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Although understanding filament formation in oxide-based memristive devices by theory has emerged, there are still fundamental unanswered questions. Importantly, for practical application of thin films the material in its amorphous state is to be considered, but mostly lacking so far, and details on sub-stoichiometry are also scarce. To gain insight into the optical and electronic properties of sub-stoichiometric amorphous tantalum oxide (TaOx), the electron energy loss spectrum (EELS) of model systems is characterized theoretically and electron transport characteristics are analyzed in detail. Calculated blue-shifts by increasing sub-stoichiometry explained the measurements, potentially suggesting estimation of oxygen vacancy concentrations through EEL spectra. Electron transport results based on TaOx material models validated by EELS measurements show that oxygen vacancy filamentary paths are initiated at low bias upon increasing sub-stoichiometry yet noting an interplay with the local amorphous structure. Contact resistances at interfaces of the TaOx switching layer and a tantalum scavenging layer or titanium nitride electrode are quantified, indicating the possibility for either oxygen vacancy- or metal cluster-based conduction mechanisms at the interface. The computational work, combined with experimental characterization for validation, provides a basis for investigating effects of sub-stoichiometry on filament formation in TaOx thin film memristive devices.

Although understanding filament formation in oxide‐based memristive devices by theory has emerged, there are still fundamental unanswered questions. Importantly, for practical application of thin films the material in its amorphous state is to be considered, but mostly lacking so far, and details on sub‐stoichiometry are also scarce. To gain insight into the optical and electronic properties of sub‐stoichiometric amorphous tantalum oxide (TaOx), the electron energy loss spectrum (EELS) of model systems is characterized theoretically and electron transport characteristics are analyzed in detail. Calculated blue‐shifts by increasing sub‐stoichiometry explained the measurements, potentially suggesting estimation of oxygen vacancy concentrations through EEL spectra. Electron transport results based on TaOx material models validated by EELS measurements show that oxygen vacancy filamentary paths are initiated at low bias upon increasing sub‐stoichiometry yet noting an interplay with the local amorphous structure. Contact resistances at interfaces of the TaOx switching layer and a tantalum scavenging layer or titanium nitride electrode are quantified, indicating the possibility for either oxygen vacancy‐ or metal cluster‐based conduction mechanisms at the interface. The computational work, combined with experimental characterization for validation, provides a basis for investigating effects of sub‐stoichiometry on filament formation in TaOx thin film memristive devices. [ABSTRACT FROM AUTHOR]

We apply density-functional theory calculations to predict dopant modulation of electrical conductivity (σo) for seven dopants (C, Si, Ge, H, F, N, and B) sampled at 18 quantum molecular dynamics configurations of five independent insertion sites into two (high/low) baseline references of σo in amorphous Ta2O5, where each reference contains a single, neutral O vacancy center (VO 0). From this statistical population (n=1260), we analyze defect levels, physical structure, and valence charge distributions to characterize nanoscale modification of the atomistic structure in local dopant neighborhoods. C is the most effective dopant at lowering Ta2Ox σo, while also exhibiting an amphoteric doping behavior by either donating or accepting charge depending on the host oxide matrix. Both B and F robustly increase Ta2Ox σo, although F does so through elimination of Ta high charge outliers, while B insertion conversely creates high charge O outliers through favorable BO3 group formation, especially in the low σo reference. While N applications to dope and passivate oxides are prevalent, we found that N exacerbates the stochasticity of σo we sought to mitigate; sensitivity to the N insertion site and some propensity to form N-O bond chemistries appear responsible. We use direct first-principles predictions of σo to explore feasible Ta2O5 dopants to engineer improved oxides with lower variance and greater repeatability to advance the manufacturability of resistive memory technologies. [ABSTRACT FROM AUTHOR]

First-principles calculations of electrical conductivity (σo) are revisited to determine the atomistic origin of its stochasticity in a distribution generated from sampling 14 ab-initio molecular dynamics configurations from 10 independently quenched models (n?=?140) of substoichiometric amorphousTa2O5, where each structure contains a neutral O monovacancy (VO0). Structuralanalysis revealed a distinct minimum Ta-Ta separation (dimer/trimer) corresponding to each VO0 location. Bader charge decomposition using a commonality analysis approach based on the σo distribution extremes revealed nanostructural signatures indicating that both the magnitude and distribution of cationic charge on the Ta subnetwork have a profound influence on σo. Furthermore, visualization of local defect structures and their electron densities reinforces these conclusions and suggests σo in the amorphous oxide is best suppressed by a highly charged, compact Ta cation shell that effectively screens and minimizes localized VO0 interaction with the a-Ta2O5 network; conversely, delocalization of VO0 corresponds to metallic character and high σo. The random network of a-Ta2O5 provides countless variations of an ionic configuration scaffold in which small perturbations affect the electronic charge distribution and result in a fixed-stoichiometry distribution of σo; consequently, precisely controlled and highly repeatable oxide fabrication processes are likely paramount for advancement of resistive memory technologies. [ABSTRACT FROM AUTHOR]

