Single intrinsic Josephson junction with double-sided fabrication technique

Abstract

We make stacks of intrinsic Josephson junctions (IJJs) imbedded in the bulk of very thin (d ≤ 100 nm) Bi2Sr2CaCu2O8+x single crystals. By precisely controlling the etching depth during the double-sided fabrication process, the stacks can be reproducibly tailor-made to be of any microscopic height (0 − 9 nm < d), i.e. enclosing a specified number of IJJ (0 − 6), including the important case of a single junction. We discuss reproducible gap-like features in the current-voltage characteristics of the samples at high bias. [1,2]. A BSCCO single crystal in the c-axis direction can thus be viewed as a large array of very thin (1.5 nm) intrinsic Josephson junctions (IJJs) in series. At present, this is perhaps the only reliable way to obtain HTS SIS junctions. These junctions appear to have high potential for high-frequency applications like heterodyne receivers and quantum voltage standards [3,4].It is difficult to isolate and study individual IJJ. Most of applications and research on IJJs use therefore stacks containing many junctions. The stacks can be formed either on the surfaces of single crystals (mesas) or etched out in the middle of BSCCO whiskers using focussed ion beams [5]. Earlier we developed a method for making a single intrinsic Josephson junction (SIJJ) enclosed in a U-shaped mesa structure [6]. An effective SIJJ could be seen in the 4-probe measurements on such a mesa. However, extra junctions under the electrodes can give rise to unwanted local heating [7,8].In this letter, we report on reproducible fabrication of uniform stacks formed inside the very thin (d ∼ 100 nm) pieces of BSCCO single crystals by using a double-sided etching technique [9]. The number of junctions in the stacks can be tailor-made to any small number N < 10 including the important case of N = 1 (a single-junction stack). As the stacks are imbedded in the superconducting material, i.e. there are no direct resistive contacts to them, heating effects can be minimized in such stacks allowing the genuine current-voltage (I-V) characteristics to be traced out. Devices like SQUIDs are feasible to make using the developed technique.The stack-fabrication routines start with evaporation of an Au thin film on a freshly cleaved crystal glued onto a sapphire substrate using polyimide. Using the conventional photolithography and Ar-ion etching, a bow-tieshaped mesa with a micro-bridge in the center is formed on the crystal. The overall thickness of this mesa d is typically about 100 nm which is controlled by etching time and rate. Then, a 40 nm d/2 deep slit is made across the bridge. In the next step, we flip the sample and glue it onto another sapphire substrate, sandwiching the single crystal between the two substrates. Separating the substrates cleaves the single crystal into two pieces, one with the mesa being flipped upside down and glued to the second substrate. We subsequently remove all material but the mesa by iteratively cleaving the former with the aid of Scotch tape and inspecting the resulting sample in optical microscope. A ne...

Temperature dependence of the bulk energy gap in underdopedBi2Sr2CaCu2O8+δKrasnov1 2009Phys. Rev. B499430View full textAdd to dashboardCite

“…The model can also explain a strange behavior of mesas with very small amount of junctions [69]. From the estimations above it follows that for a mesa 5 × 5µm 2 with only one IJJ the in-plane resistance is about 40 times larger than the mesa resistance, which make such mesas extremely prone to distortion by in-plane resistance and, probably, not suitable for ITS.…”

Small-number arrays of intrinsic Josephson junctions

“…The low-lying peaks at about ±33 mV were noted to be the most reproducible [16]. While all other conductance peaks shift in normalized voltage (V/N), i.e.…”

“…1b). When the slits overlap along the c axis, a stack of IJJ's appear in the middle and becomes gradually higher with further Ar-ion etching [16] (see Figs. 1c and 2).…”

“…By precisely controlling etching time and measuring the resulting I-V characteristics after several successive etching sessions, we managed to isolate several samples with a single IJJ [16]. Fig.…”

“…In this report, we first describe a technique [15] for making thin stacks enclosing only a few IJJ's without proximity of normal-metal electrodes [16]. These samples allow for the intrinsic-tunnelling spectroscopy with a minimum of Joule heating.…”