Titanium Nitride as a New Prospective Material for NanoSQUIDs and Superconducting Nanobridge Electronics

Faley M.I., Bikulov T.I., Bosboom V., Golubov A.A., Dunin-Borkowski R.E.

Abstract Nanometer-scale superconducting quantum interference devices (nanoSQUIDs) were fabricated within a distance of 1 µm from the corners of 2 × 2 × 0.05 mm Si cantilevers that are intended for use in a scanning nanoSQUID microscope. The nanoSQUIDs contained Josephson junctions (JJs) in the form of Nb-based nanobridges, which had widths down to 10 nm and were patterned using hydrogen silsesquioxane negative resist. Numerical simulations of the superconducting current and the spatial distribution of the order parameter in the nanobridge JJs and the nanoSQUID, as well as the current–phase relationship in the nanobridge JJs, were performed according to Ginzburg–Landau equations on one-dimensional and two-dimensional grids. Bulk micromachining of the Si cantilever was performed using reactive ion etching with SF6 gas through masks of nLOF 2020 photoresist from the front side and a quartz shadow mask from the back side of the substrate. An etch rate of 6 µmmin−1 for Si was achieved for a power of 300 W of the inductively coupled SF6 plasma. The nanoSQUIDs exhibited non-hysteretic current–voltage characteristics on the cantilever. The estimated spin sensitivity of 48 µ B (√Hz)−1 is sufficient for use of such a nanoSQUID as a magnetic field sensor for studying nanoscale objects, with a projected total distance to the object of below 100 nm.

Rodrigo R., Faley M.I., Dunin-Borkowski R.E.

Abstract We have developed nanoSQUIDs with Josephson junctions in the form of Nb nanobridges, whose thickness, width and length are in the order of the superconducting coherence length in Nb thin films, which is 15 nm at 4.2 K in 20-nm-thick films. The thin Nb films were deposited using pulsed DC magnetron sputtering. A 30-nm-thick mask of PMMA resist was formed by electron beam exposure using a dose of 50 mC/cm2 at 100 kV, at which PMMA operates as a high-resolution negative resist. Compared to the previously used HSQ resist, PMMA has a much better availability, lower health risk, a longer shelf life and a simpler development procedure, while maintaining sufficient resolution. Both Nb nanobridges with widths down to 10 nm and nanoSQUIDs with the incorporated nanobridges were fabricated using reactive ion etching with pure SF6 gas. They show non-hysteretic I(V)-characteristics and a modulation of the critical current at both directly injected or externally applied magnetic field fluxes. The obtained spin sensitivity of S n 1 / 2 ≅ 135 μ B / √ H z is very promising for the use of the nanoSQUIDs as magnetic field sensors for the investigation of nanoscale objects.

Shishkin A.G., Skryabina O.V., Gurtovoi V.L., Dizhur S.E., Faley M.I., Golubov A.A., Stolyarov V.S.

We have developed planar nanoSQUID with nanobridge-type Josephson junctions based on the oxidation resistant and high H c2 MoRe alloy. The objective of the research was to reduce size of the SQUID loop with the aim being to reduce magnetic flux noise and improve the spatial resolution of the SQUID sensors. Employing RF-magnetron sputtering, electron-beam lithography, and reactive ion etching in CHF3 + O2 plasma using Al hard masks, we have realized nanoSQUIDs with Josephson junctions in the form of 30 - 50 nm wide nanobridges and an effective magnetic flux capture radius of ∼ 95 nm. The critical temperature of the fabricated devices was T c = 7.9 K. The I(V)-characteristics demonstrated critical current I 0≃ 114 µA at 4.2 K and modulation period in magnetic fields of ∼ 700 Oe.

Torgovkin A., Chaudhuri S., Ruhtinas A., Lahtinen M., Sajavaara T., Maasilta I.J.

Shelly C.D., See P., Ireland J., Romans E.J., Williams J.M.

