5 research outputs found
Thermal Conduction across MetalāDielectric Sidewall Interfaces
The heat flow at
the interfaces of complex nanostructures is three-dimensional in part
due to the nonplanarity of interfaces. One example common in nanosystems
is the situation when a significant fraction of the interfacial area
is composed of sidewalls that are perpendicular to the principal plane,
for example, in metallization structures for complementary metal-oxide
semiconductor transistors. It is often observed that such sidewall
interfaces contain significantly higher levels of microstructural
disorder, which impedes energy carrier transport and leads to effective
increases in interfacial resistance. The impact of these sidewall
interfaces needs to be explored in greater depth for practical device
engineering, and a related problem is that appropriate characterization
techniques are not available. Here, we develop a novel electrothermal
method and an intricate microfabricated structure to extract the thermal
resistance of a sidewall interface between aluminum and silicon dioxide
using suspended nanograting structures. The thermal resistance of
the sidewall interface is measured to be ā¼16 Ā± 5 m<sup>2</sup> K GW<sup>ā1</sup>, which is twice as large as the
equivalent horizontal planar interface comprising the same materials
in the experimental sample. The rough sidewall interfaces are observed
using transmission electron micrographs, which may be more extensive
than at interfaces in the substrate plan in the same nanostructure.
A model based on a two-dimensional sinusoidal surface estimates the
impact of the roughness on thermal resistance to be ā¼2 m<sup>2</sup> K GW<sup>ā1</sup>. The large disparity between the
model predictions and the experiments is attributed to the incomplete
contact at the AlāSiO<sub>2</sub> sidewall interfaces, inferred
by observation of underetching of the silicon substrate below the
sidewall opening. This study suggests that sidewall interfaces must
be considered separately from planar interfaces in thermal analysis
for nanostructured systems
Energy-Efficient Phase-Change Memory with Graphene as a Thermal Barrier
Phase-change memory (PCM) is an important
class of data storage, yet lowering the programming current of individual
devices is known to be a significant challenge. Here we improve the
energy-efficiency of PCM by placing a graphene layer at the interface
between the phase-change material, Ge<sub>2</sub>Sb<sub>2</sub>Te<sub>5</sub> (GST), and the bottom electrode (W) heater. Graphene-PCM
(G-PCM) devices have ā¼40% lower RESET current compared to control
devices without the graphene. This is attributed to the graphene as
an added interfacial thermal resistance which helps confine the generated
heat inside the active PCM volume. The G-PCM achieves programming
up to 10<sup>5</sup> cycles, and the graphene could further enhance
the PCM endurance by limiting atomic migration or material segregation
at the bottom electrode interface
Direct Visualization of Thermal Conductivity Suppression Due to Enhanced Phonon Scattering Near Individual Grain Boundaries
Understanding
the impact of lattice imperfections on nanoscale
thermal transport is crucial for diverse applications ranging from
thermal management to energy conversion. Grain boundaries (GBs) are
ubiquitous defects in polycrystalline materials, which scatter phonons
and reduce thermal conductivity (Īŗ). Historically, their impact
on heat conduction has been studied indirectly through spatially averaged
measurements, that provide little information about phonon transport
near a single GB. Here, using spatially resolved time-domain thermoreflectance
(TDTR) measurements in combination with electron backscatter diffraction
(EBSD), we make localized measurements of Īŗ within few Ī¼m
of individual GBs in boron-doped polycrystalline diamond. We observe
strongly suppressed thermal transport near GBs, a reduction in Īŗ
from ā¼1000 W m<sup>ā1</sup> K<sup>ā1</sup> at
the center of large grains to ā¼400 W m<sup>ā1</sup> K<sup>ā1</sup> in the immediate vicinity of GBs. Furthermore, we
show that this reduction in Īŗ is measured up to ā¼10 Ī¼m
away from a GB. A theoretical model is proposed that captures the
local reduction in phonon mean-free-paths due to strongly diffuse
phonon scattering at the disordered grain boundaries. Our results
provide a new framework for understanding phononādefect interactions
in nanomaterials, with implications for the use of high-Īŗ polycrystalline
materials as heat sinks in electronics thermal management
Temperature-Dependent Thermal Boundary Conductance of Monolayer MoS<sub>2</sub> by Raman Thermometry
The electrical and
thermal behavior of nanoscale devices based
on two-dimensional (2D) materials is often limited by their contacts
and interfaces. Here we report the temperature-dependent thermal boundary
conductance (TBC) of monolayer MoS<sub>2</sub> with AlN and SiO<sub>2</sub>, using Raman thermometry with laser-induced heating. The
temperature-dependent optical absorption of the 2D material is crucial
in such experiments, which we characterize here for the first time
above room temperature. We obtain TBC ā¼ 15 MW m<sup>ā2</sup> K<sup>ā1</sup> near room temperature, increasing as ā¼ <i>T</i><sup>0.65</sup> in the range 300ā600 K. The similar
TBC of MoS<sub>2</sub> with the two substrates indicates that MoS<sub>2</sub> is the āsofterā material with weaker phonon
irradiance, and the relatively low TBC signifies that such interfaces
present a key bottleneck in energy dissipation from 2D devices. Our
approach is needed to correctly perform Raman thermometry of 2D materials,
and our findings are key for understanding energy coupling at the
nanoscale
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Light-Driven Ultrafast Polarization Manipulation in a Relaxor Ferroelectric
Relaxor ferroelectrics have been
intensely studied for
decades
based on their unique electromechanical responses which arise from
local structural heterogeneity involving polar nanoregions or domains.
Here, we report first studies of the ultrafast dynamics and reconfigurability
of the polarization in freestanding films of the prototypical relaxor
0.68PbMg1/3Nb2/3O3-0.32PbTiO3 (PMN-0.32PT) by probing its atomic-scale response via femtosecond-resolution,
electron-scattering approaches. By combining these structural measurements
with dynamic phase-field simulations, we show that femtosecond light
pulses drive a change in both the magnitude and direction of the polarization
vector within polar nanodomains on few-picosecond time scales. This
study defines new opportunities for dynamic reconfigurable control
of the polarization in nanoscale relaxor ferroelectrics