STRONG REDUCTION OF THE LATTICE THERMAL
CONDUCTIVITY IN PLANAR Si/Ge SUPERLATTICES
COCEMASOV Alexandr, NIKA Denis
Moldova State University
Reviewer: BOLDIREV Sergey, Ph.D. in Theoretical Physics and Mathematics, Assoc. Prof.
Keywords: thermal conductivity, phonons, lattice dynamics, planar superlattices
In the technologically important semiconductor devices, phonons play a dominant role in the lattice thermal
conductivity. It is well known that the thermal conductivity of semiconductor nanoscale structures is much lower
than the corresponding bulk value [1-2] due to the modification of the phonon energy spectrum, leading to a
decrease in the phonon group velocity [3-4] and an increase of the phonon scattering at the nanostructure
boundaries. As a result, the lattice thermal conductivity of Si free-standing films with thickness ~ 20 nm [5-6] and Si
nanowires with diameters 22 nm to 115 nm [7] is by 2 orders of magnitude smaller than the corresponding bulk
value.
For calculation of the phonon states in nanostructures, one often uses the continuum approach, which gives
correct dispersion relations in the long-wave limit [8-9]. However, this approach is inadequate to describe the shortlength, high-energy phonon modes that are significant for the heat transfer [10]. Therefore, for an accurate
description of the thermal properties of nanostructures, one needs to use lattice dynamic models [11-14], which are
valid for all values of the phonon wave vector.
In the present work, we developed a DC (diamond cell) model of lattice dynamics [14] for acoustic and optic
phonons in nanostructures with diamond type crystal lattice. Using our model, we investigated phonon and thermal
properties of Si/Ge planar superlattices. We have shown that in these structures a strong reduction of the lattice
thermal conductivity down to 1 W m-1 K-1 can be achieved depending on layer sizes and Si/Ge interface quality.
Financial support under the Moldova State Projects 11.817.05.10F and 10.819.05.02F are gratefully
acknowledged.
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