Strain-free epitaxial quantum dots (QDs) are fabricated by a combination of Al local droplet etching (LDE) of nanoholes in AlGaAs surfaces and subsequent hole filling with GaAs. The whole process is performed in a conventional molecular beam epitaxy (MBE) chamber. Autocorrelation measurements establish single-photon emission from LDE QDs with a very small correlation function g (2)(0)≃ 0.01 of the exciton emission. Here, we focus on the influence of the initial hole depth on the QD optical properties with the goal to create deep holes suited for filling with more complex nanostructures like quantum dot molecules (QDM). The depth of droplet etched nanoholes is controlled by the droplet material coverage and the process temperature, where a higher coverage or temperature yields deeper holes. The requirements of high quantum dot uniformity and narrow luminescence linewidth, which are often found in applications, set limits to the process temperature. At high temperatures, the hole depths become inhomogeneous and the linewidth rapidly increases beyond 640 °C. With the present process technique, we identify an upper limit of 40-nm hole depth if the linewidth has to remain below 100 μeV. Furthermore, we study the exciton fine-structure splitting which is increased from 4.6 μeV in 15-nm-deep to 7.9 μeV in 35-nm-deep holes. As an example for the functionalization of deep nanoholes, self-aligned vertically stacked GaAs QD pairs are fabricated by filling of holes with 35 nm depth. Exciton peaks from stacked dots show linewidths below 100 μeV which is close to that from single QDs.

A new type of quantum structure is discussed where the probability distributions of the charge carriers are concentrated on the shell of a cone. These GaAs cone‐shell quantum structures (CSQSs) are filled into nanoholes in AlGaAs that are fabricated in a self‐assembled fashion using local droplet etching during molecular beam epitaxy. The structural properties of the CSQSs are studied with atomic force microscopy (AFM) and the optical emission with single‐dot photoluminescence (PL). Numerical simulations of the influence of a vertical electric field predict a strong field‐dependent displacement of either the electron or the hole away from the tip of the cone shell. This displacement has several consequences. First, the Coulomb interaction is strongly reduced. Accordingly, simulations as well as PL measurements indicate a non‐parabolic Stark‐shift for CSQSs with a regime of approximately constant emission energy. Second, the calculated exciton‐recombination lifetimes establish a variability from nanoseconds up to milliseconds. Third, regarding the shape of the electron or hole probability distributions, we predict a gate‐voltage controlled transformation from a dot into a ring shape. The respective other charge carrier remains as a dot.

GaAs quantum dots (QDs) with a thin cap layer are studied as building blocks for self-aligned hybrids with a metallic nanostructure (MN). Both constituents are filled into a nanohole template that is drilled into an AlGaAs surface by self-assembled local droplet etching during molecular beam epitaxy. In a first series of samples, the interaction of a near AlGaAs surface with a single QD at varied distance is studied using microphotoluminescence (PL) spectroscopy. With decreasing distance down to 12.5 nm, surface charges cause an increase in the exciton radiative lifetime, the formation of charged excitons, and a broadening of the exciton PL peaks. The PL peak broadening is quantitatively analyzed on the basis of an analytical model assuming temporal fluctuations of the surface charge. In a second sample series, the nanoholes are filled in addition with an Au nanostructure. The optical spectra are similar to those from QDs without a metal but with a slightly stronger PL peak broadening. For a small distance of 12.5 nm clearly within the optical near-field of the MN, the QDs show a typical PL linewidth of 430 μeV that is still small enough to separate different excitonic lines.

Strain-free, vertically coupled GaAs quantum dots (QDs) with an ultra-low density below 1 × 10(7) cm(-2) are fabricated by filling of self-assembled nanoholes with a GaAs/AlGaAs/GaAs layer sequence. The sizes of the two QDs, forming a QD pair (QDP), as well as the AlGaAs tunnel-barrier between the dots are tuned independently. We present atomic force microscopy studies of the QDP formation steps. We have performed photoluminescence studies of single QDPs with varied dot size and tunnel-barrier thickness. The data indicate non-resonant tunnelling between the dots. Furthermore, we apply the quantum confined Stark effect to tune the photoluminescence energy by up to 25 meV.

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