L. Fornarini

Intense visible photoluminescence from coloured LiF films on silicon

Lithium fluoride films, about 1.5 μm thick, thermally evaporated onto silicon substrates were subjected to low energy electron bombardment. Intense photoluminescence, extending from green to red in the visible spectral range was measured at room temperature in the irradiated areas, due to the presence of the laser active F2 and F3+ colour centres.

Theoretical Modeling of Laser Ablation of Quaternary Bronze Alloys: Case Studies Comparing Femtosecond and Nanosecond LIBS Experimental Data†

A model, formerly proposed and utilized to understand the formation of laser induced breakdown spectroscopy (LIBS) plasma upon irradiation with nanosecond laser pulses at different fluences and wavelengths, has been extended to the irradiation with femtosecond laser pulses in order to control the fractionation mechanisms which heavily affect the application of laser-ablation-based microanalytical techniques. The model takes into account the different chemico-physical processes occurring during the interaction of an ultrashort laser pulse with a metallic surface. In particular, a two-temperature description, relevant to the electrons and lattice of the substrate, respectively, has been introduced and applied to different ternary and quaternary copper-based alloys subjected to fs and ns ablation both in the visible (527 nm) and in the UV (248 nm). The model has been found able to reproduce the shorter plasma duration experimentally found upon fs laser ablation. Kinetic decay times of several copper (major element) emission lines have been examined together with those relevant to the main plasma parameters. The plasma experimental temperature, derived assuming a Boltzmann distribution, and the electron density following the Saha equation have been compared with the corresponding theoretical data. A satisfactory description of plasma parameters and main matrix constituent composition has been obtained in the time window where local thermal equilibrium was assumed for LIBS data analysis. Improved analytical capabilities are predicted upon delayed detection of plasma emission in femtosecond LIBS, in relation to the better LOD achieved and to the improved data reproducibility expected. Results support the utilization of ultrafast laser sources for trace detection, despite the residual fractionation occurring in the examined range of fluences which affects the linearity of experimental calibration curves built for tin and lead after internal standardization on copper. The validation of model results by experimental data allowed highlighting, from first principles, of the ablation mechanisms for the two temporal regimes and information on how this affects the accurate microanalysis of Cu-based alloys.

Laser crystallisation of poly-SiGe for microbolometers

Abstract Bolometers based on micromachined poly-SiGe as active elements have recently been demonstrated. The advantage of using poly-SiGe relies on its low thermal conductivity, high coefficient of temperature resistance and perfect compatibility with the IC silicon technology. In order to simplify a device integration of such elements, a combination of laser-assisted low thermal budget techniques such as laser-induced chemical vapour deposition and laser-assisted crystallisation has been proposed. The present paper shows the first results obtained using this “all laser-assisted” process for producing amorphous as well as polycrystalline SiGe alloys and the simulation of the crystallisation processes via numerical analysis for tuning the parameters of the crystallisation process.

Numerical modeling of laser induced phase transitions in silicon

A new one-dimensional numerical model of laser induced phase transitions in silicon is presented. In addition to the heat flow phenomena, it includes a first order description of the nucleation and growth of the new grains. The simulations are used to assess both the relevance of different nucleation mechanisms and the numerical values of some fundamental parameters.

Influence of laser fluence in ArF-excimer laser assisted crystallisation of a-SiGe:H films

Abstract Polycrystalline silicon germanium (poly-SiGe) coatings are drawing increasing attention as active layers in solar cells, bolometers and various microelectronic devices. As a consequence, alternative low-cost production techniques, capable to produce such alloys with uniform and controlled grain size, become more and more attractive. Excimer laser assisted crystallisation, already assessed in thin film transistor production, has proved to be a valuable “low-thermal budget” technique for the crystallisation of amorphous silicon. Main advantages are the high process quality and reproducibility as well as the possibility of tailoring the grain size in both, small selected regions and large areas. The feasibility of this technique for producing poly-SiGe films has been studied irradiating hydrogenated amorphous SiGe films with spatially uniform ArF-laser pulses of different fluences. Surface morphology, structure and chemical composition have been extensively characterised, demonstrating the need of using a “step-by-step” process and a careful adjustment of both, total number of shots and laser fluence at each “step” in order to diminish segregation effects and severe damages of the film surface and of segregation effects.

