Excimer laser annealing is shown to be very promising to promote Sb incorporation in Ge up to concentrations as high as 1 x 10(21) at./cm(3). However, we demonstrate that when Ge is melted by laser irradiation, a high excess of vacancies is generated in the molten region. These vacancies induce Sb electrical deactivation at the melt depth through the formation of Sb-m-V-n complexes that act as a sink for further Sb atoms, even leading Sb to back-diffuse towards the surface, against the concentration gradient. These results are fundamental for the realization of new generation Ge-based micro and optoelectronic devices.

Metal-free and Au-catalyzed silicon nanowires (Si-NWs) grown at low temperatures have been analyzed through transmission electron microscopy (TEM) and scanning electron microscopy (SEM), and their crystalline phase studied. All the observed nanowires are crystalline, grow along two different directions, ?110? or ?112?, and contain high density of planar defects, such as stacking faults (SFs) and twins. The defect size is comparable to the wire diameter for the metal-free process whilst it is much larger than the wire diameter for the Au-catalyzed Si-NWs. In this latter case parallel SFs may re-arrange and transform in a metastable rhombohedral 9R polytype structure whose formation mechanism is discussed. © 2011 Materials Research Society.

An experimental and theoretical study of the effect of excimer laser annealing (ELA) on B redistribution and electrical activation in Ge is reported. We performed detailed structural, chemical, and electrical characterizations of Ge samples implanted with B (20 keV, 1 x 10 15, or 1 x 10(16) B/cm(2)) and processed by ELA (lambda = 308 nm) with multiple pulses (1, 3, or 10). We also developed a diffusion model, in order to simulate the B redistribution induced by the ELA process. We found an anomalous impurity redistribution in the molten phase, which causes a dopant incorporation during the melt-growth at the maximum melt depth. The investigated samples showed a partial electrical activation of the B dopant. The inactivation of B in the samples implanted with 1 x 10(15) B/cm(2) was correlated to an oxygen contamination, while the poor electrical activation of B in the samples implanted with 1 x 10(16) B/cm(2) was related to the precipitation of the dopant, in good agreement with the experimental and theoretical results.

Future system-on-panel applications require further performance improvement of circuits based on polycrystalline silicon thin film transistors (TFTs). The biggest leverage in circuit performance can be obtained by reducing channel length from the typical current values of 3-6mm to 1mm, or less. Therefore, short channel effects in scaled down polysilicon TFTs will have to be controlled in order to allow proper operation of the circuits. In this work we review a number of specific aspects of the electrical characteristics of short channel devices (channel lengths down to 0.4 mm) combining electrical characteristics measurements and two-dimensional numerical simulations.

Self-heating-related instabilities have been investigatedin p-channel polycrystalline-silicon thin-film transistor,showing an anomalous transconductance (gm) increase. The gmincrease is a fingerprint of edge effects, resulting from a buildupof positive trapped charge in the gate oxide at the channel edges.This was confirmed by the annihilation of such positive chargesobtained by sequential hot-carrier bias-stress experiments. Fromthe analysis of the edge effects in devices with different channellengths, we were able, using 2-D numerical simulations, to determinethe size of the defected edge regions to be 400 nm.

In this paper we present the design and fabrication of a fully flexible sensorial system, composed ofthree different sensor units implemented on an ultrathin polyimide substrate of 8 ¼m thick. Eachunit is composed by a capacitive chemical sensor integrated with readout electronics. The sensorsare parallel plate capacitors with the top electrode properly patterned to allow analytes diffusioninto the dielectric that acts as chemical interactive material. Three different polymers,poly(tetrafluoroethene) (PTFE), poly(methyl 2-methylpropenoate) (PMMA) and benzocyclobutene(BCB), were used as dielectrics. A ring oscillator circuit, implemented with polysilicon thin filmtransistors (PS-nTFT), was used to convert the capacitance variations into frequency shifts. Theelectronic tests show oscillating frequencies of about 211 kHz ± 2 kHz and negligible frequencyshifts under different bending radius conditions. Furthermore, system response to some alcoholsconcentrations (Methanol, Ethanol, 1-Butanol, 1-Propanol) is reported and data analysis proves thatthe system is able to discriminate methanol from ethanol.

High densities of self-catalyzed Si nanowires have been grown at temperatures down to 320 degrees C on different Si substrates, whose surfaces have been roughened by simple physical or chemical treatments. The particular substrates are Si(110) cleavage planes, chemically etched Si(111) surfaces and microcrystalline Si obtained by laser annealing thin amorphous Si layers. The NW morphology depends on the growth surface. Transmission electron microscopy indicates that the NWs are made of pure Si with a crystalline core structure. Reflectivity measurements confirm this latter finding.

