Activity Report 2010 - CNRS

CONTROL OF THERMAL
CONDUCTIVITY AT THE
NANOSCALE
The ability to precisely control the
thermal conductivity of a material is
fundamental in the development of onchip
heat management or energy
conversion. By engineering a set of
individual phonon-scattering nanodot
barriers, researchers from the University
of Bordeaux, IFW Dresden, and Liten,
have accurately tailored the thermal
conductivity of a single-crystalline SiGe
material in spatially defined regions as
short as 15 nm, attaining ultra low
thermal conductivities below 1 W/m-K.
The motivation of this study was to know
whether it is possible to achieve fully
diffusive phonon barriers in a single
crystalline material.
Previous studies on planar Si/SiGe
superlattices had reported reductions in
thermal conductivity compatible with
partially diffusive interfaces. In our case,
having dots rather than flat layers leads
to a much stronger phonon scattering,
and allows us to achieve fully diffusive
barriers.
The single crystalline nanodot samples
were grown at IFW Dresden.
Measurements of their thermal
conductivity were performed by two
different methods:
3- method (by A. Rastelli at IFW
Dresden)
time domain thermoreflectance
(TDTR), by S.Dilhaire at the University of
Bordeaux.
Theoretical modelling of flat and nanodot
based superlattices was performed at
Liten, employing atomistic Green’s
function methods.
The thermal conductivity was measured
in the cross plane direction. Singlebarrier
thermal resistances between 2
and 4x10 -9 m 2 K W -1 were attained. This
results in a room-temperature
conductivity down to about 0.9Wm -1 K -1 ,
in multilayered structures with only five
barriers.
Such low thermal conductivity is
compatible with a totally diffuse
mismatch model for the barriers, and it is
well below the amorphous limit. The
results are in agreement with atomistic
Green’s function simulations.
HIGHLIGHT : THEORY AND NANOSIMULATION
Fig. 1: Schematic of the self-assembled
nanodot multilayers fabricated by molecular
beam epitaxy.
Fig. 2: Experimental thermal conductivities of
the samples, as a function of period length
This demonstrated ability to tailor
thermal conductivity with 1 Wm -1 K -1
precision and confirmed a spatial
resolution below the 20nm range which is
very relevant to the development of
integrated miniaturized energy
harvesting or thermal management
devices, fully compatible with silicon
nanoelectronics.
CONTACTS
natalio.mingo@cea.fr
FURTHER READING
G.Pernot et al., Nature Materials, 9, 491
(2010).
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