arXiv subjects

Jonathon Cottom

Publications and source records attributed to Jonathon Cottom.

7 recordsLinked to original sources

arXiv2026

Vitriflow: calibrated amorphous structure ensembles from melt-quench simulation

Many structure-property relations in amorphous materials are encoded not only in the mean structure, but in the prevalence, correlations and spatial organisation of minority environments across length scales. Yet atomistic models are commonly validated against bulk averages and a nominal number of structures, without establishing whether the finite population resolves the motifs invoked to explain material behaviour. We present an auditable framework, implemented in the Vitriflow software package, that connects calibrated melt-quench generation to explicit populations and quantity-specific uncertainty. This approach is demonstrated on three distinct benchmark systems. In a-SiO$_2$, strain is followed from intra-tetrahedral deformation through compressed Si--O--Si bridges to primitive 3-ring topology, showing that the lower angular tail is an organised medium-range population rather than undifferentiated variation about the network mean. In matched a-Si$_3$N$_4$ refinements, DFT redistributes density and first-neighbour length scale, sharpens the force-field first shell and repairs marginal contacts, while the two DFT descendants retain essentially the same first-neighbour topology. In a-Sm$_2$O$_3$, the amount and connectivity of partially ordered domains are quantified as they emerge from the disordered parent population, while the mixed coordination characteristic of the amorphous oxide remains robust. Mean structure, distribution tails, inherited topology and connected order are therefore distinct materials variables with distinct uncertainties. There is no universal ensemble size: the Vitriflow package records what was generated, which population was analysed and what that finite population establishes, providing a reproducible route from amorphous structure datasets to quantitatively supported structure-property hypotheses.

cond-mat.mtrl-sci↗

arXiv2026

Constraint residuals, graph posteriors, and determinant-corrected full-space targets in Bayesian inverse problems

Bayesian inverse problems constrained by state equations are often sampled in a full parameter-state space by penalising the residual, rather than in a reduced space where the state is eliminated. We show that these formulations are not automatically equivalent as posterior measures. For finite-dimensional discretisations of equality-constrained inverse problems, assume the state equation $c(\theta,u)=0$ has a unique solution $u=G(\theta)$ and nonsingular state Jacobian $\D_u c$. The reduced posterior, its graph lift, and the zero-noise residual posterior are then distinct. A local change of variables shows that an uncorrected Gaussian residual penalty converges, after marginalisation over $u$, to the reduced density multiplied by $\abs{\det \D_u c(\theta,G(\theta))}^{-1}$. Thus algebraically equivalent residuals can define the same feasible set but different limiting posteriors. We derive determinant corrections for unweighted, weighted, and rescaled residual penalties that have the graph-lifted reduced posterior as their hard-constraint limit. The result separates feasibility from posterior calibration: driving the residual to zero is not sufficient for exact sampling of the graph-lifted reduced posterior unless the sampling or correction step targets the corresponding corrected density.

math.ST↗

arXiv2026

Topology-Directed Silicide Formation: An Explanation for the Growth of C49-TiSi$_2$ on the Si(100) Surface

Designing metal-semiconductor junctions is essential for optimizing the performance of modern nanoelectronic devices. A widely used material is TiSi$_2$, which combines low electronic resistivity with good endurance. However, its multitude of polymorphs continues to pose a challenge for device fabrication. In particular, the naturally occurring formation of the metastable C49-TiSi$_2$ modification remains poorly understood and is problematic due to its unfavorable electronic properties. Based on extensive DFT calculations, we present a comprehensive model of Ti adsorption on Si(100) that highlights the pivotal role of surface topology for the initial stages of the interfacial TiSi$_2$ formation process. We show that the interplay between Si surface dimers, the symmetry of the Si(100) surface, and the incorporation of Ti adsorbates below the surface drives an adsorption pattern that yields a nucleation template for the C49-TiSi$_2$ phase. Our atomistic model rationalizes experimental observations like the Stranski-Krastanov growth mode, the preferential formation of C49-TiSi$_2$ despite it being less favorable than the competing C54 phase, and why disruption of the surface structure restores thermodynamically driven growth of the latter. Ultimately, this novel perspective on the unique growth of TiSi$_2$ will help to pave the way for next-generation electronic devices.

cond-mat.mtrl-sci↗

arXiv2025

Adhesion Control through Electric Field-Induced Water Adsorption at Oxidized Silicon Interfaces