The nanoscaling of metamaterial structures represents a technological challenge toward their application in the optical frequency range. In this work we demonstrate tailored chiro-optical effects in plasmonic nanohelices, by a fabrication process providing a nanometer scale control on geometrical features, that leads to a fine tuning of operation band even in the visible range. Helicoidal 3D nanostructures have been prototyped by a bottom-up approach based on focused ion and electron beam induced deposition, investigating resolution limits, growth control and 3D proximity effects as a function of the interactions between writing beam and deposition environment. The fabricated arrays show chiro-optical properties at the optical frequencies and extremely high operation bandwidth tailoring dependent on the dimensional features of these 3D nanostructures: with the focused ion beam we obtained a broadband polarization selection of about 600 nm and maximum dissymmetry factor up to 40% in the near-infrared region, while with the reduced dimensions obtained by the focused electron beam a highly selective dichroic band shifted toward shorter wavelengths is obtained, with a maximum dissymmetry factor up to 26% in the visible range. A detailed finite difference time domain model highlighted the role of geometrical and compositional parameters on the optical response of fabricated nanohelices, in good agreement with experimental results.

Nickel enhanced amorphous Si crystallization and silicidation on polyimide were studied during multipulse excimer laser annealing (ELA) from submelting to melting conditions. A similar to 8 nm thick Ni film was deposited on a 100 nm thick alpha-Si layer at similar to 70 degrees C in order to promote partial nickel diffusion into silicon. In the submelting regime, Ni atoms distributed during deposition in alpha-Si and the thermal gradient due to the presence of the plastic substrate were crucial to induce low fluence (>= 0.08 J/cm(2)) Si crystallization to a depth which is strictly related to the starting Ni profile. Amorphous-Si crystallization is not expected on pure Si at those low fluences. Additional pulses at higher fluences do not modify the double poly-Si/alpha-Si structure until melting conditions are reached. At a threshold of similar to 0.2 J/cm(2), melting was induced simultaneously in the polycrystalline layer as well as in the residual alpha-Si due to a thermal gradient of similar to 200 degrees C. Further increasing the laser fluence causes the poly-Si layer to be progressively melted to a depth which is proportional to the energy density used. As a consequence of the complete Si melting, columnar poly-Si grains are formed above 0.3 J/cm(2). For all fluences, a continuous NiSi2 layer is formed at the surface which fills the large Si grain boundaries, with the beneficial effect of flattening the poly-Si surface. The results would open the perspective of integrating Ni-silicide layers as metallic contacts on Si during alpha-Si-crystallization by ELA on plastic substrate.

The diffusion and activation of arsenic implanted into germanium at 40 keV with maximum concentrations below and above the solid solubility (8 x 10(19) cm(-3)) have been studied, both experimentally and theoretically, after excimer laser annealing (lambda = 308 nm) in the melting regime with different laser energy densities and single or multiple pulses. Arsenic is observed to diffuse similarly for different fluences with no out-diffusion and no formation of pile-up at the maximum melt depth. The diffusion profiles have been satisfactorily simulated by assuming two diffusivity states of As in the molten Ge and a non-equilibrium segregation at the maximum melt depth. The electrical activation is partial and decreases with increasing the chemical concentration with a saturation of the active concentration at 1 x 10(20) cm(-3), which represents a new record for the As-doped Ge system. (C) 2014 AIP Publishing LLC.

We report a very simple, robust, and reliable on-chip fabrication method of a chemoresistive sensor based on silicon nanowires (NWs). Our method permits the use of nanowires without the need of their removal and transfer to a support different from the growth substrate. Our method, completely based on the silicon technology platform, exploits nanowires directly grown onto a selected area, over and between pre-patterned, interdigitated electrodes defined on oxidized silicon. The fabricated sensor is capable to detect NO2 down to a few ppb levels operating at room temperature. The sensor characteristics benefit of the presence of self-welded nanowires.

In this work we present a study of the reflectivity from highly disordered silicon nanowire films as a function of the wire size. Arrays of Au-catalyzed Si wires with length and diameter ranging from 0.15-0.2 microns and 30-50 nm up to 20-25 microns and 200-250 nm, respectively, were grown on top of either SiO2(1 microns)/Si(100) or Si(100) substrates. The integrated total reflection was measured in the 190-2500 nm spectral range. The results show that, increasing the wire size, the optical behavior of the Si wire film can be gradually tuned from that of an optical coating characterized by a graded effective refractive index to that of an ensemble of diffuse optical reflectors. In addition, we show how the optical analysis provides some important indications concerning the structural properties of the nanowires.

