Superconducting Tin Films Research Articles

Isotropic plasma-thermal atomic layer etching of superconducting titanium nitride films using sequential exposures of molecular oxygen and SF6/H2 plasma

Microwave loss in superconducting TiN films is attributed to two-level systems in various interfaces arising in part from oxidation and microfabrication-induced damage. Atomic layer etching (ALE) is an emerging subtractive fabrication method which is capable of etching with angstrom-scale etch depth control and potentially less damage. However, while ALE processes for TiN have been reported, they either employ HF vapor, incurring practical complications, or the etch rate lacks the desired control. Furthermore, the superconducting characteristics of the etched films have not been characterized. Here, we report an isotropic plasma-thermal TiN ALE process consisting of sequential exposures to molecular oxygen and an SF6/H2 plasma. For certain ratios of SF6:H2 flow rates, we observe selective etching of TiO2 over TiN, enabling self-limiting etching within a cycle. Etch rates were measured to vary from 1.1 Å/cycle at 150°C to 3.2 Å/cycle at 350°C using ex situ ellipsometry. We demonstrate that the superconducting critical temperature of the etched film does not decrease beyond that expected from the decrease in film thickness, highlighting the low-damage nature of the process. These findings have relevance for applications of TiN in microwave kinetic inductance detectors and superconducting qubits.

Quantum contributions to the magnetoconductivity of critically disordered superconducting TiN films

The onset of superconductivity in the presence of disorder is a fundamental problem of condensed matter physics. Here we investigate the magnetoconductance of disordered () superconducting TiN films above the critical temperature Tc. We show that the magnetoconductivity of moderately disordered films with is in full agreement with the perturbative theory of quantum contributions to conductivity. We demonstrate that the magnetoconductivity of films with is also in agreement with the perturbative theory down to temperatures . The quantitative discrepancy between experiment and theory develops only below temperatures for films with . This discrepancy can be eliminated if we assume steeper temperature dependence of the Larkin’s electron–electron attraction strength, . The obtained temperature dependence of electron phase breaking time is in agreement with theoretical predictions for all samples.

Reentrant Resistive Behavior and Dimensional Crossover in Disordered Superconducting TiN Films

A reentrant temperature dependence of the normal state resistance often referred to as the N-shaped temperature dependence, is omnipresent in disordered superconductors – ranging from high-temperature cuprates to ultrathin superconducting films – that experience superconductor-to-insulator transition. Yet, despite the ubiquity of this phenomenon its origin still remains a subject of debate. Here we investigate strongly disordered superconducting TiN films and demonstrate universality of the reentrant behavior. We offer a quantitative description of the N-shaped resistance curve. We show that upon cooling down the resistance first decreases linearly with temperature and then passes through the minimum that marks the 3D–2D crossover in the system. In the 2D temperature range the resistance first grows with decreasing temperature due to quantum contributions and eventually drops to zero as the system falls into a superconducting state. Our findings demonstrate the prime importance of disorder in dimensional crossover effects.

Electrodynamic response and local tunneling spectroscopy of strongly disordered superconducting TiN films

We have studied the electrodynamic response of strongly disordered superconducting TiN films using microwave resonators, where the disordered superconductor is the resonating element in a high- quality superconducting environment of NbTiN. We describe the response assuming an effective pair-breaking mechanism modifying the density of states, and compare this to local tunnelling spectra obtained using scanning tunnelling spectroscopy. For the least disordered film (kFl = 8.7, Rs = 13 {\Omega}), we find good agreement, whereas for the most disordered film (kFl = 0.82, Rs = 4.3 k{\Omega}), there is a strong discrepancy, which signals the breakdown of a model based on uniform properties.

Dual threshold diode based on the superconductor-to-insulator transition in ultrathin TiN films

We investigate transport properties of superconducting TiN films in the vicinity of the superconductor-insulator transition (SIT). We show that the current-voltage (I-V) characteristics are mirror-symmetric with respect to the SIT and can be switched to each other by the applied magnetic field. In both superconducting and insulating states, the low-temperature I-V characteristics have pronounced diode-like threshold character, demonstrating voltage/current jumps over several orders of magnitude at the corresponding critical current or threshold voltage. We have found that for both states, the theory developed for Josephson junction arrays offers a quantitative description of the experimental results.

Microwave Properties of Superconducting Atomic-Layer Deposited TiN Films

We have grown superconducting TiN films by atomic layer deposition with thicknesses ranging from 6 to 89 nm. This deposition method allows us to tune the resistivity and critical temperature by controlling the film thickness. The microwave properties are measured, using a coplanar-waveguide resonator, and we find internal quality factors above a million, high sheet inductances (5.2-620 pH), and pulse response times up to 100 \mu s. The high normal state resistivity of the films (> 100 \mu\Omega cm) affects the superconducting state and thereby the electrodynamic response. The microwave response is modeled using a quasiparticle density of states modified with an effective pair-breaker,consistently describing the measured temperature dependence of the quality factor and the resonant frequency.

