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IEEE Transactions on Applied Superconductivity

A novel method of realizing two-dimensional array systems of high-T/sub c/ step-edge Josephson junctions

C. D. Reintsema

Published Jun 1, 1995• 2 citations
Publisher

Abstract

We have developed a unique substrate terracing process to achieve two-dimensional arrays of high-T/sub c/ step-edge superconductor-normal metal-superconductor Josephson junctions. This process has been used to realize 3/spl times/3 and 3/spl times/6 antenna coupled arrays for direct detection of emission. An enhancement to this process incorporating a YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// base layer and a Nb/PdAu ground plane is described. This modification enabled the fabrication of arrays capacitively coupled to detector junctions for detection of emission at frequencies beyond 300 GHz. This report details the motivation, design, fabrication process, and preliminary results of several array experiments.>

DOI
10.1109/77.403287

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We have developed a unique substrate terracing process to achieve two-dimensional arrays of high-T/sub c/ step-edge superconductor-normal metal-superconductor Josephson junctions. This process has been used to realize 3/spl times/3 and 3/spl times/6 antenna coupled arrays for direct detection of emission. An enhancement to this process incorporating a YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// base layer and a Nb/PdAu ground plane is described. This modification enabled the fabrication of arrays capacitively coupled to detector junctions for detection of emission at frequencies beyond 300 GHz. This report details the motivation, design, fabrication process, and preliminary results of several array experiments.>
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We have fabricated EuBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// Josephson junction arrays of two different sizes to improve detection output and microwave coupling between junctions and an rf source. The arrays contain three weak link grain boundary junctions made on an MgO bicrystal substrate. It is demonstrated that both the junction array configuration and smaller junction size are effective in improving microwave coupling and hence in increasing the responsivity. The best responsivity obtained is 1085 V/W for broadband detection at 11.9 GHz.>
We propose a design for two-dimensional arrays of stacked Josephson junctions and study their properties as coherent sources of radiation in the mm and sub-mm wavelength range. This design can be implemented by using either mesas of intrinsic Josephson junctions, from crystals of high TC superconductors, or artificial stacks of low TC junctions. We outline a possible Nb/AlOx/Nb trilayer fabrication process for such devices. Our model for the dynamics of the system predicts the range of parameters over which the junctions synchronize.
We have fabricated YBCO step-edge Josephson junctions and measured the responses to millimeter and submillimeter irradiation. In the substrate milling process for the purpose of fabricating the step, the step angle was controlled using a Nb metal mask. At a step angle of 20 degrees, we obtained Josephson junctions, the characteristics of which are consistent with the RSJ model. We estimated the capacitance to be 4.4 fF using the RSJ model, but such a value for an 8 /spl mu/m wide and 300 nm thick junction seems too small. When the junctions were irradiated by 300 GHz radiation, Shapiro steps up to the 4th were observed. The junctions were used as mixers in the 100 GHz band, and as harmonic mixers in the 100 GHz LO and 700 GHz RF bands. Beat signals were observed in both cases. We also estimated the quasi-optical conversion losses of the step-edge junctions as mixers.>
Detectors made from superconducting cuprate YBa/sub 2/Cu/sub 3/O/sub x/ bicrystal Josephson junctions (BJJs) on sapphire and NdGaO/sub 3/ substrates have been fabricated and characterized in the frequency band 200-900 GHz. Junctions on sapphire substrates had a normal state resistance R/sub N//spl ap/15-60 /spl Omega/, and I/sub C/R/sub N/ product up to 2.5 mV at T=4.2 K. Junctions on NdGaO/sub 3/ substrates had lower R/sub N/=1-5 /spl Omega/ and I/sub C/R/sub N/=0.4-0.9 mV at T=77 K. Three types of detecting devices have been investigated in both the broadband and the frequency-selective detection modes. One type was patterned with log-periodic antenna, and two others with Pt-metal double-slot antenna designed for a central frequency f=300 GHz and f=400 GHz, respectively. Measurements at f=320 GHz of the reception bandwidth /spl Delta/f for a device with double-slot antenna gave a quality factor Q=f//spl Delta/f/spl ap/10. A low-noise cooled 1-2 GHz bandwidth amplifier enables a better sensitivity in the self-pumping frequency mixing mode, avoiding the 1/f noise. The dependence of the spectral density of noise on voltage was compared to the data for the Josephson emission linewidth obtained by the selective detector response method. Also discussed are measurements at 500 GHz of the NEP values carried out at different experimental conditions.
The authors have detected coherent emission from two-dimensional (2D) arrays of superconductor-insulator-superconductor (SIS) Josephson junctions. 2D arrays emit coherent radiation over a frequency range of 60 to 210 GHz, when coupled to detector junctions through DC-blocking capacitors. The detector junctions exhibit Shapiro steps at frequencies corresponding to the voltage across single-array junctions. The maximum power from a 10-by-10 junction-array coupled to a detector junction occurs at 150 GHz and is estimated to be 0.4 mu W, based on simulations of the detector circuit. By varying the number of array junctions, the array geometry, the junction critical current, and the coupling circuit, they have begun determining the essential conditions for observing coherent emission.