Femtosecond pulses are used to excite A(1g) optical phonons in Bi2Te3, Sb2Te3, and Bi2Te3/Sb2Te3 superlattice. Time-resolved reflectivity measurements show both the low-frequency and high-frequency components of A(1g) phonon modes. By comparing the phonon lifetime, it is found that the scattering rate (inverse of lifetime) in superlattice is significantly higher than those in Bi2Te3 and Sb2Te3. This represents the direct measurement of coherent phonon lifetime reduction in superlattice structures, consistent with the observed reduction in thermal conductivity in superlattices. (c) 2008 American Institute of Physics

Venkatasubramanian, Rama

Wang, Yaguo

Xu, Xianfan

English

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Purdue UniversityPurdue e-PubsBirck and NCN Publications Birck Nanotechnology CenterSeptember 2008Reduction in coherent phonon lifetime in Bi2Te3/Sb2Te3 superlatticesYaguo WangPurdue University, wang180@purdue.eduXianfan XuBirck Nanotechnology Center, School of Materials Engineering, Purdue University, xxu@purdue.eduRama VenkatasubramanianRTI Int, Ctr Solid State Energet, Res Triangle PkFollow this and additional works at: http://docs.lib.purdue.edu/nanopubThis document has been made available through Purdue e-Pubs, a service of the Purdue University Libraries. Please contact epubs@purdue.edu foradditional information.Wang, Yaguo; Xu, Xianfan; and Venkatasubramanian, Rama, "Reduction in coherent phonon lifetime in Bi2Te3/Sb2Te3 superlattices"(2008). Birck and NCN Publications. Paper 104.http://docs.lib.purdue.edu/nanopub/104Reduction in coherent phonon lifetime in Bi2Te3/Sb2Te3 superlatticesYaguo Wang,1 Xianfan Xu,1,a and Rama Venkatasubramanian21School of Mechanical Engineering and Birck Nanotechnology Center, Purdue University,West Lafayette, Indiana 47907, USA2Center for Solid State Energetics, RTI International, Research Triangle Park, North Carolina 27709, USAReceived 7 July 2008; accepted 28 August 2008; published online 19 September 2008Femtosecond pulses are used to excite A1g optical phonons in Bi2Te3, Sb2Te3, and Bi2Te3 /Sb2Te3superlattice. Time-resolved reflectivity measurements show both the low-frequency andhigh-frequency components of A1g phonon modes. By comparing the phonon lifetime, it is foundthat the scattering rate inverse of lifetime in superlattice is significantly higher than those in Bi2Te3and Sb2Te3. This represents the direct measurement of coherent phonon lifetime reduction insuperlattice structures, consistent with the observed reduction in thermal conductivity insuperlattices. © 2008 American Institute of Physics. DOI: 10.1063/1.2987518Ultrafast time-resolved optical measurement is a power-ful technique to generate and detect coherent phonons.1–3 Inabsorbing materials, coherent phonon is generated through adisplacive excitation of coherent phonon process,3 whichwas shown to be a special case of impulsive stimulated Ra-man scattering.4,5 In this study, we investigated coherentphonons in V2VI3 compounds V=Bi,Sb; VI=Se,Te,Bi2Te3 and Sb2Te3, which are narrow band-gap semiconduc-tors. These materials are used as thermoelectric materialspartly due to their low thermal conductivity.6 Recently, it wasfound that thermal conductivity in Bi2Te3 /Sb2Te3 superlat-tice structure is greatly reduced, even compared to its corre-sponding alloy, in the cross-plane direction.7 A fundamentalunderstanding of thermal conductivity reduction in theBi2Te3 /Sb2Te3 superlattice structure is important due to itsenhanced thermoelectric figure of merit.8 It has been sug-gested that the thermal conductivity reduction results frominterface scattering of phonons9,10 as well as otherprocesses.7 Even so, there has been no documented evidenceof reduction in phonon lifetimes, coherent or otherwise, thatwould begin to substantiate the basis of such thermal con-ductivity reduction in nanoscale structures. In this letter, wepresent ultrafast time-resolved measurements of coherent op-tical phonons in Bi2Te3, Sb2Te3, and Bi2Te3 /Sb2Te3 super-lattice, with the aim