R. Held

In-plane gates and nanostructures fabricated by direct oxidation of semiconductor heterostructures with an atomic force microscope

The surface of shallow Ga[Al]As heterostructures is locally oxidized with an atomic force microscope. The electron gas underneath the oxide is depleted. We demonstrate experimentally that these depleted regions of the two-dimensional electron gas can be made highly resistive at liquid nitrogen temperatures. Thus, local anodic oxidation of high electron mobility transistors with an atomic force microscope provides a novel method to define nanostructures and in-plane gates. Two examples, namely antidots and quantum point contacts as in-plane gate transistors have been fabricated and their performance at low temperatures is discussed.

Semiconductor quantum point contact fabricated by lithography with an atomic force microscope

We report on the experimental realization of a quantum point contact in a semiconductor heterostructure by lithography with an atomic force microscope (AFM). A thin, homogeneous titanium film on top of the chip surface was patterned by local anodic oxidation, induced by a current applied to an n-doped AFM tip. We demonstrate that self-aligned gate structures in the sub-micron regime can be fabricated with this technique.

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.

Fabricating tunable semiconductor devices with an atomic force microscope

We fabricated quantum wires of different geometries in Ga[Al]As heterostructures by local oxidation of the semiconductor surface with an atomic force microscope. By magnetotransport measurements at low temperatures on these wires the electronic width is determined and compared to the geometrical width. An extremely small lateral depletion length of the order of 15 nm and a high specularity of the scattering at the confining walls is found. Furthermore, we demonstrate experimentally that these quantum wires can be tuned by a combination of in-plane gates and top gates.

In-plane gate single-electron transistor in Ga[Al]As fabricated by scanning probe lithography

A single-electron transistor has been realized in a Ga[Al]As heterostructure by oxidizing lines in the GaAs cap layer with an atomic force microscope. The oxide lines define the boundaries of the quantum dot, the in-plane gate electrodes, and the contacts of the dot to source and drain. Both the number of electrons in the dot as well as its coupling to the leads can be tuned with an additional, homogeneous top gate electrode. Pronounced Coulomb blockade oscillations are observed as a function of voltages applied to different gates. We find that, for positive top-gate voltages, the lithographic pattern is transferred with high accuracy to the electron gas. Furthermore, the dot shape does not change significantly when in-plane voltages are tuned.

Electronic properties of nanostructures defined in Ga[Al]As heterostructures by local oxidation

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 and characterize the electronic properties of the resulting confinement. As an example for the versatility of this technique, we present conductance measurements on a quantum wire as a function of its position. Conductance fluctuations in real space with a characteristic period of 2 nm are observed and interpreted in terms of individual peaks in the potential landscape the wire hits as it moves through the host crystal.

Quantum wires and quantum dots defined by lithography with an atomic force microscope

Semiconductor nanostructures are realized by patterning AlGaAs/GaAs heterostructures with an atomic force microscope. Steep potential walls, precise pattern transfer and a combination of in-plane and top gates can be achieved with this technique. The electronic properties of nanostructures defined in this way are discussed on two examples, namely a quantum point contact and a single electron transistor. For the quantum point contact we demonstrate quantized conductance at temperatures of 4 K and above. This indicates the strong confinement energy in this system. For the single electron transistor, the realization of special potential shapes and the observation of high-quality Coulomb blockade is shown.

Scanning gate measurements on a quantum wire

We have performed measurements on a semiconductor quantum wire in which we induce a local potential perturbation with the metallic tip of a scanning force microscope. Measurement of the sample resistance as a function of tip position results in an electrical map of the wire in real space. We find the fingerprint of potential fluctuations in the wire which appear as local resistance fluctuations in the images. In a local transconductance measurement we observe small oscillations on the scale of the Fermi-wavelength of electrons which may arise from interference of electron waves.

Analysis of the temperature-dependent quantum point contact conductance in relation to the metal-insulator transition in two dimensions

The temperature dependence of the conductance of a quantum point contact has been measured. The conductance as a function of the Fermi energy shows temperature-independent fixed points, located at roughly multiple integers of e2/h. Around the first fixed point at e2/h, the experimental data for different temperatures can be scaled onto a single curve. For pure thermal smearing of the conductance steps, a scaling parameter of one is expected. The measured scaling parameter, however, is significantly larger than 1. The deviations are interpreted as a signature of the potential landscape of the quantum point contact, and of the source-drain bias voltage. We relate our results phenomenologically to the metal-insulator transition in two dimensions.

Shifting a Quantum Wire through a Disordered Crystal: Observation of Conductance Fluctuations in Real Space

A quantum wire is spatially displaced by suitable electric fields with respect to the scattering potentials inside a semiconductor crystal. As a function of the wire position, the low-temperature conductance shows reproducible fluctuations. Their characteristic temperature scale is a few hundred millikelvin, indicating a phase-coherent effect. One fluctuation is attributed to a single bump in the scattering potential entering or leaving the wire.