Planar thin film YBa2Cu3O7−δ Josephson junctions via nanolithography and ion damage

Abstract

We have developed a process to fabricate planar high-Tc Josephson junctions using nanolithography and a 200 keV ion implanter. Conduction occurs in the ab plane and is interface free. We can systematically tune devices to operate at temperatures between 1 K and the Tc of the undamaged superconducting material by varying the length of the weak link and by changing the amount of ion damage. All of the devices showed clear dc and ac Josephson effects. Measurement of R(T) and Ic(T) of the weak links revealed trends which were consistent with a proximity effect.

High-quality planar high-Tc Josephson junctions

“…Such an ideal behavior has not been reported in previous studies 4,5 . The period of the modulation corresponds to one flux quantum in the junction provided λ L is taken to be typical of thin films 11 (400 nm at 72K).…”

“…The atomic disorder induced by ion irradiation drives HTSc cuprates towards an insulating state ; for intermediate disorder, the T c is lowered and the resistivity is increased 2,3 . Authors already succeeded in making Superconductor-Normal-Superconductor (SNS) junctions by lowering locally the T c of a HTSc film 4 . They obtained rather good Josephson characteristics, but the overall process was rather complex still.…”

“…The geometry will be controlled firstly by the actual ion random walk within the sample, secondly by the modification of T c and the resistivity ρ of the material by defects, and thirdly by the proximity effect between the pristine region and the irradiated one. Using TRIM code 12 and suitable assumptions, we have computed a gaussian-like distribution of defects centered on the 20 nm wide slit whose standard deviation is roughly 80 nm 13 , much smaller than a crude estimate of 150 nm or so 4 , and in good agreement with experimental observations : the normal state resistance measured corresponds to the expected width of the channel within a few percents, even for the 1 µm wide ones, confirming that the straggling is much smaller than this value ; based on the actual R(T) curve of a pristine channel, we have been able to compute the one of irradiated samples with a very good accuracy (Figure 3). The increase in resistivity and the decrease in T c as a function of the created defects have been taken from the literature 3,[14][15][16] .…”

Comment on “High Tc superconducting quantum interference devices made by ion irradiation” [Appl. Phys. Lett. 89, 112515 (2006)]

“…9 High T c SQUID fabricated by oxygen ion irradiation were the first electronic device made from high-T c material flown in 1993 in space 10 on board the Space Shuttle "Discovery." After some skepticism some other groups also have successfully fabricated and investigated such weak links in Jena University, 11 KFA Julich, 12 RWTH ͑Germany͒, 13 University of Cambridge ͑UK͒, 14 University of California ͑USA͒, 15 and, recently, CNRS ͑France͒. 16 Here, I would like to point out that the ion irradiated high T c weak links have a great potential to be used not only in SQUIDs but also in other applications such as Josephson arrays for voltage standards and rapid single flux quantum ͑RSFQ͒ circuits because they can be fabricated reproducibly using only standard microelectronic methods and can be put everywhere on the film surface.…”

Very Large Scale Integration of Nano-Patterned YBa2Cu3O7-delta Josephson Junctions in a Two-Dimensional Array

“…This technique was first demonstrated by Tinchev to fabricate SQUIDs. 19 Others have followed with variations of the process to reproduce single junctions, [20][21][22][23][24] series arrays of tens of junctions, 25,26 and a series array of 280 SQUIDs. 27 Here we report the scaling up of this process by two orders of magnitude, with a 2-dimensional (2D) array of 15,820 (28 × 565) Josephson junctions or equivalently 15,255 (27 × 565) SQUIDs.…”