Search

Laser irradiation in liquid is a green, low cost, and tunable technique that can be used in order to improve solar photocatalytic activity of inorganic semiconductors. The interaction between the s...

Single crystals represent a benchmark for understanding the bulk properties of halide perovskites. We have indeed studied the dielectric function of lead bromide perovskite single crystals (MAPbBr3...

In this paper we present a computational tool for the simulation of laser annealing processes in FinFET structures. This is a complex self-consistent problem, where heating is evaluated by means of the time harmonic solution of the Maxwell equations. The main features of our computational code include: A versatile graphical user interface for the structure design; The assignment of materials and the simulation analysis; An interface with the finite element method solver for the automatic generation of the mesh and the runtime control; Parameters for numerous materials (optical/thermal properties and mass transport) as a function of temperature and phases; An efficient coupling with electromagnetic simulations for the self-consistent source estimate (i.e. the power dissipation) in nanostructured topographies; Experimental validation of nanostructured samples; Multiple-dopant models simulating dopant redistribution, including diffusion solubility and segregation; Alloy model, e.g. SiGe (where the melting point depends on the alloy fraction); Multiple phases (e.g. amorphous, liquid, crystal). As a particular application of the tool we present a study of the laser process design by varying the laser fluence, polarization of the electromagnetic field and the pitch of the devices. Results are in excellent agreement with the experiment and could serve as guidelines for the realization of targeted laser annealing processes.

In this chapter the current status of laser annealing simulation models, in the continuum formulation for the evolving fields (e.g., temperature, phases, doping density), is presented and discussed. The content is organized so as to gradually guide the reader from the basic principles to the formalisms already implemented in the Technology Computer-Aided Design (TCAD) tools for the laser processing and, finally, to the less studied features which can be the topics of future developments. The laser-matter interaction models are outlined and the thermalization approximation, on the basis of the TCAD models, is introduced in the right context. Within this approximation, self-consistent numerical solutions of the ultra-fast heating, induced by pulsed lasers in the nanoseconds time scales, are discussed highlighting the crucial differences between the solutions obtained in blanket and patterned 2D and 3D complex systems. The kinetics of the localized (in the nanometers range) phase transitions is a key modelling issue. Here we will consider both the two phases (e.g., solid-liquid) and the three phases (amorphous-liquid-crystal) evolutions whereas the latter characterizes the explosive crystallization phenomenon. The chapter discusses also critical and challenging models related to laser processing; among them: the already cited explosive phenomenon, the anomalous doping redistribution, the failure of the heat diffusion equation in the nanoscale, the accurate simulation of alloy melting.

We present a method for the simulation of the kinetic evolution in the sub µs timescale for composite materials containing regions occupied by alloys, compounds, and mixtures belonging to the Ni-Si-C ternary system. Pulsed laser irradiation (pulses of the order of 100 ns) promotes this evolution. The simulation approach is formulated in the framework of the phase-field theory and it consists of a system of coupled non-linear partial differential equations (PDEs), which considers as variables the following fields: the laser electro-magnetic field, the temperature, the phase-field and the material (Ni, Si, C, C clusters and Ni-silicides) densities. The model integrates a large set of materials and reaction parameters which could also self-consistently depend on the model variables. A parameter calibration is also proposed, specifically suited for the wavelength of a widely used class of excimer lasers (λ = 308 nm). The model is implemented on a proprietary laser annealing technology computer-aided design (TCAD) tool based on the finite element method (FEM). This integration allows, in principle, numerical solutions in systems of any dimension. Here we discuss the complex simulation trend in the one-dimensional case, considering as a starting state, thin films on 4H-SiC substrates, i.e., a configuration reproducing a technologically relevant case study. Simulations as a function of the laser energy density show an articulated scenario, also induced by the variables’ dependency of the materials’ parameters, for the non-melting, partial-melting and full-melting process conditions. The simulation results are validated by post-process experimental analyses of the microstructure and composition of the irradiated samples.

Funder: Università degli Studi di Catania, We propose a two-stage multi-objective optimization framework for full scheme solar cell structure design and characterization, cost minimization and quantum efficiency maximization. We evaluated structures of 15 different cell designs simulated by varying material types and photodiode doping strategies. At first, non-dominated sorting genetic algorithm II (NSGA-II) produced Pareto-optimal-solutions sets for respective cell designs. Then, on investigating quantum efficiencies of all cell designs produced by NSGA-II, we applied a new multi-objective optimization algorithm II (OptIA-II) to discover the Pareto fronts of select (three) best cell designs. Our designed OptIA-II algorithm improved the quantum efficiencies of all select cell designs and reduced their fabrication costs. We observed that the cell design comprising an optimally doped zinc-oxide-based transparent conductive oxide (TCO) layer and rough silver back reflector (BR) offered a quantum efficiency ($$Q_e$$ Q e ) of 0.6031. Overall, this paper provides a full characterization of cell structure designs. It derives relationship between quantum efficiency, $$Q_e$$ Q e of a cell with its TCO layer’s doping methods and TCO and BR layer’s material types. Our solar cells design characterization enables us to perform a cost-benefit analysis of solar cells usage in real-world applications.

