Three parallel algorithms for classical molecular dynamics are presented. The first assigns each processor a fixed subset of atoms; the second assigns each a fixed subset of inter-atomic forces to compute; the third assigns each a fixed spatial region. The algorithms are suitable for molecular dynamics models which can be difficult to parallelize efficiently—those with short-range forces where the neighbors of each atom change rapidly. They can be implemented on any distributed-memory parallel machine which allows for message-passing of data between independently executing processors. The algorithms are tested on a standard Lennard-Jones benchmark problem for system sizes ranging from 500 to 100,000,000 atoms on several parallel supercomputers--the nCUBE 2, Intel iPSC/860 and Paragon, and Cray T3D. Comparing the results to the fastest reported vectorized Cray Y-MP and C90 algorithm shows that the current generation of parallel machines is competitive with conventional vector supercomputers even for small problems. For large problems, the spatial algorithm achieves parallel efficiencies of 90% and a 1840-node Intel Paragon performs up to 165 faster than a single Cray C9O processor. Trade-offs between the three algorithms and guidelines for adapting them to more complex molecular dynamics simulations are also discussed.

Fast parallel algorithms for short-range molecular dynamics

Resistance switching in metal oxides could form the basis for next-generation non-volatile memory. It has been argued that the current in the high-conductivity state of several technologically relevant oxide materials flows through localized filaments, but these filaments have been characterized only indirectly, limiting our understanding of the switching mechanism. Here, we use high-resolution transmission electron microscopy to probe directly the nanofilaments in a Pt/TiO2/Pt system during resistive switching. In situ current–voltage and low-temperature (∼130 K) conductivity measurements confirm that switching occurs by the formation and disruption of TinO2n−1 (or so-called Magneli phase) filaments. Knowledge of the composition, structure and dimensions of these filaments will provide a foundation for unravelling the full mechanism of resistance switching in oxide thin films, and help guide research into the stability and scalability of such films for applications. Nanoscale filaments with a Magneli structure are shown to be responsible for resistance switching in thin films of TiO2, and the properties of the filaments are directly observed during the switching process.

Atomic structure of conducting nanofilaments in TiO2 resistive switching memory

A surface science perspective on TiO2 photocatalysis

Atomic layer deposition (ALD) is gaining attention as a thin film deposition method, uniquely suitable for depositing uniform and conformal films on complex three-dimensional topographies. The deposition of a film of a given material by ALD relies on the successive, separated, and self-terminating gas–solid reactions of typically two gaseous reactants. Hundreds of ALD chemistries have been found for depositing a variety of materials during the past decades, mostly for inorganic materials but lately also for organic and inorganic–organic hybrid compounds. One factor that often dictates the properties of ALD films in actual applications is the crystallinity of the grown film: Is the material amorphous or, if it is crystalline, which phase(s) is (are) present. In this thematic review, we first describe the basics of ALD, summarize the two-reactant ALD processes to grow inorganic materials developed to-date, updating the information of an earlier review on ALD [R. L. Puurunen, J. Appl. Phys. 97, 121301 (2005)], and give an overview of the status of processing ternary compounds by ALD. We then proceed to analyze the published experimental data for information on the crystallinity and phase of inorganic materials deposited by ALD from different reactants at different temperatures. The data are collected for films in their as-deposited state and tabulated for easy reference. Case studies are presented to illustrate the effect of different process parameters on crystallinity for representative materials: aluminium oxide, zirconium oxide, zinc oxide, titanium nitride, zinc zulfide, and ruthenium. Finally, we discuss the general trends in the development of film crystallinity as function of ALD process parameters. The authors hope that this review will help newcomers to ALD to familiarize themselves with the complex world of crystalline ALD films and, at the same time, serve for the expert as a handbook-type reference source on ALD processes and film crystallinity.

Crystallinity of inorganic films grown by atomic layer deposition: Overview and general trends

Oxygen vacancies in transition metal and rare earth oxides: Current state of understanding and remaining challenges

Al‐Doped TiO2 Films with Ultralow Leakage Currents for Next Generation DRAM Capacitors

We find, using a local density approximation $+\mathrm{\text{Hubbard}}\text{ }U$ method, that oxygen vacancies tend to cluster in a linear way in ${\mathrm{SrTiO}}_{3}$, a prototypical perovskite oxide, accompanied by strong electron localization at the $3d$ state of the nearby Ti transition metal ion. The vacancy clustering and the associated electron localization lead to a profound impact on materials properties, e.g., the reduction in free-carrier densities, the appearance of characteristic optical spectra, and the decrease in vacancy mobility. The high stability against the vacancy migration also suggests the physical reality of the vacancy cluster.

Oxygen vacancy clustering and electron localization in oxygen-deficient SrTiO(3): LDA + U study.

We report our first-principles results on point defects in $\mathrm{Ti}{\mathrm{O}}_{2}$ in the rutile phase. Both the oxygen vacancy and titanium interstitial are considered. The size effect of the supercell has been examined and the localized state associated with the oxygen vacancy turns out to be sensitive to the supercell size. We find that the oxygen vacancy does not give rise to a defect level within the energy gap while the titanium interstitial creates a localized state $0.2\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ below the conduction edge that can be related to the infrared absorption data. The charge accumulation around the oxygen vacancy is attributed to polarization of valence bands.

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First-principles study of point defects in rutile Ti O 2 − x

By using a first-principles method employing the local density approximation plus Hubbard parameter approach, we study point defects in NiO and interactions between them. The defect states associated with nickel or oxygen vacancies are identified within the energy gap. It is found that nickel vacancies introduce shallow levels in the density of states for the spin direction opposite to that of the removed Ni atom, while the oxygen vacancy creates more localized in-gap states. The interaction profiles between vacancies indicate that specific defect arrangements are strongly favored for both nickel and oxygen vacancies. In the case of nickel vacancies, defect ordering in a simple-cubic style is found to be most stable, leading to a half-metallic behavior. The ionized oxygen vacancies also show a tendency toward clustering, more strongly than neutral pairs. The microscopic origin of vacancy clustering is understood based on overlap integrals between defect states.

/pdf/interaction-and-ordering-of-vacancy-defects-in-nio-4c0orvo2gz.pdf

Interaction and ordering of vacancy defects in NiO

The effect of the carbon residue on the reliability of HfO2 thin films was investigated. HfO2 films were deposited on Si wafers by atomic layer deposition at a wafer temperature of 250°C using Hf[N(CH3)2]4 and O3 oxidant with two different densities (160 and 390g∕m3). The films deposited at the higher O3 density contained a lower concentration of carbon impurities. The leakage current density was lower and the time-dependent dielectric breakdown was improved in the higher O3 density films. First principles calculations confirmed that trap sites were generated in the band gap of HfO2 when carbon was interstitially or substitutionally present.

/pdf/effects-of-carbon-residue-in-atomic-layer-deposited-hfo2-iofhbpxb3q.pdf

Hyo-Shin Ahn

Papers

Al‐Doped TiO2 Films with Ultralow Leakage Currents for Next Generation DRAM Capacitors

Oxygen vacancy clustering and electron localization in oxygen-deficient SrTiO(3): LDA + U study.

First-principles study of point defects in rutile Ti O 2 − x

Interaction and ordering of vacancy defects in NiO

Effects of carbon residue in atomic layer deposited HfO2 films on their time-dependent dielectric breakdown reliability