Epitaxial Niobium Research Articles

Quasiparticle lifetimes in epitaxial niobium and tantalum films

By using three-terminal double tunnel junction devices at 2.2 K we have injected quasiparticles into epitaxial thin films of niobium and tantalum. The two junctions are coupled by quasiparticle diffusion and the diffusion of recombination phonons through the sapphire substrate. A comparison of our experimental results with a random walk simulation of the quasiparticle diffusion process enabled us to estimate the lifetime of the injected quasiparticles. The quasiparticle lifetime is not recombination limited in epitaxial Nb and Ta films. Furthermore Ta is less susceptible to the (as yet undetermined) loss mechanisms which govern the quasiparticle lifetime. This result indicates that tunnel junction particle spectrometers utilising Ta absorbers may display better energy resolution than those with Nb absorbers.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

The fabrication and characterization of NbCN/AlN heterostructures

Very high quality niobium nitride based superconductor-insulator-superconductor (SIS) junctions have been prepared using aluminium nitride barrier layers deposited by dc reactive magnetron sputtering. These barriers have been shown to grow epitaxially on high quality epitaxial niobium carbonitride (NbCN) base electrodes. High total gap voltages, equal to the maximum that can be expected theoretically, illustrate that there are no degraded layers at the superconductor/insulator interfaces. We have also demonstrated the first SISIS double barrier devices. Transmission electron microscopy has been used to study the growth of a thick AlN layer sandwiched between NbCN and the microstructure of NbCN/AlN multilayers prepared with several different orientation relationships.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Double tunnel junction experiments with epitaxial niobium and tantalum films

We have used three-terminal double tunnel junction devices to investigate non-equilibrium quasiparticle and phonon transport in epitaxial superconducting films of niobium and tantalum. We show that the two junctions are coupled both by quasiparticle diffusion through the film and by the diffusion of recombination phonons through the sapphire substrate. By separating these two processes and making a random walk simulation of the quasiparticle diffusion we are able to estimate the effective quasiparticle lifetime in our niobium and tantalum films. This is not limited by recombination. Furthermore the quasiparticle lifetime in tantalum is less limited by the other (as yet unidentified) loss processes than that in niobium. This suggests that superconducting tunnel junction particle spectrometers utilising tantalum absorbers may display better energy resolution than those with niobium absorbers which are currently being investigated.

Investigation of epitaxial niobium based superconducting tunnel junction detectors

Superconducting Nb/AlAlO x /Nb tunnel junctions (STJs) with epitaxial niobium base electrode are investigated as weak x-ray detectors. The recorded x-ray spectra show a two peak structure that is attributed to the absorption of 6 keV x-rays in the two junction electrodes. Low Temperature Scanning Electron Microscopy (LTSEM) proves, that the high charge output peak of the x-ray spectra is associated with x-rays absorbed in the epitaxial niobium film. The significantly reduced charge output for events in the polycrystalline niobium electrode is found to be bias voltage dependent.

The role of epitaxy in superconducting tunnel junction detectors

We have experimentally analysed the use of epitaxial and polycrystalline tantalum trapping layers in tunnel junction detectors, using epitaxial niobium based double tunnel junction devices. We have shown that the trapping rate is enhanced by having a low mean-free-path in the Ta trap film and an epitaxial Nb absorber film. This effectively increases the proportion of time spent by a quasiparticle in the trap film. Phonon-quasiparticle scattering, which reduces the effective trapping rate, has been observed.

Fabrication and characterization of all-refractory NbCN/Al/AlO/sub x//Al/Nb junctions

High-quality AlO/sub x/ tunnel barriers have been fabricated on epitaxial niobium carbonitride (NbCN) base layers by the deposition of an Al layer followed by thermal oxidation. By careful control of its uniformity, the thickness of the Al layer has been reduced to less than 3 nm, which results in an average gap voltage, V/sub g/(NbCN), of up to 2.65 mV. Using a self-aligned whole-wafer processing route, high-quality NbCN/Al/AlO/sub x//Al/Nb junctions as small as 0.6 mu m/sup 2/ have been made. These junctions offer considerable advantages over directly deposited barriers in terms of minimal subgap leakage, good control of the barrier conductance, and simple processing procedures. It was shown that submicron junctions can be fabricated with no gap smearing or reduction in quality. Using only Nb counterelectrodes total gap voltages up to 4.0 mV, with widths of 0.6 mV, have been demonstrated in high-quality junctions. >

Metallic Multilayers and Epitaxy

Figure 2. The dependence of the superconducting transition temperature of NbN films grown on three different types of substrates plotted as a function of substrate temperature during deposition. From the paper "Epitaxial Niobium Nitride/Insulator Layered Structures" by J. Talvacchio, J.R. Gavaler, and A.1. Braginski. 18,----,------,----r---..., spacer layer induces order. When spin motions within one film are excited, long range fluctuating dipolar fields are generated, exciting spin motions in neighboring films. Under these conditions, a coherent ground state is obtained for the entire structure. For alternating ferromagnetic and antiferromagnetic films, the application of a magnetic field induces spin reorientation transitions. He values were shown to vary inversely with the member of lattice planes present in each ferromagnetic film, thus suggesting the ability to tailor the structure. Incident electromagnetic energy can excite a soliton response, transmitting the impinging wave through the structure in a resonant mode. This phenomenon can result in a reflecting structure becoming specifically transparent to radiation. Recent studies suggested that increasing the incident power near the resonance level may excite the electron spin system enough to induce a non-linear magnetic susceptability to the applied field. The second session in this symposia was devoted to superconducting multilayer systems fabricated by advanced sputtering techniques. Those presentations covered amorphous metallic multilayers in fractal, quasi periodic and random layering sequences. Another study reported on the first 60 40 20 oL_......._....L._--'-_--l"-_"-----' o