Effect of disorder on magnetoresistance oscillations in nanoperforated superconducting TiN films

The low-temperature transport properties of nanoperforated superconducting TiN films have been experimentally studied. A periodic dependence of the resistance on an external magnetic field, with a period corresponding to a magnetic flux quantum per cell, is observed. An anomalous temperature dependence of the oscillation amplitude is found. The role of the effects of electron-electron interaction and localization in the physical properties of arrays of weak links, based on strongly disordered superconducting films, is discussed.

Effect of disorder on magnetoresistance oscillations in nanoperforated superconducting TiN films

Effect of disorder on magnetoresistance oscillations in nanoperforated superconducting TiN films

Quantum Metallicity on the High-Field Side of the Superconductor-Insulator Transition

We investigate ultrathin superconducting TiN films, which are very close to the localization threshold. Perpendicular magnetic field drives the films from the superconducting to an insulating state, with very high resistance. Further increase of the magnetic field leads to an exponential decay of the resistance towards a finite value. In the limit of low temperatures, the saturation value can be very accurately extrapolated to the universal quantum resistance h/e2. Our analysis suggests that at high magnetic fields a new ground state, distinct from the normal metallic state occurring above the superconducting transition temperature, is formed. A comparison with other studies on different materials indicates that the quantum metallic phase following the magnetic-field-induced insulating phase is a generic property of systems close to the disorder-driven superconductor-insulator transition.

Resistive current states in wide superconducting films in zero magnetic field

The temperature dependence of the current–voltage characteristics of high-quality thin films of tin from7 to 50 µm thick are investigated in the absence of an external magnetic field. For the first time, wehave experimentally observed phase slip centres (PSCs) and phase slip lines (PSLs) on thesame superconducting tin film with known parameters in the temperature intervalscorresponding to the mechanisms of their formation and existence. We have shown that thestates of a wide film with increasing transport current appear in the following order: thesuperconducting state for current less than critical; the resistive vortex state forcurrent more than critical, but less than maximum current for the uniform fluxflow (instability current); the critical state due to the onset of instability of thesteady pattern of viscous motion of the vortices; a vortex-free resistive state withPSLs for current more than instability current, but less than the upper criticalcurrent; and the normal state at a current higher than the upper critical current.

From quantum corrections to magnetic-field-tuned superconductor–insulator quantum phase transition in TiN films

Results of the measurements of the magnetic-field-dependent resistance of ultrathin superconducting TiN films with different degrees of disorder are presented. The main features of the isomagnetic data R ( T , B = const ) and isotherms R ( B , T = const ) observed in all samples are: fan-shaped structure of R ( T , B = const ) curves, a negative magnetoresistance at high magnetic fields, the presence of an intersection point ( B c , R c ), and the possibility of a scaling data presentation. The analysis of the data reveals that such behavior, previously regarded as the main evidence of the magnetic-field-tuned superconductor–insulator transition, can actually be observed for a usual thermodynamic superconductor–normal metal transition, provided that the behavior of this metal is controlled, to a considerable degree, by the quantum corrections to the conductivity.

Superconductivity on the localization threshold and magnetic-field-tuned superconductor-insulator transition in TiN films

Temperature- and magnetic-field dependent measurements of the resistance of ultrathin superconducting TiN films are presented. The analysis of the temperature dependence of the zero field resistance indicates an underlying insulating behavior, when the contribution of Aslamasov-Larkin fluctuations is taken into account. This demonstrates the possibility of coexistence of the superconducting and insulating phases and of a direct transition from the one to the other. The scaling behavior of magnetic field data is in accordance with a superconductor-insulator transition (SIT) driven by quantum phase fluctuations in two-dimensional superconductor. The temperature dependence of the isomagnetic resistance data on the high-field side of the SIT has been analyzed and the presence of an insulating phase was confirmed. A transition from the insulating to a metallic phase is found at high magnetic fields, where the zero-temperature asymptotic value of the resistance being equal to h/e^2.

