Robert H. Hadfield

Fabrication of nanoscale heterostructure devices with a focused ion beam microscope

A focused ion beam (FIB) microscope has been used to fabricate junctions with dimensions in the range 100–5000 nm by three-dimensional etching. We have applied this process to a variety of structures, including current-perpendicular-to-plane giant-magnetoresistive multilayer devices, superconductor–metal–superconductor Josephson junctions, where the metal is Mo, Co, or a CuxNi1−x alloy, and GaN light-emitting diodes. In addition, Tl2Ba2CaCu2O8 intrinsic Josephson junctions were also fabricated and characterized. The flexibility of the FIB technique allowed junctions of many different materials and heterostructures to be fabricated with the same process.

Capacitance as a probe of high angle grain boundary transport in oxide superconductors

We report a series of studies of grain boundary (GB) capacitance for YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// (YBCO) films grown on SrTiO/sub 3/ (STO) bicrystal substrates. By varying the film thickness and the width of the track containing the GB, we find that the substrate makes no contribution to the capacitance measured using Fiske resonances or hysteresis in most cases. This is due to the frequency dependence of the dielectric properties of SrTiO/sub 3/. We have also found that GB capacitance per unit area c/sub GB/ correlates with the resistance-area product R/sub n/A. For our own GBs and GBs reported in the literature the data is is consistent with c/sub GB//spl prop/(R/sub n/A)/sup -1/. We attribute this to variations in GB barrier properties, which reduce the active area, whilst maintaining locally the transport mechanism as tunneling.

Nanoscale SNS junction fabrication in superconductor-normal metal bilayers

We have developed a reliable and versatile technique for fabricating SNS junctions in a superconductor-normal metal bilayer using a focused ion beam microscope (FIB) in conjunction with an in-situ resistance measurement technique. This technique offers a simple method for creating multi-junction devices (SQUIDs, 3-terminal devices, arrays) with high integration densities. In this paper we discuss recent results from devices created in Nb-Cu tracks by cutting 50 nm trenches in the top Nb layer to weaken the superconducting coupling. Cuts of depths between 60 and 100% of the Nb thickness yield reproducible junctions with current voltage (I(V)) characteristics in accordance with the resistively-shunted-junction (RSJ) model, characteristic voltage I/sub C/R/sub N//spl sim/50 /spl mu/V at 4.2 K and excellent microwave response. A thorough study has been carried out of the effect on device parameters of varying the Cu layer thickness (0-175 nm). In addition transmission electron microscopy (TEM) studies have been carried out on the device structure. A two-channel model of device operation has been developed and related to the results of I/sup C/R/sub N/(T) measurements (down to 350 mK) on selected devices.

The capacitance of grain boundaries in superconducting films with strontium titanate and other substrates

Abstract Bicrystal Josephson junctions have been fabricated with coplanar shunt capacitors on a strontium titanate (SrTiO 3 ) substrate in order to investigate the origin of the junction capacitance. None of the external structures, which varied in length from 5 μm to 2 mm, appeared to have any effect on the I – V characteristic. However, a correlation was observed between the junction resistance and the capacitance obtained from both the hysteresis in the I – V characteristic and the internal junction (Fiske) resonances. This suggests that at least at low temperatures the junction capacitance is intrinsic to the grain boundary and the SrTiO 3 substrate does not contribute. The correlation between the junction resistance and the capacitance is a common feature of many grain boundaries in high T c superconductors reported in the literature, artificial or otherwise.

Nanoscale superconductor–normal metal–superconductor junctions fabricated by focused ion beam

Abstract We have developed a reliable and versatile technique for fabricating low T C superconductor–normal metal–superconductor Josephson junctions with a focused ion beam (FIB) microscope in conjunction with an in situ resistance measurement technique. This provides a simple method that allow us to create a variety of single and multi-junction superconducting devices (arrays, SQUIDs and 3 terminal devices) with desirable and well-controlled properties and high integration densities. Here we discuss the development of this technique, demonstrating the versatility of the FIB in this application with recent results from arrays intended for voltage standards, devices with novel circular junction geometry, and devices fabricated in MgB 2 .