Quantitative elemental analyses of archaeological materials by laser-induced breakdown spectroscopy (LIBS): an overview

LIBS is one of the most promising techniques for rapid, in-situ elemental analyses of artworks. It does not require sample preparation, it is almost non destructive (micro sampling) and information both about major and trace elements could be obtained simultaneously. LIBS has been used to recognize the elements present in different archaeological materials and has been also proposed for on-line monitoring during the object cleaning by lasers. Quantitative determination of the material composition can supply useful information to restorers and help the object cataloguing. However, the analytical LIBS measurements on the archaeological materials were rarely reported, mainly due to difficulties to obtain the corresponding matrix-matched standards, required for the initial calibration. Alternatively, Calibration-Free (CF) approach could be used on some class of materials if all the major sample elements are detected and if the laser plasma preserves the material stochiometry. The latter condition is sometimes missing, as in the case of bronzes under nanosecond pulse laser ablation. We have developed a theoretical model for laser ablation of quaternary copper alloys, which allows for correction of the missing plasma stochiometry in CF approach. The model also predicts the optimal calibration for this type of material. In our recent work, we also obtained quantitative LIBS results on marbles by realizing the calibration standards starting from doped CaCO3 powders and by applying the corrections on the plasma parameters, different for the laboratory standards and marbles. Semi-quantitative LIBS results have been also obtained on multi-layered renaissance ceramics by subtraction of the contribution to plasma of each ceramic layer.

Texture analysis of LiF thin films evaporated onto amorphous substrates at different temperatures

Polycrystalline thin LiF films thermally evaporated on amorphous substrates show different crystallite orientations depending on the substrate temperature during evaporation. The recording of direct diffraction pole figure shows that the LiF crystallites present a single crystal texture, which can be described as a (522) [uvw] texture for high temperature deposition (250 to 300°C) and as (16 9 7) [uvw] texture for low temperature (room temperature) substrates. Scanning electron microscopy shows the presence of a globular film structure both for high and low temperature depositions with an average grain size of about 200 to 250 and 130 to 150 nm for high and low temperature evaporations, respectively. By rising the deposition temperature the (100) LiF crystallographic direction approaches the normal to the substrate plane with an increase of the film refractive index.

Realisation and characterisation of LiF/NaF thin film planar waveguides

Guided light propagation in two-layer LiF/NaF structures grown on glass substrates is reported for the first time. LiF/NaF based waveguides have been grown at different temperatures. The films texture analysis revealed both the influence of the underlying NaF buffer layer on the structure of the guiding LiF film and the importance of the film microstructure on the light propagation properties.

Passive and active optical waveguides in LiF thin films

Lithium fluoride thin films represent an innovative material for integrated optics. Passive waveguides may be produced by using a LiF/NaF two-layer structure deposited on any substrate material. On the other hand, low-energy electron beam irradiation of LiF polycrystalline films gives rise to the efficient formation of laser active lattice defects showing intense photoluminescence and sizable optical gain in the visible spectral range from green to red at room temperature. This irradiation at the same rime induces an increase of the real part of the refractive index, thus allowing one to exploit electron lithography technique to directly write integrated optical amplifiers and lasers in LiF films. Experimental results of the characterization of both passive and active waveguides are reported, demonstrating the feasibility of more complex circuits in this material.

Laser-assisted chemical vapor deposition of thick poly-Si layers for solar cells

Abstract The growth of polycrystalline silicon on glass by laser-assisted chemical vapor deposition has been studied with the aim of identifying a light absorber layer for solar cells, with superior material quality compared to other technologies available for low-temperature substrates. One-dimensional calculations of the thermal wave produced by laser irradiation have been used to elucidate the complex interaction of the molten silicon surface layer with the substrate during the growth. The experiments show the relevant role played by the seed layer used as the growth initiator. The morphology of the laser-crystallized films has been analysed by scanning electron microscopy and X-ray diffraction. Polysilicon films, 2 μm thick, with a compact structure consisting of 1–2-μm grains that are almost monocrystalline, have been obtained.