to reveal coherent phonon lifetimes inthe superlattice. Ultimately, acoustic phonons need to becharacterized and correlated with the thermal transport prop-erties. Measurement of acoustic phonons in superlattice ispossible since the zone-folded acoustic phonons can be op-tically excited.11All the experiments were performed in a standard collin-ear two-color 400 and 800 nm pump-probe scheme. Laserpulses with 50 fs full width at half maximum are generatedby an ultrafast laser system with the center wavelength at800 nm, a repetition rate of 1 kHz, and a maximum pulseenergy about 1 mJ. A second harmonic crystal is used togenerate the pump pulses centered at 400 nm. The pump andprobe beams are focused onto the sample at normal directionwith diameters of 80 and 20 m, respectively. The fluenceof probe beam is around 0.02 mJ /cm2.Samples investigated in this paper are p-type single crys-talline Bi2Te3 film, Sb2Te3 film, and Bi2Te3 /Sb2Te3 superlat-tice, with thicknesses of 1.0, 1.6, and 1.3 m, respectively.All these films are much thicker than their absorption depthtens of nanometers at 800 and 400 nm laser wavelengths.The films were grown by the metal-organic chemical-vapordeposition technique on GaAs100 substrates along the caxis of the films.12 The superlattice has 200 periods with a 2nm Bi2Te3 layer and a 4 nm Sb2Te3 layer for each period. A150 nm Bi2Te3 buffer layer exists between the superlatticeand the substrate. These 2 nm/4 nm Bi2Te3 /Sb2Te3 superlat-tices show strong satellites in x-ray diffraction studies, neg-ligible static disorder as measured by x-ray absorption spec-troscopy, and also show high thermoelectric figure of merit.Figure 1 shows time-resolved reflectivity signals at anumber of pump fluences and their Fourier transforms. Theexperimental data consist of the following two components:the oscillatory components, which are the coherent phononvibration, and the nonoscillatory components, which are re-lated to electron excitation initial drop in reflectivity andlattice heating due to electron-lattice coupling the seconddrop around 10 ps or so.13 One observation from Figs. 1b,1d, and 1f is the two frequency components of coherentphonon vibration for each sample, corresponding to the twoA1g modes a low-frequency component—A1g1 mode and ahigh-frequency component—A1g2 mode. These modes werealso observed in Raman scattering experiments,14 but havenot been reported in any previous time-domain experimentsin literature. Table I summarizes the results from both Ramanscattering and pump-probe experiment. It is seen that thevibration frequencies of the low and high-frequency A1gmodes for both Bi2Te3 and Sb2Te3 samples agree well withthe Raman measurement results carried out in their respec-tive bulk materials. The two modes observed in the superlat-tice are very close to that in Sb2Te3 Raman data are notavailable for the bulk superlattices from literature. That is,we did not observe frequency of Bi2Te3—the coherent pho-non modes of Bi2Te3 could be suppressed or dissipate at amuch faster rate see discussions below.The oscillatory and nonoscillatory components can beseparated by applying a digital low-pass filter on the experi-mental data. The signal of coherent phonon for all thesamples, shown in Fig. 2, are obtained by filtering thenonoscillatory component. The reflectivity change R /RaTel.: 1-765-494-5639. FAX: 1-765-494-0539. Electronic mail:xxu@ecn.purdue.edu.APPLIED PHYSICS LETTERS 93, 113114 20080003-6951/2008/9311/113114/3/$23.00 © 2008 American Institute of Physics93, 113114-1Downloaded 18 Nov 2008 to 128.46.220.88. