Copyright © 2005 American Scientific Publishers Journal of All rights reserved Nanoscience and Nanotechnology Printed in the United States of America Vol.5, 1015–1022, 2005 REVIEW Nanophononics: Phonon Engineering in Nanostructures and Nanodevices Alexander A. Balandin Department of Electrical Engineering, University of California, Riverside, California, USA Phonons, i.e., quanta of lattice vibrations, manifest themselves practically in all electrical, thermal and optical phenomena in semiconductors and other material systems. Reduction of the size of electronic devices below the acoustic phononDelivered mean by free Ingenta path creates to: a new situation for phonon propagation and interaction. From one side,Alexander it complicates Balandin heat removal from the downscaled devices. From the other side, it opens up anIP exciting: 138.23.166.189 opportunity for engineering phonon spectrum in nanostructured materials and achievingThu, enhanced 21 Sep 2006 operation 19:50:10 of nanodevices. This paper reviews the development of the phonon engineering concept and discusses its device applications. The review focuses on methods of tuning the phonon spectrum in acoustically mismatched nano- and heterostructures in order to change the phonon thermal conductivity and electron mobility. New approaches for the electron–phonon scattering rates suppression, formation of the phonon stop- bands and phonon focusing are also discussed. The last section addresses the phonon engineering issues in biological and hybrid bio-inorganic nanostructures. Keywords: Phonon Engineering, Nanophononics, Phonon Depletion, Thermal Conduction, Acoustically Mismatched Nanostructures, Hybrid Nanostructures. CONTENTS semiconductors.The long-wavelength phonons gives rise to sound waves in solids, which explains the name phonon. 1. Phonons in Bulk Semiconductors and Nanostructures ........1015 Similar to electrons, one can characterize the properties 2. The Phonon Engineering Concept .......................1016 3.Engineering the Phonon Thermal Conduction in of phonons by their dispersion (q), i.e., dependence of Nanostructures ......................................1018 the phonon frequency on its wave vector q.In bulk 4.Phonon Depletion in Acoustically Mismatched semiconductors with g atoms per unit cell, there are 3g Nanostructures ......................................1019 phonon dispersion modes for every value of q.In the limit 5. Phonons in Hybrid Bio-Inorganic Nanostructures ...........1020 6. Conclusions ........................................1021 of long waves, three modes describe the motion of the Acknowledgments ...................................1022 unit cell, and form the three acoustic phonon branches. References and Notes ................................1022 The other 3(g − 1) modes describe the relative motion of atoms in a unit cell, and form the optical phonon branches. 1.PHONONS IN BULK SEMICONDUCTORS Acoustic phonons have nearly linear dispersion, which can be written as = V q (where V is the sound velocity). AND NANOSTRUCTURES S S Optical phonons, in general, are nearly dispersion-less for Phonons are quantized modes of vibration occurring in a small q values (long-wavelength approximation) and have rigid crystal lattice, such as the atomic lattice of a solid. a small group velocity VG = d/dq. One can speak of a gas of phonons, which are quasi- Spatial confinement of phonons in nanostructures and particles of the energy and quasi-momentum p = q thin films can strongly affect the phonon dispersion and obeying Bose-Einstein statistics.1 Phonons manifest them- modify phonon properties such as phonon group velocity, selves practically in all properties of materials.For exam- polarization, density of states, and affect phonon inter- ple, acoustic and optical phonons limit electrical conduc- action with electrons, point defects, other phonons, etc. tivity.Optical phonons strongly influence optical properties Figure 1 shows the phonon energy dispersion in a thin film of semiconductors while acoustic phonons are dominant and a three-layered heterostructure for of the symmetric heat carriers in insulators and technologically important (SA) and antisymmetric (AS) modes.The results are J. Nanosci. Nanotech. 