Robert J. Bondi

Electrical conductivity in oxygen-deficient phases of tantalum pentoxide from first-principles calculations

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 sp...

First-Principles Study of the Structural, Electronic, and Optical Properties of Oxide-Sheathed Silicon Nanowires

Using a density functional theory approach, we examine the dielectric function (ε(ω)) optical spectra and electronic structure of various silicon nanowire (SiNW) orientations (<100>, <110>, <111>, and <112>) with amorphous oxide sheaths (-a-SiOx) and compare the results against H-terminated reference SiNWs. We extend the same methods to investigate the effects of surface passivation on <111> SiNW properties using functional group termination (-H, -OH, and -F) and three different thicknesses of oxide sheath passivation. Oxide layer growth is evidenced in the spectra by concomitant appearance of tail oxide character with signatures of increased Si disorder. Suboxide contributions and increased Si disorder from oxidation average out the band structure dispersion observed in the reference SiNWs. Furthermore, we plot average Seraphin coefficients for <111> passivations that clearly distinguish functional group termination from surface oxidation and discuss the suboxide and disorder contributions on the characteristic intersection of these coefficients. The substantial difference in properties observed between <111>-OH and <111>-a-SiOx SiNWs emphasizes the importance of using realistic oxidation models to improve understanding of SiNW properties.

Atomistic structural description of the Si(001)/a-SiO2 interface: The influence of different Keating-like potential parameters

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]. Compariso...

First-Principles Prediction of Optical Absorption Enhancement for Si Native Defect Clusters under Biaxial Strain

We use density-functional theory calculations to qualitatively explore the effects of fourfold-coordinated vacancy V4 and interstitial I4 clusters on optical absorption spectra in crystalline Si c-Si under selected conditions of biaxial strain =� 3, 0, and 3%. While both native defect clusters enhance c-Si absorption by redshifting the absorption edge, we observe additional enhancement from biaxial strain. Increased strain magnitude tends to increase the absorption enhancement effect, but the optimal sign of strain exhibits a complementary relationship: compressive strain most effectively enhances V4 absorption, while tensile strain most effectively enhances I4 absorption. The absorption redshift as a function of strain correlates well with effective bandgap reduction, including the appearance of an intermediate band under certain conditions = �3 and 0% for V4. Our results suggest that manipulation of native defect distributions and their strain fields can be used to engineer the Si absorption spectra.

Integrated atomistic modelling of interstitial defect growth in silicon

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 (I n , 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 C 2h 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.

Prediction of the formation of stable periodic self-interstitial cluster chains [(I4)m,m=1–4] in Si under biaxial strain

Using density functional theory calculations, we examined the structure and stability of extendable self-interstitial cluster configurations (In,n=12,16) with four-atom periodicity in crystalline silicon under biaxial strain (−4%≤e≤4%) on Si(100). In the absence of strain, the ground state configurations of I12 and I16 share a common structure (I12-like) with C2h symmetry and a four-atom repeating unit; however, we identified an extended configuration based on I4 (D2d symmetry) cluster aggregates [(I4)m(m=3,4)] along ⟨110⟩ that is more favorable under certain magnitudes of strain. While both the I12-like and (I4)m configurations exhibit relative stabilities that are a function of both strain and orientation, the larger (I4)m orientation effect is the primary reason that these structures are preferred in both highly tensile and highly compressive environments. This suggests that I4 derivatives may participate in the growth transition of Si self-interstitial clusters in the compact-to-extended size regime (...

Electrical conductivity in oxygen-deficient phases of transition metal oxides from first-principles calculations.

Density-functional theory calculations, ab-initio molecular dynamics, and the Kubo-Greenwood formula are applied to predict electrical conductivity in Ta2Ox (0x5) 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+). Our calculations of DC conductivity at 300K agree well with experimental measurements taken on Ta2Ox thin films 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 and other metal oxides. Furthermore, this dopant-like behavior appears specific to neutral VO cases in both Ta2O5 and TiO2 and was not observed in other oxidation states. This suggests that reduction and oxidation reactions may effectively act as donor activation and deactivation mechanisms, respectively, for VO0 in transition metal oxides.