Searcharxiv⌕ Search

arXiv subjects

David J. Smith

Publications and source records attributed to David J. Smith.

At least 19 recordsLinked to original sources
arXiv2026

Temperature Dependent Characteristics of Quasi-vertical AlN Schottky Diodes on Bulk AlN Substrate

We report on the fabrication and temperature-dependent characterization of MOCVD-grown quasi-vertical AlN Schottky barrier diodes (SBDs) on bulk AlN substrates. The SBDs exhibited high current densities exceeding 2 kA/cm2 at 10 V, with a turn-on voltage of ~3.0 V (at 1 A/cm^2) and an on/off ratio >10^9 at room temperature. Stable rectifying operation was maintained up to 300 C (the highest measured temperature), with a pronounced increase in current density at elevated temperatures due to thermally activated carrier transport, accompanied by an increase in extracted Schottky barrier height and a reduction in ideality factor. Capacitance voltage measurements showed strong temperature dependence due to the deep donor nature of Si in AlN, resulting in an increase in the net donor concentration (ND-NA) from ~5x10^17 cm-3 at 300 K to ~1x10^18 cm-3 at 373 K. Temperature-dependent reverse-bias characteristics were consistent with Poole-Frenkel emission as the dominant leakage mechanism, with an estimated trap energy of ~0.34 eV. Characterization using transmission electron microscopy and energy-dispersive X-ray spectroscopy revealed a ~5 nm AlNxOy interfacial layer at the metal/semiconductor junction, which likely influences both forward and reverse transport. These results provide insight into carrier transport, leakage mechanisms, interface chemistry, and high-temperature characteristics, and guidance for the future development of high-performance AlN power devices.

physics.app-ph↗
arXiv2025

Demonstration of KV-Class \b{eta}-Ga2O3 Trench Junction Barrier Schottky Diodes with SpaceModulated Junction Termination Extension

In this work, we report on the design and fabrication of p-NiO/Ga2O3 trench junction barrier schottky diodes (JBSD) integrated with space-modulated junction termination extension (SM-JTE) and compare the performance with planar Ni/Ga2O3 schottky diodes (SBDs) and p-NiO/Ga2O3 heterojunction diodes (HJDs). The JBSDs achieved breakdown voltages exceeding 1.8 kV along with low leakage currents (<10-2 A/cm2), while displaying low turn on voltage (VON) of ~1V, which is similar to that of planar Ni/Ga2O3 SBDs. The fabricated devices showed excellent forward characteristics with low differential on-resistance (Ron,sp) ranging from 4-10.5 m{\Omega}-cm2, for fin width between 0.6- 1.25 microns. Best performing device with fin width of 0.85{\mu}m showed a unipolar figure of merit (FOM) of ~0.7GW/cm2. This work showcases the benefits of trench JBS design along with SM-JTE edge-termination for efficient high-performance kilovolt-class \b{eta}- Ga2O3 diodes.

physics.app-ph↗
arXiv2025

Epitaxial high-K AlBN barrier GaN HEMTs

We report a polarization-induced 2D electron gas (2DEG) at an epitaxial AlBN/GaN heterojunction grown on a SiC substrate. Using this 2DEG in a long conducting channel, we realize ultra-thin barrier AlBN/GaN high electron mobility transistors that exhibit current densities of more than 0.25 A/mm, clean current saturation, a low pinch-off voltage of -0.43 V, and a peak transconductance of 0.14 S/mm. Transistor performance in this preliminary realization is limited by the contact resistance. Capacitance-voltage measurements reveal that introducing 7 % B in the epitaxial AlBN barrier on GaN boosts the relative dielectric constant of AlBN to 16, higher than the AlN dielectric constant of 9. Epitaxial high-K barrier AlBN/GaN HEMTs can thus extend performance beyond the capabilities of current GaN transistors.

physics.app-ph↗
arXiv2024

Mathematical modelling of the vitamin C clock reaction: a study of two kinetic regimes

