1
1 1 2 1 2 1 S. Khachadorian , H. Scheel , A. Colli , A. Vierck , A. C. Ferrari and C. Thomsen 1 Institut für Festkörperphysik, TU Berlin ,Germany 2 Department of Engineering, University of Cambridge, Cambridge CB3 0FA, UK Temperature-dependent Raman Scattering of Silicon Nanowires Silicon nanowires were grown on Au-coated Si substrates by vapor transport and the first and second-order Raman spectra of the silicon nanowires studied in the temperature range from 77 to 900 K. The first-order and second-order Raman peaks were found to shift and broaden differently with increasing temperature. We believe this to be due to differences in the temperature dependence of the optical phonons at the à point and the X point. Moreover, the temperature dependence of the phonon branches of silicon nanowires differs from their bulk counterpart. The broadening of the second-order Raman peak of silicon nanowires reflects the changed phonon density of states. Our measurements also show that with increasing temperature the intensity of second-order Raman peak increases relative to the first-order Raman peak. SiNWs were grown by high-yield vapor transport (up to ~ 10 mg per run) [1]. Briefly, SiO powder is evaporated at about 1400 C in a horizontal tube furnace for 3 hours. The Si vapor condenses at about 900 C on a quartz substrate. The average wire diameter is 15 nm, consisting of an outer SiO2 shell of 2- 3 nm and a crystalline silicon core. During the synthesis process, Ar is allowed to flow (100 sccm) as carrier gas at pressures close to atmosphere (800- 1000 mbar). In parallel with SiNW growth, reducing the pressure proved to enhance the formation of Si-SiO2 nanochains, i.e., filamentary nanostructures where crystalline Si spheres are connected by SiO2 bridges of variable length . [1] Abstract: Sample: Results: 0 200 400 600 800 290 295 300 305 bulk Si SiNWs fit Raman frequency (cm -1 ) Temperature (K) Conclusion: References: 273 K 850 900 950 1000 1050 Raman Shift (cm -1 ) Raman Intensity (arb. units) 123 K 473 K (e) (d) (c) (b) (a) 873 K 673 K 273 K 400 450 500 550 600 Raman Shift (cm -1 ) Raman Intensity (arb. units) 123 K 473 K (e) (d) (c) (b) (a) 873 K 673 K 273 K 260 280 300 320 340 Raman Shift (cm -1 ) Raman Intensity (arb. units) 123 K 473 K (e) (d) (c) (b) (a) 873 K 673 K Raman Spectra of SiNWs at temperatures between77 and 800 K, measured with 632 nm excitation. Raman Spectra of SiNWs (2. order optical) at temperatures between77 and 800 K, measured with 632 nm excitation. Raman Spectra of SiNWs (2. order acoustic) at temperatures between77 and 800 K, measured with 632 nm excitation. 0 200 400 600 800 80 90 100 110 120 130 SiNWs Bulk Si Width (cm -1 ) Temperature (K) 200 400 600 800 930 940 950 960 970 frequency (cm -1 ) T (K) 200 400 600 800 0.05 0.10 0.15 0.20 0.25 SiNWs int.(2TA) / int.(1TO) SiNWs int.(2TO) / int.(1TO) Relative intensity Temperature (K) [1] [2] D. Berger and S. Selve [3] T. T. Oh and W. C. Kok, Physica Scripta. 55, 99-104, (1997) [4] Zixue Su, Jian Sha, Guowei Pan, Jianxun Liu, Deren Yang, Calum Dickinson, and Wuzong Zhou, J. Phys. Chem. B , 110, 1229, ( A. Colli and A. Fasoli and P. Beecher and P. Servati and S. Pisana and Y. Fu and A. J. Flewitt and W. I. Milne and J. Robertson and C. Ducati and S. De Franceschi and S. Hofmann and A. C. Ferrari, J. Appl. Phys. 102, 034302 (2007). Zentraleinrichtung Elektronenmikroskopie 2006). 