Defect and Diffusion Forum Vols. 258-260

Abstract: This paper presents the results of an experimental study of evaporating sessile drops in a controlled environment. The experimental setup allowed the investigation of the evaporation rate of sessile drops under reduced pressure (40 to 1000 mbar) and various ambient gases. Sessile drops of initial volume 2.5μL are deposited on substrates and left to evaporate in a controlled atmosphere. The effect of reducing pressure on the evaporation rate as well as changing the ambient gas is studied. Three different gases are used; namely Helium, Nitrogen and Carbon Dioxide. The role of vapour diffusion as a limiting mechanism for evaporation is studied. It is found that in all cases the evaporation rate is limited by the mass diffusion in the ambient gas provided that interfacial conditions are properly accounted for. This includes important evaporative cooling observed at higher evaporation rates and lower substrate thermal conductivity.

Abstract: The wetting and evaporation behaviour of methanol-water droplets deposited on a smooth silicon substrate were investigated experimentally. Contact angle and droplet shape kinetics were studied using an optical technique. Drops were deposited onto a silicon substrate and enclosed in a cell with nitrogen as the ambient gas. Besides the case of pure water and pure methanol, three different volume fractions of methanol in water were investigated: 10%, 50% and 80%. Using a Kruss DSA100 contact angle analyser, the behaviour of the contact angle, droplet volume, and base width was determined as a function of time. Results show that evaporation of the droplet takes place in successive stages for mixtures. The more volatile component seems to evaporate principally in the first stage, during which the contact angle of the binary drop is closer to that of pure methanol. Because the wetting behaviour is partly dictated by the surface tension of the liquid-vapour interface, methanol is believed to be concentrated at the interface during this first stage. After complete evaporation of the methanol, the wetting behaviour of the droplet tends towards that of pure water. The mechanisms that dictate the evaporation and wetting behaviour of such binary droplets include many effects: diffusion of methanol in water in the liquid phase; accumulation of one of the component near the interface and preferential evaporation followed by diffusion of one component in another in the vapour phase. In order to model the phenomenon, the above effects must be taken into account. Solutal Marangoni stress as well as interfacial instabilities may also play an important role in the behaviour of theses systems.

Abstract: The data are presented for Ni selfdiffusion and Au heterodiffusion in nanocrystalline Ni. Volume diffusion coefficients are much greater than those for a coarse – grained polycrystals extrapolated from high temperatures. Interface diffusion parameters were calculated based on the assumption that B – kinetic regime is realized at temperature range more than 448 K, while C – kinetic regime is realised at temperatures less than 423 K. The consistency of obtained results with the proposed cluster diffusion model is discussed. Diffusion in Au – Cu thin films (from several tens to several hundreds nanometers) was studied with the use of the Rutherford Back Scattering, RBS, under the kinetic regime B (448 – 523 K). The RBS spectra were transformed in the concentration depth profiles for both volume and grain boundary (GB) diffusion. The triple products Pn = snδDn (sn is the enrichment coefficient, while δ is the nanograin boundary width) were calculated using Whipple model. As a result of this analysis the s – value for Cu – Au system was determined to be of the order of unity. The paper is focused on a difference between GB diffusion parameters in nano – and coarse grained materials.

Abstract: In this paper, the authors review recent progress that has been made in understanding various aspects of grain boundary diffusion at the phenomenological level. We show how the mapping of phenomenological grain boundary diffusion problems onto a lattice readily permits lattice-based random walk theory to be used to address the problem, invariably by Monte Carlo computer simulation methods. Recent advances in calculating concentration profiles and effective diffusion coefficients are described in detail.

Abstract: The grain boundary (GB) wetting was investigated in the Sn – 25 at.% In alloy. It was found that the portion of GBs wetted by the melt depends on the annealing temperature. No GB completely wetted by melt was observed at 140°C, while all GBs were fully wetted after annealing at 180°C. Between 140°C and 180°C the portion of wetted GBs increases with increasing temperature. The tie-lines of GB wetting phase transition were constructed in the Sn–In bulk phase diagram.

Abstract: Possibilities of grain-boundary diffusion and segregation studies using nuclear gammaresonance spectroscopy (NGR) are considered. It is shown that the results of the Mössbauer investigations testify the necessity to specify the classical Fisher’s model of grain-boundary diffusion, and a possible way of such specification is suggested. It is demonstrated that investigation of grain boundaries using emission Mössbauer spectroscopy appreciably supplement the information obtained from the diffusion profiles analysis. In particular, Mössbauer investigations make it possible to evaluate directly the grain-boundary segregation factor, to determine the grain-boundary diffusion mechanism, to estimate the rate of the diffusant pumping from a grain boundary core into the bulk, etc.

Abstract: The continuous scaling of electron devices places strong demands on device design and simulation. The currently prevailing bulk transistors as well as future designs based on thin silicon layers all require a tight control of the dopant distribution. For process simulation, especially the correct prediction of boron diffusion and activation was always a problem. The paper describes the model developed for boron implanted into crystalline silicon and shows applications to hot-shield annealing and flash-assisted rapid thermal processing.

Abstract: An effective inter-atomic potential is proposed in order to describe structural and dynamical properties of II-VI and III-V semiconductors. The interaction potential consists of twoand three-body interactions. The two-body term takes into account steric repulsion, charge-induce dipole interaction due to the electronic polarizability of ions, Coulomb interaction due to charge transfer between ions, and dipole-dipole (van der Waals) interactions. The three-body term, which has a modified Stillinger-Weber form, describes bond-bending as well as bond-stretching effects. Here we report the fitting and the application of this interaction potential for InP in the crystalline phase and for CdTe in the crystalline and liquid phases. The structural correlations are discussed through pair distribution, coordination number and bond-angle functions. Vibrational density of states for InP and CdTe as well as the static structure factor for liquid CdTe are in very good agreement with experimental data.

Abstract: Nanocrystal memories are attractive candidate for the development of non volatile memory devices for deep submicron technologies. In a nanocrystal memory device, a 2D network of isolated nanocrystals is buried in the gate dielectric of a MOS and replaces the classical polysilicon layer used in floating gate (flash) memories. Recently, we have demonstrated a route to fabricate these devices at low cost by using ultra low energy ion implantation. Obviously, all the electrical characteristics of the device depend on the characteristics of the nanocrystal population (sizes and densities) but also on their exact location with respect to the gate and channel of the MOS transistor. It is the goal of this paper to report on the main materials science aspects of the fabrication of 2D arrays of Si nanocrystals in thin SiO2 layers and at tunable distances from their SiO2/interfaces.

Abstract: Despite its importance as a material in many domains, SiO₂ is still a very badly known material from the point of view of materials science. Experimentally the silicon and oxygen diffusion has been determined in silica as well as in quartz, but several discrepancies arise between different authors. From a theoretical point of view the various possible atomic defects have mostly been studied in an electronic perspective, so even the simplest ones remained quite poorly known till recently, the silicon related ones remaining completely unknown. The great similarity between silica and quartz properties is in favour of a common model. The determination of the precise formation and migration energies of the various defects is then of paramount importance for the understanding of the kinetic properties of SiO₂. We will present in this paper the results of a study of the formation and mobility properties of oxygen and silicon defects in the view of determining the self-diffusion mechanism(s). Our work relies on up to date ab-initio methods: total energy calculations in a DFT-LDA approach, using either plane wave or pseudo-atomic basis for the wave functions and pseudopotentials.We shall discuss the role of the various parameters controlling the kinetic behaviour: chemical potential of the species, nature of the main impurities, cristallinity, and preparation mode of the sample.