R. Reitano

Relaxation and crystallization of amorphous silicon carbide probed by optical measurements

Abstract Optical spectroscopy in the visible (300–1100 nm) and in the infrared (400–4000cm−1) regions was used to monitor the relaxation and crystallization processes of pure amorphous silicon carbide (a-SiC) thin films upon annealing at temperatures between 200 and 1000°C. These films were obtained by ion implantation of crystalline material with 200keVkr+ at a fluence of 2 × 10 ions cm−2. The refractive index n and the absorption index k were calculated from the ultraviolet-visible transmittance and reflectance, and information on the vibration modes of the Si-C bonds was detected from infrared transmittance. Thermal treatment changes the optical properties of a-SiC; in particular, annealing at temperatures lower than 800°C resulted in a continuous variation in both the refractive index and the absorption index and in a decrease in the infrared silicon-carbon peak width. Annealing at higher temperatures produces sudden variations in the shape of the refractive index and in the infrared silicon-carbon pe...

Graphitic clusters in hydrogenated amorphous carbon induced by keV‐ion irradiation

Energy gap and hydrogen concentration have been measured in hydrogenated amorphous carbon (a‐C:H) after bombardment with 100 keV He+ and 300 keV Ar+ ion beams, up to ion fluences 3×1016 ions/cm2. a‐C:H films have been obtained by ion irradiation at low ion fluences of polystyrene layers and with a subsequent low‐temperature annealing (400 °C, 10 min). Experimental values for the energy gap (2.5–0.5 eV) and hydrogen concentration (7.4×1022–1.8×1022 H atoms/cm3) have been interpreted in terms of a graphitic cluster structure with a different size (3–20 A). We were able to determine experimentally the value of the bond integral ‖β‖ and we found it in good agreement with the calculated one for graphite.

Fast Melting of Amorphous Silicon Carbide Induced by Nanosecond Laser Pulse

We report the first experimental detailed study of laser induced surface melting on the nanoscale time scale of amorphous silicon carbide layers produced by ion implantation. Time-resolved reflectivity has been used to observe the fast liquid–solid–liquid transition features, and transmission electron microscopy (TEM) was used in order to study the structure resulting after the fast solidification following the laser induced melting. By means of the evaluation of the laser fluences required to induce melting of amorphous layers of different thickness on top of a crystalline substrate, we evaluated the thermal diffusion coefficient and the melting point of the amorphous material which occurred much lower than for crystalline material. Moreover, we give evidence of amorphous-to-crystal transitions occurring in the solid phase on the nanosecond time scale, for laser irradiation at fluences below the melting threshold. A quite different crystalline structure is observed for crystallization from the liquid phase than from the solid phase.

Transition from relaxed to derelaxed amorphous silicon: Optical characterization

Optical constants of relaxed, derelaxed, and partially relaxed amorphous silicon (a‐Si) in the range 0.4–0.9 μm are reported. The thermodynamical state of amorphous silicon (a‐Si) has been changed either by thermal treatments or low dose ion implantation. Ellipsometry has been used to evaluate the complex refractive index for several amorphous states with enthalpy content between that of the fully relaxed and fully derelaxed a‐Si. We observed a strong correlation between the electronic structure as probed by our optical measurements and the topological short‐range order as probed by Raman scattering.

Growth and characterization of SiC layers obtained by microwave-CVD

Abstract Silicon carbide is a wide band gap semiconductor of interest for its application in many electronic devices. In recent years, a large research activity has been devoted to growth techniques for amorphous, polycrystalline or even epitaxial structures. In this paper, we have reported results on microcrystalline and polycrystalline SiC layers grown by high temperature ECR-CVD over 4″ (100) silicon wafer in SiH4+CH4 gas mixtures. The structure of the films has been investigated by X-ray diffractometry, micro-Raman spectroscopy and transmission electron microscopy (TEM). Stoichiometric SiC films containing a complete chemical order, 3C–SiC crystals with orientation close to that of Si substrate and lateral dimensions larger than 100 nm have been obtained.