0 200 400 600 800 2 4 6 8 10 12 SiNWs Bulk Si Fit FWHM (cm -1 ) Temperature (K) The width of 2. order optical Raman spectrum for SiNWs (black solid circles) and bulk Si (white circles). Inset: the center of the 2. order optical Raman spectrum. Raman frequency of SiNWs (blak solid circles ) and bulk Si (white circles )as a function of temperature. Raman frequencies of SiNWs (black solid cyrcles) and bulk Si (whitesolid circles) at temperatures between 77 and 800 K. 1. order optical (LTO) 2. order optical (LTO) 2. order acoustic (LTA) The full width at half maximum of Raman peak for SiNWs (black solid cyrcles) and bulk Si (white cyrcles) as a function of temperature. TEM image of SiNWs [2] TEM image of an individual SiNW core [2] The relative intensity int.(2TO)int/ int.(2TA) decreases with temperature, from about 4 at 123 K to 1.30 at 873 K for SiNWs (bulk Si). The optical phonon mode seems to have lower scattering efficiency than the acoustic mode with the decreasing temperature. The dependency of SiNWs matches with the bulk counterpart. This dependency was reported to be not the same for SiNWs and bulk Si[4]. (4 at 181 K) (1.38 at 665.15K ) 100 200 300 400 500 600 700 800 900 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 SiNWs int.(2TO) / int.(2TA) Bulk int.(2TO) / int.(2TA) Relative intensity Temperature (K) Sample Äù/ÄT ù(0) SiNWs -0.0232 4) ( 525.1(2) Bulk Si -0.0214(3) 528.2(1) Sample Äù/ÄT ù(0) flank first second third first second third SiNWs -0.069(3) -0.025(4) -0.034 8) ( 937(2) 1015(3) 1057(5) Bulk Si -0.056(1) -0.021(2) -0.017(4) 941(1) 1012(1) 1043(2) Sample Äù/ÄT ù(0) SiNWs -0.0149(5) 306.0(2) Bulk Si -0.0119(4) 308.2(1) The relative intensity int.(2TA) / int.(1TO) and Int.(2TO) int/ Int.(1TO) for SiNWs increases with temperature. This can be explained as following. The temperature-dependent hot activated phonon population of the vibrational state is given by the Bose-Einstein distribution function, n(ù, T) =[ exp(ħ ù/kâT – 1)]-1. For one-phonon (two-phonon) processes, relations between the Raman scattering section and temperature is proportional to n(ù, T) (n(ù, T) n(ù', T)). 0.0 0.2 0.4 0.6 0.8 1.0 0 1 2 3 4 5 6 D =0.3060 D =0.1561 D =0.2364 D Frequency (THz) [00k]- direction at low temp. [00k]- direction at high temp. G Reduced wave-vector X D =0.3060 Analytical phonon dispersion relations for diamond-like structures [3] (black curves) and the effect of temperature via decreased force-constant by 10% (red curves). 1. flank 3. flank 2. flank Sample dù/dT Mode O(Ã) 2 TA(X) 2 TO(X) 2 O(Ã) -1 Frequency (cm ) 520 300 923 1038 Theory 0.3060 2×0.1561 2×0.3060 2×0.0306 SiNWs (exp.) -0.0232(4) -0.0149(5) -0.069(3) -0.034(8) Bulk Si (exp.) -0.0214(3) -0.0119(4) -0.056(1) -0.017(4) 0.2 μm 5 nm 0.0 0.1 0.2 0.3 0.4 0.5 0 1 2 3 4 5 6 D =0.2208 D =0.1080 D = 0.2726 D =0.3077 D =0.3060 L Frequency (THz) [kkk]- direction at low temp. [kkk]- direction at high temp. L G Reduced wave-vector The temperature dependence of Raman frequencies of SiNWs was studied. The temperature dependent Raman frequencies of 1. order optical phonons differs from those of 2. order optical and acoustic. By use of a simple analytical model [3] these differences were explained. The relative intensity Int.(2TA) / Int.(1TO) and Int.(2TO) int/ Int.(1TO) increases with temperature. The second order optical phonon mode seems to have lower scattering efficiency than the second order acoustic mode with the decreasing temperature. Email: [email protected] 0 200 400 600 800 505 510 515 520 525 bulk Si SiNWs fit linear fit Raman frequency (cm -1 ) Temperature (K)