Chemically reacting systems exhibiting a repeatable delay period before a visible and sudden change are referred to as clock reactions; they have a long history in education and provide an idealisation of various biochemical and industrial processes. We focus on a purely substrate-depletive clock reaction utilising vitamin C, hydrogen peroxide, iodine and starch. Building on a recent study of a simplified two-reaction model under high hydrogen peroxide concentrations, we develop a more detailed model which breaks the slow reaction into two steps, one of which is rate-limiting unless hydrogen peroxide levels are very high. Through asymptotic analysis, this model enables the effect of hydrogen peroxide concentration to be elucidated in a principled way, resolving an apparent discrepancy with earlier literature regarding the order of the slow reaction kinetics. The model is analysed in moderate- and high-hydrogen peroxide regimes, providing approximate solutions and expressions for the switchover time which take into account hydrogen peroxide concentration. The solutions are validated through simultaneously fitting the same set of parameters to several experimental series, then testing on independent experiments across widely varying hydrogen peroxide concentration. The study thereby presents and further develops a validated mechanistic understanding of a paradigm chemical kinetics system.

math.DS↗
arXiv2024

Formation of Beta-Indium Selenide Layers Grown via Selenium Passivation of InP(111)B Substrate

Indium selenide, In2Se3, has recently attracted growing interest due to its novel properties, including room temperature ferroelectricity, outstanding photoresponsivity, and exotic in-plane ferroelectricity, which open up new regimes for next generation electronics. In2Se3 also provides the important advantage of tuning the electrical properties of ultra-thin layers with an external electrical and magnetic field, making it a potential platform to study novel two-dimensional physics. Yet, In2Se3 has many different polymorphs, and it has been challenging to synthesize single-phase material, especially using scalable growth methods, as needed for technological applications. In this paper, we use aberration-corrected scanning transmission electron microscopy to characterize the microstructure of twin-free single-phase ultra-thin layers of beta-In2Se3, prepared by a unique molecular beam epitaxy approach. We emphasize features of the In2Se3 layer and In2Se3/InP interface which provide evidence for understanding the growth mechanism of the single-phase In2Se3. This novel approach for forming high-quality twin-free single phase two-dimensional crystals on InP substrates is likely to be applicable to other technologically important substrates.

cond-mat.mtrl-sci↗
arXiv2023

Optical and Acoustic Phonons in Turbostratic and Cubic Boron Nitride Thin Films on Diamond Substrates

We report an investigation of the bulk optical, bulk acoustic, and surface acoustic phonons in thin films of turbostratic boron nitride (t-BN) and cubic boron nitride (c-BN) grown on B-doped polycrystalline and single-crystalline diamond (001) and (111) substrates. The characteristics of different types of phonons were determined using Raman and Brillouin-Mandelstam light scattering spectroscopies. The atomic structure of the films was determined using high-resolution transmission electron microscopy (HRTEM) and correlated with the Raman and Brillouin-Mandelstam spectroscopy data. The HRTEM analysis revealed that the cubic boron nitride thin films consisted of a mixture of c-BN and t-BN phases, with c-BN being the dominant phase. It was found that while visible Raman spectroscopy provided information for characterizing the t-BN phase, it faced challenges in differentiating the c-BN phase either due to the presence of high-density defects or the overlapping of the Raman features with those from the B-doped diamond substrates. In contrast, Brillouin-Mandelstam spectroscopy clearly distinguishes the bulk longitudinal and surface acoustic phonons of the c-BN thin films grown on diamond substrates. Additionally, the angle-dependent surface Brillouin-Mandelstam scattering data show the peaks associated with the Rayleigh surface acoustic waves, which have higher phase velocities in c-BN films on diamond (111) substrates. These findings provide valuable insights into the phonon characteristics of the c-BN and diamond interfaces and have important implications for the thermal management of electronic devices based on ultra-wide-band-gap materials.

cond-mat.mtrl-sci↗
arXiv2021

Compositional Control and Optimization of Molecular Beam Epitaxial Growth of (Sb2Te3)x(MnSb2Te4)y Magnetic Topological Insulators

