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Thickness-dependent phonon renormalization and enhanced Raman scattering in ultrathin silicon nanomembranes Seonwoo Lee, Kangwon Kim, Krishna P. Dhakal, Hyunmin Kim, Won Seok Yun, Jae Dong Lee, Hyeonsik Cheong, and Jong-Hyun Ahn Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.7b03944 • Publication Date (Web): 14 Nov 2017 Downloaded from http://pubs.acs.org on November 16, 2017
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Nano Letters
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Thickness-dependent phonon renormalization and enhanced Raman
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scattering in ultrathin silicon nanomembranes
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Seonwoo Lee†*, Kangwon Kim‡*, Krishna P. Dhakal†, Hyunmin Kim⊥, Won Seok Yun§, JaeDong Lee§, and Hyeonsik Cheong‡**, Jong-Hyun Ahn†** †
School of Electrical and Electronic Engineering, Yonsei University, Seoul, 03722, Republic of Korea. ⊥Companion
Diagnostics & Medical Technology Convergence Research Lab, DGIST, Daegu, 42988, Republic of Korea.
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§
Department of Emerging Materials Science, DGIST, Daegu, 42988, Republic of Korea.
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‡
Department of Physics, Sogang University, Seoul, 04107, Republic of Korea.
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*
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**
These authors (Lee S. and Kim K.) contributed equally to this work Corresponding Authors:
[email protected] and
[email protected] 1
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Nano Letters
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ABSTRACT
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We report on the thickness-dependent Raman spectroscopy of ultrathin silicon (Si)
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nanomembranes (NMs) whose thicknesses range from 2 to 18 nm using several excitation
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energies. We observe that the Raman intensity depends on the thickness and the excitation
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energy due to the combined effects of interference and resonance from the band-structure
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modulation. Furthermore, confined acoustic phonon modes in the ultrathin Si NMs were
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observed in ultralow-frequency Raman spectra, and strong thickness dependence was observed
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near the quantum limit, which was explained by calculations based on a photoelastic model. Our
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results provide a reliable method to accurately determine the thickness of Si NMs with
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thicknesses of less than a few nanometers.
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KEYWORDS
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Silicon nanomembranes, Raman spectroscopy, band-structure modulation, Ultralow-frequency
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Raman spectra
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Nano Letters
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Bulk silicon (Si) is the most widely used semiconductor in electronic1 and photonic2
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applications. Recently, as their dimensions have been reduced to the scale of a few nanometers,
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extensive research on Si nanomembranes (NMs) has been performed focusing on their
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controllable optical3,4, mechanical5-7, and electrical8,9 applications. Studies have demonstrated
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that for Si NM thicknesses below 5 nm, quantum confinement effect (QCE) is induced, which
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enhances photoluminescence (PL)10-13 and electroluminescence (EL) efficiencies14. This finding
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implies a high possibility of implementing Si-based optoelectronic devices. On the other hand,
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thickness reduction also modulates the electron-phonon interaction, which appears as different
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vibrational bands in Raman spectra. Thus, multiple studies on Raman scattering have been
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performed to investigate the thickness dependence of the phonon vibration in Si NMs15-21 and
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other Si nanostructures22-24. However, most of these optical investigations of Si NMs were only
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performed on commercially available silicon-on-insulator (SOI) wafers (i.e., thin top
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Si/SiO2/base Si). In this case, optical signals from the bottom base Si layer interfere with the PL
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and Raman spectra of the top Si layer. In fact, the penetration depths of excitation laser beams
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vary depending on their wavelength; for example, laser excitation wavelengths of 325, 532, and
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784.8 nm have penetration depths of 5, 1000, and 12000 nm, respectively, for single-crystal Si.25
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Of these, 325-nm UV lasers have mainly been adopted because the small penetration depth
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reduces the effect of the base Si layer. Mizuno et al. studied the thickness dependence of the
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Raman intensity of the optical mode in a thin Si layer for thicknesses of 0.2-10 nm at 520 cm−1
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using a 325-nm laser.16 However, top Si layer thicknesses of less than 5 nm resulted in unwanted
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interference with the base Si layer because the penetration depth of a 325-nm laser in Si is 5 nm.
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Therefore, accurate optical analysis of an ultrathin (