This paper investigates the feasibility of using weak link nanobridges as Josephson junction elements for the purpose of creating Josephson circuits. We demonstrate the development of a single-step electron beam lithography procedure to fabricate niobium nanobridges with dimensions down to . The single-step process facilitates fabrication that is scalable to complex circuits that require many junctions. We measure the IV-characteristics (IVC) of the nanobridges between temperatures of and and find agreement with numerical simulations and the analytical resistively shunted junction (RSJ) model. Furthermore, we investigate the behaviour of the nanobridges under rf irradiation and observe the characteristic microwave-induced Shapiro steps. Our simulated IVC under rf irradiation using both the RSJ model and circuit simulator JSIM are in agreement with the experimental data. As a potential use of nanobridges in circuits requiring many junctions, we investigate the theoretical performance of a nanobridge-based Josephson comparator circuit using JSIM.

Makise K., Sun R., Terai H., Wang Z.

We investigated full-epitaxial TiN/AlN/TiN Josephson junctions on MgO substrates for superconducting qubit applications. The critical temperature (T C ) of TiN film is relatively low at around 5 K, but its lattice constant is about 0.424 nm, which is close to the lattice constant of MgO of 0.421 nm. TiN and AIN films were prepared by DC magnetron sputtering in a load-lock sputtering system with an ultra-high vacuum chamber. The deposition temperature was varied from ambient temperature to 1073 K. In XRD analysis, (200) peaks were observed in both the TiN single layer film and the TiN/AIN/TiN trilayer film. No other XRD peaks were observed in the single layer or trilayer films. The lattice constant of TiN was determined to be 0.4242 nm from XRD analysis, close to the value of 0.4212 nm for MgO. The 150 nm-thick single-layer TiN film on the MgO substrate showed a T C of 5.3 K and a resistivity of 3.5 μΩcm at 10 K. Based on these epitaxial TiN films, we fabricated TiN/AIN/TiN Josephson junctions and measured their current-voltage characteristics. At 1.9 K, the electrical parameters of junctions with J C = 50 A/cm 2 showed that the gap voltage and the ratio of R sg /R N were about 2.5 mV and 2.1, respectively.

Chang J.B., Vissers M.R., Córcoles A.D., Sandberg M., Gao J., Abraham D.W., Chow J.M., Gambetta J.M., Beth Rothwell M., Keefe G.A., Steffen M., Pappas D.P.

We demonstrate enhanced relaxation and dephasing times of transmon qubits, up to ∼60 μs, by fabricating the interdigitated shunting capacitors using titanium nitride (TiN). Compared to qubits made with lift-off aluminum deposited simultaneously with the Josephson junction, this represents as much as a six-fold improvement and provides evidence that surface losses from two-level system (TLS) defects residing at or near interfaces contribute to decoherence. Concurrently, we observe an anomalous temperature dependent frequency shift of TiN resonators, which is inconsistent with the predicted TLS model.

Wenner J., Barends R., Bialczak R.C., Chen Y., Kelly J., Lucero E., Mariantoni M., Megrant A., O’Malley P.J., Sank D., Vainsencher A., Wang H., White T.C., Yin Y., Zhao J., et. al.

Losses in superconducting planar resonators are presently assumed to predominantly arise from surface-oxide dissipation, due to experimental losses varying with choice of materials. We model and simulate the magnitude of the loss from interface surfaces in the resonator and investigate the dependence on power, resonator geometry, and dimensions. Surprisingly, the dominant surface loss is found to arise from the metal-substrate and substrate-air interfaces. This result will be useful in guiding device optimization, even with conventional materials.

Sage J.M., Bolkhovsky V., Oliver W.D., Turek B., Welander P.B.

Superconducting coplanar waveguide (SCPW) resonators have a wide range of applications due to the combination of their planar geometry and high quality factors relative to normal metals. However, their performance is sensitive to both the details of their geometry and the materials and processes that are used in their fabrication. In this paper, we study the dependence of SCPW resonator performance on materials and geometry as a function of temperature and excitation power. We measure quality factors greater than 2 × 106 at high excitation power and 6 × 105 at a power comparable to that generated by a single microwave photon circulating in the resonator. We examine the limits to the high excitation power performance of the resonators and find it to be consistent with a model of radiation loss. We further observe that while in all cases the quality factors are degraded as the temperature and power are reduced due to dielectric loss, the size of this effect is dependent on resonator materials and geometry. Finally, we demonstrate that the dielectric loss can be controlled in principle using a separate excitation near the resonance frequencies of the resonator.

