Temperature dependence of Raman spectra for individual silicon nanowires Gregory S. Doerk, Carlo Carraro, and Roya Maboudian Department of Chemical Engineering, University of California, Berkeley, California 94720, USA Received 6 May 2009; revised manuscript received 12 July 2009; published 27 August 2009 The temperature dependence of the Stokes first-order optical phonon frequency has been measured for individual silicon nanowires with diameters between 33 to 180 nm in the temperature range of 20 to 300 ° C. The nanowires were synthesized via both the vapor-liquid-solid method and electrochemical etching of bulk silicon. Significant laser-induced local heating was avoided by using a laser power of 0.5 mW or less, corre- sponding to fluxes of 0.7 mW/ m 2 . For both types of nanowires the slope of Raman frequency vs tem- perature closely matches the value of bulk Si,  d dT  =-0.022  .001 cm -1 °C -1 , across the entire diameter range, indicating no change in lattice anharmonicity. These results have important implications for understand- ing nanowire lattice thermal conductivity and extending the domain for Raman thermometry of silicon nanostructures. DOI: 10.1103/PhysRevB.80.073306 PACS numbers: 78.30.Am, 63.22.Gh, 65.80.+n, 81.70.Fy Semiconductor nanowires are promising for integration into thermoelectric materials as they may reduce lattice ther- mal conductivity while maintaining high electrical conductivity. 1 Single-crystal silicon nanowires NWs grown by the vapor-liquid-solid process VLS for instance, show a strong dependence of thermal conductivity on diameter, which has been attributed to boundary scattering. 2 More re- cently it was demonstrated that single-crystal silicon nano- wires synthesized by Ag-catalyzed electrochemical etching of Si wafers possess thermal conductivities five to eight times lower than those of VLS Si NWs of the same diameter, enabling orders of magnitude improvement over bulk Si in the thermoelectric figure of merit. 3 These surprising results were attributed to the rough surfaces of the electrochemically etched EE Si NWs, though the available theories for deter- mining thermal conductivity of crystalline materials that ac- count for surface roughness could not provide quantitative agreement with the experimental data. 4,5 As a probe of phonons in crystals, Raman spectrometry may provide valuable insight into their thermal properties. Specifically, the anharmonicity of the vibrational potential energy provides a route to precisely measure local tempera- ture in the crystal through Raman spectrometry. 6 Previous Raman investigations done on ensembles of Si NWs with small diameters D  20 nm have assumed bulk anhar- monic behavior in order to isolate the effects of quantum confinement. 7–9 Yet one report has claimed anharmonic con- stants relating the temperature dependence of the first-order optical phonon frequency from an ensemble of 20 nm di- ameter Si NWs that are different from those for bulk Si. 10 Therefore, it is unclear whether anharmonic constants for silicon nanowires change with decreasing diameter and at what diameter this becomes significant. Furthermore, in the case of EE Si NWs, recent theoretical modeling of roughened Si NWs indicate the need to include the entire phonon frequency range in calculations. 11,12 In par- ticular, disordered surfaces may substantially decrease the lifetime of propagating modes by at least one order of mag- nitude, and result in a larger proportion of heat being trans- mitted via higher frequency 70 cm -1 , nonpropagating, diffusive including optical modes similar to those in amor- phous Si. 12 To date there have been no temperature related Raman studies of EE Si NWs, which would potentially aid in the clarification and verification of emerging models for heat transfer in roughened nanostructures, and will improve our understanding of the behavior of their high-frequency phonons in general. In this Brief Report we study the temperature dependence of the Raman spectra of individual VLS Si NWs with diam- eters in the range of about 30 to 180 nm and of EE Si NWs with diameters in the range of about 50 to 130 nm, in the temperature ranges of 20–250 °C and 20–300 °C, respectively. The growth of VLS Si NWs was achieved by Au- catalyzed atmospheric pressure chemical vapor deposition following the procedure described previously. 13 A key fea- ture is that the Au catalyst was deposited as a film via gal- vanic displacement from potassium tetrachloroaurate KAuCl 4 . 14,15 Inhomogeneous dewetting and alloying of the Au film with the Si substrate immediately before growth results in a dense NW array with a broad diameter distribution, 16–18 and Si NWs greater than 30 nm in diameter grow preferentially in the 111 crystallographic direction. 19 A scanning electron microscope SEM image of as- synthesized VLS Si NWs is shown in Fig. 1a. Electrochemically etched Si NWs were synthesized di- rectly following the methods of Ref. 3, originally demon- strated by Peng et al. 20,21 Briefly, n type  =1–5  cm single-crystal on-axis Si111 dice were sonicated in acetone and isopropyl alcohol, rinsed with DI H 2 O, and dried in N 2 . The sample dice were then immersed in a solution of 40 mM AgNO 3 and 5 M HF for 2 h or less, at an etch rate of ap- proximately 5 m / hr. After etching, the samples were rinsed with deionized DI H 2 O to remove Ag dendrites that grow during the etching process, and immersed into concen- trated nitric acid for at least one hour to remove any remain- ing Ag. Finally, the samples were immersed in concentrated HF for 1 min to remove any SiO 2 . Figure 1b shows an SEM image of as-synthesized EE Si NWs. Wires were dispersed from suspension in ethanol onto Si111 dice, each covered with an evaporated tungsten film to mask the substrate. Since both the wires synthesized by PHYSICAL REVIEW B 80, 073306 2009 1098-0121/2009/807/0733064 ©2009 The American Physical Society 073306-1