Interplay between phonon confinement effect and anharmonicity in silicon nanowires

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Interplay between phonon confinement effect and anharmonicity in silicon nanowires

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Physica E 38 (2007) 109–111 Interplay between phonon confinement effect and anharmonicity in silicon nanowires M.J. Konstantinovic ´ a,b,Ã a SCK CEN, Studiecentrum voor Kernenergie/Centre d’Etude de l’Energie Nucle ´ aire, Boeretang 200, B-2400 Mol, Belgium b Institute of Physics, P.O. Box 68, 11080 Belgrade, Serbia Available online 16 December 2006 Abstract Getting light out of silicon is a difficult task since the bulk silicon has an indirect energy electronic band gap structure. It is expected that this problem can be circumvented by silicon nanostructuring, since the quantum confinement effect may cause the increase of the silicon band gap and shift the photoluminescence into the visible energy range. The increase in resulting structural disorder also causes the phonon confinement effect, which can be analyzed with a Raman spectroscopy. The large phonon softening and broadening, observed in silicon nanowires, are compared with calculated spectra obtained by taking into account the anharmonicity, which is incorporated through the three and four phonon decay processes into Raman scattering cross-section. This analysis clearly shows that the strong shift and broadening of the Raman peak are dominated by the anharmonic effects originating from the laser heating, while confinement plays a secondary role. r 2007 Elsevier B.V. All rights reserved. PACS: 78.30.Am; 78.20.Àe; 78.66.Db Keywords: Nanowires; Silicon; Raman 1. Introduction For the past 10 years, researchers have tried to coax light out of silicon, with varying degrees of success. The main problem is that the indirect energy band gap electronic structure of bulk silicon makes it not suitable for optoelectronic applications. It is expected that this problem can be circumvented by silicon nanostructuring, since the quantum confinement effect may cause the increase in the silicon band gap and shift the photoluminescence into the visible energy range. The expectation that reducing dimensions of silicon structures would turn this material from indirect into direct band gap system triggered a lot of research in the field of opt oelectronics. However, despite a large amount of research, the exact origin of the increased luminescence and a strong Raman phonon softening, reported in previous works on Si clusters [1–8], are not fully unde rstood. Recently, it was shown [12] that anharmonicity, due to the local heating effect, represents the main source of phonon softening and broadening, while the phonon confinement plays a secondary role. Here, I extend this investigation to the silicon nanowires, reanalyze the local heating effect that is always present in these kind of experiments, and compare the results with those in silicon nanoclusters. 2. Experiment The sample used in this investigation is made of an array of silicon nanowires (nanopillars, nanorods) obtained by electrochemical etching process [9]. Fig. 1 shows a scanning electron micrograph of a typical part of the sample. Nanowires are vertically aligned with a typical length of about 10 mm and a diameter of about 50–500 nm. Some nanorods are found to be detached from the non-reacted part of the silicon crystal, lying in the horizontal position on the top of the sample. Micro-Raman spectra were taken in ambient conditions with excitation from the 514.5 nm ARTICLE IN PRESS www.elsevier.com/locate/physe 1386-9477/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.physe.2006.12.011 Ã Institute of Physics, P.O. Box 68, 11080 Belgrade, Serbia. Tel.: +381 11 3162190; fax: +381 11 3160346. E-mail address: konst@phy.bg.ac.yu. line of an Ar laser, using powers at the sample surface that varied from 10 to 500 mW. The Raman spectra were measured in the backscattering configuration and analyzed using a DILOR triple spectrometer with liquid-nitrogen- cooled charge-coupled-device detector. 3. Results and discussion Fig. 2 shows typical Raman spectra taken from silicon nanowires. A dramatic change in the spectra is observed for the moderate increase in the laser power. There is a strong red shift of the first-order phonon mode at 520 cm À1 , which is accompanied by a substantial broad- ening, as evident from a series of Stokes and anti-Stokes Raman spectra taken with laser powers ranging from 10 to 500 mW. The softening and broadening are large, up to 30 and 20 cm À1 , respectively. It can be also seen in Figs. 2 and 3 that the intensity ratio between the Stokes and anti- Stokes part of the spectrum decreases as the laser power increases. This implies that a dramatic change in the local temperature of the nanowires takes place during the measurements, as expected in micro-Raman experiments where the laser light is focused on the micrometer-size area. Typically, the silicon bulk samples do not exhibit any shifts and broadening of the first-order phonon mode at 520 cm À1 for the laser powers