Vissers M.R., Gao J., Wisbey D.S., Hite D.A., Tsuei C.C., Corcoles A.D., Steffen M., Pappas D.P.

Thin films of TiN were sputter-deposited onto Si and sapphire wafers with and without SiN buffer layers. The films were fabricated into RF coplanar waveguide resonators, and internal quality factor measurements were taken at millikelvin temperatures in both the many photon and single photon limits, i.e. high and low power regimes, respectively. At high power, internal quality factors ($Q_i$'s) higher than $10^7$ were measured for TiN with predominantly a (200)-TiN orientation. Films that showed significant (111)-TiN texture invariably had much lower $Q_i$'s, on the order of $10^5$. Our studies show that the (200)-TiN is favored for growth at high temperature on either bare Si or SiN buffer layers. However, growth on bare sapphire or Si(100) at low temperature resulted in primarily a (111)-TiN orientation. Ellipsometry and Auger measurements indicate that the (200)-TiN growth on the bare Si substrates is correlated with the formation of a thin, $\approx 2$ nm, layer of SiN during the pre-deposition procedure. In the single photon regime, $Q_i$ of these films exceeded $8\times10^5$, while thicker SiN buffer layers led to reduced $Q_i$'s at low power.

Troeman A.G., van der Ploeg S.H., Il’Ichev E., Meyer H.-., Golubov A.A., Kupriyanov M.Y., Hilgenkamp H.

The current-phase relationship has been measured as a function of temperature for niobium nanobridges with different widths. A deformation from Josephson-like sinusoidal characteristics at high temperatures to sawtooth shaped curves at intermediate and multivalued relationships at low temperatures was observed. Based on this, possible hysteresis in the current-voltage characteristics of niobium nanobridge superconducting quantum interference devices can be attributed to phase slippage.

Vershinin N., Filonov K., Straumal B., Gust W., Wiener I., Rabkin E., Kazakevich A.

Decorative and protective TiN coatings were vacuum arc deposited in an industrial installation ‘Nikolay’ allowing coating of strips with maximum size 2100×1300×8 mm. Titanium nitride (TiN) coatings on stainless steel strips have been characterized in terms of microstructure and corrosion resistance . The vacuum arc deposited TiN coatings have a higher corrosion resistance compared with TiN coatings on steel produced by plasma assisted chemical vapour deposition , glow discharge deposition, direct current magnetron sputtering or magnetron sputter deposition .

Oh U.C., Je J.H.

The effects of strain energy on the preferred orientation of TiN thin films were investigated. In the TiN film deposited by plasma-enhanced chemical-vapor deposition with a power of 50 W, the overall energy of the film mainly depended on the surface energy because its strain energy was relatively small. The preferred orientation of the film corresponded to the plane with the lowest surface energy, i.e., (200). However, in the TiN film deposited by rf sputtering with a power of 200 W, the overall energy of the film was largely controlled by strain energy due to its large strain energy, and its growth orientation corresponded to the plane with the lowest strain energy, i.e., (111). Furthermore, the preferred orientation of the TiN film was changed from (200) to (111) with the film thickness. It is considered that this phenomenon is due to the increase of strain energy with its thickness.

Narayan J., Tiwari P., Chen X., Singh J., Chowdhury R., Zheleva T.

We report epitaxial growth of TiN films having low resistivity on (100) silicon substrates using pulsed laser deposition method. The TiN films were characterized using x-ray diffraction, Rutherford backscattering, four-point-probe ac resistivity, high resolution transmission electron microscopy and scanning electron microscopy techniques and epitaxial relationship was found to be 〈100〉 TiN ∥ 〈100〉 Si. TiN films showed 10%–20% channeling yield. In the plane, four unit cells of TiN match with three unit cells of silicon with less than 4.0% misfit. This domain matching epitaxy provides a new mechanism of epitaxial growth in systems with large lattice misfits. Four-point-probe measurements show characteristic metallic behavior of these films as a function of temperature with a typical resistivity of about 15 μΩ cm at room temperature. Implications of low-resistivity epitaxial TiN in silicon device fabrication are discussed.

Pelleg J., Zevin L.Z., Lungo S., Croitoru N.

