Paola Alippi

Strained tetragonal states and bain paths in metals

Paths of tetragonal states between two phases of a material, such as bcc and fcc, are called Bain paths. Two simple Bain paths can be defined in terms of special imposed stresses, one of which applies directly to strained epitaxial films. Each path goes far into the range of nonlinear elasticity and reaches a range of structural parameters in which the structure is inherently unstable. In this Letter we identify and analyze the general properties of these paths by density functional theory. Special examples include vanadium, cobalt, and copper, and the epitaxial path is used to identify an epitaxial film as related uniquely to a bulk phase. [S0031-9007(97)03190-6]

A phase-field approach to the simulation of the excimer laser annealing process in Si

We present a phase-field methodology applied to the simulation of dopant redistribution in Si during an excimer laser annealing process. The kinetic model derived in the framework of the Ginsburg–Landau thermodynamic formalism is made up of three coupled equations that rule the concurrent evolution of the thermal, phase, and impurity fields. The model was solved numerically by considering, as the initial conditions, the generic material modification due to an ion implant process, i.e., the implanted impurity profile in a SiO2/a–Si/c–Si stack. The model is parametrized for the cases of As and B doping, considering the thermal properties of the materials in the stack and the impurity-dependent diffusivity in the solid, liquid, and interfacial regions (the latter is characterized by a finite dimension). Simulated profiles are compared with the experimental results that have been obtained by secondary ion mass spectrometry and spreading resistance profiling. These comparisons demonstrate the reliability of th...

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.

A hybrid zinc phthalocyanine/zinc oxide system for photovoltaic devices: a DFT and TDDFPT theoretical investigation

By combining ab initio density functional theory (DFT) and time-dependent density functional perturbation theory (TDDFPT) methods, we investigate the structural, electronic and optical properties of a zinc phthalocyanine (ZnPc) molecule interacting with the zinc oxide (ZnO) wurtzite (100) surface. Our results reveal the existence of a strong molecule–surface coupling whose major effect is the appearance of a new unoccupied electronic level, deriving from an intimate mixing of ZnPc and ZnO electronic states and strategically located within the ZnO conduction band and below the ZnPc LUMO. This level induces appreciable changes in the ZnPc absorption spectrum and is expected to significantly favor a molecule-to-surface transfer of photo-excited electrons, a key process in the functioning of hybrid photovoltaic devices. The molecule–surface interactions are also characterized by significant van der Waals forces and by the formation of molecule–surface chemical bonds, thus resulting in appreciable molecular adhesion to the surface.

Role of light scattering in excimer laser annealing of Si

We have studied, by means of simulations and experiments, the interaction between the electromagnetic field, generated by excimer laser, and Si device. This study strictly refers to laser annealing process, recently attracting a broad interest as an alternative thermal treatment. Our numerical methodology is based on coupling the simulation of the electromagnetic field, for the calculation of the heat source distribution, and the simulations of the thermal, phase, and impurity fields. Simulations of laser irradiation in metal-oxide-semiconductor transistor structures are discussed and compared to the corresponding experimental analysis. Our results are useful to understand problematics and perspectives of the laser annealing application in the fabrication of scaled devices.

How much room for BiGa heteroantisites in GaAs1−xBix?

We addressed the issue of bismuth heteroantisite defects (BiGa) in GaAs1−xBix/GaAs epilayers by coupling x-ray absorption spectroscopy at the bismuth edge with density functional theory calculations of the defect structure. Calculations predict a large relaxation of the Bi-As interatomic distances when Bi atoms substitute Ga, however we found no experimental evidence of it. Quantitative analysis of the x-ray absorption spectra allows us to establish a maximum concentration limit for BiGa, which corresponds to about 5% of the total Bi atoms. BiGa do not account for the modifications in the spectra previously attributed to short range ordering.

Tetragonal states from epitaxial strain on metal films

The tetragonal states produced by isotropic pseudomorphic epitaxial strain in the (001) plane on a tetragonal phase of a crystal are calculated for V, Ti, Rb, Li, K, Sr from first-principles electronic theory. It is shown that each metal has two tetragonal phases corresponding to minima of the total energy with respect to tetragonal deformations, hence are equilibrium phases, and that the equilibrium phases are separated by a region of inherent instability. The equilibrium phase for any strained tetragonal state can thus be uniquely identified. Lattice constants and relative energies of the two phases and the saddle point between them are tabulated, as well as the tetragonal elastic constants of each phase.

Structural characterization of Ni2Si pseudoepitaxial transrotational structures on [001] Si

The formation of pseudoepitaxial transrotational structures has been observed during the early stage of the reaction of thin Ni layers on [001] Si substrates. During the reaction, large Ni(2)Si domains, characterized by single bending contours, establish a close relationship with the silicon lattice. The silicide domain consists of a core region, along the bending contour, where the silicide layer has grown epitaxially with silicon. Outside the core, the planes, at first parallel to the bending contour, continuously bend over the range 15-20 degrees , whilst those at 90 degrees remain aligned with silicon across the interface. Owing to the cylindrical symmetry of those transrotational structures, transmission electron microscopy analyses provided direct evidence of the bending phenomenon and allowed a complete description of the fully relaxed domain structure. A non-conventional mechanism of strain relaxation has been proposed, which is competitive with respect to the usual formation of misfit dislocations. The competitive phenomenon consists of Ni(2)Si lattice bending and rearrangement of the interface to minimize the Gibbs free energy of the domain.

Role of the indium–carbon interaction on In diffusion and activation in Si

We present a theoretical approach to the study of C influence on In diffusion and activation in Si. We consider a kick-out type model for the defect driven migration of both species, while we assume that InC complexes form by the interaction between the defect-impurity pairs and the substitutional species. Moreover, we introduce a double-level activation model in order to calculate the active fraction of the diffusion profiles. In parameters are derived both from ab initio calculations and from fitting the experimental diffusion profiles. The modeling is used in order to explain experimental data relative to In implanted into Si substrates containing different C contamination background.

Electrical activation phenomena induced by excimer laser annealingin B-implanted silicon

The activation process induced by excimer laser annealing (ELA) has been investigated in 10keV B-implanted samples. It is found that for energy densities inducing melt depths of the order or larger of the implanted region the junction depth is controlled by the melt depth, with activation approaching 100% and box-shaped carrier density distributions with abrupt junction profile. For energy densities inducing a melting shallower than the implanted region, two different activation mechanisms have been identified: the first occurring in the molten region and leading to complete B activation; the second occurring in the region immediately below the molten zone and leading to thermal activation of B, induced by the heat wave propagating into the Si wafer. This last process is characterized by an activation energy of 5eV and is not accompanied by B diffusion. As a consequence, a deep tail of active B is produced, preventing the possibility to form abrupt and ultrashallow junctions. These results suggest that fo...