We present a composite design methodology for the simulation and optimization of the solar cell performance. Our method is based on the synergy of different computational techniques and it is especially designed for the thin-film cell technology. In particular, we aim to efficiently simulate light trapping and plasmonic effects to enhance the light harvesting of the cell. The methodology is based on the sequential application of a hierarchy of approaches: (a) full Maxwell simulations are applied to derive the photon’s scattering probability in systems presenting textured interfaces; (b) calibrated Photonic Monte Carlo is used in junction with the scattering matrices method to evaluate coherent and scattered photon absorption in the full cell architectures; (c) the results of these advanced optical simulations are used as the pair generation terms in model implemented in an effective Technology Computer Aided Design tool for the derivation of the cell performance; (d) the models are investigated by qualitative and quantitative sensitivity analysis algorithms, to evaluate the importance of the design parameters considered on the models output and to get a first order descriptions of the objective space; (e) sensitivity analysis results are used to guide and simplify the optimization of the model achieved through both Single Objective Optimization (in order to fully maximize devices efficiency) and Multi Objective Optimization (in order to balance efficiency and cost); (f) Local, Global and “Glocal” robustness of optimal solutions found by the optimization algorithms are statistically evaluated; (g) data-based Identifiability Analysis is used to study the relationship between parameters. The results obtained show a noteworthy improvement with respect to the quantum efficiency of the reference cell demonstrating that the methodology presented is suitable for effective optimization of solar cell devices.

This paper introduces PareDA (ParetoDesignAutomation), a composite automated methodology for the optimization of analog circuits and solar cell devices. The PareDA framework combines randomized algorithms, domain and constraints sensitivity analysis, epsilon-dominance and global robustness analysis in order to perform simulation-based, multi-scenario and multi-objective optimization. PareDA is evaluated on the problems of designing a three-stage operational amplifier, a yield-aware optimization of a folded-cascode operational amplifier (requiring multiple operating conditions) and a model for selective emitter solar cells.Comparisons with a selection of state-of-the-art techniques (such as NSGA-II and YdIRCO) highlight the effectiveness of PareDA both in terms of Pareto optimality of the solutions found and time-to-converge. The solutions obtained by PareDA dominate those of comparative techniques, in particular, the proposed technique shows a significant average performance improvement (ranging from 35% to 49%) with respect to such techniques. Moreover, the CPU time required by PareDA to converge is smaller of at least 75% if compared with the other methodologies here analyzed (e.g. significantly improved designs for folded-cascode operational amplifier are found in just 320s). Finally, the PareDA algorithm can also benefit from parallelization, which leads to a significant speed-up with respect to the nonparallel version.

Current semiconductor device manufacturing often needs the integration of annealing process steps with a low thermal budget; and, among them, pulsed laser annealing (LA) is a reliable option. Consequently, the use of LA specialized Technology Computer Aided Design (TCAD) models is emerging as a support for the development of this particular heating methods. Anyway, models already implemented in academic or commercial packages usually consider some approximations which can lead to inaccurate predictions if they are applied in rather common configurations of nano-device: i.e. structures with nm wide elements where amorphous pockets are also present. In particular, in these cases non-diffusive thermal transport and explosive crystallization could take place. Here we present upgrades of the LA TCAD models allowing the simulation of these phenomena. We will demonstrate that these models can be reliably integrated in the current TCAD packages discussing the key features of the numerical solutions features in some particular cases.

The electronic properties of the graphene (Gr) Schottky junction with an Al0.22Ga0.78N/GaN heterostructure on silicon have been investigated, both by experiment and with use of ab initio DFT calculations. A peculiarly high n-type doping (1.1 x 10(13) cm(-2)), observed for Gr in contact with AlGaN, was explained by the combined effect of Fermi level pinning by AlGaN surface states and charge transfer. Spatially uniform current injection across the Gr/AlGaN/GaN heterojunction was revealed by nanoscale resolution conductive atomic force microscopy (CAFM) analyses. Furthermore, a Gr/AlGaN/GaN Schottky diode with excellent rectifying behavior has been demonstrated and used as the key building block for a hot electron transistor (HET) with a 10 nm Al2O3 base-collector barrier. Thanks to the highly efficient hot electron injection from the AlGaN/GaN emitter, this transistor exhibits high on-state current density (J(C,ON) approximate to 1 A/cm(2)), high on-state over off-state current density ratio (J(C,ON)/J(C,OFF) approximate to 10(6)), and a common-base current gain alpha approximate to 0.15, solely limited by the high Al2O3 base-collector barrier. The excellent performances of the Gr/AlGaN/GaN Schottky junction represent an important step toward the development of a HET technology compatible with the state-of-the-art GaN high electron mobility transistors.

