Steuerung von Nb-2DEG-Nb Josephson-Kontakten mittels Strominjektion
Since early eighties when first superconductor/semiconductor hybrid structures JOFET (Josephson Field Effect Transistor) had been created there has been a huge interest in controlled Josephson junctions. The idea of the JOFET is to control the supercurrent by changing the electron concentration in the two-dimensional electron gas (2DEG) by the gate voltage. However the required voltages are relatively high effectively resulting in attenuation of a signal. Later it was shown that injection of hot carriers into the 2DEG region also makes it possible to control the Josephson current. In this approach the injection current is smaller than the supercurrent, i.e. the system works as a current amplifier. This thesis is concerned with the control of the supercurrent in Nb-InGaAs/InP-Nb Josephson junctions by current injection into the 2DEG region in 3- and 4-terminal configurations. It is shown that control efficiency depends on the junction width. The measurements in applied magnetic field B show recovery of the critical current at a value of B that corresponds to the first minimum in Ic-B dependence. Both above experimental observations can be explained by a local character of the influence of the injected current. Significant part of this thesis is devoted to details of sample processing. Our studies have shown that highly electrically transparent Nb/2DEG interfaces can be prepared by cleaning the semiconductor surface by He plasma. The results of charge transport studies of single Nb/2DEG interfaces are also presented. The last part of the thesis deals with attempts to realize Andreev levels spectroscopy by means of carrier injection through a quantum point contact controlled by in-plane gates. Such quantum point contacts have been realized in InGaAs/InP heterostructures, however their coupling to S/2DEG/S junctions was too weak to result in any experimentally observable charge injection.
info:eu-repo/classification/ddc/530, Physik, weak link, Josephson-Kontakt, Hybridstruktur
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