S. Scalese

Substitutional carbon incorporation in epitaxial Si1−yCy alloys on Si(001) grown by molecular beam epitaxy

We show that C incorporation kinetics depend significantly on the carbon concentration. The carbon substitutionality (fraction of substitutional incorporated carbon atoms) is strongly influenced by the growth conditions, such as growth temperature and Si growth rate. In addition, reduction in the growth temperature and increase of the growth rate can both increase the substitutional carbon fraction. This behavior is well described by a kinetic model, with the energy barrier and preexponential frequency factor decreasing with increasing carbon concentration. Very low carbon concentrations (in the dopant range) can be predominantly incorporated substitutionally, independent of the growth temperature. At higher C concentrations (in the percentage range), the substitutional carbon fraction is shifted to lower values. We predict the existence of an upper limit for substitutional C incorporation, with its value depending on the specific growth conditions.

Thin crystalline 3C-SiC layer growth through carbonization of differently oriented Si substrates

The growth of thin cubic silicon carbide (3C-SiC) buffer layers in an horizontal hot-wall chemical vapor deposition reactor, through the carbonization of differently oriented Si surfaces, is presented. A qualitative and quantitative study has been performed on statistical parameters related to voids due to the buffer layer growth on the different substrate orientations emphasizing shape, size, and density as a function of the substrate orientation. Variation in the void parameters can be attributed to the atomic packing density related to the substrate orientations, which were (100) Si, (111) Si, and (110) Si in this study. Scanning electron microscopy and transmission electron microscopy were performed to analyze the surface and the crystalline quality of the 3C-SiC films grown and, eventually, an empirical model for the carbonization of Si surfaces formulated. Large platens characterize the 3C-SiC films with shapes related to the orientations of the substrate. These platens derive from the two-dimension...

Complete suppression of the transient enhanced diffusion of B implanted in preamorphized Si by interstitial trapping in a spatially separated C-rich layer

A method for completely suppressing the transient enhanced diffusion (TED) of boron implanted in preamorphized silicon is demonstrated. Boron is implanted in a molecular beam epitaxy (MBE) grown silicon sample that has been previously amorphized by silicon implantation. The sample is then annealed in order to epitaxially regrow the amorphous layer and electrically activate the dopant. The backflow of silicon interstitials released by the preamorphization end-of-range (EOR) damage is completely trapped by a carbon-rich silicon layer interposed by MBE between the damage and the implanted boron. No appreciable TED is observed in the samples up to complete dissolution of the EOR damage, and complete electrical activation is obtained. The method might be considered for the realization of ultrashallow junctions for the far future complementary metal–oxide–semiconductor technology nodes.

Oxidation rate enhancement of SiGe epitaxial films oxidized in dry ambient

We present a study on thin oxides obtained by rapid thermal oxidation of Si1−xGex epitaxial layers. The oxidation processes were performed in dry O2 at 1000 °C for times up to 600 s. Our data show an oxide growth rate enhancement with respect to pure Si. Except for a very small amount of GeO2 that is found at the surface, all the Ge is rejected towards the SiO2/SiGe interface, forming a Ge-enriched layer free of extended defects. The comparison of our results for dry processes with those reported in the literature for wet ambient supports the idea that the kinetics of SiGe oxidation is controlled by similar mechanisms in both cases, in contrast with models and interpretations so far proposed.

Laser assisted green synthesis of free standing reduced graphene oxides at the water?air interface

A single step, scalable and green strategy has been developed to obtain reduced graphene oxide layers in water dispersion through nanosecond laser pulse irradiation of carbon targets. The layers spontaneously migrate at the water-air interface, forming sheets of several tens of micrometers and show intense ultraviolet photoluminescence. This unique condition offers an intriguing environment where opposing dielectric media meet and can be used in all those processes where molecular interactions such as hydrogen bonding and electrostatic interactions are greatly enhanced.

