15 research outputs found
Strain and bond length dynamics upon growth and transfer of graphene by NEXAFS spectroscopy from first principles and experiment
Monitoring Deformation in Graphene Through Hyperspectral Synchrotron Spectroscopy to Inform Fabrication
The promise from
graphene to produce devices with high mobilities
and detectors with fast response times is truncated in practice by
strain and deformation originating during growth and subsequent processing.
This work describes effects from graphene growth, multiple layer transfer,
and substrate termination on out of plane deformation, critical to
device performance. Synchrotron spectroscopy data was acquired with
a state-of-the-art hyperspectral large-area detector to describe growth
and processing with molecular sensitivity at wafer length scales.
A study of methodologies used in data analysis discouraged dichroic
ratio approaches in favor of orbital vector approximations and data
mining algorithms. Orbital vector methods provide a physical insight
into mobility-detrimental rippling by identifying ripple frequency
as main actor, rather than intensity; which was confirmed by data
mining algorithms, and in good agreement with electron scattering
theories of corrugation in graphene. This work paves the way to efficient
information from mechanical properties in graphene in a high throughput
mode throughout growth and processing in a materials by design approach
Microscopic Relaxation Channels in Materials for Superconducting Qubits
Despite mounting evidence that materials imperfections are a major obstacle
to practical applications of superconducting qubits, connections between
microscopic material properties and qubit coherence are poorly understood.
Here, we perform measurements of transmon qubit relaxation times in
parallel with spectroscopy and microscopy of the thin polycrystalline niobium
films used in qubit fabrication. By comparing results for films deposited using
three techniques, we reveal correlations between and grain size, enhanced
oxygen diffusion along grain boundaries, and the concentration of suboxides
near the surface. Physical mechanisms connect these microscopic properties to
residual surface resistance and through losses arising from the grain
boundaries and from defects in the suboxides. Further, experiments show that
the residual resistance ratio can be used as a figure of merit for qubit
lifetime. This comprehensive approach to understanding qubit decoherence charts
a pathway for materials-driven improvements of superconducting qubit
performance
Ultrathin Magnesium-based Coating as an Efficient Oxygen Barrier for Superconducting Circuit Materials
Scaling up superconducting quantum circuits based on transmon qubits
necessitates substantial enhancements in qubit coherence time. Among the
materials considered for transmon qubits, tantalum (Ta) has emerged as a
promising candidate, surpassing conventional counterparts in terms of coherence
time. However, the presence of an amorphous surface Ta oxide layer introduces
dielectric loss, ultimately placing a limit on the coherence time. In this
study, we present a novel approach for suppressing the formation of tantalum
oxide using an ultrathin magnesium (Mg) capping layer deposited on top of
tantalum. Synchrotron-based X-ray photoelectron spectroscopy (XPS) studies
demonstrate that oxide is confined to an extremely thin region directly beneath
the Mg/Ta interface. Additionally, we demonstrate that the superconducting
properties of thin Ta films are improved following the Mg capping, exhibiting
sharper and higher-temperature transitions to superconductive and magnetically
ordered states. Based on the experimental data and computational modeling, we
establish an atomic-scale mechanistic understanding of the role of the capping
layer in protecting Ta from oxidation. This work provides valuable insights
into the formation mechanism and functionality of surface tantalum oxide, as
well as a new materials design principle with the potential to reduce
dielectric loss in superconducting quantum materials. Ultimately, our findings
pave the way for the realization of large-scale, high-performance quantum
computing systems
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.Comment: 22 pages, 7 figures, will be submitted to Superconductor Science and
Technology Special Focus Issue Journa
Diamond Surface Functionalization via Visible Light-Driven C-H Activation for Nanoscale Quantum Sensing
Nitrogen-vacancy centers in diamond are a promising platform for nanoscale
nuclear magnetic resonance sensing. Despite significant progress towards using
NV centers to detect and localize nuclear spins down to the single spin level,
NV-based spectroscopy of individual, intact, arbitrary target molecules remains
elusive. NV molecular sensing requires that target molecules are immobilized
within a few nanometers of NV centers with long spin coherence time. The inert
nature of diamond typically requires harsh functionalization techniques such as
thermal annealing or plasma processing, limiting the scope of functional groups
that can be attached to the surface. Solution-phase chemical methods can be
more readily generalized to install diverse functional groups, but they have
not been widely explored for single-crystal diamond surfaces. Moreover,
realizing shallow NV centers with long spin coherence times requires highly
ordered single-crystal surfaces, and solution-phase functionalization has not
yet been shown to be compatible with such demanding conditions. In this work,
we report a versatile strategy to directly functionalize C-H bonds on
single-crystal diamond surfaces under ambient conditions using visible light.
This functionalization method is compatible with charge stable NV centers
within 10 nm of the surface with spin coherence times comparable to the state
of the art. As a proof of principle, we use shallow ensembles of NV centers to
detect nuclear spins from functional groups attached to the surface. Our
approach to surface functionalization based on visible light-driven C-H bond
activation opens the door to deploying NV centers as a broad tool for chemical
sensing and single-molecule spectroscopy
Chemical profiles of the oxides on tantalum in state of the art superconducting circuits
Over the past decades, superconducting qubits have emerged as one of the
leading hardware platforms for realizing a quantum processor. Consequently,
researchers have made significant effort to understand the loss channels that
limit the coherence times of superconducting qubits. A major source of loss has
been attributed to two level systems that are present at the material
interfaces. We recently showed that replacing the metal in the capacitor of a
transmon with tantalum yields record relaxation and coherence times for
superconducting qubits, motivating a detailed study of the tantalum surface. In
this work, we study the chemical profile of the surface of tantalum films grown
on c-plane sapphire using variable energy X-ray photoelectron spectroscopy
(VEXPS). We identify the different oxidation states of tantalum that are
present in the native oxide resulting from exposure to air, and we measure
their distribution through the depth of the film. Furthermore, we show how the
volume and depth distribution of these tantalum oxidation states can be altered
by various chemical treatments. By correlating these measurements with detailed
measurements of quantum devices, we can improve our understanding of the
microscopic device losses
Confinement-Induced Reduction in Phase Segregation and Interchain Disorder in Bulk Heterojunction Films
The effects of thin-film confinement on the material properties of ultrathin polymer (electron donor):fullerene (electron acceptor) bulk heterojunction films can be important for both fundamental understanding and device applications such as thin-film photovoltaics. We use variable angle spectroscopic ellipsometry and near edge X-ray absorption fine structure spectroscopy to measure the optical constants, donor–acceptor volume fraction profile, and the degree of interchain order as a function of the thickness of a poly(3-hexythiophene-2,5-diyl) and phenyl-C61-butyric acid methyl ester bulk heterojunction film. We find that as the thickness of the bulk heterojunction film is decreased from 200 nm to the thickness confinement regime (less than 20 nm), the vertical phase segregation gradient of the donor and acceptor phases becomes less pronounced. In addition, observing the change in exciton bandwidth and the shift of absorption resonances (0–0 and 0–1) relative to neat donor and acceptor films, we find that the conjugation length and disorder in ultrathin films (20 nm) are less affected than thicker (200 nm) films by the addition of fullerene into the polymer. We believe that these findings could be important for discovering methods of precisely controlling the properties of bulk heterojunction films with crucial implications for designing more efficient organic-based photovoltaics