ISBN 9783843938082

978-3-8439-3808-2, Reihe Physik

Achim Küster
Photoluminescence of nanostructures in droplet-etched nanoholes

102 Seiten, Dissertation Universität Hamburg (2018), Softcover, A5

Zusammenfassung / Abstract

GaAs/AlGaAs quantum dots and quantum dot molecules are grown using molecular beam epitaxy and investigated with excitation power as well as electric field dependent photoluminescence spectroscopy. The dots are fabricated by filling of local droplet etched nanoholes. In detail, aluminum droplets are deposited on an AlGaAs substrate to etch nanoholes into the surface of the semiconductor. Depth and areal density of the nanoholes can be tuned via the etching parameters. To form the dots, a thin GaAs layer is deposited, which partially fills the nanoholes. Afterwards, the dots are capped with an AlGaAs layer. For molecule fabrication the nanoholes are filled with two dots separated by an tunnel barrier.

In the first part of this thesis the influence of etching parameters is studied in detail and a set of parameters is determined that is optimal for dot and molecule fabrication. In the second part, optical properties of dots and molecules are studied.

The excitonic luminescence of dots and molecules exhibits sharp linewidths of about 70 µeV. We studied the energy dependence of the exciton ground state on the dot size and determined a tuning range of 1.55 eV to 1.85 eV. Our molecule fabrication technique allows us to adjust the height of the constituent dots and the thickness of the tunnel barrier independently.

The exciton ground-state energies of both molecule dots are investigated dot-size dependent. The lifetimes of excitonic complexes in single dots are investigated on samples with varied aluminum concentration in the AlGaAs barrier. Furthermore, exciton lifetimes are determined for dots in molecules.

The electric field dependence of the luminescence is studied. For these measurements, the dots and molecules are embedded in a Schottky diode. The red shift of the dot luminescence, caused by the quantum-confined Stark effect, is analyzed and compared to simulations. The electronic states of the two dots in a molecule are energetically shifted against each other via the electric field, in this connection the tunnel barrier acts as an electrostatic lever arm. In field-dependent spectra of molecules so called anticrossings are detected. This observation is a clear sign for resonant coupling between electron states of the constituent dots. Molecule states are reported here for the first time in molecules of such dot systems.