Gennaro Picardi

Determining the stress tensor in strained semiconductor structures by using polarized micro-Raman spectroscopy in oblique backscattering configuration

We present a characterization technique for the determination of the stress tensor as well as of the crystallographic orientation of strained semiconductor structures. The technique is based on a polarized oblique incidence micro-Raman experiment in a backscattering configuration. A methodology relating the stress-induced frequency shifts and linewidths of the phonon peak to the stress tensor components within the adopted experimental configuration was developed. The method consists in monitoring the variations of the stress-sensitive peak frequencies and linewidths while rotating stepwise the sample about its normal. The practical application of the technique is illustrated on a Si∕SiGe sample microelectronic structure demonstrating a full plane stress tensor determination.

Assessing individual radial junction solar cells over millions on VLS-grown silicon nanowires

Silicon nanowires (SiNWs) grown on low-cost substrates provide an ideal framework for the monolithic fabrication of radial junction photovoltaics. However, the quality of junction formation over a random matrix of SiNWs, fabricated via a vapor-liquid-solid (VLS) mechanism, has never been assessed in a realistic context. To address this, we probe the current response of individual radial junction solar cells under electron-beam and optical-beam excitations. Excellent current generation from the radial junction units, compared to their planar counterparts, has been recorded, indicating a high junction quality and effective doping in the ultra-thin SiNWs with diameters thinner than 20 nm. Interestingly, we found that the formation of radial junctions by plasma deposition can be quite robust against geometrical disorder and even the crossings of neighboring cell units. These results provide a strong support to the feasibility of building high-quality radial junction solar cells over high-throughput VLS-grown SiNWs on low-cost substrates.

Laser heating versus phonon confinement effect in the Raman spectra of diamond nanoparticles

Nanodiamond particles with typical diameters of 20 and 6xa0nm produced by high pressure high temperature or detonation processes have been studied by micro-Raman spectroscopy. We show that the frequency downshift and broadening of the first-order diamond phonon band is not uniquely related to phonon confinement, as commonly assumed. Local heating caused by the focused laser light must be also taken in account, since it may affect the Raman spectrum in a similar fashion, even at relatively low laser power levels. A combined theoretical model considering both effects (quantum confinement and local heating) on the excited phonon modes is presented and adopted for the simulation of the experimental data. We observe different heating behaviours upon laser illumination depending on the particles origin, thus underscoring the importance to compensate for this effect before retrieving structural parameters.

Exchange of Methyl‐ and Azobenzene‐Terminated Alkanethiols on Polycrystalline Gold Studied by Tip‐Enhanced Raman Mapping

Mixed thiol self-assembled monolayers (SAMs) presenting methyl and azobenzene head groups were prepared by chemical substitution from the original single-component n-decanethiol or [4-(phenylazo)phenoxy]hexane-1-thiol SAMs on polycrystalline gold substrates. Static contact-angle measurements were carried out to confirm a change in the hydrophobicity of the functionalized surfaces following the exchange reaction. The mixed SAMs presented contact-angle values between those of the more hydrophobic n-decanethiol and the more hydrophilic [4-(phenylazo)phenoxy]hexane-1-thiol single-component SAMs. By means of tip-enhanced Raman spectroscopy (TERS) mapping experiments, it was possible to highlight that molecular replacement takes place easily and first at grain boundaries: for two different mixed SAM compositions, TERS point-by-point maps with <50 nm step sizes showed different spectral signatures in correspondence to the grain boundaries. An example of the substitution extending beyond grain boundaries and affecting flat areas of the gold surface is also shown.

Nanophotonics for ultrathin crystalline silicon photovoltaics: When photons (actually) meet electrons

Nanopatterning has recently demonstrated to be an efficient method for boosting the light absorption of thin films (< 50 μm) of crystalline silicon. However, convincing solar cell results are still missing. The goal of the European project PhotoNVoltaics is to investigate the impacts that nanopatterns have on thin crystalline silicon solar cells and to identify the conditions for their efficient integration. The present contribution presents the main findings of the consortium so far. Optical modeling and optimization of nanopatterned thin-film c-Si cells have indicated a few trends regarding the design of the optimum pattern for 1-40 μm thin foils: merged inverted nanopyramids, with their progressive profile seem to provide the best combination of antireflective and light trapping properties, together with negligible surface damage and best coating template for subsequent process steps. But despite the high Jsc enhancement that these nanopatterns are expected to bring, and despite the higher efficiency of these nanophotonic structures for 1-2 μm-thin foils, the absolute Jsc values indicate that thicker foils will have to be preferred if direct competition with wafer-based and thin-film technologies is concerned. An ideal structure is taking shape as an IBC heterojunction cell, with a frontside nanopattern and a thickness of 40 μm.

Polarization-Sensitive Tip-Enhanced Raman Scattering

Tip-Enhanced Raman Spectroscopy (TERS) is a very promising analytical technique for high sensitivity, high spatial resolution analysis of the physical and chemical properties of nanoscale materials including nanocrystals, biomolecules, carbon-based nanostructures and nanometer-size devices. Polarized TERS, in addition, offers novel opportunities for high contrast spectroscopy and imaging of semiconductor crystals and crystalline nanostructures. This chapter reviews the current state-of-the-art in polarization-sensitive TERS focusing the attention on the experimental implementations of the technique, on the light scattering properties of the metallic probes, on the Raman signal enhancement mechanisms and, finally, on the applications of this technique.

Determination of the biaxial stress in strained silicon nano-stripes through polarized oblique incidence Raman spectroscopy

We report on the experimental determination of the biaxial stress characteristic of the strain state present in strained silicon nano-stripes on insulator structures. Conventional confocal backscattering Raman spectroscopy being insensitive to the tensorial nature of strain, a methodology based on the use of polarized oblique incidence backscattering Raman spectroscopy is employed. The stress component values thus obtained are compared with those provided by grazing incidence x-ray diffraction as a reference technique. By combining the oblique backscattering configuration with polarization control of the incident and scattered beams, an efficient method for the accurate measurement of biaxial stress in patterned silicon structures results.

LIGHT DEPOLARIZATION EFFECTS IN TIP ENHANCED RAMAN SPECTROSCOPY OF SILICON (001) AND GALLIUM ARSENIDE (001)

ETURCHE, c M. LAMY DE LA CHAPELLE, d D. BARCHIESI, c F. BONACCORSO, a♮ C. DANDREA, a♯ M. CHAIGNEAU, e G. PICARDI, e† AND R. OSSIKOVSKI e ABSTRACT. We report on the effects of light depolarization induced by sharp metallic tips in Tip-Enhanced Raman Spectroscopy (TERS). Experiments on Si(001) and GaAs(001) crystals show that the excitation field depolarization induces a selective enhancement of specific Raman modes, depending on their Raman tensor symmetry. A complete polar- ization analysis of the light backscattered from the tip confirms the TERS findings. The spatial confinement of the depolarization field is studied and its dependence on the excita- tion wavelength and power are explored.