Influence of mechanisms of nonequilibrium quasiparticle scattering on the properties of phase-slip centers

Electrical characteristics of homogeneous superconducting tin films with phase-slip centers (PSC) and with structures of the superconductor–normal metal–superconductor (SNS) type are studied experimentally. It is found that near the superconducting transition temperature, elastic scattering of nonequilibrium quasiparticles can play a significant role in PSC by reducing the effective charge disbalance time, and hence the diffusion length for quasiparticles as compared to similar processes in the SNS structure. The application of an external electromagnetic field of frequency ∼1010 Hz in the vicinity of the superconducting transition temperature reduces the resistance of the SNS structure to zero, indicating a superconducting transition in the N region. An increase in the transport current results in the formation of the first PSC just in this region. Non-Josephson oscillations appear in this region at certain temperatures, currents, and radiation power.

On dynamic regime of phase-slip lines in wide superconducting films

Resistive current states in wide superconducting tin films with a dynamic phase slippage have been investigated experimentally. We studied simultaneously the processes of the phase-slip lines' evolution and the heat exchange between the sample and a surrounding medium. Steps on the voltage dependence of the thermal flow from the sample were observed. They demonstrate an instantaneous decrease of the thermal flow from the sample at voltages when a destruction of the stable phase-slip state occurs due to the thermal instability. This phenomenon was analyzed in view of the theory predicting the pulsed phonon fluxes from the active phase-slip regions. We explained the discovered peculiarities by a fast disappearance of the phase-slip lines' structure that operated as a source of phonons. The expression for the thermal energy drop was derived for our case. A comparison of the experimental and calculated dependencies demonstrated a qualitative coincidence.

On excess conductivity of wide superconducting films with phase slippage

Properties of wide superconducting tin films in the resistive current state with phase slippage have been studied experimentally. We have observed a region of excess conductivity on the current-voltage characteristics of the samples. Experimental results were discussed in view of the theory establishing the relationship between an interference term of the total current and excess current in weak superconductors. We derived the equation to evaluate the inelastic scattering time of superconductor τ E using the excess current of wide films with phase slippage. Our τ E values are in a good agreement with those obtained by other methods.

Tunnel junction with the thin-film electrodes in a resistive phase-slip state

Resistive current state of wide superconducting tin films with the phase-slip lines has been studied experimentally. We measured the current-voltage characteristics (CVCs) of the junction with the electrodes-wide films in the resistive phase-slip state. Features related to the formation and evolution of the phase-slip lines in wide films have been discovered on the CVCs of the tunnel junction.

Current-driven plasma instabilities in superconducting systems

Experimental evidence is presented for the existence of a current-driven plasma instability in superconducting films. Anomalous dissipation has been observed in a current-carrying superconducting YBa 2Cu 3O 7 microbridge film which is separated from a superconducting tin film by an insulating spacer layer. This is consistent with the instability predicted for a system of two superconductors separated by an ideal interface and carrying oppositely directed currents. New calculations show that in spite of the presence of the insulating layer, the instability should still occur due to proximity effects.

The cooldown process of superconducting tin films in contact with cold sapphire

Momentary current voltage characteristics of tin-tunnel junctions evaporated on an a-cut sapphire were measured during the cooldown process which starts immediately after stopping the steady-state heating from a constantan heater which was evaporated at the top of the tin diode with a SiO x -layer in between. The measured momentary current-voltage characteristics cannot be distinguished within experimental error from thermal characteristics taken at a higher substrate temperature. Hence, at the tunnel junction, the electron system can be described at each time during the cooldown process by an electron temperatureT 1 el (t). It is shown that the decay time of the electron system is far too long to be in accordance with a one-temperature model combined with the acoustic mismatch model, which has been verified in pulse-heating experiments during the heatup and steady-state time interval in a preceeding paper.1 We observed a pure exponential decay of the electron temperature starting at the onset of superconductivity atT C =3.72 K and ending at the bath temperature which is usually aboutT C /2. The measured decay constant τ is within a 10% maximum deviation equal to the effective recombination time τeff of the injected quasiparticles, well-known from other investigations. We found the same dependence of τ on bath temperature, on the thickness of the tin film, and on oxygen content as was observed for τeff in earlier experiments. Simple models for the behavior of the electron and phonon system during the cooldown process are investigated to explain this unexpected decay behavior at temperatures far away from the bath temperature.

Quasiparticle recombination time of superconducting tin films in a parallel magnetic field

The influence of a parallel magnetic field on the quasiparticle recombination time of superconducting tin tunneling junctions has been determined experimentally. The results are compared with the predictions of a theoretical model that takes into account the modification of the quasiparticle density of states by the applied magnetic field.

THE INHOMOGENEOUS STATES OF SUPERCONDUCTING TIN FILMS UNDER PHONON INJECTION

Using Sn-I-Sn-I-Sn-I-Sn triple tunnel junctions, some spatial inhomogeneous states in superconducting tin films were observed under strong phonon injection.