A possible effect of solidification velocity on activation of surface segregated Ga, In, and Al in high Ge content SiGe after UV melt laser anneal has been discussed. The experiment has shown that this effect depends on the atomic size of the dopants, implying that the solidification velocity may dominate the incorporation of the dopants into active substitutional sites. The control of the solidification velocity may play a key role for improving dopant activation over an equilibrium solid solubility limit.

We investigated the complex interaction between a nickel layer and a 4H-SiC substrate under UV-laser irradiation since the early stages of the atomic inter-diffusion. An exhaustive description is still lacking in the literature. A multimethod approach based on Transmission Electron Microscopy, Energy Dispersive Spectroscopy and Diffraction (electron and X-ray) techniques has been implemented for a cross-correlated description of the final state of the contact after laser irradiation. They detailed the stoichiometry and the lattice structure of each phase formed as well as the Ni/Si alloy profile along the contact for laser fluences in the range 2.4-3.8 J/cm2. To make a bridge between process conditions and post-process characterizations, time dependent ultra-fast phenomena (laser pulse about 160ns), such as intermixing driven melting and Ni-silicides reactions, have been simulated by a modified phase fields approach in the proper many-compounds formulation. We disclose that raising laser fluence has multiple effects: 1) the thickness of the silicide layer formed at the interface with 4H-SiC expands; 2) the silicon content into the reacted layer increases; 3) a Ni2Si phase is promoted at the highest fluence 3.8 J/cm2; 4) silicon atomic diffusion into a topmost residual nickel layer occurs, with the Ni/Si ratio increasing towards the contact surface., Comment: 12 pages, 8 figures

Notwithstanding the graphitization of SiC under high thermal treatment can take place for all SiC surfaces, the quality of the resulting graphene as well as its structural and electrical characteristics strongly depend on the SiC face where growth has taken place. In this paper we use the density functional theory to analyze the structural and electronic properties of epitaxial graphene grown on three different SiC planes. Calculations are presented for the (6√3×6√3)R30°-reconstructed SiC(0001) surface (Si face) as well as the nonpolar SiC(11-20) and SiC(1-100) planes. We argue that the formation of a strongly-bound interface buffer layer is an exclusive property of the SiC(0001) surface. Moreover, our results indicate that nonpolar planes give rise to graphene with a nearly ideal low-energy spectrum.

Kinetic lattice Monte Carlo simulations, implementing ab initio calibrated energetics, are applied to study the evolution of vacancy systems in graphene. The evolution of the under-coordinated atomic layer proceeds, in general, by island nucleation and growth. However, the first stage (nucleation) is strongly influenced by the stability of small aggregates (e.g. di-vacancies) and by the effective coalescence/attachment barriers. Quantitative predictions of the system kinetics in terms of crystal state and defects' morphology as a function of the initial state and the temperature can be obtained and readily compared with experimental structural characterization of processed samples.

The diffusion of oxygen and its interactions with dopants during laser thermal annealing (LTA) in the melting regime is investigated. O is shown to penetrate from the surface and to diffuse during LTA both into germanium un- and also implanted with arsenic or boron. In the latter cases, the diffusion is unaffected by As, whereas it is significantly reduced by B. Accordingly, the electrical activation of As and B are respectively un-and anti-correlated with O. Therefore, it is concluded that O does not interact with As nor in the melt, nor in the solid phase during the cooling transient. On the other hand, the B–O clustering reported in literature to explain the B deactivation during LTA [Impellizzeri et al., J. Appl. Phys. 113(2013) 113505] occurs already in the liquid phase during the regrowth. Thence, the present data provide noteworthy results for the LTA implementation in future complementary metal-oxide devices.

Pulsed laser annealing is one of the promising low thermal budget approaches to overcome process limitations and develop alternative schemes to achieve better device performance and enable 3D architectures. Its applications range from the Front End Of the Line (doping, contacts, strain engineering) to Back End Of the Line (Cu grain engineering) in logic and memory devices. One key enabler for integrating this disruptive technology in the coming highly challenging technology nodes is an accurate time-resolved modeling of laser matter interaction, thermal diffusion, phase change and species diffusion at the nanosecond timescale, all to be solved self-consistently. In this paper, we will present the TCAD simulation package of the laser annealing process (LASSE Innovation Application Booster or LIAB), with a specific focus on the phase field model and calibration of relevant materials. The coupled partial differential equation system is described and a methodology for materials calibration, especially challenging in the melting regime, is detailed with results shown for Ge and SiGe, with a application on a typical p-type finFET contact region anneal 2D use case.

In this work we report on the development of back-gated carbon nanotube-field effect transistors (CNT-FETs), with CNT layers playing the role of the channel, and on their electrical characterisation for sensing applications. The CNTs have been deposited by electrophoresis on an interdigitated electrode region created on a SiO2/Si substrate. Different kinds of CNTs have been used (MWCNTs by arc discharge in liquid nitrogen and MWCNTs by chemical vapour deposition, CVD) and the electrical characterisation of the devices was performed in a NH3- and NO2-controlled environment. Preliminary data have shown an increase in the channel resistance under NH3 exposure, whereas a decrease is observed after exposure to NO2, and the sensitivity to each gas depends on the kind of CNTs used for the device. Furthermore, the defect formation by Si ion implantation on CNTs was investigated by high-resolution transmission electron microscopy (TEM) and Raman analysis. The behaviour observed for the different devices can be explained in terms of the interaction between structural or chemical defects in CNTs and the gas molecules.