in the range used in experiment. Moreover, the Raman spectra show the existence of the symmetric phonon line shapes, regardless of the frequency shifts and the bro adening. This observa- tion is in clear contradiction with a strong asymmetric line shape expected in the case of quantum phonon confine- ment [10]. A comparison between calculated and measured Raman spectra is shown in Fig. 3. The calculated curve is obtained by taking the Lorentz line shape that includes the anharmonic effects via three and four phonon decay processes [11,12]: oðk; TÞ¼oðkÞþDðTÞ, DðTÞ¼A 1 þ 2 e _o=2k B T À 1  þ B 1 þ 3 e _o=3k B T À 1 þ 3 ðe _o=3k B T À 1Þ 2  , oðkÞ¼ ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ð1:7 þ cosðpk=2ÞÞ10 5 q , where o(k) is the silicon phonon dispersion at 300 K. The phonon line width is given by GðTÞ¼C 1 þ 2 e _o=2k B T À 1  þ D 1 þ 3 e _o=3k B T À 1 þ 3 ðe _o=3k B T À 1Þ 2  , where A, B, C and D are anharmonic constants. The temperature difference between spectra 1 and 2 presented ARTICLE IN PRESS Fig. 1. Scanning electron microscopy picture of the Si-nanowires. -540 -520 -500 -480 -460 460 480 500 520 540 Raman shift (cm) -1 Laser power Intensity (arb.units) anti-Stokes Stokes Fig. 2. The Stokes and anti-Stokes Raman spectra of silicon nanowires measured with different laser line power densities. anti-Stokes Fig. 3. The Stokes and anti-Stokes Raman spectra of silicon nanowires measured at two different laser powers. The full lines are calculated spectra. M.J. Konstantinovic ´ / Physica E 38 (2007) 109–111110 in Fig. 3 is estimated from the Stokes and anti-Stokes intensity ratio to be around 600 K. The agreement between calculation and experimental data is very good, showing that the shift and broadening arise mainly due to local laser heating effect. The expected peak asymmetry, due to phonon confinement effect, is not observed. Similar results are obtained in the case of silicon nanoclusters [12]. The small peak asymmetry observed in the case of silicon nanoclusters at the low-frequency side of the peak, is in the spectra of Fig. 3 represented by a low- intensity hump. The fact that this feature is suppressed in nanowires in comparison to nanoclusters suggests that it might originate from the Raman scattering of amorphous silicon. This can be understood as being the consequence of the difference between the preparation techniques. The silicon nanoclusters were produced by the laser vaporiza- tion techn ique, which resulted in the formation of nanoclusters on the top of the amorphous film. On the other hand, the nanowires are produced by starting from the silicon crystalline material (eching of the crystalline bulk sample) so the amorphous signal is expected to be much smaller. The Raman spectra of silicon nanowires point to the main problem related to the optical characterization of nanostructures: the hea t dissipation during the experiment. It is, however, expected that the heat dissipation depends on the actual size of the wire. Moreover, the size irregularity of the wire sample might enhance the contribution of the anharmonic decay as well. In this type of experiments, one usually measures the averaged signal from various nano-sized structures, consistent with certain temperature distribution for different wires. Because of that, the question of individual wire contribution cannot be addressed since the laser spot size is much larger than the size of a single wire. 4. Conclusion This work shows that strong anharmonic effects exist in the silicon sample consisting of an array of nanowires. It is found that the shift and broadening of the first-order Raman peak are dominated by the local heating effect, while the confinement plays a secondary role. References [1] L.T. Canham, Appl. Phys. Lett. 57 (1990) 1046. [2] L.T. Canham, Phys. Stat. Sol. B 190 (1995) 9. [3] M.V. Wolkin, et al., Phys. Rev. Lett. 82 (1999) 197. [4] H. Richter, Z.P. Wang, L. Ley, Solid State Commun. 39 (1981) 625. [5] Z. Iqbal, et al., Appl. Phys. Lett. 36 (1980) 163. [6] Z. Sui, et al., Appl. Phys. Lett. 60 (1992) 2086. [7] Y. Kanemitsu, et al., Phys. Rev. B 48 (1993) 2827. [8] P. Mishra, K.P. Jain, Phys. Rev. B 62 (14) (2000) 790. [9] S. Bersiere, et al., in preparation. [10] R. Shuker, R.W. Gammon, Phys. Rev. Lett. 25 (1970) 222. [11] M. Balkanski, R.F. Wallis, E. Haro, Phys. Rev. B 28 (1983) 1928. [12] M.J. Konstantinovic, et al., Phys. Rev. B 66 (2002) 161311(R). ARTICLE IN PRESS M.J. Konstantinovic ´ / Physica E 38 (2007) 109–111 111 . Physica E 38 (2007) 109–111 Interplay between phonon confinement effect and anharmonicity in silicon nanowires M.J. Konstantinovic ´ a,b,Ã a SCK CEN, Studiecentrum. circumvented by silicon nanostructuring, since the quantum confinement effect may cause the increase in the silicon band gap and shift the photoluminescence into the visible

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  • Interplay between phonon confinement effect and anharmonicity in silicon nanowires

    • Introduction

    • Experiment

    • Results and discussion

    • Conclusion

    • References

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