Size effects in near-ultraviolet Raman spectra of few-nanometer-thick silicon-oninsulator nanofilms

We have fabricated Si-on-insulator (SOI) layers with a thickness h1 of a few nanometers and examined them by Raman spectroscopy with 363.8 nm excitation. We have found that phonon and electron confinement play important roles in SOI with h1<10 nm. We have confirmed that the first-order longitudinal optical phonon Raman band displays size-induced major homogeneous broadening due to phonon lifetime reduction as well as minor inhomogeneous broadening due to wave vector relaxation (WVR), both kinds of broadening being independent of temperature. Due to WVR, transverse acoustic (TA) phonons become Raman-active and give rise to a broad band in the range of 100–200 cm 1. Another broad band appeared at 200–400 cm 1 in the spectrum of SOI is attributed to the superposition of 1st order Raman scattering on longitudinal acoustic phonons and 2nd order scattering on TA phonons. Suppression of resonance-assisted 2-nd order Raman bands in SOI spectra is explained by the electron-confinement-induced direct band gap enlargement compared to bulk Si, which is confirmed by SOI reflection spectra. Published by AIP Publishing. [http://dx.doi.org/10.1063/1.4947021

The H\"older-Poincar\'e Duality for Lq,pL_{q,p}-cohomology

We prove the following version of Poincare duality for reduced $L_{q,p}$-cohomology: For any $1<q,p<\infty$, the $L_{q,p}$-cohomology of a Riemannian manifold is in duality with the interior $L_{p',q'}-cohomology for$1/p+1/p'=1$,$1/q+1/q'=1$.Comment: 21 page

Size effects in near-ultraviolet Raman spectra of few-nanometer-thick silicon-oninsulator nanofilms

We have fabricated Si-on-insulator (SOI) layers with a thickness h1 of a few nanometers and examined them by Raman spectroscopy with 363.8 nm excitation. We have found that phonon and electron confinement play important roles in SOI with h1<10 nm. We have confirmed that the first-order longitudinal optical phonon Raman band displays size-induced major homogeneous broadening due to phonon lifetime reduction as well as minor inhomogeneous broadening due to wave vector relaxation (WVR), both kinds of broadening being independent of temperature. Due to WVR, transverse acoustic (TA) phonons become Raman-active and give rise to a broad band in the range of 100–200 cm 1. Another broad band appeared at 200–400 cm 1 in the spectrum of SOI is attributed to the superposition of 1st order Raman scattering on longitudinal acoustic phonons and 2nd order scattering on TA phonons. Suppression of resonance-assisted 2-nd order Raman bands in SOI spectra is explained by the electron-confinement-induced direct band gap enlargement compared to bulk Si, which is confirmed by SOI reflection spectra. Published by AIP Publishing. [http://dx.doi.org/10.1063/1.4947021

Electron and Acoustic Phonon Confinement in Ultrathin-Body Ge on Insulator

International audienceRecent findings show significant enhancement of both the 488nm-excitation Raman signal and the electron mobility as ultrathin-body Ge-on-insulator (UTB GeOI) thickness (TGeOI) scaling below 13nm [1,2]. These phenomena could be attributed to a quantum size effect causing a change in the Ge energy band structure. Here, we study the characteristics of electron and acoustic phonon confinement in UTB GeOI with TGeOI = 1-20 nm. We observe electron band gap enlargement with a decrease in TGeOI and extract E1 exciton transverse effective mass value ~ 0.07 m0. Another size effect is the GeOI absorption enhancement associated with the increase in the electron density of states (DOS). For TGeOI < 5 nm, the E1(TGeOI) dependence suggests the influence of size-induced changes in the electron-phonon interaction on the E1 gap. This idea is supported by low-frequency Raman spectroscopy of GeOI confined acoustic phonons showing a significant change in acoustic phonon spectrum at TGeOI < 5 nm

Optical study of oriented double-Se₈-ring clusters and luminescent Se₂⁻ anions in LTA at extremely high selenium loading density

Abstract Recently, LTA-Se(1–8) samples with 1–8 Se atoms per cavity (simplified unit cell, large cavity + sodalite cage) obtained via adsorption at the temperature of ∼450 °C were reported. It was shown that single Se8 or single Se₁₂ ring are formed in the large LTA cavities, Se₈/Se₁₂ ring concentration ratio decreasing with an increase in the Se loading density. Contrary, in the present work, using Se vapour adsorption at ∼550 °C, we succeeded in encapsulation of ∼17 Se atoms per cavity (LTA-Se(17)) with a significant increase in the Se₈/Se₁₂ concentration ratio manifesting double Se₈-ring cluster formation in the most of the LTA large cavities, which is a step towards cluster crystal fabrication. According to our polarization/orientation Raman spectroscopic study of LTA-Se(17) single crystals, the orientations of the Se₈ and Se₁₂ appeared to be similar to those in previously investigated LTA-Se(1–8). Importantly, luminescent Se₂⁻ anions, oriented along the LTA 4-fold axes and located in the sodalite cages, are detected via Raman polarization/orientation dependencies of LTA-Se(17). Bright Se₂⁻ light emission with a maximum at ∼1.56 eV and vibronic structure is observed in the 1.3–1.8 eV spectral range. We show that the anions experience a compression in LTA which is slightly relaxing with a decrease in temperature producing an anomalous Raman band downshift. The compression of Se₂⁻ in LTA is weaker/stronger than that in sodalite/cancrinite, luminescence band photon energy depending on its strength. High concentration of regularly arranged Se₂⁻ in LTA suggests considering LTA-Se(17) as an important novel light-emitting material

Raman microscopy and infrared optical properties of SiGe Mie resonators formed on SiO 2 via Ge condensation and solid state dewetting

International audienceAll-dielectric photonics is a promptly developing field of optics and material science. The main interest atvisible and near-infrared frequencies is light management using high-refractive-index Mie-resonant dielectricparticles. Most work in this area of research was done exploiting Si-based particles. Here, we study monocrys-talline Mie-resonant particles made of Ge-rich SiGe alloys with refractive index higher than that of Si. Theseislands are formed via solid state dewetting of SiGe flat layers by using two different processes: i) dewetting ofmonocrystalline SiGe layers (60-80% Ge content) obtained via Ge condensation of SiGe on silicon on insulator,ii) dewetting of a SiGe layer deposited via molecular beam epitaxy on silicon on insulator and ex-situ Ge con-densation, forming a Ge-rich shell surrounding a SiGe-core. Using high-spatial-resolution Raman microscopywe monitor Ge content x and strain of flat layers and SiGe-islands. We observe strain relaxation associatedwith formation of trading dislocations in the SiGe islands compared to the starting SiGe layers, as confirmed byTEM images. For initial high Ge concentration in the flat layers, the corresponding Ge content in the dewettedislands is lower, owing to diffusion of Si atoms from Si or SiO 2 into SiGe islands. The Ge content also variesfrom particle to particle on the same sample. Size and shape of the dewetted particles depend on the fabricationprocess: thicker initial SiGe layers lead to larger particles. Samples with narrow island size distribution displayrather sharp Mie resonances in the 1000-2500 nm spectral range. Larger islands display Mie resonances atlonger wavelength. Positions of the resonances are in agreement with the theoretical calculations in the discretedipole approximation