Sputter-deposited TiN thin films on glass substrate were investigated by X-ray diffraction analysis. The lattice parameter determined on the basis of (200) and (220) peaks is smaller than that determined on the basis of (111) peaks. A decrease in lattice parameter with increasing temperature was observed on annealing. The films exhibited preferred orientation. The tendency towards a specific preferred orientation is discussed on the basis of strain and surface energies. At low substrate temperatures and/or at small film thicknesses surface energy controls growth and a (100) preferred orientation is expected. At large film thicknesses and at high substrate temperatures the strain energy predominates and the (111) orientation is expected.

Faley M.I., Vas J.V., Lu P.-., Dunin-Borkowski R.E.

Colletta G., Johny S., Collins J.A., Casaburi A., Weides M.

In this work, we present a numerical model specifically designed for 3D multilayer devices, with a focus on nanobridge junctions and coplanar waveguides. Unlike existing numerical models, ours does not approximate the physical layout or limit the number of constituent materials, providing a more accurate and flexible design tool. We calculate critical currents, current–phase relationships, and the energy gap where relevant. We validate our model by comparing it with published data. Through our analysis, we found that using multilayer films significantly enhances control over these quantities. For nanobridge junctions in particular, multilayer structures improve qubit anharmonicity compared to monolayer junctions, offering a substantial advantage for qubit performance. For coated multilayer microwave circuits, it allows for better studies of the proximity effect, including their effective kinetic inductance.

Bugu S., Biradar S., Blake A., Liu C., Myronov M., Duffy R., Fagas G., Petkov N.

Skryabina O.V., Bakurskiy S.V., Ruzhickiy V.I., Shishkin A., Klenov N.V., Soloviev I.I., Kupriyanov M.Y., Stolyarov V.S.

Abstract We study the effect of electrode width on superconducting current transport in Nb–Au–Nb Josephson bridges. The critical current as well as the normal bridge resistance drop with decreasing electrode width on scales of a few μ m , which are orders of magnitude larger than the estimated coherence length of the Au strip. We consider several physical reasons for such an anomalous influence of the width W of the superconducting electrode on the critical current I c (AIWIc) and provide model fits for the resistive and superconducting properties of the bridges. The smooth dependence of the Nb–Au–Nb bridge parameters on the electrode width can be used to optimize the design of superconducting devices for specific applications.

Yadav S., Gajar B., Aloysius R.P., Sahoo S.

The interplay between superconducting fluctuations (SFs) and weak localization (WL) has been probed by temperature dependent resistance [R(T)] and magnetoresistance (MR) measurements in two-dimensional disordered superconducting TiN thin films.

Fell J., Bickel S., Fiedler C., Delan A., Junghähnel M.

Yadav S., Saravanan M.P., Sahoo S.

AbstractThe association of quantum Griffiths singularity (QGS) to the magnetic-field-induced superconductor-metal transition predicts the unconventional diverging behaviour of dynamical critical exponent in low disorder crystalline two-dimensional superconductors. But whether this state exists in the superconducting systems exhibiting superconductor-insulator transition remains elusive. Here, we report the emergence of quantum Griffiths singularity in ultrathin disordered TiN thin films with more than two orders of magnitude variation in their normal state resistance. For both superconductor-metal transition and superconductor-insulator transition types, a diverging critical exponent is observed while approaching the quantum phase transition. Further, the magnetoresistance isotherms obey a direct activated scaling governed by an infinite-randomness fixed critical point. Finally, this work establishes the robustness of the QGS phenomenon towards a wide range of temperature and also towards a wide range of disorder strength as correlated with the normal state resistance.

Dang M.N., Singh S., King H.J., Navarro-Devia J.H., Le H., Pattison T.G., Hocking R.K., Wade S.A., Stephens G., Papageorgiou A., Manzano A., Wang J.