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jspdue to the coherent phonon can be approximated asRR=R/RQQ =R/RQAQ0, 1where A is the amplitude of coherent phonon vibration andQ0 is the normalized coordinate of coherent phonon. Q0 canbe modeled as a chirped damping harmonic oscillator:13,15,16Q0 = exp− tcos + tt +  , 2where , , ,  are phonon scattering rate, angular fre-quency, chirping coefficient, and initial phase of phonon vi-bration, respectively. Since the vibration for the A1g2 mode ismuch weaker and decades much faster than the A1g1 mode, itscontribution in the fitting process is negligible. Therefore, inthe model and the discussion below, we only consider thecontribution from the A1g1 mode.One observation from Fig. 2 is that at the same laserfluence, the initial amplitudes of coherent phonon oscilla-tions in the superlattice are a factor of 2–3 smaller than thosein either Bi2Te3 or Sb2Te3 films. On the other hand, it is seenfrom Fig. 1 that the initial electron excitation the initialreflectivity drop for all samples, including the superlatticeare close at the same laser fluence, indicating that the laserenergies absorbed by the two components as well by thesuperlattice are similar. Therefore, the weaker phonon oscil-lation in the superlattice could be caused by a weaker cou-pling between electrons and the lattice in Bi2Te3 or by rapidquenching of coherent phonons in Bi2Te3 in a superlatticestructure, which are supported by the phonon frequency mea-surement. The weaker electron-lattice coupling could also be-10123450 2 4 6 8 10 12R/R(x10-2)Delay(ps)Pump Fluence(mJ/cm2)0.690.580.470.360.25Bi2Te3(a)0 1 2 3 4 5 6 70123456NormalizedFTAmplitudeFrequency(THz)Pump Fluence(mJ/cm2)0.690.580.470.360.251.86THz 4.0THzx10Bi2Te3(b)-10123450 2 4 6 8 10 12Delay(ps)Pump Fluence(mJ/cm2)0.690.580.470.360.25R/R(x10-2)Sb2Te3(c)012340 2 4 6 8 10Pump Fluence(mJ/cm2)Sb2Te30.690.580.470.360.25Frequency(THz)NormalizedFTAmplitude 2.05THz 4.98THzx10(d)-0.500.511.522.530 2 4 6 8 10 12Pump Fluence(mJ/cm2)0.690.580.470.360.25Delay(ps)R/R(x10-2)(e) Superlattice012340 2 4 6 8 10Frequency(THz)Pump Fluence(mJ/cm2)NormalizedFTAmplitudex104.97THz2.05THz 0.690.580.470.360.25(f ) SuperlatticeFIG. 1. a, c, and e Reflectivity change for Bi2Te3, Sb2Te3, andBi2Te3 /Sb2Te3 superlattice at different pump fluences. b, d. and f Thecorresponding frequency spectrum calculated by FFT. Part of the spectrumcurves are magnified ten times to see the A1g2 mode clearly. All the curves arevertically translated and labeled with the pump fluence.TABLE I. Comparison of A1g phonon frequencies from Raman scatteringand pump-probe experimentModeBi2Te3 Sb2Te3 SuperlatticeFrequency THz Frequency THz Frequency THzRaman Pump-probe Raman Pump-probe Raman Pump-probeA1g1 1.88 1.86 2.07 2.05 ¯ 2.05A1g2 4.02 4.00 4.95 4.98 ¯ 4.970123450 2 4 6 8 10 12Delay(ps)Bi2Te3 Pump Fluence (mJ/cm2)0.690.580.470.360.25(a)R/R(x10-2)00.511.522.533.50 2 4 6 8 10 12Delay(ps)Sb2Te3 Pump Fluence (mJ/cm2)0.690.580.470.360.25R/R(x10-2)(b)00 .20 .40 .60 .811 .21 .41 .60 2 4 6 8 10 12P um p F lue nce (m J/cm2)D e la y(p s)0 .6 90 .5 80 .4 70 .3 60 .2 5(c) Su perla tticeR/R(x10-2)FIG. 2. Coherent phonon vibration signal for Bi2Te3, Sb2Te3, andBi2Te3 /Sb2Te3 superlattice. The dots are experimental data, and lines arefitted results.113114-2 Wang, Xu, and Venkatasubramanian Appl. Phys. Lett. 93, 113114 2008Downloaded 18 Nov 2008 to 128.46.220.88. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jspaided by the electrons and holes, forming a miniband in asuperlattice.8 In any case, the absence of coherent phononsfrom the minority component of the superlattice and theoverall initial amplitude are telling of interesting physicalphenomena that warrant further investigations.The phonon scattering rate—the inverse of coherentphonon dephasing time fitted from Eq. 2—has a linear de-pendence on the pump fluence, as shown in Fig. 3. At higherlaser fluences, photoexcited electrons contribute to the pro-cess of shortening coherent phonon lifetime or increasing thescattering. After photon excitation, the excited electrons re-lease their excess kinetic energy by emitting incoherentphonons. Both the photoexcited electrons and the resultingincoherent phonons have a linear dependence on pump flu-ence, which results in the linear dependence of scattering rateon pump fluence. If the linear relation between the pumpfluence and the scattering rate is extrapolated to the zeropump fluence, the phonon scattering rate under no