International audience; We present a comprehensive investigation of laser induced damage in silicon by implementing a methodology allowing the identification and the localization of the defects. PL analyses combined with TEM reveals that laser annealed samples reveal the presence of Carbon and Oxygen related defects, in the form of C-lines and G-lines (including the associated G satellite lines). Additional Oxygen related defects (Si-O-Si complexes) were revealed by FTIR analysis. Based on SIMS analyses combined with chemical etch, it is found that the optical defects observed in laser annealed samples originate from the oxygen atoms and carbon impurities contained in the surface native oxide from which they are released and subsequently diffuse into the substrate during anneal down to the liquid/solid interface.

Two-dimensional (2D) materials are currently object of many interests both from a basic and a technological standpoint. In particular, graphene (Gr) and the semiconducting transition metal dichalcogenides (including MoS2, WS2, MoSe2, WSe2) have been widely investigated for transistors applications. As a matter of fact, 2D materials present peculiar nanoscale structural and electrical inhomogeneities, related to the specific synthesis mechanisms and to the interaction with the substrate, which are ultimately reflected in the macroscopic electrical behaviour of electronic devices based on these systems. In this context, conductive atomic force microscopy (CAFM) is the method of choice to investigate the mechanisms of current injection between contacts and 2D materials and/or the lateral homogeneity of 2D materials electrical properties. This paper will discuss some case studies of CAFM applications to Gr and MoS2, to illustrate the potentiality of this characterization method for 2D materials investigation. The results of these nanoscale analyses will be used to explain the electrical behaviour of field effect transistors fabricated on these materials.

Using quantum mechanical methods, in the framework of non-equilibrium Green’s function (NEGF) theory, we discuss the effects of the real space distribution of hydrogen adatoms on the electronic properties of graphene. Advanced methods for the stochastic process simulation at the atomic resolution are applied to generate system configurations in agreement with the experimental realization of these systems as a function of the process parameters (e.g., temperature and hydrogen flux). We show how these Monte Carlo (MC) methods can achieve accurate predictions of the functionalization kinetics in multiple time and length scales. The ingredients of the overall numerical methodology are highlighted: the ab initio study of the stability of key configurations, on lattice matching of the energetic configuration relation, accelerated algorithms, sequential coupling with the NEGF based on calibrated Hamiltonians and statistical analysis of the transport characteristics. We demonstrate the benefit to this coupled MC-NEGF method in the study of quantum effects in manipulated nanosystems.

In this paper, the structural and electronic properties of epitaxial graphene (EG) grown on 8 degrees-off 4H-SiC (0001) by high temperature thermal processes have been extensively investigated by a combination of several high resolution characterization techniques. The increase in the number of graphene layers with the growth temperature (from 1600 to 1700 degrees C) was studied by microRaman spectroscopy and high resolution transmission electron microscopy (HRTEM) on cross sectioned samples. The few layers of graphene reside on a stepped SiC surface with alternating (0001) terraces and (11 - 2n) facets. Peculiar corrugations (wrinkles) in the graphene membrane preferentially oriented perpendicularly to the substrate steps were also observed. Motivated by recent atomic resolution studies of the EG/SiC interface revealing a local delamination of the interfacial C buffer from the (11 - 2n) facets, we searched for a correlation of these interfacial structural properties with the macroscopic electronic transport in EG field effect transistors (FETs). In particular, electrical characterization of EG Lop gated FETs fabricated with the channel length parallel or perpendicular to the substrate steps revealed a peculiar anisotropy of the channel conductance with respect to the steps orientation. This effect was explained in terms of a local enhancement of EG resistance on the (11-2n) facets with respect to the (0001) basal plane, which is consistent with a reduced doping due to the local buffer layer delamination from those facets. Furthermore, scanning probe microscopy-based local electron mean free path measurements on EG showed a similar to 3 x enhancement of mean free path On the buffer-layer-free (11-2n) facets with respect to (0001) terraces, probably associated to a strong reduction of Coulomb scattering effects on graphene's electrons. (C) 2013 Elsevier B.V. All rights reserved.

International audience; Damage evolution and dopant distribution during nanosecond laser thermal annealing of ion implanted silicon have been investigated by means of transmission electron microscopy, secondary ion mass spectrometry, and atom probe tomography. Different melting front positions were realized and studied: nonmelt, partial melt, and full melt with respect to the as-implanted dopant profile. In both boron and silicon implanted silicon samples, the most stable form among the observed defects is that of dislocation loops lying close to (001) and with Burgers vector parallel to the [001] direction, instead of conventional {111} dislocation loops or {311} rod-like defects, which are known to be more energetically favorable and are typically observed in ion implanted silicon. The observed results are explained in terms of a possible modification of the defect formation energy induced by the compressive stress developed in the nonmelted regions during laser annealing.