This study investigates the mechanical properties, surface integrity, and chemical configuration of PVD-coated high-speed steel (HSS) cutting tools, with a particular focus on titanium nitride (TiN) and titanium aluminium nitride (TiAlN) coatings. A range of characterisation methodologies were employed to examine the impact of pre-coating surface conditions on the resulting coatings. This impact includes the effects of gas bubble production and unequal distribution of elements, which are two unwanted occurrences. Notwithstanding these difficulties, coatings applied on surfaces that were highly polished exhibited more consistency in their mechanical and elemental characteristics, with a thickness ranging from 2 to 4 µm. The study of mechanical characteristics confirms a significant increase in hardness, from an initial value of roughly 1000 HV0.5 for untreated tools to 1300 HV0.5 for tools with physical vapour deposition (PVD) coatings. Although PVD coatings produced on an industrial scale might not exceed the quality of coatings manufactured in a laboratory, they do offer substantial enhancements in terms of hardness. This study highlights the significant importance of thorough surface preparation in achieving enhanced coating performance, hence contributing to the efforts to prolong the lifespan of tools and enhance their performance even under demanding operational circumstances.

Jiang Y., Shiojiri M., Shyue J., Chen M.

This study demonstrates the atomic layer epitaxial growth of titanium nitride (TiN) with a record-low resistivity (8.2×10−8Ω∙m) by hydrogen-manipulated chemical reaction on each monolayer. The incorporation of hydrogen plasma at a specific time during atomic layer deposition is critical to activate the epitaxial growth at only 300 °C, as evidenced by X-ray diffraction pole figure and high-resolution/scanning transmission electron microscopy. The lattice misfit is relaxed within just few monolayers away from the TiN/substrate interface. An "island plus layer" mode is proposed to explain the growth of TiN, which is intrinsically composed of twins. The low resistivity and high crystallinity of the TiN epitaxial layer manifest the significant impact of the time-manipulated hydrogen tailoring on material properties. Furthermore, the hydrogen-manipulated atomic layer epitaxy benefits from large-area uniformity, low growth temperature, and no need for high-vacuum operation, which are more advantageous over molecular beam epitaxy and so exhibit promising prospects in diverse applications.

Haeri S.Z., Khiadani M., Ramezanzadeh B., Mohammed H.A., Zargar M.

Faley M.I.

The fabrication methods, principle of operation, microstructural, and electron transport properties, as well as the application of nanoscale superconducting quantum interference devices (nanoSQUIDs) are considered. NanoSQUIDs by definition have a submicrometer loop size, which limits dimensions of Josephson junctions to about 100 nm or less. This makes the use of tunnel junctions problematic, but encourages the use of nanobridges and other types of Josephson junctions with high critical current densities. The use of nitride superconductors helps to optimize the superconducting parameters and enhances the corrosion resistance of the nanobridge Josephson junctions and nanoSQUIDs. Additional passivation by a Si layer improves thermal sink and protects against the mechanical and electrical fragility of the devices. Nanosculpturing of cantilevers makes it possible to position the nanoSQUID at a distance of 10–100 nm from the objects under study in the nanoSQUID scanning measurement system. Several promising applications of nanoSQUIDs are briefly reviewed.

Ruzhickiy V., Bakurskiy S., Kupriyanov M., Klenov N., Soloviev I., Stolyarov V., Golubov A.

In this paper, we present a theoretical study of electronic transport in planar Josephson Superconductor–Normal Metal–Superconductor (SN-N-NS) bridges with arbitrary transparency of the SN interfaces. We formulate and solve the two-dimensional problem of finding the spatial distribution of the supercurrent in the SN electrodes. This allows us to determine the scale of the weak coupling region in the SN-N-NS bridges, i.e., to describe this structure as a serial connection between the Josephson contact and the linear inductance of the current-carrying electrodes. We show that the presence of a two-dimensional spatial current distribution in the SN electrodes leads to a modification of the current–phase relation and the critical current magnitude of the bridges. In particular, the critical current decreases as the overlap area of the SN parts of the electrodes decreases. We show that this is accompanied by a transformation of the SN-N-NS structure from an SNS-type weak link to a double-barrier SINIS contact. In addition, we find the range of interface transparency in order to optimise device performance. The features we have discovered should have a significant impact on the operation of small-scale superconducting electronic devices, and should be taken into account in their design.

Faley M.I., Williams J., Lu P., Dunin-Borkowski R.E.