Structure and bonding in ion irradiated polystyrene

Abstract In this paper we report the Raman spectra of polystyrene films irradiated with 240 keV Ar+ ions at fluences between 1015 and 4 × 1016 cm−2. The presence of two broad bands ID, and IG at 1380 and 1560 cm−1 clearly indicates that ion bombardment induces a transition from the original molecular solid to hydrogenated amorphous carbon. Computer analysis of these spectra shows that the ratio I D I G increases with ion fluence till a saturation value of about 2. We explain this behaviour with the formation and growth of ordered clusters.

Excimer laser induced thermal evaporation and ablation of silicon carbide

Abstract Generally laser ablation of materials is performed at laser fluences above a definite threshold for which a dense plume of energetic particles is ejected from the irradiated target and considerably high ablation rate is achieved. No particular effort has been devoted to correlate the film quality to the employed laser fluence, also in correlation with different possible ablation mechanisms of ablation. We have performed measurements of ablation rate on silicon carbide single crystal targets by XeCl laser pulses as function of the laser fluence and we have investigated the angular distribution of the emitted material through the measurements of the thickness of the deposited films as well as the morphology of the craters in the irradiated targets as a function of the laser fluence. We have also modelled the melting dynamics and calculated the surface temperature and the consequent ‘pure thermal’ evaporation following each laser pulse by using the well known ‘thermal model’ often used for the laser annealing. In the presented paper we show that SiC ablation behaviour depends on the laser fluence so that three phenomenologically different energy density ranges may be defined.

Hydrogenated amorphous carbon synthesis by ion beam irradiation

keV ion beam irradiation at high fluence of polystyrene produces carbon films with interesting optical, electrical and mechanical properties. Very hard films with a very low energy gap of 0.6 eV or less are easily obtained with 250 keV argon beams at 1016 ions/cm2. Reflection energy loss and optical measurements have been used to study the structure and the bondings in amorphous carbon films (a-C:H) and carbon clusters of about 3 nm have been detected. The variation of the optical gap in ion irradiated polystyrene shows that a great variety of hydrogenated amorphous carbon films can be obtained by ion implantation by changing ion fluence and ion energy loss mechanism.

Tyrosine capped silver nanoparticles: A new fluorescent sensor for the quantitative determination of copper(II) and cobalt(II) ions.

Nanoparticles have been increasingly used as sensors for several organic and inorganic analytes. In this work, we report a study on the synthesis of novel highly fluorescent l-Tyr capped silver nanoparticles (AgNPs) and their use for the determination of metal ions. The AgNPs have been characterized by TEM, UV-Vis and Photoluminescence (PL) spectroscopy and dynamic light scattering (DLS) measurements and used for the quantitative determination of Co(II) and Cu(II) ions. In the l-Tyr capped AgNPs, the α-amino and α-carboxyl groups of the surface-confined amino acid can coordinate the entitled metal ions, giving rise to a decrease of the silver surface plasmon absorption, that is linearly correlated with the metal ions concentrations. The addition of Co(II) and Cu(II) solutions to the l-Tyr AgNPs also induces a paramagnetic quenching of the fluorescence in the PL spectra and the related Stern Volmer plots highlight a linear correlation over the whole concentration range for both metal ions, with a more pronounced effect for the copper(II) ion.

Enhancement of Er optical efficiency through bismuth sensitization in yttrium oxide

The process of energy transfer (ET) between optically active ions has been widely studied to improve the optical efficiency of a system for different applications, from lighting and photovoltaics to silicon microphotonics. In this work, we report the influence of Bi on the Er optical emission in erbium-yttrium oxide thin films synthesized by magnetron co-sputtering. We demonstrate that this host permits to well dissolve Er and Bi ions, avoiding their clustering, and thus to stabilize the optically active Er3+ and Bi3+ valence states. In addition, we establish the ET occurrence from Bi3+ to Er3+ by the observed Bi3+ PL emission decrease and the simultaneous Er3+ photoluminescence (PL) emission increase. This was further confirmed by the coincidence of the Er3+ and Bi3+ excitation bands, analyzed by PL excitation spectroscopy. By increasing the Bi content of two orders of magnitude inside the host, though the occurrence of Bi-Bi interactions becomes deleterious for Bi3+ optical efficiency, the ET process be...