Magnetic topological insulators such as MnBi2Te4 and MnSb2Te4 are promising hosts of novel physical phenomena such as quantum anomalous Hall effect and intrinsic axion insulator state, both potentially important for the implementation in topological spintronics and error-free quantum computing. In the bulk, the materials are antiferromagnetic but appropriate stacking with non-magnetic layers or excess Mn in the crystal lattice can induce a net ferromagnetic alignment. In this work we report the growth of (Sb2Te3)x(MnSb2Te4)y layers with varying Mn content by molecular beam epitaxy. The Mn flux fraction provided during growth controls the percent of MnSb2Te4 that is formed in the resulting layers by a self-assembly process. Highly crystalline layers with compositions varying between Sb2Te3 (y=0) and MnSb2Te4 (x=0) were obtained. The results show that Mn incorporates as a structural component to form MnSb2Te4, and as an impurity element both in Sb2Te3 and in MnSb2Te4. Two modifications of the growth conditions were implemented to enhance the incorporation of Mn as a structural element to form MnSb2Te4. Annealing of a thin portion of the layer at the beginning of growth (pre-anneal step), and increasing the growth temperature, both result in a larger percent of MnSb2Te4 for similar Mn flux fractions during growth. Samples having at least a few percent of MnSb2Te4 layers exhibit ferromagnetic behavior likely due to the excess Mn in the system which stabilizes on Sb sites as MnSb antisite defects.

cond-mat.mtrl-sci↗
arXiv2021

Squirmer hydrodynamics near a periodic surface topography

The behaviour of microscopic swimmers has previously been explored near large scale confining geometries and in the presence of very small-scale surface roughness. Here we consider an intermediate case of how a simple microswimmer, the tangential spherical squirmer, behaves adjacent to singly- and doubly-periodic sinusoidal surface topographies that spatially oscillate with an amplitude that is an order of magnitude less than the swimmer size and wavelengths within an order of magnitude of this scale. The nearest neighbour regularised Stokeslet method is used for numerical explorations after validating its accuracy for a spherical tangential squirmer that swims stably near a flat surface. The same squirmer is then introduced to the different surface topographies. The key governing factor in the resulting swimming behaviour is the size of the squirmer relative to the surface topography wavelength. When the squirmer is much larger, or much smaller, than the surface topography wavelength then directional guidance is not observed. Once the squirmer size is on the scale of the topography wavelength limited guidance is possible, often with local capture in the topography troughs. However, complex dynamics can also emerge, especially when the initial configuration is not close to alignment along topography troughs or above topography crests. In contrast to sensitivity in alignment and the topography wavelength, reductions in the amplitude of the surface topography or variations in the shape of the periodic surface topography do not have extensive impacts on the squirmer behaviour. Our findings more generally highlight that detailed numerical explorations are necessary to determine how surface topographies may interact with a given swimmer that swims stably near a flat surface and whether surface topographies may be designed to provide such swimmers with directional guidance.

physics.flu-dyn↗
arXiv2020

The Venus Life Equation

Ancient Venus and Earth may have been similar in crucial ways for the development of life, such as liquid water oceans, land-ocean interfaces, favorable chemical ingredients and energy pathways. If life ever developed on, or was transported to, early Venus from elsewhere, it might have thrived, expanded and then survived the changes that have led to an inhospitable surface on Venus today. The Venus cloud layer may provide a refugium for extant life that persisted from an earlier more habitable surface environment. We introduce the Venus Life Equation - a theory and evidence-based approach to calculate the probability of extant life on Venus, L, using three primary factors of life: Origination, Robustness, and Continuity, or L = O x R x C. We evaluate each of these factors using our current understanding of Earth and Venus environmental conditions from the Archaean to the present. We find that the probability of origination of life on Venus would be similar to that of the Earth and argue that the other factors should be nonzero, comparable to other promising astrobiological targets in the solar system. The Venus Life Equation also identifies poorly understood aspects of Venus that can be addressed by direct observations with future exploration missions.

astro-ph.EP↗
arXiv2020

Doing more with less: the flagellar end piece enhances the propulsive effectiveness of human spermatozoa

Spermatozoa self-propel by propagating bending waves along a predominantly active elastic flagellum. The organized structure of the "9 + 2" axoneme is lost in the most-distal few microns of the flagellum, and therefore this region is unlikely to have the ability to generate active bending; as such it has been largely neglected in biophysical studies. Through elastohydrodynamic modeling of human-like sperm we show that an inactive distal region confers significant advantages, both in propulsive thrust and swimming efficiency, when compared with a fully active flagellum of the same total length. The beneficial effect of the inactive end piece on these statistics can be as small as a few percent but can be above 430%. The optimal inactive length, between 2-18% of the total length, depends on both wavenumber and viscous-elastic ratio, and therefore is likely to vary in different species. Potential implications in evolutionary biology and clinical assessment are discussed.

