arXiv subjects

Ekta Bhatia

Publications and source records attributed to Ekta Bhatia.

13 recordsLinked to original sources

arXiv2026

Characterization of Hydroxyls in Surface Oxide of Superconducting Tantalum and Their Mitigation in Quantum Circuits

Recently, tantalum (Ta) has gained attention in superconducting quantum circuits due to the longer coherence times achieved when replacing niobium (Nb) in capacitor pads. Previous literature shows that surface oxides that form upon ambient exposure on superconducting metals such as Ta, Al, and Nb host two-level system (TLS) defects, which are a leading source of microwave loss and decoherence. While the surface oxides of Nb and Al have been extensively studied, Ta oxides remain less well understood. Using secondary ion mass spectrometry of alpha-Ta films deposited at 300 mm wafer scale, we show for the first time that hydroxyls accumulate in the Ta suboxide region above the underlying Ta. Angle-resolved X-ray photoelectron spectroscopy shows that the surface region is dominated by Ta2O5, with sub-stoichiometric TaOx present in between the Ta2O5 and underlying Ta. The thickness of the tantalum oxide is confirmed by transmission electron microscopy. We demonstrate that [OH] incorporation can be suppressed by replacing the native oxide with an oxide formed during chemical mechanical planarization of alpha-Ta films. Our findings support the hypothesis that TLS defects are non-uniform within the oxide thickness and suggest hydroxyls as a probable molecular origin of these loss channels. Furthermore, we show the feasibility of plasma nitridization as a method to decrease hydroxyl loading on alpha-Ta surfaces. The modulation of hydroxyl content through surface engineering of alpha-Ta can enable the fabrication of more robust, high-coherence superconducting quantum circuits by addressing a potential TLS source.

cond-mat.supr-con↗

arXiv2026

Tantalum Damascene Coplanar Waveguide Resonators Fabricated Using 300 mm Scale Processes

Surface oxides contribute to losses in superconducting transmon devices resulting in degraded performance. We explore the use of the damascene process to replace the sidewall native oxide of a device with a metal/substrate interface. We simulate sidewall oxidation by burying an oxide layer during fabrication. We observe a modest improvement between the two types of devices, which is suggestive of a reduction in the surface participation ratio.

quant-ph↗

arXiv2026

Temperature-Dependent Dielectric Function of Tantalum Nitride Formed by Atomic Layer Deposition for Tunnel Barriers in Josephson Junctions

We report the dielectric functions of insulating tantalum nitride (TaN) films, deposited using atomic layer deposition (ALD) on 300 mm Si/SiO2 substrates, to demonstrate their suitability as tunnel barriers in tantalum-based Josephson junctions (JJ) for superconducting quantum circuits. The temperature-dependent ellipsometric angles were measured using ALD TaN films with nominal thicknesses of 13 nm and 25 nm at an incidence angle of 70 degrees, across photon energy ranges of 0.03 eV to 0.7 eV (80-300 K) and 0.5 eV to 6.5 eV (80-600 K). This data was used to develop a dispersion model for insulating ALD TaN films that incorporates a Tauc-Lorentz oscillator with a band gap of 1.5-1.8 eV to model the interband optical transitions. The extracted dielectric function of ALD TaN films shows an insulating behavior (mid-infrared transparency) at all temperatures and for both film thicknesses tested. ALD TaN does not exhibit infrared absorption due to free carriers, even at elevated temperatures, demonstrating its insulating nature, which is required for the tunnel barrier of the JJ in quantum applications. The results of transmission electron microscopy, including selected area electron diffraction, and X-ray diffraction are also discussed. Sputter depth-profile X-ray photoelectron spectroscopy (XPS) shows an N/Ta ratio of ~1.2 throughout the film. The lower band gap, low roughness, and thermal stability of ALD TaN compared to AlOx suggest the possibility of fabricating JJs with thicker barriers while achieving critical current densities required for qubits, better control of thickness and composition, reduced topography, and resistance to aging.

cond-mat.supr-con↗

arXiv2026

Effects of Integrated Heatsinking on Superconductivity in Tantalum Nitride Nanowires at the 300 Millimeter Scale