The electrical activation of B+ implanted at 20keV with a fluence of 1x10(15)cm(-2) in crystalline Ge, following a laser annealing (=308nm) with multipulses (1, 3, or 10), was studied. Incomplete activation was observed for all the irradiated samples. The inactivation of B was correlated to the presence of oxygen, coming from the native germanium oxide. The formation of B-O complexes occurs during the solidification of the Ge, hampering the substitutionality and the electrical activation of the dopant. We estimated the diffusivity of oxygen in liquid Ge, approximate to 3x10(-5)cm(2)s(-1), by fitting the experimental O concentration profiles. These studies clarify the key role played by oxygen on the electrical activation of B in Ge by laser annealing, and have to be considered for the fabrication of junctions in advanced scaled Ge-based devices. (C) 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

We report the fabrication of silica nanowires (NWs) decorated with Au or Ag nanoparticles (NPs) by dewetting thin metal films evaporated on the NWs. The Au or Ag NPs, displaced along the NWs, form a three-dimensional (3D) ensemble of metallic NPs in a macroporous structure. Their optical behavior results from the combination of the high white-light scattering of silica NWs with the selective absorption of the localized surface plasmon resonances (LSPRs) of the NPs, causing light trapping just at the LSPR wavelengths. Such a 3D plasmonic structure shows a strong dependence of the LSPR wavelength on the refractive index of the environment in which the 3D NP ensemble is immersed, a feature that makes them morphologically and optically peculiar materials appealing for sensing applications.

Ni-Si reaction and alpha-Si crystallization on polyimide were simultaneously induced by excimer laser annealing. A similar to 8 nm Ni film was deposited on Si in such a way that Ni atoms were also distributed within the alpha-Si layer. The role of Ni atoms during crystallization and surface silicidation was studied in the submelting regime and modeled by diffusion-reaction equations. It has been found that the starting Ni distribution in alpha-Si and the thermal gradient due to the plastic were crucial to induce Si crystallization. At a threshold of similar to 0.2 J/cm(2) melting is induced in the polycrystalline silicon layer and in the residual alpha-Si.

In this work, we experimentally investigate the chiro-optical properties of 3D metallic helical systems at optical frequencies. Both single and triple-nanowire geometries have been studied. In particular, we found that in single-helical nanostructures, the enhancement of chiro-optical effects achievable by geometrical design is limited, especially with respect to the operation wavelength and the circular polarization conversion purity. Conversely, in the triple-helical nanowire configuration, the dominant interaction is the coupling among the intertwined coaxial helices which is driven by a symmetric spatial arrangement. Consequently, a general improvement in the g-factor, extinction ratio and signal-to-noise-ratio is achieved in a broad spectral range. Moreover, while in single-helical nanowires a mixed linear and circular birefringence results in an optical activity strongly dependent on the sample orientation and wavelength, in the triple-helical nanowire configuration, the obtained purely circular birefringence leads to a large optical activity up to 8°, independent of the sample angle, and extending in a broad band of 500 nm in the visible range. These results demonstrate a strong correlation between the configurational internal interactions and the chiral feature designation, which can be effectively exploited for nanoscale chiral device engineering.

The threshold voltage (V-T) variations induced by the drain bias (V-ds) are investigated in polycrystalline silicon thin film transistors (TFTs), with channel length ranging from 20 to 0.4 mu m, by combining experimental measurements and two-dimensional (2D) numerical simulations. A careful analysis of the electrical characteristics in both subthreshold and off regime is performed, by taking in account also the effects of the leakage current field enhanced mechanisms on the overall generation-recombination rate. We show that the main causes of V-T variations are the drain induced barrier lowering (DIBL) and floating body effects (FBEs), induced by impact ionization. The relative influence of FBEs and DIBL is analyzed by performing numerical simulations with or without including the impact ionization model. A detailed analysis of the 2D Poisson equation has allowed to identify and evaluate the contributions of DIBL and FBEs to the threshold voltage variation when both are present. It is found that, in short channel TFTs at high drain bias, the V-T variations can't be attributed to DIBL effect alone and there is a noticeable contribution of the FBEs to the threshold voltage reduction. From the numerical simulations, the influence of FBEs and DIBL on the electrostatic barrier at source junction and its reduction for increasing V-ds is analyzed for long and short channel TFTs.

Three dimensional helical chiral metamaterials resulted in effective manipulation of circularly polarized light in the visible infrared for advanced nanophotonics. Their potentialities are severely limited by the lack of full rotational symmetry preventing broadband operation, high signal-to-noise ratio and inducing high optical activity sensitivity to structure orientation. Complex intertwined three dimensional structures such as multiple-helical nanowires could overcome these limitations, allowing the achievement of several chiro-optical effects combining chirality and isotropy. Here we report three dimensional triple-helical nanowires, engineered by the innovative tomographic rotatory growth, on the basis of focused ion beam-induced deposition. These three dimensional nanostructures show up to 37% of circular dichroism in a broad range (500-1,000 nm), with a high signal-to-noise ratio (up to 24 dB). Optical activity of up to 8 degrees only due to the circular birefringence is also shown, tracing the way towards chiral photonic devices that can be integrated in optical nanocircuits to modulate the visible light polarization.