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Phonon Engineering: From Concepts to Device Applications

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 phonon group 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 concept, see the extracts from the invited plenary talk given by Professor Balandin at the 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 Nano-Device Laboratory (NDL) 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 and formulated conditions for achieving phonon band gaps in semiconductor quantum dot superlattices. 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 Nano-Device Laboratory (NDL) 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 HERE. A home built 3w-thermal conductivity measurement setup combined with cryogenic equipment allows NDL researchers to study phonon transport 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 HERE.

Figure 4. Raman spectra of TMV virus under visible laser excitation. NDL, 2004.

Another thrust of the nanophononics research in the Nano-Device 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 found HERE. 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.

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