We report the superconducting properties of tantalum nitride (TaN) nanowires and TaN/copper (TaN/Cu) bilayer nanowires fabricated on 300 mm silicon wafers using CMOS-compatible processes. We evaluate how an integrated Cu heatsink modifies the superconducting response of TaN nanowires by improving thermal dissipation without significantly compromising key superconducting parameters. Through analysis of hysteresis in current-voltage curves, we demonstrate that Cu integration improves heat dissipation, supporting expectations of faster reset times in superconducting nanowire single-photon detectors (SNSPDs), consistent with enhanced heat transfer away from the hot spot. Using the Skocpol-Beasley-Tinkham (SBT) hotspot model, we quantify the Cu-enabled improvement in heat transfer as an approximately 100x increase in the SBT slope parameter beta and effective interfacial heat-transfer efficiency compared to TaN nanowires. The near-unity ratio of critical to retrapping current in TaN/Cu bilayer nanowires provides another evidence of efficient heat removal enabled by the integrated Cu layer. Our results show a zero-temperature Ginzburg-Landau coherence length of 7 nm and a critical temperature of 4.1 K for 39 nm thick TaN nanowires. The nanowires show <5% variation in critical dimensions, room-temperature resistance, residual resistance ratio, critical temperature, and critical current across the 300 mm wafer for all measured linewidths, demonstrating excellent process uniformity and scalability. These results indicate the trade-offs between superconducting performance and heat-sinking efficiency in TaN/Cu bilayer nanowires. They also underscore the viability of wafer-scale fabrication for fast, large-area SNSPD arrays for applications in photonic quantum computing, cosmology, and neuromorphic computing devices.

cond-mat.supr-con↗

arXiv2025

Ta-based Josephson junctions using insulating ALD TaN tunnel barriers

Josephson junctions form the core circuit element in superconducting quantum computing circuits, single flux quantum digital logic circuits, and sensing devices such as SQUIDs. Aluminum oxide has typically been used as the tunnel barrier. Its formation by exposure to low oxygen pressures at room temperature for short periods of time makes it susceptible to aging and limits the thermal budget of downstream processes. In this paper, we report the first demonstration of α-Ta/insulating TaN/a-Ta superconductor/insulator/superconductor Josephson junctions fabricated on 300 mm wafers using CMOS-compatible processes. The junctions were fabricated on high-resistivity silicon substrates using standard processes available at 300 mm scale, including 193 nm optical lithography, ALD of TaN in a cluster tool, and chemical mechanical planarization to enable highly planar interfaces. Junction areas ranging from 0.03 um2 to 9 um2 with ALD TaN thickness between 2 nm and 7 nm were characterized. A critical current density of 76 uA/um2 was observed in junctions using 4 nm ALD TaN in the tunnel barrier. The dependence of Jc on ALD TaN layer thickness is analyzed, and the influence of junction geometry, packaging, and temperature on I-V characteristics is discussed. Junctions were retested after a period of 4 months to quantify junction aging. The potential of this novel material system and a 300 mm superconducting junction process flow to fabricate thermally and environmentally stable junctions is discussed. The vision of a Superconducting Quantum Process Design Kit for a Multi-Project Wafer program to enable rapid development and proliferation of superconducting quantum and digital digital logic systems is presented. This work represents the first step towards establishing such a Quantum Foundry, providing access to high quality qubits and single-flux quantum logic circuits at 300 mm wafer scale.

cond-mat.supr-con↗

arXiv2024

Oxidation Kinetics of Superconducting Niobium and a-Tantalum in Atmosphere at Short and Intermediate Time Scales

The integration of superconducting niobium and tantalum into superconducting quantum devices has been increasingly explored over the past few years. Recent developments have shown that two-level-systems (TLS) in the surface oxides of these superconducting films are a leading source of decoherence in quantum circuits, and understanding the surface oxidation kinetics of these materials is key to enabling scalability of these technologies. We analyze the nature of atmospheric oxidation of both niobium and a-tantalum surfaces at time scales relevant to fabrication, from sub-minute to two-week atmospheric exposure, employing a combination of x-ray photoelectron spectroscopy and transmission electron microscopy to monitor the growth of the surface oxides. The oxidation kinetics are modeled according to the Cabrera-Mott model of surface oxidation, and the model growth parameters are reported for both films. Our results indicate that niobium surface oxidation follows a consistent regime of inverse logarithmic growth for the entire time scale of the study, whereas a-Ta surface oxidation shows a clear transition between two inverse logarithmic growth regimes at time t = 1 hour, associated with the re-coordination of the surface oxide as determined by x-ray photoelectron spectroscopy analysis. Our findings provide a more complete understanding of the differences in atmospheric surface oxidation between Nb and a-Ta, particularly at short time scales, paving the way for the development of more robust fabrication control for quantum computing architectures.