Effect of O:Er concentration ratio on the structural, electrical, and optical properties of Si:Er:O layers grown by molecular beam epitaxy

The structural, electrical, and optical properties of crystalline Si codoped with Er and O by molecular beam epitaxy (MBE) have been investigated in detail. Si:Er:O layers (∼250 nm thick) have been grown by MBE, realizing uniform dopant concentrations in the range 8×1018–1.5×1020 cm−3 for Er and up to 5×1020 cm−3 for O. The O:Er ratio was varied between 0 and ∼20. Samples have been subsequently annealed at 900 °C for 1 h. We observed that clear constraints to the Er and O contents exist in order to incorporate them in a good quality single crystal. We also found that the O:Er ratio represents the main parameter in determining the properties of this system. For instance, Er is observed to behave as a donor in MBE grown samples and the donor concentration increases with the O:Er ratio until a saturation regime is achieved for a ratio higher than 6–8. All the samples emit light at 1.54 μm and similar behavior is also found for the optical activation of the Er ions. The thermal process usually increases the n...

Rapid thermal oxidation of epitaxial SiGe thin films

The oxidation of epitaxial thin Si 1-x Ge x layers (0.06 < x < 0.30) at 1000 °C in dry O 2 , for times between 20 and 240 s, has been investigated. The analysis of the thin oxides ( 4-20 nm) has been performed using X-ray photoelectron spectroscopy and Rutherford backscattering spectrometry. Although most of the Ge is found to pile-up at the oxide/Si 1-x Ge x interface, our data indicate the formation of both SiO 2 and GeO 2 for all investigated samples and oxidation times. Moreover, the oxidation rate, enhanced with increasing the Ge concentration in the alloy, is reported. To our knowledge, this is the first experimental evidence of GeO 2 formation and rate enhancement in the regime of high temperature oxidation in dry O 2 and Si 1-x Ge x alloys with x < 0.5. The differences could be peculiar to the initial stage of oxidation, as well as to the rapid thermal process used in our case, but a clear answer is currently unavailable.

Controlled synthesis of carbon nanotubes and linear C chains by arc discharge in liquid nitrogen

Arc discharge between two graphite rods in liquid nitrogen has been investigated in order to identify the main factors ruling the formation of carbon nanotubes (CNTs) and linear C chains. The influence of the experimental parameters on the structural properties of the produced materials was evaluated and interpreted, taking into account the existing models. We found that the electrode size and discharge current values greatly influence the structural quality of the nanotubes (e.g., presence of carbonaceous impurities, innermost tube diameter) and a proper combination of these parameters allows one to control the synthesis of CNTs and/or CNT-linear C chain hybrid systems.

Modification of graphene oxide by laser irradiation: a new route to enhance antibacterial activity.

The antibacterial activity and possible toxicity of graphene oxide and laser-irradiated graphene oxide (iGO) were investigated. Antibacterial activity was tested on Escherichia coli and shown to be higher for GO irradiated for at least three hours, which seems to be correlated to the resulting morphology of laser-treated GO and independent of the kind and amount of oxygen functionalities. X-ray photoelectron spectroscopy, Raman spectroscopy, dynamic light scattering and scanning electron microscopy (SEM) show a reduction of the GO flakes size after visible laser irradiation, preserving considerable oxygen content and degree of hydrophilicity. SEM images of the bacteria after the exposure to the iGO flakes confirm membrane damage after interaction with the laser-modified morphology of GO. In addition, a fish embryo toxicity test on zebrafish displayed that neither mortality nor sublethal effects were caused by the different iGO solutions, even when the concentration was increased up to four times higher than the one effective in reducing the bacteria survival. The antibacterial properties and the absence of toxicity make the visible laser irradiation of GO a promising option for water purification applications.

Material modifications induced by laser annealing in two-dimensional structures

The effects of the laser irradiation on metal-oxide-semiconductor structures are investigated by means of a phase-field methodology. We numerically solved the model equations in one- and two-dimensional structures also containing SiO2/amorphous-Si/crystalline-Si stacks. The simulated laser annealing processes are discussed in detail, pointing out the influence of the geometrical constraints on the irradiation effects in the samples. The simulation results are compared with the experimental two-dimensional delineation of dopant profiles. These comparisons show the importance of the joint theoretical and experimental investigations in order to fully understand the phenomena occurring in submicron sized laser irradiated structures.