T. Heinzel

Nanolithography by local anodic oxidation of metal films using an atomic force microscope

Abstract We use an atomic force microscope (AFM) to pattern metal surfaces in the nanometer range. Our technique is based on an electrochemical process called anodic oxidation. By applying a voltage between the AFM-tip and the sample substrate an electrochemical reaction is induced. With this technique several metals and semiconductors can be oxidized locally, i.e. in close vicinity of the tip scanning over the surface. We show how the formation of these oxide structures depends on key parameters, such as humidity and writing speed. Instead of a voltage source, we are using a constant current source to drive the oxidation. By means of this method, deficiencies related to the voltage source technique can be avoided. As a result, we are able to write structures on thin titanium films with excellent electronic properties. We focus on the patterning of titanium, since titanium is suited as a gate material on Ga[Al]As-heterostructures.

Micro-photoluminescence of self-assembled quantum dots in the presence of an electron gas

Abstract InGaAs/GaAs self-assembled quantum dots in the presence of a two-dimensional electron gas were studied by micro-photoluminescence. Several sharp optical emission lines attributed to individual quantum dots were observed. These lines exhibit a blue shift and a broadening as the electron gas density is increased. We discuss the origin of the blue shift considering the variation of the built-in electric field in the asymmetric geometry of the dots. The observed broadening of the quantum dot emission lines is an indication of coupling between the two-dimensional electron gas and the electrons in the quantum dot.

TEM study of InAs self-assembled quantum dots in GaAs

InAs self-assembled quantum dots grown on GaAs(001) substrate were investigated by TEM as well as by electrical transport measurements. The TEM analysis revealed the presence of a complicated triple-layer structure instead of simple dots for high In-concentrations. The electrical measurement suggest that the dots act as controllable scattering centers. A saturation of the mobility is observed for the highest dot density samples.

Zeeman Splitting in Quantum Dots

Coulomb blockade resonances in quantum dots are investigated as a function of magnetic fields applied in the plane of the electron gas. For weak coupling of the dot to the leads, where the Coulomb resonances are basically dominated by individual quantum states, we observe strongly fluctuating Coulomb peak positions, which can be qualitatively explained by linear Zeeman shifts. For strong coupling the peak positions are quite well described by the linear Zeeman shift in spite of the fact that transport occurs through multiple levels. From the point of view of a single particle picture the spin degrees of freedom are remarkably stable.

Electronic properties of AFM-defined semiconductor nanostructures: quantum wires and single electron transistors

The electron gas in AlGaAs heterostructures can be depleted below regions which are patterned with an atomic force microscope. This leads to laterally insulating regions across which in-plane gate voltages can be applied in order to tune the electronic nanostructures. In long quantum wires we find that the wire potential can be laterally shifted in real space while keeping the general transport characteristics of the wire unchanged. With an additional front gate electrode the wire potential and location can be changed over large parameter ranges. The wire walls are found to be steep and smooth on the length scales relevant for electronic transport. In single electron transistors fabricated with the same technology we find that the charge on the dot can be tuned by up to 70 electrons. Because of the steep walls the electronic dot shape at the Fermi energy resembles the lithographic pattern with high accuracy.

Nano-oxidation of semiconductor heterostructures with atomic force microscopes: technology and applications

Tunable nanostructures can be patterned in Ga[Al]As heterostructures with an atomic force microscope (AFM). Oxidizing the GaAs cap layer locally by applying a voltage to the AFM tip leads to depletion of the electron gas underneath the oxide. Here, we describe this type of AFM lithography as a tool to fabricate tunable nanostructures. Novel technological options are discussed, and the electronic properties of the resulting confinement is characterized. As an example for the versatility of this technique, we present electronic transport measurements on quantum wires.

Metal-insulator transition at B=0 in an AlGaAs/GaAs two-dimensional electron gas under the influence of InAs self-assembled quantum dots

We report the observation of a metal-insulator transition in zero magnetic eld in a two dimensional electron gas under the influence of a plane of InAs self-assembled quantum dots located at 30 A below the AlGaAs/GaAs heterointerface. The transition is observed as a function of temperature and electric field at B=0. A scaling analysis yields exponents similar to those obtained for Si MOSFETs . We suggest that the disorder introduced by the quantum dots plays a crucial role.