Rosa Ruggeri

Crystallization of implanted amorphous silicon during millisecond annealing by infrared laser irradiation

We investigated the homogenous nucleation of crystalline grains in amorphous Si during transient temperature pulse of few milliseconds IR laser irradiation. The crystallized volume fraction is ∼80%. Significant crystallization occurs in nonsteady regime because of the rapid temperature variation (106 °C/s). Our model combines the time evolution of the crystal grain population with the consumption of the amorphous volume due to the growth of grains. Thanks to the experimental approach based on a laser source to heat α-Si and the theoretical model we extended the description of the spontaneous crystallization up to 1323 K or 250 K above the temperature investigated by conventional annealing.

Luminescence properties of SiOxNy irradiated by IR laser 808 nm: The role of Si quantum dots and Si chemical environment

We investigated optical, structural, and chemical properties of SiOxNy layers irradiated by CW IR laser during a time lapse of few milliseconds. We observed tunable photoluminescence signal at room temperature in the range 750–950 nm, without Si/SiO2 phase separation, depending on the IR laser power irradiation. Furthermore, no photoluminescence signal was recorded when the IR laser power density was high enough to promote phase separation forming Si quantum dots. By chemical analysis the source of the luminescence signal has been identified in a change of silicon chemical environment induced by IR laser annealing inside the amorphous matrix.

Crystallization of Deposited Amorphous Silicon by Infrared Laser Irradiation

We investigated the effect of infrared continous wave laser irradiation on amorphous silicon layers deposited by plasma enhanced chemical vapor deposition. Crystallization occurs via spontaneous nucleation at high temperature or by layer melting and solidification. Amorphous layers of thickness in the range 50-1000 nm have been crystallized up to 80% of their volume. We observed a two-dimensional growth when the thickness is 50 nm, whereas a three-dimensional growth occurs for thicker layers with an average grain size of < 30 nm, weakly dependent on the thickness. We also found that hydrogen desorption occurs without layer degradation, regardless of the layer thickness. The coalescence of hydrogen before outdiffusion produces voids only for layers thicker than 200 nm, whose presence reduces the average grain size by a factor of 2-3.

Giant photoluminescence emission in crystalline faceted Si grains

Empowering an indirect band-gap material like Si with optical functionalities, firstly light emission, represents a huge advancement constantly pursued in the realization of any integrated photonic device. We report the demonstration of giant photoluminescence (PL) emission by a newly synthesized material consisting of crystalline faceted Si grains (fg-Si), a hundred nanometer in size, assembled in a porous and columnar configuration, without any post processing. A laser beam with wavelength 632.8 nm locally produce such a high temperature, determined on layers of a given thickness by Raman spectra, to induce giant PL radiation emission. The optical gain reaches the highest value ever, 0.14 cm/W, representing an increase of 3 orders of magnitude with respect to comparable data recently obtained in nanocrystals. Giant emission has been obtained from fg-Si deposited either on glass or on flexible, low cost, polymeric substrate opening the possibility to fabricate new devices.

Raman and photoluminescence spectroscopy of Si nanocrystals: Evidence of a form factor

We investigated the quantum confinement in Si nanocrystals embedded in a SiO2 matrix. The size was accurately controlled in the range 3–8 nm by annealing at high temperature Si/SiO2 multilayers fabricated by chemical vapour deposition. Raman shift and line width were compared with existing theoretical models for each cluster size. We found evidence of uni-dimensional confinement in 3 nm crystals, whereas for 4.5 nm crystals the confinement appears three-dimensional. This conclusion is supported by the luminescence spectra shifting towards higher wavelengths for the smaller size, in opposite direction for larger sizes.

Strong infrared photoluminescence in highly porous layers of large faceted Si crystalline nanoparticles.

Almost all physical processes in solids are influenced by phonons, but their effect is frequently overlooked. In this paper, we investigate the photoluminescence of large silicon nanoparticles (approximately 100 nm size, synthesized by chemical vapor deposition) in the visible to the infrared detection range. We find that upon increasing laser irradiance, an enormous photoluminescence emission band appears in the infrared. Its intensity exhibits a superlinear power dependence, increasing over four orders of magnitude in the investigated pump power range. Particles of different sizes as well as different shapes in porous layers are investigated. The results are discussed taking into account the efficient generation of phonons under high-power pumping, and the reduced capability, porosity dependent, of the silicon nanoparticles to exchange energy with each other and with the substrate. Our findings are relevant for heat management strategies in silicon.

Octahedral faceted Si nanoparticles as optical traps with enormous yield amplification

We describe a method for the creation of an efficient optical scatter trap by using fully crystalline octahedral Silicon nanoparticles (Si-NPs) of approximately 100 nanometres in size. The light trapping, even when probing an isolated nanoparticle, is revealed by an enormous amplification of the Raman yield of up to 108 times that of a similar Si bulk volume. The mechanism conceived and optimised for obtaining such a result was related to the capability of a Si octahedron to trap the light because of its geometrical parameters. Furthermore, Si-NPs act as very efficient light scatterers not only for the direct light beam but also for the trapped light after it escapes the nanoparticle. These two effects are observed, either superimposed or separated, by means of the Raman yield and by photoluminescence enhancements. The inductively coupled plasma synthesis process performed at a temperature of only 50°C allows for the ubiquitous use of these particles on several substrates for optical and photovoltaic applications.

Structural properties of relaxed thin film germanium layers grown by low temperature RF-PECVD epitaxy on Si and Ge (100) substrates

We report on unusual low temperature (175 °C) heteroepitaxial growth of germanium thin films using a standard radio-frequency plasma process. Spectroscopic ellipsometry and transmission electron microscopy (TEM) reveal a perfect crystalline quality of epitaxial germanium layers on (100) c-Ge wafers. In addition direct germanium crystal growth is achieved on (100) c-Si, despite 4.2% lattice mismatch. Defects rising from Ge/Si interface are mostly located within the first tens of nanometers, and threading dislocation density (TDD) values as low as 106 cm−2 are obtained. Misfit stress is released fast: residual strain of −0.4% is calculated from Moire pattern analysis. Moreover we demonstrate a striking feature of low temperature plasma epitaxy, namely the fact that crystalline quality improves with thickness without epitaxy breakdown, as shown by TEM and depth profiling of surface TDD.

Electrical Properties of Ultrathin SiO

We found a strong correlation between the layer porosity and electrical properties of a SiO2 layer deposited by inductively coupled plasma chemical vapor deposition. At 50 °C the SiO2 layers have a double structure: a dense layer in contact with the substrate and a porous layer on top of it. The critical thickness at which voids appear depends on the deposition rate. Breakdown voltage and charge trapping performances of SiO2 layers are very good if the thickness is below the critical value and deteriorate significantly in thicker, porous, layers. Layer porosity is absent when the sample is deposited at 250 °C.

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We investigated the role of Nitrogen in the luminescence emission of amorphous SiOxNy layers irradiated by Infrared Laser. Variable content of Nitrogen (0-22%) has been obtained by magnetron sputtering or plasma enhanced chemical vapor deposition techniques. We demonstrate that emission is obtained from the amorphous matrix and the emission peak shifts towards longer wavelengths with increasing the N concentration. The peak shift is associated with the formation of O-Si-N chemical combinations. In contrast, no red shift was observed when N is absent.