なかのにっき

Ferroelectric materials can create an electric field the way iron magnets create a magnetic field. And because they're so easily miniaturized, ferroelectrics are being used to permanently store data in a new generation of so-called smart cards, used for financial transactions in some countries. But condensed matter physicists still don't fully understand how these promising materials work. When an isolated ferroelectric crystal becomes polarized (the electric field equivalent of "magnetized"), some decades-old theories predict that its surface should conduct electricity, even though the crystal itself is an insulator (nonconducting). Now a Japanese team reports in the 8 January PRL that they have finally verified this surprising prediction. But not all experts are convinced.

# Device                Mountpoint      FStype  Options         Dump    Pass#
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/dev/wd0s1a             /               ufs     rw              1       1
/dev/wd0s1g             /home           ufs     rw              2       2
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#!/bin/sh
rsync -vv --recursive \
        --archive --delete \
        --hard-links --sparse \
        --exclude-from=/usr/local/etc/rsync.exclude \
        / /backup/

/backup/
/proc/*
/var/tmp/
/tmp/
/usr/ports/
/usr/src/
/usr/obj/
/usr/X11R6/

An empirical model based on cascade "quenching" and epitaxial recrystallization has been developed to describe the accumulation of the amorphous fraction during ion beam irradiation experiments. The model is based on the assumption that the amorphous fraction that remains after the formation of a cascade is related to a crystallization efficiency parameter A. For low values of A, as would be expected at low temperatures, for heavy-ion irradiations, or for materials that are good glass formers, the accumulation of the amorphous fraction as a function of dose is an exponential function. For high values of A, as would be expected at elevated temperatures, for light-ion irradiations, or for materials that are poor glass formers, the accumulation of the amorphous fraction as a function of dose is a sigmoidal function. Amorphization dose varies as a function of temperature and is reflected by the temperature-dependent crystallization efficiency. The effects of ion mass and energy on critical amorphization dose and temperature are discussed in terms of the cascade size. The dose-rate effect on the critical temperature of amorphization is derived considering the thermal annealing of the damaged material.

Needle-shaped bcc nucleation centers in fcc films of Fe on Cu(100) are observed by scanning tunneling microscopy. They form virtually without mass transfer and nearly under conservation of volume, which causes a large strain within the nascent bcc grain. The corresponding strain energy almost equals the gain in structural energy, rendering the bcc nucleation very sensitive to any effect influencing this subtle balance. We suggest that modifying the film by straining, alloying, or surface adsorption may inhibit the bcc nucleation and lead to thick metastable fcc films.

A model of surfactant-mediated epitaxy is investigated using the kinetic Monte Carlo method. This model assumes that (1) adatom-adatom interaction on the surfactant layer is weakly repulsive for dimers, and (2) a concerted atomic exchange of adatoms with surfactant atoms occurs when adatom clusters above the surfactant layer reach a threshold size. All essential features observed in a recent study of Ge/Pb/Si(111) reported by Hwang, Chang, and Tsong can be satisfactorily explained with this model.

A scaling law describing the aspect ratio of submonolayer epitaxial islands is established by applying kinetic Monte Carlo simulations. Anisotropic detachment of atoms from island borders is identified to be responsible for the transition from nearly isotropic islands at low temperatures over one-dimensional chainlike structures to elongated compact islands at high temperatures. The devised scaling law reflects the influence of the energy barriers, growth temperature, and flux very precisely. A comparison with experimental data demonstrates quantitative agreement and allows the determination of kinetic parameters.

Scanning tunneling microscopy (STM) studies have revealed that when a small fraction of a monolayer of Al, Ga, or In is deposited on Si(001) it forms one-dimensional lines of ad-dimers on the surface. No similar structures have been reported for the case of group V elements. We have studied the adsorption of group III and group V metals on Si(001) by first principles total energy calculations starting with the adsorption of a single atom up to a full surface coverage. Different adsorption sites are found for single group III and V adatoms. When a second atom is adsorbed, in both cases, there is a strong preference of the two atoms to dimerize. However, the orientation and position of the ad-dimer is determined by the binding site of the first atom. The formation of the long lines in the case of group III and not for group V metals is a direct consequence of these differences.