In this paper, the electronic transport in few layer graphene epitaxially grown on the Si face of 8 degrees off-axis 4H-SiC has been investigated using conductive atomic force microscopy (CAFM). The comparison between the two dimensional morphology and current maps in epitaxial graphene (EG) residing over the stepped SiC surface revealed a local resistance increase of EG over the (11 - 2n) facets with respect to the(0001) basal plane. This is consistent with a significant reduction of the carrier concentration, possibly approaching to the neutrality, in EG residing on facets with respect to the high n-type doping (similar to 10(13) cm(-2)) commonly found in EG on the basal plane. The observed difference can be ascribed to a different interfacestructure between EG and SiC on the two faces. The results of these nanoscale measurements allow us to explain the typically observed macroscopic anisotropy in EG conductivity with respect to the substrate steps orientation. (C) 2013 Elsevier B. V. All rights reserved.

Epitaxial graphene fabricated by thermal decomposition of the Si-face of silicon carbide (SiC) forms a defined interface to the SiC substrate. As-grown monolayer graphene with buffer layer establishes an ohmic interface even to low-doped (e. g. [N] ≈ 1015cm-3) SiC, and a specific contact resistance as low as ρC= 5.9×10-6Ωcm2can be achieved on highly n-doped SiC layers. After hydrogen intercalation of monolayer graphene, the so-called quasi-freestanding graphene forms a Schottky contact to n-type SiC with a Schottky barrier height of 1.5 eV as determined fromC-Vanalysis and core level photoelectron spectroscopy (XPS). This value, however, strongly deviates from the respective value of less than 1 eV determined from I-V measurements. It was found from conductive atomic force microscopy (C-AFM) that the Schottky barrier is locally lowered on other crystal facets located at substrate step edges. For very small Schottky contacts, the barrier height extracted fromI-Vcurves approaches the value of 1.5 eV fromC-Vand XPS.

The impact of solidification front velocity (SFV) on dopant segregation and activation surpassing the equilibrium solid solubility limit was investigated in high Ge content SiGe using UV nanosecond melt laser annealing (MLA). We first simulated the SFV evolution in the MLA-induced solidification of a SiGe binary system. It was then combined with the near-surface atomic and electrically active dopant concentrations measured in the Ga, In, and Al-implanted SiGe after MLA. Solute trapping phenomenon and self-compensation effect of the dopants were discussed to capture a part of their segregation and activation dynamics in the presented SiGeX ternary systems.

Despite its structural and chemical stability, graphene is often subjected to n- or p-type doping when interacting with substrates, gate oxides, or environmental molecules. Such interaction shifts the Fermi level of the system away from the Dirac point and alters the intrinsic electronic and transport characteristics of the graphene sheet. The density functional theory is used herein to show that the Fermi level of a graphene/AlN or graphene/Al2O3 heterostructure can be extensively tuned through the polarity and surface reconstruction of either the nitride or the oxide layer. Hence, Fermi-level engineering through the manipulation of confining materials can become a viable route for enhancing the selectivity and optimizing the properties of graphene-based devices.

In this work, we present a model describing the boron redistribution during laser thermal annealing in the melting regime based on the adsorption of boron atoms at the solid-liquid interface. To validate the model, we performed SIMS measurements on silicon samples implanted with boron with an energy of 3 keV and doses of 3 × 1013 cm-2 and 4 × 1014 cm-2 annealed with a XeCl excimer laser with a wavelength of 308 nm, a pulse duration of 160 ns, and up to 10 consecutive pulses. After calibration, our model is able to reproduce the measured profiles for the different process conditions. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Processes inducing micro- and nano-structural modifications of materials play a crucial role in the advanced technology. In particular, laser irradiation attracts a great interest as a tool for the localized thermal treatment (laser thermal annealing) when the global heating of the sample could be detrimental. With the expansion of the application fields, a research effort is required to understand the kinetics in systems subjected to extreme transient excitations induced by the interaction with the laser light. Modeling and simulations cover a relevant part of this research effort. In this paper we review the current status of the laser annealing modeling. The theoretical analysis will be restricted to the heating stage of the irradiation, i.e. the ultrafast energy transfer from the electronic states to the lattice vibrations will not be considered. Therefore, the timescale of the process must be larger than ~1 ns. Models, numerical methods and applications to the experiments will be presented to deal with different questions. Among them: the simulation of the thermal field when the heat absorption depends consistently on the temperature, the prediction of a phase change kinetics in a melting process including the redistribution of alloyed systems, the damage evolution in the transient non-uniform thermal field promoted by the irradiation induced kinetics, the related dopant activation kinetics. Numerical techniques include: continuum, kinetic Monte Carlo and molecular dynamics approaches.