2005, Vol. 5, No. 7 1533-4880/2005/5/1015/008/$17.00+.25 doi:10.1166/jnn.2005.175 1015 Nanophononics: Phonon Engineering in Nanostructures and Nanodevices Balandin shown for the 6 nm-wide AlN slab and the three-layer het- history.In 1950s, Rytov published a series of theoret- 3 erostructure with the core layer thickness d2 = 4 nm.One ical papers where he analyzed acoustic vibrations in can see that the phonon dispersion in these structures is “artificial thinly-laminated media,” a structure, which now very different from the bulk phonon modes.Modification would be referred to as a superlattice, and described of the acoustic phonon dispersion is particularly strong folded acoustic phonons in such media.The folded in freestanding thin films or in nanostructures embed- phonons were later observed in quantum well superlat- ded into elastically dissimilar materials.Such modification tices made of GaAs/AlGaAs and other semiconductors.4 may turn out to be desirable for some applications while In 1980s and early 1990s there have been large amount detrimental for others.Thus, nanostructures offer a new of theoretical work done aimed at calculating the con- REVIEW way of controlling phonon transport via tuning its disper- fined acoustic phonon–electron scattering rates in free- 2 sion relation, i.e., phonon engineering. The concept of standing thin films and nanowires.Some notable exam- engineering (or rather re-engineering) the phonon disper- ples of this work include papers from the research groups sion in nanostructures has the potential to be as powerful of Stroscio,5 Mitin,6 Nishiguchi7 and Bandyopadhyay.8 as the concept of the band-gap engineering for electrons, Most papers on the subject used the elastic continuum which revolutionized the electronic industry. approach for calculating phonon dispersion and adopted In this paper I focus on acoustic phonons in hetero- solution techniques developed in acoustics and mechan- nanostructures.The optical phonon confinement in quan- tum dots, Raman scattering from localized optical phonons ics.The prime motivation was to see if the spatial con- in nanostructures, phonon bottleneck and related issues finement and quantization of the acoustic phonon modes Delivered by Ingentain freestanding to: thin films or nanowires produces notice- deserve a special analysis, which goes beyondAlexander the scope Balandin of the present review.In the remainder of the paper, I pro- able effect on the deformation potential scattering of elec- IP : 138.23.166.189trons.The opinions were split about how important the vide a brief description of the phonon engineeringThu, 21 Sep concept 2006 19:50:10 (Section 2), discuss the thermal conductivity in nanostruc- acoustic phonon confinement in the description of elec- tures (Section 3), describe a possibility of phonon depletion tron transport in low-dimensional structures.Some theo- in acoustically mismatched heterostructures (Section 4), rists argued that the phonon-confinement induced changes and present results related to phonons in biological systems for macroscopic characteristics, such as carrier mobility, and hybrid bio-inorganic nanostructures (Section 5). are not pronounced.7 9 Others have found that the defor- mation potential scattering can be substantially suppressed for certain electron energies.6 8 This earlier work focused 2.THE PHONON ENGINEERING CONCEPT on the effects of the acoustic phonon confinement on elec- The idea of looking at the changes that acoustic phonon tron transport in free-standing quantum wells and quantum spectrum experiences in heterostructures has a rather long wires did not lead to experimental claims of significantly Professor Alexander A. Balandin received his M.S. degree in Applied Physics and Math- ematics from the Moscow Institute of Physics and Technology (MIPT), Russia in 1991.He received his second M.S. and Ph.D. degrees in Electrical Engineering from the University of Notre Dame, USA in 1995 and 1996, respectively.He worked at the Electrical Engineering Department, UCLA from 1997 to 1999.In 1999, he joined the Department of Electrical Engineering of the University of California–Riverside (UCR) as an Assistant Professor. Currently, he is a Professor of Electrical Engineering and Director of the Nano-Device Laboratory (NDL), which he organized in 2001.Professor Balandin’s research interests are in the area of electronic materials, nanostructures and nanodevices.Current research topics in his group include phonon engineering at nanoscale, electron–phonon transport in nanostructures and nanodevices, hybrid bio-inorganic nanostructures, wide band-gap semiconductor materials and nanostructures, electronic noise phenomena, thermal management and reliability of nanoscale devices.He carries both theoretical and experimental research.Professor Balandin is an author or coauthor of more than 80 journal publications, ten invited book chapters, and a book Nanophononics.He edited books Noise and Fluctuations Control in Electronic Devices, Handbook of Semiconductor Nanostructures and Nanodevices, and others.He chaired many international conference sessions and served as a SPIE conference chairman.Professor Balandin is an Editor-in- Chief of the Journal of Nanoelectronics and Optoelectronics (JNO).He also serves on the editorial board of the Journal of Nanoscience