cond-mat.supr-con↗

arXiv2023

Engineering of Niobium Surfaces Through Accelerated Neutral Atom Beam Technology For Quantum Applications

A major roadblock to scalable quantum computing is phase decoherence and energy relaxation caused by qubits interacting with defect-related two-level systems (TLS). Native oxides present on the surfaces of superconducting metals used in quantum devices are acknowledged to be a source of TLS that decrease qubit coherence times. Reducing microwave loss by surface engineering (i.e., replacing uncontrolled native oxide of superconducting metals with a thin, stable surface with predictable characteristics) can be a key enabler for pushing performance forward with devices of higher quality factor. In this work, we present a novel approach to replace the native oxide of niobium (typically formed in an uncontrolled fashion when its pristine surface is exposed to air) with an engineered oxide, using a room-temperature process that leverages Accelerated Neutral Atom Beam (ANAB) technology at 300 mm wafer scale. This ANAB beam is composed of a mixture of argon and oxygen, with tunable energy per atom, which is rastered across the wafer surface. The ANAB-engineered Nb-oxide thickness was found to vary from 2 nm to 6 nm depending on ANAB process parameters. Modeling of variable-energy XPS data confirm thickness and compositional control of the Nb surface oxide by the ANAB process. These results correlate well with those from transmission electron microscopy and X-ray reflectometry. Since ANAB is broadly applicable to material surfaces, the present study indicates its promise for modification of the surfaces of superconducting quantum circuits to achieve longer coherence times.

physics.app-ph↗

arXiv2023

Chemical Mechanical Planarization for Ta-based Superconducting Quantum Devices

We report on the development of a chemical mechanical planarization (CMP) process for thick damascene Ta structures with pattern feature sizes down to 100 nm. This CMP process is the core of the fabrication sequence for scalable superconducting integrated circuits at 300 mm wafer scale. This work has established the elements of the various CMP-related design rules that can be followed by a designer for the layout of circuits that include Ta-based coplanar waveguide resonators, capacitors, and interconnects for tantalum-based qubits and single flux quantum (SFQ) circuits. The fabrication of these structures utilizes 193 nm optical lithography, along with 300 mm process tools for dielectric deposition, reactive ion etch, wet-clean, CMP and in-line metrology, all tools typical for a 300 mm wafer CMOS foundry. Process development was guided by measurements of physical and electrical characteristics of the planarized structures. Physical characterization such as atomic force microscopy across the 300 mm wafer surface showed local topography was less than 5 nm. Electrical characterization confirmed low leakage at room temperature, and less than 12% within wafer sheet resistance variation, for damascene Ta line-widths ranging from 100 nm to 3 μm. Run-to-run reproducibility was also evaluated. Effects of process integration choices including deposited thickness of Ta are discussed.

physics.app-ph↗

arXiv2021

Unconventional domain wall magnetoresistance of patterned Ni/Nb bilayer structures below superconducting transition temperature of Nb

Scattering of spin-up and spin-down electrons while passing through a ferromagnetic domain wall leads to an additional resistance for transport current, usually observed prominently in constricted magnetic structures. In this report, we use the resistance of the domain wall as a probe to find indirect signatures of the theoretically predicted spin-singlet supercurrent to spin-triplet supercurrent conversion effect of ferromagnetic domain walls. Here we examine the domain wall induced resistance in Ni stripe in a bilayer Ni/Nb geometry in the normal state and in the superconducting state of Nb. By making a 6um wide gap in the top Nb layer we routed the transport current through the Ni layer in the normal state and in the superconducting state of Nb. In the normal state of Nb, in-field transport measurements showed a clear domain wall magneto-resistance (DWMR) peak near the coercive field, where the domain wall density is expected to be maximum. Interestingly, however, below the superconducting transition temperature of Nb, the DWMR peak of the Ni layer showed a sharp drop in the field range where the number of domain walls becomes maximum. This observation may be a possible signature of magnetic domain wall induced spin-triplet correlations in the Ni layer due to the direct injection of spin-singlet Cooper pairs from Nb into the magnetic domain walls.