Doping and stability of monolayer low defect content graphene transferred on a silicon dioxide substrate on silicon are investigated by micro-Raman spectroscopy and atomic force microscopy (AFM) during thermal treatments in oxygen and vacuum controlled atmosphere. The exposure to molecular oxygen induces graphene changes as evidenced by a blue-shift of the G and 2D Raman bands, together with the decrease of I2D/IG intensity ratio, which are consistent with a high p-type doping (∼1013 cm-2) of graphene. The successive thermal treatment in vacuum does not affect the induced doping showing this latter stability. By investigating the temperature range 140-350 °C and the process time evolution, the thermal properties of this doping procedure are characterized, and an activation energy of ∼56 meV is estimated. These results are interpreted on the basis of molecular oxygen induced ∼1013 cm-2 p-type doping of graphene with stability energy >49 meV and postdoping reactivity in ambient atmosphere due to reaction of air molecules with oxygen trapped between graphene and substrate.

This article considers a graphene-based quasi-one-dimensional system and explores the impact of structural perturbations on the electronic and transport properties of the material. Two phenomenologically different quantum mechanical approaches are used to describe the perturbation, namely the spectral weight of the unfolded band structure and the transmission coefficient of the propagated electrons. We show that these two descriptions present strong qualitative similarities and yield complementary information for the understanding of the induced electronic alterations.

The local atomic configuration of graphene oxide (GO) was investigated by identifying the different oxygen functionalities and following their evolution induced by thermal treatments in various environments (vacuum, nitrogen or argon flow). X-ray photoelectron spectroscopy and scanning transmission electron microscopy analyses were performed, and electron energy-loss (EEL) spectra were a) acquired in different regions of GO and thermally reduced GO flakes. Experimental results show a series of characteristic peaks related to C and O K-edge shells and different features of GO thermally annealed at the same temperature but in different environments. In order to understand the experimental results, density functional theory calculations of core-loss EEL spectra of GO (C and O K-edges) in the presence of oxygen functional groups have been performed for different combinations and/or concentrations. Such calculations have allowed for the association of the observed experimental peaks to the presence of specific oxygen functional groups, giving the opportunity to establish the atomic configurations that prevail in different ranges of annealing temperatures and environments.

The minisymposium was concerned with the mathematical modeling and simulation of charge transport in graphene and other 2D materials and in structures, like double gate MOSFETs, nano-ribbons and nano-wires, where the presence of confinement effects allows for the formal description of the carrier flow as that of a two dimensional or one dimensional electron gas.

The effects of temperature and atmosphere (air and O2) on the doping of monolayers of graphene (Gr) on SiO2 and Si substrates, and on the doping of MoS2 multilayer flakes transferred on the same substrates have been investigated. The investigations were carried out by in situ micro-Raman spectroscopy during thermal treatments up to 430 °C, and by atomic force microscopy (AFM). The spectral positions of the G and 2D Raman bands of Gr undergo only minor changes during treatment, while their amplitude and full width at half maximum (FWHM) vary as a function of the temperature and the used atmosphere. The thermal treatments in oxygen atmosphere show, in addition to a thermal effect, an effect attributable to a p-type doping through oxygen. The thermal broadening of the line shape, found during thermal treatments by in situ Raman measurements, can be related to thermal phonon effects. The absence of a band shift results from the balance between a red shift due to thermal effects and a blue shift induced by doping. This shows the potential of in situ measurements to follow the doping kinetics. The treatment of MoS2 in O2 has evidenced a progressive erosion of the flakes without relevant spectral changes in their central zone during in situ measurements. The formation of MoO3 on the edges of the flakes is observed indicative of the oxygen-activated transformation.

The precise control of the doping characteristics of graphene-based systems is a key parameter for successful integration in devices and applications. Doping can tune graphene’s carrier density, which is important for the optimization of graphene-based flexible and transparent electrodes. Moreover, if doping is confined within one out of the two equivalent graphene sublattices, considerable band gaps can open that could serve for switching devices on and off. We use the density functional theory to explore the conditions of sublattice symmetry breaking in nitrogen-doped graphene. We show that the nucleation of graphene grains and their growth through the propagation of unpassivated zigzag edges could give rise to a highly unbalanced doping, due to exothermic substitutions on edge sites. Based on our ab initio results, we calibrate an event-driven kinetic Monte Carlo model that simulates the growth kinetics and investigates the N incorporation mechanism in real-process conditions. We demonstrate that the degree of symmetry breaking strongly depends on the growing conditions.

International audience; Heavy doping of Ge is crucial for several advanced micro-and optoelectronic applications, but, at the same time, it still remains extremely challenging. Ge heavily n-type doped at a concentration of 1 X 10(20) cm(-3) by As ion implantation and melting laser thermal annealing (LTA) is shown here to be highly metastable. Upon post-LTA conventional thermal annealing As electrically deactivates already at 350 degrees C reaching an active concentration of similar to 4 x 10(19) cm(-3). No significant As diffusion is detected up to 450 degrees C, where the As activation decreases further to similar to 3 x 10(19) cm(-3). The reason for the observed detrimental deactivation was investigated by Atom Probe Tomography and in situ High Resolution X-Ray Diffraction measurements. In general, the thermal stability of heavily doped Ge layers needs to be carefully evaluated because, as shown here, deactivation might occur at very low temperatures, close to those required for low resistivity Ohmic contacting of n-type Ge.

