Phonons are quantized modes of vibration occurring in a rigid crystal lattice, such as the atomic lattice of a solid.
Phonons manifest themselves in all properties of materials: phonons
limit electrical conductivity, optical phonons strongly influence optical response, while
acoustic phonons carry heat in insulators and semiconductors. Long-wavelength
phonons gives rise to sound in solids (hence the name phonon).
Spatial confinement of phonons in nanostructures
can strongly affect the phonon spectrum and modify phonon properties such as
velocity, polarization, density of states and
electron - phonon interaction. Thus,
nanostructures offer a new way of
controlling phonon transport and electron - phonon interaction
via tuning phonon dispersion relation, i.e.
phonon engineering. The idea of engineering phonon dispersion in
nanostructures has the potential to be as powerful as the idea
of the band-gap engineering for electrons, which is now utilized in
a variety of devices (see Figure 1).
For an overview of the development of the phonon engineering
see the extracts from the invited
plenary talk given by Professor Balandin
International Conference on Phonon Scattering 2004, St. Petersburg, Russia and
at the California Nanosystems Institute,
University of California, Los Angeles.
Figure 1. Illustration of the phonon engineering concept in hetero- and nanostructures.
After A.A. Balandin, Plenary Talk, PHONONS 2004 International Conference.
Professor Balandin's Nano-Device
Laboratory (NDL) research group
offers a unique blend of both theoretical and experimental research in
phonon engineering. The problems of acoustic phonon transport
are closely related to heat removal and thermal management
of elecetronic devices and circuits.
As the feature size of the devices continues to decrease and the amount of dissipated
power continues to increase the problem of thermal management becomes
extremely important for further development of electronic industry.
The heat removal issue is becoming more complicated due to a number of
additional factors such as (i) introduction of new materials (alternative dielectrics, SOI, etc.),
which have low thermal conductivity; (ii) increase in the number of interconnects and layers of
different materials in chip designs with corresponding increase in the total thermal boundary
resistance; (iii) higher switching speeds; (iv) increased device integration
(vertical MOSFETs, proposed 3D integration, etc.); and size effects that lead to
decrease in the thermal conductivity of the material itself (phonon – boundary scattering, etc.).
NDL researchers carry out experimental investigation of thermal transport in
nanostructured and layered semiconductor materials (see Figure 2).
Figure 2. Measured thermal conductivity in quantum dot superlattices, NDL 2004.
In recent years, Professor Balandin and researchers of the
worked on generalization of the concept of phonon engineering with
the goal to achieve the “customer-tailored” phonon spectrum for the
enhancement of operation of the
electronic, thermoelectric and optoelectronic devices. In a recently published
Applied Physics Letter
it was shown that
phonon spectrum in acoustically mismatched nanostructures can be
engineered in such a way that the phonon group velocity increases along a chosen direction (see
Figure 3), thus imporving
the heat removal. In several other papers (see the Publication List)
NDL researchers proposed
a method of phonon depletion in certain regions of the device
conditions for achieving phonon band gaps in semiconductor quantum dot
Some thrusts of the
phonon engineering research in NDL are partially supported by the
National Science Foundation and
the DARPA-SRC funded
MARCO Center on Functional Engineered Nano
Artichectonics (FENA) .
Figure 3. Phonon group velocity increase due to the "acoustically hard" boundaries.
Researchers of the
carry out experimental nanophononic research frequently crossing
traditional boarder lines among the disciplines: from semiconductor nanostructures
to carbon nanotubes and viruses. Raman and Brillouin spectroscopies are used to investigate
confined optical and acoustic phonons in quantum dots and other nanostructures.
More information about Raman spectroscopy in NDL can be found
A home built 3w-thermal conductivity measurement
setup combined with
cryogenic equipment allows NDL researchers to study
in nanostructural materials over the wide
temperature range experimentally.
Cylindrically shaped viruses, such as TMV, have been used
as templates for self-assembly of nanostructures and elements
of nanoelectronic circuits. NDL researchers study phonons in
functionalized TMV and TMV-based hybrid nanostructures
both experimentally (see Figure 4) and theoretically.
More about calculation of phonon modes in viruses and
bio-nanotechnology research conducted in NDL can be found
Figure 4. Raman spectra of TMV virus under visible laser excitation. NDL, 2004.
Another thrust of the nanophononics research in
Laboratory (NDL) is theoretical and experimental investigation of
optical phonons in wurtzite ZnO and GaN quantum dots (see Figure 5).
Dr. Fonoberov and Professor Balandin derived
an exact integral equation, which defines
interface and confined polar optical-phonon modes in wurtzite nanocrystals.
It has been shown theoretically, that while the frequency of confined polar optical phonons in
zincblende nanocrystals is equal to that of the bulk crystal phonons,
the confined polar optical phonons in wurtzite nanocrystals
have a discrete spectrum of frequencies different from those of the bulk crystal. Details
of this investigation have been reported in a series of papers published in
Physical Review B and Journal of Applied Physics (see
the Publication List)
Figure 5. Calculated optical phonon modes in wurtzire semiconductor quantum dots.
Results are after V.A. Fonoberov and A.A. Balandin, Phys. Rev. B (2004).
GaN and GaN-based III-V alloys are promising materials for the next
generation of high-power electronic, microwave and optoelectronic devices.
For all envisioned
applications of GaN materials it is important to effectively
remove the generated heat.
Thus, the thermal conductivity of GaN and AlGaN alloys, used
in GaN/AlGaN heterostructure field-effect transistors (HFETs)
is a very important characteristic. NDL is the first group to
carry out systematic experimental and theoretical
study of thermal conduction in AlGaN films (see Figure 6).
Figure 6. Thermal conductivity in AlGaN alloy as a function of Al mole fraction.
Results are after W.L. Liu and A.A. Balandin, J. Appl. Phys., 2005.
More information on the PHONON ENGINEERING
and other projects
currently under way in the Nano-Device Laboratory (NDL) can be
To join NDL as a graduate student or postdoctoral research visit
the web-page HERE.
To learn more about course offering in the field of Materials,
Devices and Circuits visit the web-page HERE.