A Monte Carlo program for the simulation of the growth of two-dimensional polycrystalline films onto amorphous substrates is proposed. The coordinate of the atoms are quasicontinuous, allowing islands with different orientations to be included in the same sample. Enhanced diffusion along grain boundaries does not need to be modeled ad hoc since, in our model, it is a simple geometrical consequence of grain misorientation. Adsorption sites are partly fixed and partly dynamically generated at run time. The energy barriers are calculated taking into account the environment of the diffusing atoms. Island and grain size distributions estimated over samples obtained with simulated depositions performed at various substrate temperature are reported. The effect of the substrate temperature on the average grain size in films with coverages from 0.2 to 1 have been simulated and compared with the experimental data available in the literature.

The effects of a patterned substrate on island nucleation are investigated using kinetic Monte Carlo simulations. Two different models are formulated by incorporating an inhomogeneous energy surface into the basic solid-on-solid model of epitaxial growth to describe surface diffusion and consequent island nucleation on a patterned substrate. These models are related to two examples of real systems in which preferential nucleation at specific sites is encountered. Growth on a patterned substrate produces quite uniformly sized islands, which are are found to order into regular arrays displaying the periodicity of the underlying substrate. Confinement due to the patterned substrate is observed to be strongly dependent on the growth conditions. We demonstrate that there exists an optimal set of growth conditions determined by the length scale of the substrate pattern. In addition, the influence of the patterned substrate on the process of Ostwald ripening is discussed.

The contributions of atomically flat terraces as well as of SA, SB, and DB steps to the optical anisotropy of Si(001) surfaces have been calculated using a real-space multigrid method together with ab initio pseudopotentials. Our results for ideal (1×2), p(2×2), and c(2×4) reconstructed surfaces show a distinct influence of the dimer arrangement on the optical spectra. The calculated spectrum for the Si(001)c(2×4) surface agrees best with the signal measured for atomically smooth terraces. The significant optical anisotropy around 3 eV observed for vicinal surfaces, however, is induced by surface steps. Both electronic transitions directly at the surface as well as in deeper layers contribute to the optical anisotropy. We identify two mechanisms causing anisotropy signals from layers beneath the surface: the influence of the anisotropic surface potential on the bulk wave functions as well as minor contributions from atomic relaxations caused by surface-induced stress.

The kinetics of fast island decay on Ag(111) close to steps has been studied in scanning tunneling microscopy experiments and atomistic total energy calculations. We show that a breakdown of the step-edge barrier is not sufficient to explain the decay rates observed during fast decay events. From the interplay between experiments and theory, we instead propose that fast decay originates from concerted diffusion processes, which bypass the detachment barrier of atoms from kinks and step sites.

The yields of negatively charged carbon cluster ions C (2n21) and C (n = 8 and 10) emitted from graphite irradiated by 14.5 keV Cs+ ions were monitored during the initial stages of Cs incorporation in the near-surface region of the specimen. The associated work-function variations were determined in situ from the shifts of the sputtered-ions' emission-energy spectra; the total change amounted to ~2.7 eV. The lowering of induces an exponential increase of the ionization probability P^2 of the sputtered cluster ions. For C with n<=9, these variations of P^2 were monitored for several selected emission energies (from 0.5 to 15 eV), in order to investigate a possible dependence of P2 on the ion's emission velocity. For C+ and C_2- ions, such an influence of the velocity on the ionization probability was observed, whereas for larger clusters no distinct dependence was found in the velocity range accessible (7×10^5 cm/s). The results indicate that a resonant electron-transfer process may effect the ionization of the sputtered cluster species.