excitationcondition 0 is obtained. These scattering rates for Bi2Te3,Sb2Te3, and Bi2Te3 /Sb2Te3 superlattice are found to be0.188, 0.295, and 0.357 THz, respectively. Apparently, thescattering rate of the superlattice is higher than any of itscomponents, 90% higher than that in Bi2Te3 and 20% higherthan that in Sb2Te3. This result supports the existence ofextra phonon lifetime reduction in superlattice, which maystem from a variety of scattering processes at the interfacesbetween the constituent layers of the superlattice. If suchbehaviors also exist for acoustic phonons, particularly thelong wavelength acoustic phonons that have similar smallwave vector as the optical phonons, that would explain thereduction in heat transport in superlattice.In conclusion, we observed both the low-frequency andhigh-frequency components of A1g phonon in Bi2Te3,Sb2Te3, and Bi2Te3 /Sb2Te3 superlattice. The coherent opti-cal phonon lifetime in the superlattice is shorter than those inBi2Te3 and Sb2Te3; and the phonon vibration modes in su-perlattice are very similar to those in Sb2Te3. The phononlifetime reduction in superlattice suggests phonon-interfaceinteractions. This could form the basis for phonon-blockingand electron-transmitting characteristics of the superlattices.8Further studies are needed to elucidate the nature and mecha-nism of such enhanced scattering processes in a variety ofnanoscale materials.We would like to acknowledge the support to this workby the National Science Foundation, the Sandia NationalLaboratory, and the Air Force Office of Scientific Research.The work at RTI International was carried out with DARPA/DSO funded efforts, ONR U.S. Navy Contract No.N00014-04-C-0042 and ARO U.S. ARMY Contract No.W911NF-08-C-0058. These program supports are gratefullyacknowledged.1T. K. Cheng, S. D. Brorson, A. S. Kazeroonian, J. S. Moodera, G.Dresselhaus, M. S. Dresselhaus, and E. P. Ippen, Appl. Phys. Lett. 57,1004 1990.2T. K. Cheng, J. Vidal, H. J. Zeiger, G. Dresselhaus, M. S. Dresselhaus, andE. P. Ippen, Appl. Phys. Lett. 59, 1923 1991.3H. J. Zeiger, J. Vidal, T. K. Cheng, E. P. Ippen, G. Dresselhaus, and M. S.Dresselhaus, Phys. Rev. B 45, 768 1992.4G. A. Garrett, T. F. Albrecht, J. F. Whitaker, and R. Merlin, Phys. Rev.Lett. 77, 3661 1996.5T. E. Stevens, J. Kuhl, and R. Merlin, Phys. Rev. B 65, 144304 2002.6F. J. DiSalvo, Science 285, 703 1999.7R. Venkatasubramanian, Phys. Rev. B 61, 3091 2000.8R. Venkatasubramanian, E. Siivola, T. Colpitts, and B. O’Quinn, NatureLondon 413, 597 2001.9G. Chen, Phys. Rev. B 57, 14958 1998.10M. N. Touzelbaev, P. Zhou, R. Venkatasubramanian, and K. E. Goodson,J. Appl. Phys. 90, 763 2001.11T. Dekorsy, A. Bartels, H. Kurz, K. Mizoguchi, M. Nakayama, and K.Köhler, Phys. Status Solidi B 215, 425 1999.12R. Venkatasubramanian, T. Colpitts, E. Watko, M. Lamvik, and N.ElMasry, J. Cryst. Growth 170, 817 1997.13A. Q. Wu and X. Xu, Appl. Phys. Lett. 90, 251111 2007.14W. Richter, H. Köhler, and C. R. Becker, Phys. Status Solidi B 84, 6191977.15O. V. Misochko, M. Hase, K. Ishioka, and M. Kitajima, Phys. Rev. Lett.92, 197401 2004.16A. Q. Wu, X. Xu, and R. Venkatasubramanian, Appl. Phys. Lett. 92,011108 2008.0.20.210.220.230.240.250.260.2 0.3 0.4 0.5 0.6 0.7ScatteringRate(THz)F luence (mJ/cm2)0.188 + 0.086 FBi2Te3(a)0.30.320.340.360.380.40.2 0.3 0.4 0.5 0.6 0.70.295+ 0.148 FScatteringRate(THz)F luence (mJ/cm2)Sb2Te3(b)0.340.360.380.40.420.440.2 0.3 0.4 0.5 0.6 0.7Fluence (mJ/cm2)ScatteringRate(THz)0.357 + 0.089 FSuperlattice(c)FIG. 3. Scattering rate of A1g1 mode with different pump fluences. The dotsare experimental data, and the lines are fitted results.113114-3 Wang, Xu, and Venkatasubramanian Appl. Phys. Lett. 93, 113114 2008Downloaded 18 Nov 2008 to 128.46.220.88. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jsp

Reduction in coherent phonon lifetime in Bi2Te3/Sb2Te3 superlattices

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Reduction in coherent phonon lifetime in Bi2Te3/Sb2Te3 superlattices

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