Design and fabrication of internally shunted tantalum nitride barrier Josephson junctions for RSFQ logic applications

Superconductor/normal-conductor/superconductor (SNS) junctions are the preferred choice for rapid single flux quantum logic (RSFQ) because of their internally shunted current-voltage (I-V) characteristics. Most implementations of SNS junctions have not shown an advantage over the widely used externally shunted Nb/AlOx/Nb tunnel junctions, mainly due to their low barrier resistivity resulting in extremely low I cRn product, a parameter that determines the circuit speed.; The design rule of next generation RSFQ circuits requires ∼1 mum junction size, an IcRn of >500 muV, and an Ic of ∼100 muA. Assuming a barrier thickness of 25--100 nm, barrier material is required to have resistivity of ∼5--20 mO-cm. Many materials including semiconductors, metal/oxide composites and defective nitride materials undergo the metal-insulator transition. It is, however, not clear whether any of these barriers can be used to make junctions which support the large, uniform and reproducible critical current needed for RSFQ circuits. In particular, carrier dynamics and the percolative nature of carrier transport through materials near the metal-insulator transition are not well understood and their influence on the junction properties needs to be addressed.; We have chosen to explore junctions with TaxN as the barrier because its resistivity can be readily tuned in the desired range by using enhanced nitrogen partial pressures during reactive growth and TaxN does not require the extreme levels of purity and epitaxy to be consistently produced. Our experimental and theoretical investigation discovered that the mechanism responsible for the wide range of resistivities produced can be attributed to the formation of tantalum vacancies (VTa). Being a penta-valent acceptor, V Ta will "freeze out" carriers and result in metal-insulator transition.; Utilizing the near metal-insulator transition resistivity of Ta xN, NbN/TaxN/NbN sandwich junctions made on MgO substrates showed unexpectedly large IcR n products of >1 mV at 4.2 K. The high cost and limited size of MgO substrates prevented those junctions from being useful for industry. In order to design TaxN barrier junctions as a drop-in replacement for Nb/AlOx/Nb tunnel junctions in the existing fabrication lines, NbTiN/TaxN/NbTiN and Nb/TaxN/Nb sandwiches are fabricated on oxidized 100 mm diameter Si wafers. Nb0.62Ti0.38N is used as electrode because of the higher Tc and the lower penetration depth than the best epitaxial grown NbN. The junction properties, i.e. temperature dependence of IcRn and xin, and thickness dependence of Jc, are consistent with well-known SNS theories if a longer than expected normal metal coherence length is assumed. The junctions are found to have the desired Jc, Rn , and IcRn values for the next generation RSFQ technology.; To develop a more suitable mathematical model for SNS junctions with a barrier near metal-insulator transition and to study the influence of free carrier dynamics in the barrier, the resistively shunted junction (RSJ) model was extended to incorporate a frequency dependent dielectric response. The new model is capable of including response of the free carriers, influence of excitation from shallow defects, very soft phonon modes, and boundary resistances.