Letter pubs.acs.org/NanoLett
Thickness-Dependent Phonon Renormalization and Enhanced Raman Scattering in Ultrathin Silicon Nanomembranes Seonwoo Lee,†,⊥ Kangwon Kim,∥,⊥ Krishna P. Dhakal,† Hyunmin Kim,‡ Won Seok Yun,§ JaeDong Lee,§ Hyeonsik Cheong,*,∥ and Jong-Hyun Ahn*,† †
School of Electrical and Electronic Engineering, Yonsei University, Seoul 03722, Republic of Korea Companion Diagnostics & Medical Technology Convergence Research Lab and §Department of Emerging Materials Science, DGIST, Daegu 42988, Republic of Korea ∥ Department of Physics, Sogang University, Seoul 04107, Republic of Korea ‡
S Supporting Information *
ABSTRACT: We report on the thickness-dependent Raman spectroscopy of ultrathin silicon (Si) nanomembranes (NMs), whose thicknesses range from 2 to 18 nm, using several excitation energies. We observe that the Raman intensity depends on the thickness and the excitation energy due to the combined effects of interference and resonance from the band-structure modulation. Furthermore, confined acoustic phonon modes in the ultrathin Si NMs were observed in ultralow-frequency Raman spectra, and strong thickness dependence was observed near the quantum limit, which was explained by calculations based on a photoelastic model. Our results provide a reliable method with which to accurately determine the thickness of Si NMs with thicknesses of less than a few nanometers. KEYWORDS: Silicon nanomembranes, Raman spectroscopy, band-structure modulation, ultralow-frequency Raman spectra
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intensity of the optical mode in a thin Si layer for thicknesses of 0.2−10 nm at 520 cm−1 using a 325 nm laser.16 However, top Si layer thicknesses of less than 5 nm resulted in unwanted interference with the base Si layer because the penetration depth of a 325 nm laser in Si is 5 nm. Therefore, accurate optical analysis of an ultrathin (