cond-mat.supr-con↗

arXiv2020

Study of magnetization reversal in Neel and Bloch regime of Ni and Py stripes using Kerr microscopy

We present a systematic study of the magnetization reversal of nickel and permalloy micro-stripes with Neel and Bloch domain walls using Kerr microscopy. Magnetic field driven domain propagation was observed from higher width to lower width stripes for magnetic fields applied along the length of micro-stripes. Stripe like domains were observed with nucleation starting in lower width region followed by their propagation to higher width regions for magnetic fields applied along the width of micro-stripes. The comparison of magnetization reversal in Bloch and Néel domain wall regime showed higher domain wall density in Bloch regime for both nickel and permalloy stripes.

cond-mat.mes-hall↗

arXiv2020

Magnetic field tunable superconducting transition in Nb/Co/Py/Nb exchange spring multilayers

Over the last decade it has been shown that magnetic non-collinearity at a s-wave superconductor/ferromagnet interface is a key ingredient for spin-singlet to spin-triplet pair conversion. This has been verified in several synthetic non-collinear magnetic structures. A magnetically soft and hard ferromagnetic layer combination in a bi-layer structure can function as a field tunable non-collinear magnetic structure which may offer magnetic-field tuneability of singlet-to-triplet pair conversion. From magnetization measurements of Nb/Co/Py/Nb multilayers we demonstrate a reversible enhancement of the superconducting critical temperature of 400 mK by measuring Tc with and without a non-collinear magnetic structure between Co and Py. The sensitivity of Tc in these structures offers the potential for realizing magnetic field tunable Josephson junctions in which pair conversion and Josephson critical currents are controllable using modest magnetic fields.

cond-mat.supr-con↗

arXiv2019

Magnetic-field-orientation-dependent triplet supercurrents in Josephson junctions with symmetric and asymmetric exchange-spring interfaces

Josephson junctions with s-wave superconductors (S) and multiple ferromagnetic (F) layers carry spin-triplet supercurrents in the presence of magnetically inhomogeneous spin-mixer interfaces. Here, we report magnetic Josephson junctions with exchange-spring and double exchange-spring ("spin-mixer") Co/Py interfaces. At the Co/Py interface, exchange coupling is strong with respect to the magnetic anisotropy of Py and so, depending on the direction and magnitude of an external magnetic field, a non-collinear magnetic structure forms in Py creating the necessary magnetic inhomogeneity for singlet-to-triplet pair conversion. We detect supercurrents through Py with a thickness exceeding 10 nm, which is much larger than the singlet pair coherence length, suggesting the propagation of magnetically tuneable triplet supercurrents.

cond-mat.supr-con↗

arXiv2019

Domain wall induced modulation of low field H-T phase diagram in patterned superconductor-ferromagnet stripes

We present a systematic study of the magnetic domain wall induced modulation of superconducting transition temperature (Tc) in Nb/Ni bilayer stripes. By varying the thickness of the Ni layer from 20 nm to 100 nm we have been able to measure the low field Tc-H phase diagram spanning the Neel domain wall and Bloch domain wall range of thicknesses. Micromagnetic simulations and magnetic force microscopy measurements confirmed a stronger out-of-plane stray field in the Bloch domain walls compared to the Neel walls. A suppression in Tc was observed in the magnetization reversal region of the Ni film, the magnitude of which followed linearly to the strength of the out-of-plane stray field due to the domain walls. The magnitude of the stray field was quantified by comparing the Tc of the suppressed region of H-Tc phase diagrams with the unaffected part of the H-Tc curve. With Bloch domain walls a change in Tc of more than 60 mK was observed which is much more compared to the earlier reports. We believe that the narrow stripe geometry of the bilayers and the transverse external field maximized the effect of the domain walls in the Ni layer on the overlying superconducting film, leading to a larger change in Tc. This observation may be useful for domain wall controlled switching devices in superconducting spintronics.

cond-mat.supr-con↗