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978-3-8439-1083-5, Reihe Physik
Nanostructures Grown on Metal-Droplet Etched Holes
102 Seiten, Dissertation Universität Hamburg (2013), Softcover, A5
In this thesis the formation and the properties of GaAs quantum dots and nanopillars are studied. Both types of nanostructures are fabricated by filling of nanoholes etched using the local droplet etching technique in molecular beam epitaxy. This technique bases on the fact that beneath Al or Ga droplets on semiconductor surfaces self-assembled nanoholes are drilled into semiconductor surfaces. This process takes place at a very low arsenic background pressure and high temperatures.
In the first part I present nanoholes partially filled with GaAs, which are then capped to form GaAs quantum dots in an AlGaAs matrix. By precisely controlling the process parameters, especially the arsenic flux during droplet deposition, I was able to fabricate ultra-low density (ULD) nanoholes. This is a decrease of up three orders of magnitude to LDE nanoholes fabricated under standard arsenic pressure. I fabricated ULD GaAs QDs of different sizes, depending on the filling level of the holes. We studied the optical properties of single ULD GaAs QDs at low and high excitation powers and demonstrate the high optical quality of these QDs and their versatility in terms of emission energy and structural properties. Furthermore, I expanded the ULD process to form vertically stacked GaAs quantum dot molecules in an AlGaAs matrix with an AlGaAs tunnel barrier. We studied the optical properties of these new QDMs and could prove coupling between the individual dots by non-resonant tunnelling in dependence of the tunnel barrier thickness. Furthermore, by using the quantum confined Stark effect we shifted their emission energies up to 25 meV.
In the second part, I present nanopillars, that are formed by completely filling nanoholes, which were drilled through a few nanometer thin AlAs or AlGaAs barrier between thick GaAs layers. These nanopillars connect two reservoirs of GaAs. If the surrounding AlAs layer is removed, one has nano sized point contacts between two GaAs reservoirs, through which the phonon transport is ballistic. Thorben Bartsch could show that the top layer thickness is irrelevant for this effect. Furthermore, we used the ULD holes to fabricate nanopillars with a very low density to study the first electronic transport through a small number of pillars in a defined mesa through a few nanometer thick AlGaAs barrier.