Kinetics of silicide formation by thin films of V on Si and SiO_2 substrates

The reaction rate of vacuum‐evaporated films of V of the order of 1000 Å thick is investigated by MeV He backscattering spectrometry. On substrates of single‐crystal Si and for anneal times up to several hours in the temperature range 570–650°C, VSi_2 is formed at a linear rate in time. The activation energy of the process is 1.7±0.2 eV. The presence of oxygen in amounts of 10% can significantly decelerate the reaction. On substrates of SiO_2 in the temperature range 730–820°C and anneal times of several hours or less, V_3Si is formed at a square‐root rate in time. The activation energy of this process is 2.0±0.2 eV

A global method for coupling transport with chemistry in heterogeneous porous media

Modeling reactive transport in porous media, using a local chemical equilibrium assumption, leads to a system of advection-diffusion PDE's coupled with algebraic equations. When solving this coupled system, the algebraic equations have to be solved at each grid point for each chemical species and at each time step. This leads to a coupled non-linear system. In this paper a global solution approach that enables to keep the software codes for transport and chemistry distinct is proposed. The method applies the Newton-Krylov framework to the formulation for reactive transport used in operator splitting. The method is formulated in terms of total mobile and total fixed concentrations and uses the chemical solver as a black box, as it only requires that on be able to solve chemical equilibrium problems (and compute derivatives), without having to know the solution method. An additional advantage of the Newton-Krylov method is that the Jacobian is only needed as an operator in a Jacobian matrix times vector product. The proposed method is tested on the MoMaS reactive transport benchmark.Comment: Computational Geosciences (2009) http://www.springerlink.com/content/933p55085742m203/?p=db14bb8c399b49979ba8389a3cae1b0f&pi=1

PRODUCTION OF SMALL OHMIC CONTACTS BY LASER PROCESSING FOR Si AND GaAs DEVICES

Le recuit local de contacts de petite dimension sur une couche de Si et GaAs a été réalisé à l'aide d'un laser continu focalisé (Ø = 3 µm, 0,5W) sans affecter les contacts Schottky situés quelques µm à coté des parties irradiées. Des contacts à base de Ti/Pt sur du silicium du type N ont montré une résistance inférieure à 10-6Ωcm2, des résistances de contact du Ti/AuGe/Pt sur une couche de GaAs du type N sont de l'ordre de 1.5.10-5 Ωcm2. Les résultats sont comparables à ceux obtenus pour des contacts recuit par une méthode conventionelle.Small contact patterns on Si and GaAs have been alloyed locally with a focused cw laser beam (Ø = 3µm, 0,5W), without affecting Schottky contacts few µm apart the irradiated areas. Contact resistances of less than 10-6 Ωcm2 for Ti/Pt contacts on n-Si and 1.5.10-5 Ωcm2 for Ti/AuGe/Pt on contacts on n-GaAs have been obtained. Conventional alloyed contacts show similar values