We demonstrate the self-assembled creation of a novel type of strain-free semiconductor quantum dot (QD) by local droplet etching (LDE) with Al to form nanoholes in AlGaAs or AlAs surfaces and subsequent filling with GaAs. Since the holes are filled with a precisely defined filling level, we achieve ultrauniform LDE QD ensembles with extremely narrow photoluminescence (PL) linewidth of less than 10 meV. The PL peaks agree with a slightly anisotropic parabolic potential. Small QDs reveal indications for transitions between electron and hole states with different quantization numbers. For large QDs, a very small fine-structure splitting is observed.

We study the formation of nanoholes and rings on GaAs and AlGaAs surfaces by local droplet etching (LDE) with gallium and indium. The nanohole properties are tuned by variation in etching temperature and time as well as by the etchant. Nanoholes fabricated by In LDE are larger and have an about ten times lower density compared to Ga LDE, which allows the fabrication of nanoholes with ultralow density of less than 5×106 cm−2. Furthermore, the nanohole borders are surrounded by distinct walls. The walls are crystallized from droplet material and serve as quantum rings with tunable size and band gap.

Photoluminescence from GaAs nanodisks fabricated by using combination of neutral beam etching and atomic hydrogen-assisted molecular beam epitaxy regrowth Independent wavelength and density control of uniform GaAs/AlGaAs quantum dots grown by infilling selfassembled nanoholes

Experimental results of the local droplet etching technique for the self-assembled formation of nanoholes and quantum rings on semiconductor surfaces are discussed. Dependent on the sample design and the process parameters, filling of nanoholes in AlGaAs generates strain-free GaAs quantum dots with either broadband optical emission or sharp photoluminescence (PL) lines. Broadband emission is found for samples with completely filled flat holes, which have a very broad depth distribution. On the other hand, partly filling of deep holes yield highly uniform quantum dots with very sharp PL lines.

Semiconductor quantum dots containing two electrons, also called artificial quantum-dot helium atoms, are model structures to investigate the most fundamental many-particle states induced by Coulomb interaction and the Pauli exclusion principle. Here, electronic excitations in quantum-dot helium are investigated by resonant Raman spectroscopy in magnetic fields. We observe transitions from the ground state into the excited singlet state and, in the depolarized Raman configuration which allows spin-flip processes, into the triplet state.