S. I. Raider

Low‐frequency noise in low 1/f noise dc SQUID’s

We demonstrate that the low‐frequency noise in our edge junction dc superconducting quantum interference devices, with a basic 1/f flux noise of 2×10−12 Φ20/Hz at 1 Hz, can all be accounted for in terms of junction critical current fluctuations. A novel modulation readout scheme is able to cancel the effect of junction critical current fluctuations and reduce our total noise to 1×10−12 Φ20/Hz at 0.1 Hz, a level that is three times lower than the lowest flux noise ever previously reported at this frequency.

In situ measurements of SiO(g) production during dry oxidation of crystalline silicon

We report in situ measurements of SiO(g) evolution during the oxidation of silicon by O2 for a range of experimental conditions including the transition from active to passive oxidation. The results show that this transition occurs when the SiO(g) partial pressure reaches the equilibrium vapor pressure for the reaction Si(s)+SiO(s)⇄2SiO(g). During the growth of a SiO2 film, there is no significant transport of SiO molecules into the gas phase.

Nitrogen reaction at a silicon–silicon dioxide interface

Evidence is provided indicating that a nitridation reaction occurs at a silicon–silicon dioxide interface during high‐temperature annealing in nitrogen. The factors affecting nitrogen‐reaction kinetics are similar to those affecting oxidation of silicon with dry oxygen. The nitridation reaction is significantly slower than the oxidation reaction, and trace amounts of oxidants can effectively compete with nitrogen for silicon reaction sites.

Josephson integrated circuit process for scientific applications

We have developed and are regularly practicing a seven mask-level Josephson integrated circuit fabrication process tailored to dc SQUID requirements and intended for SQUID studies and other scientific applications of Josephson technology. The process incorporates low capacitance Nb/Nb 2 O 5 /PbAuIn edge junctions, PdAu shunt resistors, and a wiring pitch of 5 μm for the SQUID input coil level (which is PbAuIn). The junctions can be made as small as 2μm by 0.3μm, with a capacitance (including parasitics) of ∼0.14 pF. This process yields stable and reliable junctions and integrated circuits.

Nb/Nb oxide/Pb-alloy Josephson tunnel junctions

A procedure is described for fabricating thin film Nb/Nb oxide/Pb-alloy Josephson tunnel junctions that satisfies the principal requirements for integrated circuit design and fabrication. A deposited Nb film, evaporated from an e-gun heated source, was patterned by chemical etching to form a base electrode. A junction was completed by plasma etching and plasma oxidizing the Nb junction area to form a tunnel barrier and by depositing a Pb alloy counterelectrode. Josephson tunnel junctions with Nb/Nb oxide/Pb-Au-In structures were prepared with low excess subgap currents in the current-voltage (I-V) curve and with reproducible and stable I-V characteristics. Variations in junction current density from run-run were ± 15%. Seven 3-junction interferomeeters out of a population of 50,000 were shorted (99.99% yield) for causes not immediately attributable to photoresist-related defects. No changes in Josephson current were detected after thermal cycling 17,000 interferometers 1,800 × between room temperature and 4.2°K, after storing them for 2 years at 5°C, or after annealing 5,000 interferometers for 4.5 hours at 70°C.

Time‐dependent breakdown of silicon dioxide films

Na ion drift through intentionally contaminated SiO2 films is observed at room temperature when voltages of the appropriate polarity are applied across the oxide. Enhanced electronic conduction and time‐dependent breakdowns of Al/SiO2/Si capacitors electrically stressed at constant field strengths were correlated with this Na ion drift. The applied field strengths, E, and times, t, to breakdown of contaminated capacitors were found to be linearly related by Peeks law, in which E ≈ t−1/4.

Structure of a Nb oxide tunnel barrier in a Josephson junction

Oxides grown on the surface of a polycrystalline Nb thin film using rf plasma cleaning and plasma oxidation processes were examined by transmission electron microscopy. The oxides were prepared on the Nb surface using the same rf plasma processes developed for fabricating the tunnel barriers of Nb/Nb oxide/Pb alloy Josephson tunnel junctions. Transmission electron diffraction patterns obtained from the tunnel barrier formed on the Nb surface indicated the presence of a crystalline NbC0.25O0.75 layer between the amorphous Nb2O5 tunnel barrier layer and the Nb base electrode. The thickness of this transition layer was found to be sensitive to the rf plasma cleaning conditions prior to the tunnel barrier formation. The formation of this NbC0.25O0.75 layer of proper thickness is correlated to good junction quality. An epitaxial relationship between the NbC0.25O0.75 layer and the underlying Nb crystal was also observed.

All‐Nb low‐noise dc SQUID with 1‐μm tunnel junctions

Tunnel‐junction dc SQUIDs are being fabricated from Nb films using electron‐beam lithography to define linewidths as small as 0.5 μm. The barriers in the Nb‐NbOx‐Nb junctions are grown in an oxygen plasma discharge, and the Pd2Si resistive shunts are formed by annealing Pd on the Si substrates. These devices have excellent storage and recycling properties. At a measurement frequency of 40 kHz, a SQUID with an inductance of about 1 nH, a critical current of about 4 μA, and a parallel shunt resistance of about 15 Ω had an intrinsic energy resolution of 2.5×10−32 J Hz−1 at 4.2 K and 1.1×10−32 J Hz−1 at 1.6 K, in good agreement with a model of the dc SQUID.

Shapiro steps on current-voltage curves of dc SQUIDs

We have studied in detail the current‐voltage (I‐V) curves of dc SQUIDs in the presence of rf fields in the frequency range 2–18 GHz. Strong voltage locking at half‐integral as well as the usual integral Shapiro step spacing is observed when the applied dc‐magnetic flux is close to half‐integral numbers of flux quanta. Numerical simulations are in excellent agreement with the experimental I‐V curves. The half‐integral steps are found to be associated with the flip‐flop between two adjacent fluxoid states of the SQUID synchronized to the rf field.

Spatial variation of the observed energy gap in granular superconducting NbN films

We have simultaneously measured the surface topography and the tunneling current‐voltage (I‐V) characteristics of thin films of NbN as a function of lateral position using a low‐temperature scanning tunneling microscope. We have imaged the superconducting energy gap by fitting the I‐V’s with an energy gap for each spatial position. Such images showed significant variation of the gap, including areas with reduced gap when a magnetic field (up to 6 T) was applied. The features in the gap image were often correlated with features in the surface topographic images.