The role of quantum confinement (QC) in Si and Ge nanostructures (NSs) including quantum dots, quantum wires, and quantum wells is assessed under a wide variety of fabrication methods in terms of both their structural and optical properties. Structural properties include interface states, defect states in a matrix material, and stress, all of which alter the electronic states and hence the measured optical properties. We demonstrate how variations in the fabrication method lead to differences in the NS properties, where the most relevant parameters for each type of fabrication method are highlighted. Si embedded in, or layered between, SiO2, and the role of the sub-oxide interface states embodies much of the discussion. Other matrix materials include Si3N4 and Al2O3. Si NSs exhibit a complicated optical spectrum, because the coupling between the interface states and the confined carriers manifests with varying magnitude depending on the dimension of confinement. Ge NSs do not produce well-defined luminescence due to confined carriers, because of the strong influence from oxygen vacancy defect states. Variations in Si and Ge NS properties are considered in terms of different theoretical models of QC (effective mass approximation, tight binding method, and pseudopotential method). For each theoretical model, we discuss the treatment of the relevant experimental parameters.

Quantum confinement in Si and Ge nanostructures: Theory and experiment

The red shift and the broadening of the Raman signal from microcrystalline silicon films is described in terms of a relaxation in the q-vector selection rule for the excitation of the Raman active optical phonons. The relationship between width and shift calculated from the known dispersion relation in c-Si is in good agreement with available data. An increase in the decay rate of the optical phonons predicted on the basis of the same model is confirmed experimentally.

The one phonon raman spectrum in microcrystalline silicon

We report on the uniaxial strain and thermal conductivity of well-ordered, suspended silicon nanowire arrays between 10 to 20 nm width and 22 nm half-pitch, fabricated by extreme-ultraviolet (UV) interference lithography. Laser-power-dependent Raman spectroscopy showed that nanowires connected monolithically to the bulk had a consistent strain of $\ensuremath{\sim}0.1%$, whereas nanowires clamped by metal exhibited variability and high strain of up to 2.3%, having implications in strain engineering of nanowires. The thermal conductivity at room temperature was measured to be $\ensuremath{\sim}1\phantom{\rule{0.28em}{0ex}}\mathrm{W}/\mathrm{m}\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ for smooth nanowires and $\ensuremath{\sim}0.1\phantom{\rule{0.28em}{0ex}}\mathrm{W}/\mathrm{m}\phantom{\rule{4pt}{0ex}}\mathrm{K}$ for rougher ones, similar to results by other investigators. We found no modification of the bulk properties in terms of intrinsic scattering, and therefore, the decrease in thermal conductivity is mainly due to boundary scattering. Different types of surface roughness, such as constrictions and line-edge roughness, may play roles in the scattering of phonons of different wavelengths. Such low thermal conductivities would allow for very efficient thermal energy harvesting, approaching and passing values achieved by state-of-the-art thermoelectric materials.

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Strain and thermal conductivity in ultrathin suspended silicon nanowires

........................................................................................................................................................................... 3 Zusammenfassung ....................................................................................................................................................... 4 Acknowledgements ..................................................................................................................................................... 5 Table of

Materials Properties Of Ultra-Thin Silicon Nanowire Arrays Fabricated By Euv Interference Lithography

The oxide-assisted growth (OAG) mechanism for the growths of one-dimensional and quasi-two-dimensional nanomaterials has been described. The basics and the experimental base of the OAG mechanism have been presented. The general hypothesis of the growths by the OAG mechanism has been examined. High-temperature reaction, formation of clusters, and their impacts in the growths by the OAG mechanism have been investigated. Characteristic features of it have been elaborated. These features suggest that it is primarily employed for nanowire growths. Nanowire growths by it have therefore been illustrated and reviewed. The roles of metals and the roles of sulfur in the growths by the OAG mechanism have been accounted for. The formation of oxide sheaths in nanowires by the OAG mechanism has been demonstrated. Strengths and weaknesses of it in mediating nanowire growths have been stated. The importance and feasibility of it have been documented.

Oxide-Assisted Growth Mechanism

Silicon nanowires were successfully synthesized by a thermal evaporation method. We have observed a strong and broad emission band centered at 670?nm, which is attributed to the quantum confinement effect related to Si nanostructures embedded in the complex SiOx matrix. By fitting an experimental Raman spectrum, we confirm that the as-received wires possess crystalline silicon cores whose sizes were around 5?nm. Furthermore, the abnormal dependence of integral photoluminescence intensity on measured temperature was investigated.

Toan Thang Pham

Papers

Raman photoluminescence spectra of silicon nanowires synthesized by a vapor phase transport method