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IEEE Transactions on Applied Superconductivity

Microwave responses of an insular intrinsic Josephson junction stack fabricated from Bi-Sr-Ca-Cu-O single crystal

H.B. Wang, Peiheng Wu, Takashi Yamashita

Published Mar 1, 2001• 10 citations

Abstract

The conventional c-axis intrinsic Josephson junction (IJJ) stack is usually fabricated from a piece of single crystal in such a way that it looks like a small mesa sitting on a big pedestal. In some cases the effects of this pedestal should be taken into account in order to explain the experimental observations. To avoid this problem we have developed a novel technique by which we are able to remove a small sample from the single crystal and place it on a substrate with low microwave loss. This enable us to couple it to other microwave devices such as antenna etc. This small sample, or junction stack, measures a few micrometers by a few micrometers in the a-b plane and a few hundred angstroms along the c-axis and is plated with normal metal on both the top and the bottom. To distinguish the structure from the conventional one, we call it an insular intrinsic Josephson junction stack. We carefully study current-voltage characteristics, microwave responses, frequency mixing properties, and discuss the possible applications.

DOI: 10.1109/77.919564

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The conventional c-axis intrinsic Josephson junction (IJJ) stack is usually fabricated from a piece of single crystal in such a way that it looks like a small mesa sitting on a big pedestal. In some cases the effects of this pedestal should be taken into account in order to explain the experimental observations. To avoid this problem we have developed a novel technique by which we are able to remove a small sample from the single crystal and place it on a substrate with low microwave loss. This enable us to couple it to other microwave devices such as antenna etc. This small sample, or junction stack, measures a few micrometers by a few micrometers in the a-b plane and a few hundred angstroms along the c-axis and is plated with normal metal on both the top and the bottom. To distinguish the structure from the conventional one, we call it an insular intrinsic Josephson junction stack. We carefully study current-voltage characteristics, microwave responses, frequency mixing properties, and discuss the possible applications.

We propose a new method for fabricating resistively shunted intrinsic Josephson junctions (RS-IJJs). Two mesas, measuring 6 × 6μm 2 and 100 × 100 μm 2 respectively in the a - b plane, have been fabricated in a Bi 2 Sr 2 CaCu 2 O 8+ x single crystal with a SiO layer evaporated in-between as an insulator. A gold stripe was sputtered onto the insulator to connect the mesas, thus effectively shunting the smaller one. The current–voltage ( I – V ) curves of such RS-IJJs have shown the typical behaviour of a resistively shunted junction with greatly reduced hysteresis and voltage jump. Under microwave irradiation at a 3 mm waveband, we have registered three types of Shapiro steps: conventional, sub-harmonic and zero-crossing. In this rapid communication, we discuss the fabrication details, as well as the observations of the Shapiro steps.

The microwave distribution inside a rectangular stack (15 μm×0.72 μm×60 nm) of Bi2Sr2CaCu2O8+x intrinsic Josephson junctions (IJJs) was studied. The stack was microfabricated into a transmission-line geometry, with a-few-hundred-nm-thick Au layers deposited on the top and bottom of the stack. The microwave distribution was monitored by measuring the anomalous suppression of the tunneling critical current of the IJJs with varied microwave power at frequencies in the W band. This technique can provide valuable information on the microwave transmission modes inside the sandwiched stack of IJJs, which is utterly important for the high-frequency device applications using IJJs, such as fluxon-flow THz oscillators.

The microwave response properties of Bi 2 Sr 2 CaCu 2 O 8+x long intrinsic Josephson junctions have been experimentally and theoretically investigated. We fabricated a mesa structure consisting of 14 junctions, 50 μm long and 4 μm wide, and observed the microwave-induced steps in the I-V characteristics under irradiation with microwaves in the range 5–8 GHz. The step voltages were proportional to the square root of the microwave power, implying that the steps were produced by the motion of fluxons created by the microwaves. The Josephson frequency corresponding to the maximum step voltage was estimated to be 570 GHz. The step voltage decreased with increasing temperature, which can be explained by the decrease of the Swihart velocity. We performed a numerical simulation using coupled sine-Gordon equations and obtained a power dependence of the step voltage similar to the experimental result, and demonstrated standing waves of electric fields with in-phase plasma oscillation in all the junctions. These facts suggest that the steps observed in the experiment were produced by plasma resonance excited by the external microwaves.

We measured the low-frequency-voltage noise properties of a small-sized stack of intrinsic Josephson junctions formed on the surface of a cleaved Bi2Sr2CaCu2Oy (Bi-2212) single crystal, containing a few intrinsic Josephson junctions. A stack with an area of 2×2μm was fabricated by electron-beam lithography and argon-ion-milling. The current–voltage (I–V) characteristics along the c-axis direction of the stack were measured by a three-terminal method. The stack showed (I–V) and dV∕dI-V curves without backbending due to heating effects. Five discrete resistive branches with hysteresis were observed in the low-voltage region of these curves. The measured noise-voltage spectral density SV(f) had a 1∕f dependence on frequency. We estimated the magnitude of the 1∕f noise parameter η from SV(f) in a Bi-2212 stack and found that above the sum-gap voltage in a small-sized stack η was almost the same as that for low-Tc Nb∕Al+AlOx∕Nb high-quality tunnel junctions.

Using a newly developed double-side fabrication method, an IJJ stack plus a bow-tie antenna and chokes were integrated in a slice 200 nm thick and singled out from inside a bulk Bi2Sr2CaCu2O8+x (BSCCO) single crystal. The junctions in the fabricated stack were very uniform, and the number of junctions involved was rather controllable. In addition to this method, which can be used to fabricate integrated circuits based on intrinsic Josephson junctions in high temperature (Tc) superconductors, also reported will be terahertz responses of IJJs, and the possible applications in quantum voltage standard, spectroscopy, and so on.