We fabricated superconducting and ferromagnetic nanostructures, which are intended for applications in transmission electron microscopy (TEM), in a commercial sample holder that can be cooled using liquid helium. Nanoscale superconducting quantum-interference devices (nanoSQUIDs) with sub-100 nm nanobridge Josephson junctions (nJJs) were prepared at a distance of ~300 nm from the edges of a 2 mm × 2 mm × 0.05 mm substrate. Thin-film TiN-NbN-TiN heterostructures were used to optimize the superconducting parameters and enhance the oxidation and corrosion resistance of nJJs and nanoSQUIDs. Non-hysteretic I(V) characteristics of nJJs, as well as peak-to-peak quantum oscillations in the V(B) characteristics of the nanoSQUIDs with an amplitude of up to ~20 µV, were obtained at a temperature ~5 K, which is suitable for operation in TEM. Electron-beam lithography, high-selectivity reactive ion etching with pure SF6 gas, and a naturally created undercut in the Si substrate were used to prepare nanoSQUIDs on a SiN membrane within ~500 nm from the edge of the substrate. Permalloy nanodots with diameters down to ~100 nm were prepared on SiN membranes using three nanofabrication methods. High-resolution TEM revealed that permalloy films on a SiN buffer have a polycrystalline structure with an average grain dimension of approximately 5 nm and a lattice constant of ~0.36 nm. The M(H) dependences of the permalloy films were measured and revealed coercive fields of 2 and 10 G at 300 and 5 K, respectively. These technologies are promising for the fabrication of superconducting electronics based on nJJs and ferromagnetic nanostructures for operation in TEM.

Draher T., Polakovic T., Li J., Li Y., Welp U., Jiang J.S., Pearson J., Armstrong W., Meziani Z., Chang C., Kwok W., Xiao Z., Novosad V.

AbstractTitanium nitride is a material of interest for many superconducting devices such as nanowire microwave resonators and photon detectors. Thus, controlling the growth of TiN thin films with desirable properties is of high importance. This work aims to explore effects in ion beam-assisted sputtering (IBAS), were an observed increase in nominal critical temperature and upper critical fields are in tandem with previous work on Niobium nitride (NbN). We grow thin films of titanium nitride by both, the conventional method of DC reactive magnetron sputtering and the IBAS method, to compare their superconducting critical temperatures $$T_{c}$$ T c as functions of thickness, sheet resistance, and nitrogen flow rate. We perform electrical and structural characterizations by electric transport and x-ray diffraction measurements. Compared to the conventional method of reactive sputtering, the IBAS technique has demonstrated a 10% increase in nominal critical temperature without noticeable variation in the lattice structure. Additionally, we explore the behavior of superconducting $$T_c$$ T c in ultra-thin films. Trends in films grown at high nitrogen concentrations follow predictions of mean-field theory in disordered films and show suppression of superconducting $$T_c$$ T c due to geometric effects, while nitride films grown at low nitrogen concentrations strongly deviate from the theoretical models.

Yakovlev D.S., Nazhestkin I.A., Ismailov N.G., Egorov S.V., Antonov V.N., Gurtovoi V.L.

We study operation of a superconducting quantum interference devices (SQUIDs) based on a new bilayer material. They can be used for the ultra-sensitive detection of magnetic momentum at temperatures down to milliKelvin range. Typically, thermal origin hysteresis of the symmetric SQUID current-voltage curves limits operating temperatures to T>0.6Tc. We used a new bilayer material for SQUID fabrication, namely proximity-coupled superconductor/normal-metal (S/N) bilayers (aluminum 25 nm / platinum 5 nm). Because of the 5 nm Pt-layer, Al/Pt devices show nonhysteretic behavior in a broad temperature range from 20 mK to 0.8 K. Furthermore, the Al/Pt bilayer devices demonstrate an order of magnitude lower critical current compared to the Al devices, which decreases the screening parameter (βL) and improves the modulation depth of the critical current by magnetic flux. Operation at lower temperatures reduces thermal noise and increases the SQUID magnetic field resolution. Moreover, we expect strong decrease of two-level fluctuators on the surface of aluminum due to Pt-layer oxidation protection and hence significant reduction of the 1/f noise. Optimized geometry of Al/Pt symmetric SQUIDs is promising for the detection of single-electron spin flip.