Browsing by Author "Göktas, Oktay"

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Small alloyed ohmic contacts to 2DES and submicron scale corbino devices in strong magnetic fields : observation of a zero bias anomaly and single electron charging
(2009) Göktas, Oktay; von Klitzing, Klaus (Prof. Dr.)
The Quantum Hall Effect (QHE) was a big surprise discovered on a two-dimensional electron system (2DES) in 1980. The surprising result was that the Hall resistance develops plateaus with simultaneously vanishing longitudinal resistance for a certain range of magnetic field. The value of Hall resistance is quantized at h/(ie2) where i is an integer number. The accuracy of the quantization made the possibility of defining an international resistance standard. Klaus von Klitzing was honored by the physics Nobel prize in 1985 for his discovery. The original discovery was made on a 2DES at the Si-SiO2 interface in a metal-oxide-semiconductor field effect transistor. However, 2DES realized in molecular beam epitaxy grown modulation doped GaAs/AlGaAs heterostructures become the base system for the investigation of the QHE due to their superior electron mobility. The 2DES forms at the GaAs/AlGaAs heterojunction which lies typically 35 nm to 200 nm below the surface. Making a reliable, reproducible and low resistive ohmic contacts to the 2DES in these heterostructures is an important issue. Alloyed Indium ohmic contacts have been used to contact the 2DES on these heterostructures. Since, Indium is not a suitable material for lithographic purposes - it is hard to evaporate - Au/Ge/Ni contacts, which are used to contact n-GaAs, have been adopted to contact the 2DES in GaAs/AlGaAs heterostructures. However, every group developed and used its own recipe and there are not much systematic studies available in the literature. A detailed study of alloyed ohmic contacts to 2DES is necessary since they become an important ingredient of the QHE for the microscopic understanding of the phenomena. The present understanding of QHE is still under debate. Several theories has been developed to explain the effect. Of them the edge state picture was very successful to account for most of the experimental findings. However, recent experiments on the Hall potential profiles in quantum Hall samples by scanning force microscopes reveal convincing evidences for the existence of compressible and incompressible stripes in the depletion region at the sample edges supporting the theories that take screening effects into account. Moreover, these experiments have shown that compressible and incompressible stripes also exist at the border between ohmic contacts and the 2DES. Based on the fact that incompressible stripes have insulating properties, we propose a submicron Corbino devices as a new type of single electron-charging device in this work. Single-electron charging is a phenomena that is observable when the electrostatic charging energy for adding an electron to a conducting island becomes larger than the thermal energy. The effect is observable for the devices in micron or submicron size at temperatures in the range of one Kelvin or below. The basic ingredients of a single-electron charging device is a small island coupled to the leads via tunneling barriers. When Fermi wavelength of electrons become comparable to the island size, which is the case for electrons in a 2DES in GaAs/AlGaAs heterostructures with a few hundred nanometer device size, the effect of the size confinement starts to play a role and one speaks of quantum dot systems. In a Corbino device, at low temperatures and under strong magnetic field, there are incompressible stripes isolating the main compressible bulk from the ohmic contacts for certain magnetic field range. When the size of Corbino device is small enough - in submicron scale - single-electron charging should be observable. This makes the base of our proposal. During this work we have investigated electrical and structural properties of alloyed Au/Ge/Ni contacts to the 2DES in GaAs/AlGaAs heterostructures in great detail and developed a model for the ohmic contact formation on these systems based on the experimental findings. We have focused on contacting the 2DES in GaAs/AlGaAs heterostructures on submicron scale and succeeded to make successfully working contacts down to a diameter of 0.2 µm. Using these small contacts we have prepared successfully working Corbino devices in submicron scale. As characterizing the devices at low temperatures we have observed a zero bias anomaly - a differential conductance dip at zero bias. Applying a small magnetic field partly suppress the zero bias anomaly. This zero bias anomaly seems to be an interference effect with some unusual properties. Under strong magnetic field we have observed magnetic field periodic oscillations and signature of single-electron charging. We propose models for the explanation of the observed phenomena based on compressible and incompressible stripes.