In this paper we discuss the advanced methods for the stochastic process simulation at the atomic resolution, showing how they can achieve accurate predictions of the functionalization kinetics in multiple time and length scales. Focusing on processing of graphene based systems, the ingredients of the numerical method are highlighted: ab-initio study of key configurations stability, “on lattice” matching of the energetic-configuration relationship, super-lattice and parallel-lattice formulation of complex kinetics, accelerate algorithms. Several examples of applications to graphene synthesis and manipulation processes are presented. Finally, we demonstrate that the fully potential of the technique is achieved when optical, magnetic and electronic properties of the simulated atomic configurations are analyzed by quantum mechanics based methods.

Ion irradiation of a graphene sheet can give rise to a wide range of point and extended defects on the ideal honeycomb lattice. Here we perform first-principles calculations for the determination of the electronic and transport properties of damaged graphene nanoribbons based on nonequilibrium Green function techniques. Considering a wide range of defects (vacancies, di-vacancies, Stone–Wales, sp 3 -type) we study the conductive characteristics, showing that the common feature in all cases is the presence of transport gaps induced by local perturbations of the wavefunction around the defected areas. However, the resonances of these pseudogaps are intrinsically related to the defect type, making possible a structural characterization of a defected graphene system based on its electrical behavior.

Multi-wall carbon nanotubes (MWCNTs) produced by the arc discharge between two graphite rods in liquid nitrogen have been investigated with the use of Raman spectroscopy and transmission electron microscopy (TEM). The effects of the applied voltage on the structural properties of the produced MWCNTs have been observed, in particular, as it concerns the size of the innermost and outermost diameters. The apparent discrepancies observed between the Raman and TEM results are explained through the observation of a selective response of the MWCNTs to the excitation laser energy in Raman spectroscopy, similar to the case of single-wall CNTs. Electronic structure calculations correlate such behavior with optical transitions, in the presence of strong electronic localizations, spatially confined in a few neighboring walls. Copyright © 2011 John Wiley & Sons, Ltd.

This Colloquium discusses the coherent electron transport properties of non-ideal quasi-one-dimensional carbon-based systems, i.e. graphene nanoribbons, carbon nanotubes and linear chains. Results are based on numerical simulations that implement Schroedinger/Poisson as well as non-equilibrium polaronic methodologies for the calculation of quantities relative to the conduction. Attention is paid on the role of localized or extended modifications of the structural and electronic symmetry within atomistic descriptions that go beyond phenomenological approaches. Three different types of perturbations are considered: (a) perturbations induced by defects/impurities in the atomic lattice, (b) perturbations induced by the coupling with the metallic contacts, and (c) perturbations induced by the application of bias. The objective is to elucidate the potentiality as well as the possible drawbacks of future all-carbon-based electronics.

The integration of laser annealing in SiGe and Ge based MOS devices is investigated by means of numerical simulations. Our simulation code is based on two modules: the former simulates the interaction between the laser light and the transistor structure to estimate the heating, the latter simulates heat diffusion, phase changes and material redistribution under irradiation. The model is calibrated in the case of different atomic species (namely Si, Ge and common dopant impurities), considering the thermal properties of the materials and the impurity depending diffusivity in the solid, liquid and interfacial region. We present several simulation results obtained by varying materials, implanted impurity profiles and geometry of the CMOS-like structures. With the support of the simulation results we discuss the possible perspectives of the excimer laser annealing application to the fabrication of post-Si CMOS devices. In particular, we show that by using Ge and SiGe materials the process window for a melting process is larger with respect to the case of traditional Si based devices.

Explosive crystallization is a well known phenomenon occurring due to the thermodynamic instability of strongly under-cooled liquids, which is particularly relevant in pulsed laser annealing processes of amorphous semiconductor materials due to the globally exothermic amorphous-toliquid-to-crystal transition pathway. In spite of the assessed understanding of this phenomenon, quantitative predictions of the material kinetics promoted by explosive crystallization are hardly achieved due to the lack of a consistent model able to simulate the concurrent kinetics of the amorphous-liquid and liquid-crystal interfaces. Here, we propose a multi-well phase-field model specifically suited for the simulation of explosive crystallization induced by pulsed laser irradiation in the nanosecond time scale. The numerical implementation of the model is robust despite the discontinuous jumps of the interface speed induced by the phenomenon. The predictive potential of the simulations is demonstrated by means of comparisons of the modelling predictions with experimental data in terms of in situ reflectivity measurements and ex-situ micro-structural and chemical characterization. Published by AIP Publishing.

Electron quantum transport is theoretically studied for finite-size armchair graphene nanoribbons biased within source and drain metallic electrodes, using an extended-Huckel-based Green's function coupled to a three-dimensional Poisson solver. The analysis evidences dynamic nonequilibrium electron charging phenomena that can affect the conduction mechanism by provoking electronic structure alterations. The origin of such process can be traced in a tracking relationship between the device's local density of states and the electrochemical potentials of the contacts. Such effect has no equivalent in the semiclassical limit.