physics.flu-dyn↗
arXiv2020

Modelling oral adrenal cortisol support

A simplified mathematical model of oral hydrocortisone delivery in adrenal insufficiency is described; the model is based on three components (gastric hydrocortisone, free serum cortisol and bound serum cortisol) and is formulated in terms of linear kinetics, taking into account the dynamics of glucocorticoid-protein binding. Motivated by the need to optimise cortisol replacement in the situations of COVID-19 infection, the model is fitted to recently-published data on 50 mg dosing and earlier data on 10 mg dosing. The fitted model is used to predict typical responses to standard dosing regimes, which involve a larger dose in the morning and 1 or 2 smaller doses later in the day, and the same regimes with doses doubled. In all cases there is a circadian-like response, with early morning nadir. The model is also used to consider an alternative dosing strategy based on four equal and equally-spaced doses of 10, 20 or 30 mg per 24 h, resulting in a more even response resembling a response to sustained inflammatory stress.

q-bio.TO↗
arXiv2020

Passively parallel regularized stokeslets

Stokes flow, discussed by G.G. Stokes in 1851, describes many microscopic biological flow phenomena, including cilia-driven transport and flagellar motility; the need to quantify and understand these flows has motivated decades of mathematical and computational research. Regularized stokeslet methods, which have been used and refined over the past twenty years, offer significant advantages in simplicity of implementation, with a recent modification based on nearest-neighbour interpolation providing significant improvements in efficiency and accuracy. Moreover this method can be implemented with the majority of the computation taking place through built-in linear algebra, entailing that state-of-the-art hardware and software developments in the latter, in particular multicore and GPU computing, can be exploited through minimal modifications ('passive parallelism') to existing MATLAB computer code. Hence, and with widely-available GPU hardware, significant improvements in the efficiency of the regularized stokeslet method can be obtained. The approach is demonstrated through computational experiments on three model biological flows: undulatory propulsion of multiple C. Elegans, simulation of progression and transport by multiple sperm in a geometrically confined region, and left-right symmetry breaking particle transport in the ventral node of the mouse embryo. In general an order-of-magnitude improvement in efficiency is observed. This development further widens the complexity of biological flow systems that are accessible without the need for extensive code development or specialist facilities.

physics.flu-dyn↗
arXiv2019

Efficient Implementation of Elastohydrodynamics via Integral Operators

The dynamics of geometrically non-linear flexible filaments play an important role in a host of biological processes, from flagella-driven cell transport to the polymeric structure of complex fluids. Such problems have historically been computationally expensive due to numerical stiffness associated with the inextensibility constraint, as well as the often non-trivial boundary conditions on the governing high-order PDEs. Formulating the problem for the evolving shape of a filament via an integral equation in the tangent angle has recently been found to greatly alleviate this numerical stiffness. The contribution of the present manuscript is to enable the simulation of non-local interactions of multiple filaments in a computationally efficient manner using the method of regularized stokeslets within this framework. The proposed method is benchmarked against a non-local bead and link model, and recent code utilizing a local drag velocity law. Systems of multiple filaments (1) in a background fluid flow, (2) under a constant body force, and (3) undergoing active self-motility are modeled efficiently. Buckling instabilities are analyzed by examining the evolving filament curvature, as well as by coarse-graining the body frame tangent angles using a Chebyshev approximation for various choices of the relevant non-dimensional parameters. From these experiments, insight is gained into how filament-filament interactions can promote buckling, and further reveal the complex fluid dynamics resulting from arrays of these interacting fibers. By examining active moment-driven filaments, we investigate the speed of worm- and sperm-like swimmers for different governing parameters. The MATLAB(R) implementation is made available as an open-source library, enabling flexible extension for alternate discretizations and different surrounding flows.

physics.flu-dyn↗
arXiv2019

Simulations of particle tracking in the oligociliated mouse node and implications for left-right symmetry breaking mechanics