First-principles density-functional theory calculations are used to study the atomistic structure, structural energetics, and electron density near the O monovacancy (VO n; n=0,1+,2+) in both bulk, amorphous tantalum pentoxide (a-Ta2O5), and also at vacuum and metallic Ta interfaces. We calculate multivariate vacancy formation energies to evaluate stability as a function of oxidation state, distance from interface plane, and Fermi energy. VO n of all oxidation states preferentially segregates at both Ta and vacuum interfaces, where the metallic interface exhibits global formation energy minima. In a-Ta2O5, VO n is characterized by structural contraction and electron density localization, while VO2+ promotes structural expansion and is depleted of electron density. In contrast, interfacial VO 0 and VO 2+ show nearly indistinguishable ionic and electronic signatures indicative of a reduced VO center. Interfacial VO 2+ extracts electron density from metallic Ta, indicating that VO 2+ is spontaneously reduced at the expense of the metal. This oxidation/reduction behavior suggests careful selection and processing of both oxide layer and metal electrodes for engineering memristor device operation. [ABSTRACT FROM AUTHOR]

We apply first-principles density-functional theory (DFT) calculations, ab-initio molecular dynamics, and the Kubo-Greenwood formula to predict electrical conductivity in Ta2Ox (0 ≤ x ≤ 5) as a function of composition, phase, and temperature, where additional focus is given to various oxidation states of the O monovacancy (VOn; n = 0,1+,2+). In the crystalline phase, our DFT calculations suggest that VO0 prefers equatorial O sites, while VO1+ and VO2+ are energetically preferred in the O cap sites of TaO7 polyhedra. Our calculations of DC conductivity at 300 K agree well with experimental measurements taken on Ta2Ox thin films (0.18 ≤ x ≤ 4.72) and bulk Ta2O5 powder-sintered pellets, although simulation accuracy can be improved for the most insulating, stoichiometric compositions. Our conductivity calculations and further interrogation of the O-deficient Ta2O5 electronic structure provide further theoretical basis to substantiate VO0 as a donor dopant in Ta2O5. Furthermore, this dopant-like behavior is specific to the neutral case and not observed in either the 1+ or 2+ oxidation states, which suggests that reduction and oxidation reactions may effectively act as donor activation and deactivation mechanisms, respectively, for VOn in Ta2O5. [ABSTRACT FROM AUTHOR]

We investigate the influence of force field parameterization on the atomic-level description of the interface structure between Si(001) and its amorphous oxide [Si(001)/a-SiO2] with systematic application of continuous random network model-based Metropolis Monte Carlo (CRN-MMC) simulations. Particular emphasis is given to the nature of the potentials in both the crystalline Si and a-SiO2 phases, especially the quantifiable relative rigidity between phases. To assess their reliability, the energetics and mechanical properties of the interface models generated from the CRN-MMC approach with different Keating-like potential parameters were compared with those calculated using density functional theory. We statistically characterized the structural parameters and interface abruptness from various potential models of varying interface O coverage ratio in terms of bond angle, ring size, and suboxide distributions; lateral Si-O-Si bridge bond interface densities; and strain energy profiles along [001]. Comparison of our simulation results and existing experimental observations shows that a sufficiently hard character of the a-SiO2 phase parameterization is essential in generation of atomically accurate depictions of the Si(001)/a-SiO2 interface. [ABSTRACT FROM AUTHOR]

Details of Hickory Street Bridge replacement project, completed in Pennsylvania, are presented. Surrounding-sensitive designs and cost effectiveness are adverted as major highlights of this project.

A new theoretical approach that combines Metropolis Monte Carlo, tight-binding molecular dynamics, and density functional theory calculations is introduced as an efficient technique to determine the structure and stability of native defects in crystalline silicon. Based on this combined approach, the growth behaviour of self-interstitial defects in crystalline Si is presented. New stable structures for small interstitial clusters (In, 5 ≤ n ≤ 16) are determined and show that the compact geometry appears favoured when the cluster size is smaller than 10 atoms (n < 10). The fourfold-coordinated dodeca-interstitial (I12) structure with C2h symmetry is identified as an effective nucleation centre for larger extended defects. This work provides the first theoretical support for earlier experiments that suggest a shape transition from compact to elongated structures around n = 10. We also provide some theoretical evidence that suggests that {3 1 1} extended defects grow slowly along 〈2 3 3〉 and relatively faster along 〈1 1 0〉, which is consistent with typical defect aspect ratios observed through transmission electron microscopy. [ABSTRACT FROM AUTHOR]