiconductor quantum ring (QR) structures created in a self-assembled fashion with the local droplet etching (LDE) method. LDE is fully compatible with conventional molecular beam epitaxy (MBE) and utilizes liquid Ga or Al droplets which drill nanoholes into semiconductor surfaces. The nanohole openings are surrounded by walls composed of Arsenides of the droplet material. Here the nanohole and wall formation mechanisms and the tunability of their structural properties are discussed. Three different concepts for QR generation by LDE are addressed. In the first concept, GaAs recrystallized during LDE with Ga droplets on AlGaAs substrates forms directly GaAs quantum rings. The second concept is based on the wave-function tuning of V-shaped GaAs QDs by an applied gate-voltage. Here, either the electron or the hole wave function can be transformed into a ring-shape, whereas the respective other charge carrier type remains in a zero-dimensional QD state. The third concept considers the partial depletion of a near surface GaAs quantum well (QW) due to tunneling. The LDE-related wall increases locally the distance to the surface which reduces tunneling and generates a ring-shaped charge-carrier concentration in the QW below the wall. The fabrication and structural properties of these three types of QRs, simulations of the quantized electronic levels and wave functions, and first optical data are discussed.