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Journal of Applied Physics

Josephson junction oscillators as probes of electronic nanostructures

A. S. Adourian, Scott Yang, Robert M. Westervelt, K. L. Campman, Arthur C. Gossard

Published Nov 15, 1998• 3 citations
Publisher

Abstract

We have fabricated high-quality planar Nb/AlOx/Nb Josephson junctions on-chip adjacent to quantum dots in a near surface two-dimensional electron gas in a GaAs/AlGaAs heterostructure. When used as a voltage-tunable oscillator coupled capacitively to a quantum dot, the Josephson junction can produce a localized time-dependent potential of 200 μV across the dot at frequencies in excess of 300 GHz. The fabrication process involves five separate patterning and processing steps to define the multilayer integrated device.

DOI
10.1063/1.368847

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We have fabricated high-quality planar Nb/AlOx/Nb Josephson junctions on-chip adjacent to quantum dots in a near surface two-dimensional electron gas in a GaAs/AlGaAs heterostructure. When used as a voltage-tunable oscillator coupled capacitively to a quantum dot, the Josephson junction can produce a localized time-dependent potential of 200 μV across the dot at frequencies in excess of 300 GHz. The fabrication process involves five separate patterning and processing steps to define the multilayer integrated device.
We present measurements on microwave spectroscopy on a double quantum dot with an on-chip microwave source. The quantum dots are realized in the two-dimensional electron gas of an AlGaAs/GaAs heterostructure and are weakly coupled in series by a tunnelling barrier forming an `ionic' molecular state. We employ a Josephson oscillator formed by a long Nb/Al-AlO x /Nb junction as a microwave source. We find photon-assisted tunnelling sidebands induced by the Josephson oscillator, and compare the results with those obtained using an externally operated microwave source.
Rugged, high current density NbN1−xCx/MgO/NḃN1−xCx tunnel junctions have been fabricated and tested as voltage tunable Josephson junction terahertz oscillators. The emitted radiation from these junctions is detected on chip by a second junction which is capacitively coupled to the first. For oscillator junctions with a critical current density of Jc∼3.5×104 A/cm2 we find that the junction oscillates with a voltage amplitude of ∼1.5 mV. The detected rf voltage level remains essentially constant from 300 GHz to above 1 THz. The oscillator junction produces 0.5 μW of terahertz radiation of which, due to impedance mismatch, 0.01 μW is coupled into the detector junction.
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Overdamped Nb/Al-AlOx/Nb Josephson junctions are an intermediate state between the SIS and SNS Josephson junctions. With a proper choice of fabrication parameters, stable and reproducible nonhysteretic curent-voltage characteristics have been obtained Featuring critical current densities up to 70 kA/cm2and characteristic voltages up to 450 μV at 4.2 K, overdamped junctions are suitable for many aplications ranging from RSFQ electronic circuits to programmable and ac voltage standards. Tests on single junctions arrays of series-connected junctions have shown a suitable behaviour when driven by a microwave with frequencies around 75 GHz. The quantized steps have proven to be stable and flat at nV level. These junctions also shown an improved temperature dependence, which can be interesting in view of the development instrumentation with an integrated quantum voltage reference operated in a closed cycle refrigerator.
Sub-μm\mu\hbox{m}μm Josephson Junctions for Superconducting Quantum Devices
For high-performance superconducting quantum devices based on Josephson junctions (JJs), decreasing lateral sizes is of great importance. Fabrication of sub-μm JJs is challenging, due to nonflat surfaces with step heights of up to several 100 nm generated during the fabrication process. We have refined a fabrication process with significantly decreased film thicknesses, resulting in almost flat surfaces at intermediate steps during the JJ definition. In combination with a mix-and-match process, combining electron-beam lithography and conventional photolithography, we can fabricate JJs with lateral dimensions down to 0.023 μm2. We propose this refined process as an alternative to the commonly used chemical-mechanical polishing procedure. Transport measurements of JJs, having critical-current densities ranging from 50 to 104A/cm2, are presented at 4.2 K. Our JJ process yields excellent quality parameters, Rsg/RNup to ~ 50, Vmfrom 15 to 80 mV and Vgapup to 2.81 mV, and also allows the fabrication of high-quality, sub-μm wide, long JJs (LJJs) for the study of Josephson vortex behavior. The developed technique can also be used for similar multilayer processes and is very promising for fabricating sub- μm JJs for quantum devices such as SQUIDs, qubits, and SIS mixers.