A high-density surface-wave magnetized argon plasma operated in the very low pressure regime together with a rf biased system is used to study the pure physical etching characteristics of platinum thin films. It is shown that, for a given dc self-bias voltage, the platinum etch rate strongly decreases as the operating pressure increases, which results from a decrease of the ion density at the sheath edge and from enhanced redeposition. It is found that using a high-density plasma in the very low pressure regime yields high etch rates with a good selectivity over resist. Fence-free features can also be achieved at bias voltages that, in contrast with reactive ion etching reactors, are only slightly above the platinum sputtering threshold.

We analyze a real-space expression for the local stress tensor. This tensor rigorously satisfies conservation of linear momentum. From this expression a coarse-grained tensor is obtained for use in atomistic simulations of solids. Our formulation is then validated by considering both a homogeneously strained crystalline solid and one containing an oversized inclusion. In the latter case a direct comparison is made with results from anisotropic elasticity theory. We find that we are able to obtain good agreement with the suitably averaged continuum solutions in the far-field regime. Moreover, the coarse-grained tensor derived here appears to offer superior accuracy as compared to a stress tensor that has been widely used in atomistic analysis.

Based on our previous Monte Carlo simulation model of electron interactions with solids, including cascade secondary electron production, in which an optical dielectric function was used to describe electron energy loss and the associated secondary electron excitation, we have systematically investigated secondary electron generation and emission for 19 metals. The calculated secondary yield curve for primary beam energy ranging from 100 eV to 2 keV was found to correspond with the experimental universal curve. The dependence of the secondary yield on the work function was studied numerically, leading to a remarkable scattered deviation from Baroody's relationship. This deviation shows that the secondary yield relates to different aspects of behavior by electrons in a metal, such as the cascade production process, the stopping power and specific energy loss mechanism for a sample, and the dependence on the electron density of states. The results provide an explanation for the scattered data on the experimental yield versus the work function. The calculations indicate that the characteristic energy loss of primaries may result in a corresponding feature in the energy distribution of secondaries.

The control of reactive sputter processes has been dynamically simulated by integrating the Larsson differential equations. This was done by employing a fast Runge?Kutta step control algorithm, allowing us to simulate sputtering with more than 20-fold real time speed on a pentium 166 Mhz. A simple proportional integral differential (PID) algorithm was implemented to simulate (i) the partial pressure control via reactive gas flow at a fixed current and (ii) the partial pressure control via current at a fixed reactive gas flow. The control cycle time was varied with respect to real life process control. These simulations show that arbitrary setpoints on the stationary s curve resulting from the steady state Larsson equations can be stabilized. However, the cycle time of the PID controller has to be small enough, e.g., less than 600 ms, for a reliable control. The setpoints in the transition mode are highly unstable, so that the process drifts immediately into one of the two corresponding stable steady states (typically within about 3?15 s) after freezing the control. In addition these computations were compared with experimental control results of reactively sputtered TiO2 and Nb2O5 films deposited by the midfrequency technique. In both cases the total s curve was stabilized at a constant oxygen flow. The process stabilization was performed at power densities of up to 5 W/cm2, limited by the generator output. For the oxygen partial pressure measurements a -probe with optimized speed was used.

An expansion of a bicomponent laser plume into a dilute ambient gas is simulated using a combined direct simulation-random trajectory Monte Carlo method. The stoichiometry of thin films deposited from laser-desorbed material on a flat substrate is examined. In the case of energy-dependent particle adsorption probability, the dependencies of deposition rate on the background pressure are shown to be nonmonotonic with maximums at low gas pressure. In addition, an increase in the ratio of light to heavy species was obtained at low pressure. We demonstrate that these results can be attributed to the interplay between the effects of collisions with the background gas on the fluxes of particles arriving at the substrate and on the adsorption probability of the species. The calculation results are consistent with recent experiments. The study is of interest for the optimization of the experimental conditions during pulsed laser deposition of multicomponent materials.