Adhesion plays a pivotal role in computer chip manufacturing, directly affecting the precision and durability of positioning components such as wafer stages. Electrical biasing is widely employed to eliminate floating potential and to enable electrostatic clamping. However, upon electrical grounding adhesion can persist and there is limited knowledge about the nature of this adhesion hysteresis. Here, we investigate potential causes underlying electric field-induced adhesion hysteresis at the interface between an n-type AFM tip and a p-type silicon sample using atomic force microscopy. Our findings reveal that neither charge trapping nor siloxane bond formation significantly impacts the measured adhesion. Surprisingly, we show that adhesion can be tuned through electric field-induced water adsorption under low relative humidity (RH < 10%). Our results provide new insights into adhesion hysteresis and opportunities for adhesion control.

cond-mat.soft↗

arXiv2023

Enhanced dendrite nucleation and Li-clustering at vacancies on graphene

An ever present challenge for Li-ion batteries is the formation of metallic dendrites on cycling that dramatically reduces cycle life and leads to the untimely failure of the cell. In this work we investigate the modes of Li-cluster formation on pristine and defective graphene. Firstly, we demonstrate that on a defect free surface the cluster formation is impeded by the thermodynamic instability of \ce{Li_2} and \ce{Li_3} clusters. In contrast, the presence of a vacancy dramatically favours clustering. This provides insights into the two modes of Li-growth observed: for the pristine basal plane if the Li-Li repulsion of the small clusters can be overcome then plating type behaviour would be predicted (rate / voltage dependent and at any point on the surface); whilst dendritic growth would be predicted to nucleate from vacancy sites, either pre-existing in the material or formed as a result of processing.

cond-mat.mtrl-sci↗

arXiv2019

The origin of negative charging in amorphous Al$_2$O$_3$ films: The role of native defects

Amorphous aluminum oxide Al$_2$O$_3$ (a-Al$_2$O$_3$) layers grown by various deposition techniques contain a significant density of negative charges. In spite of several experimental and theoretical studies, the origin of these charges still remains unclear. We report the results of extensive Density Functional Theory (DFT) calculations of native defects - O and Al vacancies and interstitials, as well as H interstitial centers - in different charge states in both crystalline $α$-Al$_2$O$_3$ and in a-Al$_2$O$_3$. The results demonstrate that both the charging process and the energy distribution of traps responsible for negative charging of a-Al$_2$O$_3$ films [M. B. Zahid et al., IEEE Trans. Electron Devices 57, 2907 (2010)] can be understood assuming that the negatively charged O$_{\textrm{i}}$ and V$_{\textrm{Al}}$ defects are nearly compensated by the positively charged H$_{\textrm{i}}$, V$_{\textrm{O}}$ and Al$_{\textrm{i}}$ defects in as prepared samples. Following electron injection, the states of Al$_{\textrm{i}}$, V$_{\textrm{O}}$ or H$_{\textrm{i}}$ in the band gap become occupied by electrons and sample becomes negatively charged. The optical excitation energies from these states into the oxide conduction band agree with the results of exhaustive photo-depopulation spectroscopy (EPDS) measurements [M. B. Zahid et al., IEEE Trans. Electron Devices 57, 2907 (2010)]. This new understanding of the origin of negative charging of a-Al$_2$O$_3$ films is important for further development of nanoelectronic devices and solar cells.

cond-mat.mtrl-sci↗

arXiv2017

Electrically detected magnetic resonance of carbon dangling bonds at the Si-face 4H-SiC/SiO$_2$ interface

SiC based metal-oxide-semiconductor field-effect transistors (MOSFETs) have gained a significant importance in power electronics applications. However, electrically active defects at the SiC/SiO$_2$ interface degrade the ideal behavior of the devices. The relevant microscopic defects can be identified by electron paramagnetic resonance (EPR) or electrically detected magnetic resonance (EDMR). This helps to decide which changes to the fabrication process will likely lead to further increases of device performance and reliability. EDMR measurements have shown very similar dominant hyperfine (HF) spectra in differently processed MOSFETs although some discrepancies were observed in the measured $g$-factors. Here, the HF spectra measured of different SiC MOSFETs are compared and it is argued that the same dominant defect is present in all devices. A comparison of the data with simulated spectra of the C dangling bond (P$_\textrm{bC}$) center and the silicon vacancy (V$_\textrm{Si}$) demonstrates that the P$_\textrm{bC}$ center is a more suitable candidate to explain the observed HF spectra.

cond-mat.mtrl-sci↗