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 core-shell nanowires, VFF model, thermal conductivity
Thermal transport in nanosized structures is widely investigated around the world [1-4]. Simultaneously it’s
conducted theoretical and experimental search of new types of nanostructures 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 [3,4]. Coefficient of thermoelectric quality factor “figure of merit” ZT in
the denominator contains lattice and electronic thermal conductivities, therefore 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 Si is a poor thermoelectric material with ZT ~ 0.001 at room temperature [5]. At the same time, Si
nanowires (NWs) with a diameter of several tens of nm are considered perspective for thermoelectric applications
[4]. Experimental work [4] shows that in ultra-thin Si NWs with irregular (rough) edges it’s possible to achieve ZT ~
0.3-0.6 at room temperature, due to a fiftyfold drop in 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 NWs. Even greater thermal conductivity reduction it is
possible to achieve in hetero-NWs – NWs coated by a shell material with lower than in Si the speed of sound, such
as Ge or plastic. In such NWs the decrease in thermal conductivity compared with homogeneous NWs 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 NWs with a rectangular cross-section is carried
out within VFF model of crystal lattice vibrations. From the comparison of phonon average group velocities and
temperature dependences of lattice thermal conductivity in Si/Ge NWs and in homogeneous Si NWs with crosssection sizes of a few nm it is shown that the addition of Ge coating on a Si core modifies the phonon spectrum and
leads to a drop in the velocity of phonons and lattice thermal conductivity compared with homogeneous Si NWs. In
lattice thermal conductivity theory for NWs, 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 hetero-NWs with the Ge thickness of a few monolayers. A further increase in the
thickness of Ge weakens this effect. The theoretical results show the promise of ultra-thin hetero-NWs for
thermoelectric and thermal insulation applications.
References:
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transport in few-layer graphene. Nature Mater. 2010, vol.9, p.555.
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p.569.
3. Nika D., Pokatilov E., Balandin A., Fomin V., Rastelli A., and Schmidt O. Reduction of lattice thermal
conductivity in one-dimensional quantum-dot superlattices due to phonon filtering. Phys. Rev. B. 2011, vol.84,
p.165415.
4. Hochbaum A. et al. Enhanced thermoelectric performance of rough Si nanowires. Nature. 2008, vol.451, p.163.
5. Weber L. and Gmelin E. Transport properties of silicon. Appl. Phys. A. 1991, vol.53, p.136.
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.