Nanofabricated SNS junction series arrays in superconductor-normal metal bilayers

We have applied our existing focused ion beam based nanoscale planar SNS bridge junction technology to the fabrication of series arrays, with a view to lumped array applications. Single junctions are created in a micrometre-width bilayer track (125 nm Nb on 75 nm Cu) by milling a trench 50 nm wide in the upper superconducting layer. Individual junctions with non-hysteretic, resistively-shunted current-voltage characteristics, critical current I C ∼ 1 mA and characteristic voltage I C R N ∼ 50 μV at 4.2 K can be fabricated routinely. The characteristics of 10-junction series arrays at junction spacings of 0.2 μm to 1.6 μm have been studied at 4.2 K and above. Locking of all the junctions in the array under an applied microwave field is observed as the transition temperature is approached. This effect is achieved at a lower temperature for shorter junction spacings, suggesting a penetration depth-dependent electromagnetic coupling mechanism. Measurements of differential resistance versus current reveal the I C distribution of individual junctions within the array. Spreads of critical current δI C = I Cmax /I Cmin ∼ 1.5 at 4.2 K are typically observed. The spread in normal state resistance, R N between junctions is negligible in comparison, as the unbroken normal metal layer shunts all of the junctions in the array. This allows locking to be achieved in spite of the appreciable spread in I C .

Asymmetry modulated SQUIDs made by direct focused ion beam milling

Abstract We have developed a fabrication method using a focused ion beam microscope that allows us to produce SNS junctions in which thermally energetic electrons can enter the normal metal region. The SNS junctions are fabricated in a normal metal/low TC superconductor bilayer by removing narrow (∼50 nm) region of the superconductor. Connecting either a normal metal resistive heater of a photon-absorbing pixel with a lateral connection to the normal metal layer of the bilayer in the vicinity of the junction will inject hot electrons into the junction, thus reducing the critical current. In a superconducting quantum interference device (SQUID) such an effect is manifested as an adjustable asymmetry parameter which results in a change in the flux–modulation curve. We are developing SQUIDs utilising this concept for application as high energy resolution spectrometers. In order to realise these devices, we are fabricating SNS junctions with critical current (IC)–normal resistance (RN) products in excess of 0.1 mV and temperature dependent critical currents operating at 0.3 K. The use of the focused ion beam affords great flexibility in the design of our devices. Here we discuss the parameters for the optimum design and selection of materials and present our latest results from our prototype devices.

Josephson junctions with hysteretic current voltage characteristics at high temperatures

The properties of bicrystal Josephson junctions with external capacitors are reported. It was found that the hysteresis in the current voltage (IV) characteristic was very sensitive to the wavelength of the Josephson oscillation in the dielectric and thermal noise suppression of the critical current. A McCumber parameter /spl beta//sub c/ of 1.2 at 65 K has been achieved. In addition intrinsic grain boundary capacitance has been found to give a large /spl beta//sub c/ value of 9.6 at 4.2 K.

Novel Josephson junction geometries in NbCu bilayers fabricated by focused ion beam microscope

Abstract We explore novel junction configurations as an extension of our established focused ion beam-based low Tc SNS junction fabrication technique [1] . By milling a circular trench (diameter 1 μm, width 50 nm) in a 125 nm Nb 75 nm Cu bilayer we define a superconducting island connected to the bulk of the film by a normal metal barrier and entirely enclosed in-plane by the superconducting film. The properties of this Corbino geometry Josephson junction can be probed by depositing an insulating layer over the device and drilling a 0.3 μm diameter hole down to the island to allow a Nb via to be deposited. Behavior of such devices has been studied in a Helium bath at 4.2 K. An SNS-like current–voltage characteristic and Shapiro steps are observed. It is in terms of magnetic field behavior that the device exhibits novel characteristics: as the device is entirely enclosed in type II superconductor, when a magnetic field is applied perpendicular to the plane of the film, only quantized flux can enter the junction. Hence as applied magnetic field is increased the junction critical current is unchanged, then abruptly suppressed as soon as a flux quantum enters.