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International Conference on Infrared, Millimeter, and Terahertz Waves (IRMMW-THz)

Performance of SIS mixers at 205 Ghz employing submicron Nb and NbN tunnel junctions

Hamid Javadi

Published Dec 1, 1990• 4 citations
Publisher
DOI
10.1117/12.2301470

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We present the noise properties of a mixer with the NbN-MgO-NbN quasiparticle tunnel junctions. Our work is based on experiment in the 120-180 GHz range with the NbN tunnel junctions in the mixer. Mixer printed circuit is totally made of NbN. The mixer operates at 5.4 K temperature unacceptable with Nb junctions. The minimum DSB receiver noise temperature is about 65 K at 162 GHz and approaches the Nb SIS mixer performance in mm band. It has been found that the noise sources in the NbN junctions are comparable to the Nb junctions and that the receiver noise with the NbN SIS mixer may be only few times more than the quantum limit of noise in the frequency range below the gap frequency. Output noise of the SIS mixer has been found constant in a wide frequency band and within an important range of the local oscillator amplitudes.

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At frequencies above the gap frequency of niobium (710 GHz), the performance of Nb/A10x/Nb SIS mixers degrades and the losses in the niobium films of the superconductive Nb/SiO/Nb microstrip circuits used to provide a good RF impedance match severely limit the performance. Therefore, we are investigating the properties of NbN mixers, and in particular of NbN/SiO/NbN microstrip circuits integrated with submicron IlbN/Mg0/NbN tunnel junctions. The very large penetration depth of NbN (3000-4000 A) makes the circuit design difficult and also puts critical constraints on alignment during fabrication. We present here the design of several rf tuning circuits for 630 GHz including novel symmetric designs which ease the alignment tolerances. In addition, a new approach for determining experimentally the penetration depth in the NbN films is presented.

Softwaretechnik-Trends•Mar 1, 1995•1 citation

Fabrication and characterization of high current-density, submicron, NbN/MgO/NbN tunnel junctions

Jeff A. Stern, Henry George LeDuc, A. Judas

At near-millimeter wavelengths, heterodyne receivers based on SIS tunnel junctions are the most sensitive available. However, in order to scale these results to submillimeter wavelengths, certain device properties should be scaled. The tunnel-junction's current density should be increased to reduce the RC product. The device's area should be reduced to efficiently couple power from the antenna to the mixer. Finally, the superconductor used should have a large energy gap to minimize RF losses. Most SIS mixers use Nb or Pb-alloy tunnel junctions; the gap frequency for these materials is approximately 725 GHz. Above the gap frequency, these materials exhibit losses similar to those in a normal metal. The gap frequency in NbN films is as-large-as 1440 GHz. Therefore, we have developed a process to fabricate small area (down to 0.13 sq microns), high current density, NbN/MgO/NbN tunnel junctions.

•Jan 1, 1992•12 citations
We present the noise properties of a mixer with the NbN-MgO-NbN quasiparticle tunnel junctions. Our work is based on experiment in the 120-180 GHz range with the NbN tunnel junctions in the mixer. Mixer printed circuit is totally made of NbN. The mixer operates at 5.4 K temperature unacceptable with Nb junctions. The minimum DSB receiver noise temperature is about 65 K at 162 GHz and approaches the Nb SIS mixer performance in mm band. It has been found that the noise sources in the NbN junctions are comparable to the Nb junctions and that the receiver noise with the NbN SIS mixer may be only few times more than the quantum limit of noise in the frequency range below the gap frequency. Output noise of the SIS mixer has been found constant in a wide frequency band and within an important range of the local oscillator amplitudes.
At frequencies above the gap frequency of niobium (710 GHz), the performance of Nb/A10x/Nb SIS mixers degrades and the losses in the niobium films of the superconductive Nb/SiO/Nb microstrip circuits used to provide a good RF impedance match severely limit the performance. Therefore, we are investigating the properties of NbN mixers, and in particular of NbN/SiO/NbN microstrip circuits integrated with submicron IlbN/Mg0/NbN tunnel junctions. The very large penetration depth of NbN (3000-4000 A) makes the circuit design difficult and also puts critical constraints on alignment during fabrication. We present here the design of several rf tuning circuits for 630 GHz including novel symmetric designs which ease the alignment tolerances. In addition, a new approach for determining experimentally the penetration depth in the NbN films is presented.
At near-millimeter wavelengths, heterodyne receivers based on SIS tunnel junctions are the most sensitive available. However, in order to scale these results to submillimeter wavelengths, certain device properties should be scaled. The tunnel-junction's current density should be increased to reduce the RC product. The device's area should be reduced to efficiently couple power from the antenna to the mixer. Finally, the superconductor used should have a large energy gap to minimize RF losses. Most SIS mixers use Nb or Pb-alloy tunnel junctions; the gap frequency for these materials is approximately 725 GHz. Above the gap frequency, these materials exhibit losses similar to those in a normal metal. The gap frequency in NbN films is as-large-as 1440 GHz. Therefore, we have developed a process to fabricate small area (down to 0.13 sq microns), high current density, NbN/MgO/NbN tunnel junctions.