Stress-Induced Self-Organization of Nanoscale Structures in SiGe/Si Multilayer Films

The design and fabrication of phononic crystals (PnCs) hold the key to control the propagation of heat and sound at the nanoscale. However, there is a lack of experimental studies addressing the impact of order/disorder on the phononic properties of PnCs. Here, we present a comparative investigation of the influence of disorder on the hypersonic and thermal properties of two-dimensional PnCs. PnCs of ordered and disordered lattices are fabricated of circular holes with equal filling fractions in free-standing Si membranes. Ultrafast pump and probe spectroscopy (asynchronous optical sampling) and Raman thermometry based on a novel two-laser approach are used to study the phononic properties in the gigahertz (GHz) and terahertz (THz) regime, respectively. Finite element method simulations of the phonon dispersion relation and three-dimensional displacement fields furthermore enable the unique identification of the different hypersonic vibrations. The increase of surface roughness and the introduction of sho...

Two-Dimensional Phononic Crystals: Disorder Matters

We report the changes in dispersion relations of hypersonic acoustic phonons in free-standing silicon membranes as thin as \sim 8 nm. We observe a reduction of the phase and group velocities of the fundamental flexural mode by more than one order of magnitude compared to bulk values. The modification of the dispersion relation in nanostructures has important consequences for noise control in nano and micro-electromechanical systems (MEMS/NEMS) as well as opto-mechanical devices.

Phonons in Slow Motion: Dispersion Relations in Ultra-Thin Si Membranes

We report the changes in dispersion relations of hypersonic acoustic phonons in free-standing silicon membranes as thin as ∼8 nm. We observe a reduction of the phase and group velocities of the fundamental flexural mode by more than 1 order of magnitude compared to bulk values. The modification of the dispersion relation in nanostructures has important consequences for noise control in nano- and microelectromechanical systems (MEMS/NEMS) as well as opto-mechanical devices.

Phonons in slow motion: dispersion relations in ultrathin Si membranes.

We report on fabrication and characterization of ultra-thin suspended single crystalline flat silicon membranes with thickness down to 6 nm. We have developed a method to control the strain in the membranes by adding a strain compensating frame on the silicon membrane perimeter to avoid buckling after the release. We show that by changing the properties of the frame the strain of the membrane can be tuned in controlled manner. Consequently, both the mechanical properties and the band structure can be engineered, and the resulting membranes provide a unique laboratory to study low-dimensional electronic, photonic, and phononic phenomena.

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Ultra-thin free-standing single crystalline silicon membranes with strain control

We report on inelastic light scattering (ILS) by longitudinal acoustic phonons in thin $\mathrm{Si}(001)$ layers (thickness $\ensuremath{\approx}30\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$). Calculations based on the photoelastic model are presented for unsupported and supported layers. We consider ILS by standing longitudinal acoustic modes along [001]. Our calculations take into account the spatial modulations of acoustic, optical, and photoelastic properties. We successively identify their contributions to the scattering efficiency and find that there is a strong interplay between acoustic, optical, and photoelastic cavity effects. The need to consider optical cavity effects is pointed out. It is shown here that they can be included in a convenient way in the scattered electromagnetic fields, by solving the wave equation in the presence of the polarization induced by the photoelastic effect. A detailed analysis of the scattering efficiency (peak frequencies, intensities, and widths) is presented. The dependence of the ILS spectra on film thickness and on substrate characteristics are addressed. Calculations are successfully compared to experimental data for thin Si membranes and silicon-on-insulator structures. It is shown that the inelastic light scattering involves a set of discrete quantized acoustic modes for membranes and a continuum of acoustic modes for silicon-on-insulator structures.

Inelastic light scattering by longitudinal acoustic phonons in thin silicon layers: From membranes to silicon-on-insulator structures

The local order around carbon atoms in epitaxial ${\mathrm{Si}}_{1\ensuremath{-}y}{\mathrm{C}}_{y}$ alloys (with $y$ around 1%) grown by Si molecular-beam epitaxy and thermal decomposition of ${\mathrm{C}}_{2}{\mathrm{H}}_{4}$ on Si(001) or Si(111) has been studied as a function of growth temperature. To this end, information from local probes such as x-ray photoelectron spectroscopy C $1s$ binding energies, experimental and simulated C $1s$ core-level x-ray photoelectron diffraction (XPD) distributions, and Raman spectroscopy are compared. Between the growth conditions leading to a solid solution of substitutional C at lower temperature and those leading to SiC precipitation at higher temperature, original metastable and ordered C-rich alloy phases appear, which may be coherently embedded in Si without degrading crystal quality. The results for such phases probed by the two latter techniques are indicative of a crystalline order implying concentrated third-nearest-neighbor (NN) carbon pairs. The comparison of experimental XPD distributions recorded in different azimuthal planes with simulated ones with C either in substitutional or in interstitial sites is in favor of substitution with a local contraction of the first-neighbor Si-C bond length between 10% and 20%. If we admit that the surface ordering of the C atoms in the Si(001) surface layers is the extension of such particular bulk arrangements in third NN C pairs we are able to explain an experimentally observed $c\ensuremath{-}(4\ifmmode\times\else\texttimes\fi{}4)$ low-energy electron diffraction pattern.

Third-nearest-neighbor carbon pairs in epitaxial Si 1 − y C y alloys: Local order for carbon in silicon characterized by x-ray photoelectron diffraction and Raman spectroscopy

Raman measurements on a thick GaAsN layer and on GaAsN/GaAs quantum well structures are reported. The scattering was excited close to resonance with the N-induced E+ transition, and detected in both Stokes and anti-Stokes regions including the low-frequency range around the Rayleigh line. A broad continuous scattering due to acoustic phonons is observed on the thick GaAsN layer. Calculations of the Raman efficiency showed that localization and mixing of the resonant electronic states well account for the measured spectral lineshapes. The localization length around a single nitrogen impurity is estimated and the band impurity formation discussed. Periodic oscillations of the scattered intensity are clearly observed on the quantum well structures. They are analyzed in terms of Raman interference effects due to spatial coherence of the resonant electronic states. We found that layering of the electronic density along the growth axis well accounts for the observed oscillations period, spectral envelope and interference contrast. The experimental data and the calculations support the formation of an impurity band.

https://hal.archives-ouvertes.fr/docs/00/02/81/07/PDF/submitprbBachelier021112.pdf

Resonant Raman scattering in GaAsN: Mixing, localization, and impurity band formation of electronic states

We present a new tool to measure island dimensions and spatial correlations in quantum dot (QD) multilayers. This approach is based on interference phenomena between the Raman scattering probability amplitudes associated with each QD. From the Raman interference envelopes we deduced mean island heights in different Si/Ge multilayer structures. From the interference contrast, we deduced the vertical correlation degree. The latter compare very well with previous transmission electron microscopy (TEM) and X-ray measurements.

Jesse Groenen

Papers

Inelastic light scattering by longitudinal acoustic phonons in thin silicon layers: From membranes to silicon-on-insulator structures

Third-nearest-neighbor carbon pairs in epitaxial Si 1 − y C y alloys: Local order for carbon in silicon characterized by x-ray photoelectron diffraction and Raman spectroscopy

Resonant Raman scattering in GaAsN: Mixing, localization, and impurity band formation of electronic states

A new tool for measuring island dimensions and spatial correlations in quantum dot multilayers: Raman scattering interferences

Resonant Raman Scattering by Acoustic Phonons in Quantum Dots