John M. Rowell

Experimental study of MgB2 decomposition

The thermal stability of MgB2 has been studied experimentally to determine the role of thermodynamic and kinetic barriers in the decomposition process. The MgB2 decomposition rate approaches one monolayer per second at 650 °C and has an activation energy of 2.0 eV. The evaporation coefficient is inferred to be ∼10−4, indicating that this process is kinetically limited. These values were inferred from in situ measurements using a quartz crystal microbalance and a residual gas analyzer, in conjunction with ex situ measurements of redeposited material by Rutherford backscattering spectroscopy and secondary ion mass spectroscopy. The presence of a large kinetic barrier to decomposition indicates that the synthesis of MgB2 thin films conditions may be possible with vacuum processing, albeit within a narrow window in the reactive growth conditions.

Internally shunted sputtered NbN Josephson junctions with a TaNx barrier for nonlatching logic applications

We report on the growth, fabrication, and device characterization of NbN internally shunted Josephson junctions with a TaNx barrier. The resistivity of TaNx films could be varied from a few hundred micro-ohms to a few hundred milliohms by increasing the N2 pressure during reactive sputtering. The temperature dependence of IcRn of the junctions with ∼13 mΩ cm barrier resistivity was measured for various barrier thicknesses. The coherence length of the barrier was determined to be 5 nm. By adjusting the barrier thickness, IcRn values >500 μV were observed up to 8.3 K, with Ic and Rn of magnitudes that are suitable for single-flux-quantum digital circuits.

Thermochemistry of MgB/sub 2/ thin film synthesis

We have investigated the thermodynamic and kinetic barriers involved in the synthesis of MgB/sub 2/ films. This work refines our initial conjectures predicting optimal MgB/sub 2/ thin film growth conditions as a consequence of the unusually large kinetic barrier to MgB/sub 2/ decomposition. The small Mg sticking coefficient at temperatures greater than 300/spl deg/C prevents high temperature synthesis with traditional vacuum growth methods. However, as a result of the large kinetic barrier to MgB/sub 2/ decomposition, in-situ thermal processing can be used to enhance the crystallinity and the superconductivity of MgB/sub 2/ films. We used these methods to produce MgB/sub 2/ thin films with relatively high transition temperatures (/spl sim/37 K) by pulsed laser deposition (PLD).

Low temperature synthesis of MgB2

We report the synthesis of MgB2 with bulk superconducting properties by conventional solid state methods at temperatures as low as 550 °C. Mg deficiencies of the type Mg1−xB2 were tested. Tc was found to decrease by about 1 K at large x, though the amount of nonstoichiometry, if any, is likely to be very small. For specific processing conditions, indications of the 25–30 K transition often seen in thin films were seen in the bulk materials. The lower temperature transition may be associated with the grain boundaries. These results indicate that it should be possible to fabricate MgB2 with bulk properties in in situ thin films at temperatures of 600 °C or less.

Low Temperature Fabrication of MgB2

We report the synthesis of MgB2 with bulk superconducting properties by conventional solid state methods at temperatures as low as 550 °C. Mg deficiencies of the type Mg1−xB2 were tested. Tc was found to decrease by about 1 K at large x, though the amount of nonstoichiometry, if any, is likely to be very small. For specific processing conditions, indications of the 25–30 K transition often seen in thin films were seen in the bulk materials. The lower temperature transition may be associated with the grain boundaries. These results indicate that it should be possible to fabricate MgB2 with bulk properties in in situ thin films at temperatures of 600 °C or less.

Reactivity of MgB2 with common substrate and electronic materials

The reactivity of MgB2 with powdered forms of common substrate and electronic materials is reported. Reaction temperatures between 600 and 800 °C, encompassing the range commonly employed in thin-film fabrication, were studied. The materials tested for reactivity were ZrO2, yttria stabilized zirconia, MgO, Al2O3, SiO2, SrTiO3, TiN, TaN, AlN, Si, and SiC. At 600 °C, MgB2 reacted only with SiO2 and Si. At 800 °C, however, reactions were observed for MgB2 with Al2O3, SiO2, Si, SiC, and SrTiO3. The Tc of MgB2 decreased in the reactions with SiC and Al2O3.

Engineering issues in high-frequency RSFQ circuits

Abstract This paper reports progress on several projects that contribute to advancing the state of the art of rapid single flux quantum (RSFQ) logic. The first project is aimed to demonstrate, with true digital testing, the performance of RSFQ circuits of significant size and importance at a frequency that challenges the best semiconductor circuits, with only a miniscule fraction of their power dissipation. The second is a demonstration of an internally shunted SNS junction that has a high I c R n product and is intended as a drop-in replacement for the now-common resistively shunted tunnel junction; the advantage of this device is reduction of size, minimization of parasitic inductances, as well as high I c R n product for higher frequency operation. In the third project, we are trying to break the memory bottleneck that has long plagued superconductor digital electronics by using a hybrid of Josephson and CMOS technologies.

Measurement of the coherence length of sputtered Nb/sub 0.62/Ti/sub 0.38/N thin films

Superconducting and normal state properties of the useful film material NbxTi/sub 1-x/N have been characterized. In particular, the coherence length of reactively sputtered Nb/sub 0.62/Ti/sub 0.38/N thin films is determined to be 2.4/spl plusmn/0.3 nm. The results are inferred from fitting the de Gennes-Werthamer theory to experimental measurements of the proximity-induced depression of the transition temperature in Nb-Nb/sub 0.62/Ti/sub 0.38/N-Nb structures. The coherence length, as defined by this theory, can be used to infer the zero-temperature Ginzburg-Landau coherence length, /spl xi//sub GL/ (0), of 3.8 nm for the Nb/sub 0.62/Ti/sub 0.38/N synthesized in this study. The shorter coherence length and lower resistivity of these films, when compared to NbN, indicates that they are an appealing choice for electrodes in rapid single flux quantum circuits.

Modified superconductor–insulator–normal metal–insulator– superconductor Josephson junctions with high critical parameters

Superconductor–insulator–normal metal–insulator–superconductor (SINIS) and SINS′NIS junctions (here S, I, and N denote superconductor, insulator, and normal metal, respectively) were fabricated using Nb/Al technology and characterized at low temperatures. It is shown that asymmetric SINIS junctions with different transparency of the two tunnel barriers may have higher critical voltages than analogous symmetric junctions at 4.2 K. Also, SINS′NIS junctions were fabricated and investigated. If the thickness of the S′ layer is very thin, these junctions have quasiparticle current–voltage characteristics similar to those of conventional SINIS junctions at 4.2 K, but the Josephson critical current densities, and therefore, the critical voltages, are much higher than the corresponding values for conventional SINIS junctions.

Multiple Andreev reflection in MgB2/MgO/MgB2 Josephson junctions

The current-voltage and conductance-voltage characteristics of MgB2/MgO/MgB2 junctions made with MgB2 electrodes grown by hybrid physical-chemical vapor deposition were systematically analyzed. In the junctions with different sizes and critical current densities, we found excess current and subharmonic gap structure indicative of multiple Andreev reflection. An apparent link between multiple Andreev reflection and substantial Josephson current at high temperatures suggests that the barrier is dominated by high-transparency channels.