THERMAL CONDUCTIVITY OF SI/GE NANOWIRES WITHIN THE VALENCE FORCE FIELD
MODEL OF CRYSTAL LATTICE VIBRATIONS
CRISMARI Dmitrii
Moldova State University
Reviewer: NIKA D., dr.
Keywords: Si/Ge nanowires, Valence Force Field model, core-shell heterostructures, thermal conductivity
Thermal transport in nanosized structures is widely investigated around the world [1-6]. Simultaneously it’s
conducted theoretical and experimental search of new types of nanosized structures both with high thermal
conductivity [1,2] for applications in micro- and nanoelectronics as effective heat-removing materials and with low
thermal conductivity for thermoelectric applications [4,5]. Coefficient of thermoelectric quality factor “figure of
merit” ZT in the denominator contains the lattice and electronic thermal conductivities, therefore the thermal
conductivity reduction under the condition of good conductive properties’ preservation (electrical conductivity and
the Seebeck coefficient) allows to increase ZT over a wide temperature range.
Bulk silicon is a poor thermoelectric material with ZT ~ 0.001 at room temperature [7]. At the same time,
silicon nanowires with a diameter of several tens of nanometers are considered perspective for thermoelectric
applications [5]. Experimental work [5] shows that in ultrathin silicon nanowires with irregular (rough) edges it’s
possible to achieve ZT ~ 0.3-0.6 at room temperature, due to a fiftyfold drop in the lattice thermal conductivity.
Reduction of lattice thermal conductivity in these structures is due to the modification of the phonon energy
spectrum in comparison with the bulk case and strong phonon scattering on the surface of nanowires. Even greater
thermal conductivity reduction it is possible to achieve in heterowires – nanowires coated by a shell material with
lower than in silicon the speed of sound, such as germanium or plastic. In such nanowires the decrease in thermal
conductivity compared with homogeneous nanowires occurs due to additional modification of the phonon energy
spectrum and the reduction of phonon group velocities.
In the present paper the phonon properties’ research of Si/Ge heteronanowires with a rectangular cross-section
is carried out within the Valence Force Field model of crystal lattice vibrations. From the comparison of phonon
average group velocities and temperature dependences of the lattice thermal conductivity in Si/Ge heteronanowires
and in homogeneous silicon nanowires with cross-section sizes of a few nanometers it is shown that the addition of
germanium coating on a silicon core modifies the phonon spectrum and leads to a drop in the velocity of phonons
and the lattice thermal conductivity compared with homogeneous Si nanowires. In the lattice thermal conductivity
theory for nanowires, in addition to the phonon-phonon Umklapp processes, in this study it was taken into
consideration also and the mechanism of surface phonon scattering.
It has been demonstrated that the maximum effect of a decrease in phonon average group velocity and lattice
thermal conductivity is achieved in nanoheterowires with the germanium thickness of a few monolayers. A further
increase in the thickness of germanium weakens this effect. The theoretical results show the promise of ultra-thin
heterostructure nanowires for thermoelectric and thermal insulation applications.
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The author is grateful to the Academy of Sciences of Moldova for the financial support of researches within the
Projects 11.817.05.10F and 12.819.05.18F.