We present self-consistent quantum transport calculations for device structures based on metallic and semiconducting carbon nanotubes. We combine the non-equilibrium Green's function formalism with an extended Huckel Hamiltonian and a Poisson solver for a realistic representation of the device and contacts’ chemistry and the system's electrostatics. We consider pure nanotubes and Au, Pd and Al metallic electrodes and reflect on their transparency in terms of non-equilibrium charging effects. Our attention focuses on the variations of the system's transport features when tuning the terminal potentials, which represent the external parameters of our semiempirical model. Results reveal a non-generalized dependence of the conduction mechanism on helicity, electrode type and bias factors. We finally demonstrate that self-consistency is essential for quantum transport calculations of semiconducting carbon nanotubes.

Graphene (Gr) was grown on the C face of 4H-SiC under optimized conditions (high annealing temperatures ranging from 1850 to 1950°C in Ar ambient at 900 mbar) in order to achieve few layers of Gr coverage. Several microscopy techniques, including optical microscopy (OM), ?Raman spectroscopy, atomic force microscopy (AFM) and atomic resolution scanning transmission electron microscopy (STEM) have been used to extensively characterize the lateral uniformity of the as-grown layers at different temperatures. ?Raman analysis provided information on the variation of the number of layers, of the stacking-type, doping and strain.

A Kinetic Monte Carlo scheme is applied to simulate with atomic resolution the synthesis of mono (few) layer(s) graphene (Gr) from a silicon carbide (SiC) substrate by selective evaporation of silicon (Si) atoms. The simulation computes the individual dynamics of the residual carbon (C) atoms which diffuse and reconfigure starting from the positions occupied in the SiC hexagonal lattice to the final Gr honeycomb structure. During the transition they gradually modify hybridization (from sp3 to sp2) and bond partners (from Si-C to C-C). We demonstrate that our method is able to recover the complex evolution steps of the epitaxial Gr on SiC in large systems for large time intervals. Moreover, the simulation results can be validated directly by means of comparison with experimental data when varying the material (e.g. initial surface configuration or polarity) or process (e.g. temperature and pressure) conditions.

We discuss the strategies for the calculation of quantum transport in disordered graphene systems from the quasi-one-dimensional to the two-dimensional limit. To this end, we employ real- and momentum-space versions of the non-equilibrium Green’s function formalism along with acceleration algorithms that can overcome computational limitations when dealing with two-terminal devices of dimensions that range from the nano- to the micro-scale. We apply this formalism for the case of rectangular graphene samples with a finite concentration of single-vacancy defects and discuss the resulting localization regimes.

This paper presents an investigation of the morphological and structural properties of graphene (Gr) grown on SiC(000-1) by thermal treatments at high temperatures (from 1850 to 1950 °C) in Ar at atmospheric pressure. Atomic force microscopy and micro-Raman spectroscopy showed that the grown Gr films are laterally inhomogeneous in the number of layers, and that regions with different stacking-type (coupled or decoupled Gr films) can coexist in the same sample. Scanning transmission electron microscopy and electron energy loss spectroscopy shoed that a nm-thick C-Si-O amorphous layer is present at the interface between Gr and SiC. Basing on these structural results, the mechanisms of Gr growth on the C-face of SiC under these annealing conditions and the role of this disordered layer in the suppression of epitaxy between Gr and the substrate have been discussed.

We have developed a kinetic Monte Carlo numerical scheme, specifically suited to simulate structural transitions in crystalline materials, and implemented it for the case of epitaxial graphene on SiC. In this process, surface Si atoms selectively sublimate, while the residual C atoms rearrange from a position occupied in the SiC hexagonal lattice to the graphene honeycomb structure, modifying their hybridization (from sp(3) to sp(2)) and bond partners (from Si-C to C-C). The model is based on the assumption that the Monte Carlo particles follow the evolution of their reference crystal until they experience a thermally activated reversible transition to another crystal structure. We demonstrate that, in a formulation based on three parallel lattices, the method is able to recover the complex evolution steps of epitaxial graphene on SiC. Moreover, the simulation results are in noteworthy agreement with the overall experimental scenario, both when varying the structural properties of the material (e.g., the initial surface configuration or polarity) as well as the process conditions (e.g., the temperature and pressure).

We have theoretically investigated the role of the interfacial arrangement on the transport properties of finite armchair and zigzag single-walled carbon nanotubes attached to various metallic contacts. We have applied the non-equilibrium Green functional formalism with a Tight-Binding and an Extended Huckel Hamiltonian. Our calculations evidence that the contacts block conduction channels for certain types of nanotubes, leading to a reduction of the transmission with respect to the ideal case. We have moreover identified distance between nanotubes and contacts as one of the most important factors of influence for the conduction mechanism. On the other hand, geometrical reconstructions of different types of symmetries between carbon and metal atoms result in having a minor impact on transport.