The concept of internal anatomical asymmetry is familiar; usually in humans the heart is on the left and the liver is on the right, however how does the developing embryo know to produce this consistent laterality? Symmetry breaking initiates with left-right asymmetric cilia-driven fluid mechanics in a small fluid-filled structure called the ventral node in mice. However the question of what converts this flow into left-right asymmetric development remains unanswered. A leading hypotheses is that flow transports morphogen containing vesicles within the node, the absorption of which results in asymmetrical gene expression. To investigate how vesicle transport might result in the situs patterns observed in wildtype and mutant experiments, we extend the open source Stokes flow package, NEAREST, to consider the hydrodynamic and Brownian motion of particles in a mouse model with flow driven by one, two, and 112 beating cilia. Three models for morphogen-containing particle released are simulated to assess their compatibility with observed results in oligociliated and wildtype mouse embryos: uniformly random release, localised cilium stress induced release, and localised release from motile cilia themselves. Only the uniformly random release model appears consistent with the data, with neither localised-release model resulting in significant transport in the oligociliated embryo.

q-bio.TO↗
arXiv2018

Self-assembled Bismuth Selenide (Bi2Se3) quantum dots grown by molecular beam epitaxy

We report the growth of self-assembled Bi2Se3 quantum dots (QDs) by molecular beam epitaxy on GaAs substrates using the droplet epitaxy technique. The QD formation occurs after anneal of Bismuth droplets under Selenium flux. Characterization by atomic force microscopy, scanning electron microscopy, X-ray diffraction, high-resolution transmission electron microscopy and X-ray reflectance spectroscopy is presented. The quantum dots are crystalline, with hexagonal shape, and have average dimensions of 12 nm height (12 quintuple layers) and 46 nm width, and a density of $8.5 \cdot 10^9 cm^{-2}$. This droplet growth technique provides a means to produce topological insulator QDs in a reproducible and controllable way, providing convenient access to a promising quantum material with singular spin properties.

cond-mat.mtrl-sci↗
arXiv2018

Non-identifiability of parameters for a class of shear-thinning rheological models, with implications for haematological fluid dynamics

Choosing a suitable model and determining its associated parameters from fitting to experimental data is fundamental for many problems in biomechanics. Models of shear-thinning complex fluids, dating from the work of Bird, Carreau, Cross and Yasuda, have been applied in highly-cited computational studies of heamodynamics for several decades. In this manuscript we revisit these models, first to highlight a degree of uncertainty in the naming conventions in the literature, but more importantly to address the problem of inferring model parameters by fitting rheology experiments. By refitting published data, and also by simulation, we find large, flat regions in likelihood surfaces that yield families of parameter sets which fit the data equally well. Despite having almost indistinguishable fits to experimental data these varying parameter sets can predict very different flow profiles, and as such these parameters cannot be used to draw conclusions about physical properties of the fluids, such as zero-shear viscosity or relaxation time of the fluid, or indeed flow behaviours. We verify that these features are not a consequence of the experimental data sets through simulations; by sampling points from the rheological models and adding a small amount of noise we create a synthetic data set which reveals that the problem of parameter identifiability is intrinsic to these models.

physics.flu-dyn↗
arXiv2018

Sharp quadrature error bounds for the nearest-neighbor discretization of the regularized stokeslet boundary integral equation

The method of regularized stokeslets is a powerful numerical method to solve the Stokes flow equations for problems in biological fluid mechanics. A recent variation of this method incorporates a nearest-neighbor discretization to improve accuracy and efficiency while maintaining the ease-of-implementation of the original meshless method. This method contains three sources of numerical error, the regularization error associated from using the regularized form of the boundary integral equations (with parameter $\varepsilon$), and two sources of discretization error associated with the force and quadrature discretizations (with lengthscales $h_f$ and $h_q$). A key issue to address is the quadrature error: initial work has not fully explained observed numerical convergence phenomena. In the present manuscript we construct sharp quadrature error bounds for the nearest-neighbor discretisation, noting that the error for a single evaluation of the kernel depends on the smallest distance ($\delta$) between these discretization sets. The quadrature error bounds are described for two cases: with disjoint sets ($\delta>0$) being close to linear in $h_q$ and insensitive to $\varepsilon$, and contained sets ($\delta=0$) being quadratic in $h_q$ with inverse dependence on $\varepsilon$. The practical implications of these error bounds are discussed with reference to the condition number of the matrix system for the nearest-neighbor method, with the analysis revealing that the condition number is insensitive to $\varepsilon$ for disjoint sets, and grows linearly with $\varepsilon$ for contained sets. Error bounds for the general case ($\delta\geq 0$) are revealed to be proportional to the sum of the errors for each case.

physics.flu-dyn↗