arXiv subjects

Nobuhiko P. Kobayashi

Publications and source records attributed to Nobuhiko P. Kobayashi.

13 recordsLinked to original sources

arXiv2026

Individual Vanadium Dopants Form Deep In-Gap States in Monolayer WS2

Point defects in atomically thin materials have a strong impact on physical properties and those that induce in-gap states are advantageous for quantum information science and engineering (QISE). However, dopant engineering consisting of well-controlled synthesis and robust identification of in-gap states is challenging. In this work, we addressed this challenge by first using finely tuned chemical vapor deposition to incorporate vanadium dopants into a monolayer WS2 (V-WS2). Next, we utilized a suite of scanned probe microscopy techniques to identify and characterize individual dopants. The latter included conductive atomic force microscopy (cAFM), low temperature scanning tunneling microscopy and spectroscopy (STM/STS), and scanning transmission electron microscopy and unambiguously revealed that vanadium dopants form deep in-gap states 0.35 eV above the valence band maximum in V-WS2. Our experimental results are well supported by first principles calculations and taken together demonstrate that V-WS2 is a promising platform for QISE applications.

cond-mat.mtrl-sci↗

arXiv2025

A Morphologically Self-Consistent Phase Field Model for the Computational Study of Memristive Thin Film Current-Voltage Hysteresis

A multiphysics phase field model is used for the computational study of memristive thin film morphology and current-voltage hysteresis. In contrast to previous computational methods, no requirements are made on conducting filament geometry. Our method correctly predicts conducting filaments evolve on thermodynamic paths that are energetically favored due to stochastic structural and chemical variations naturally occurring at the atomic-level, due to both latent and intentional fabrication effects. These results have significant implications for the computational design of a broad class of memristive thin films, enabling practical wafer-scale mapping, uniformity, and endurance analysis and optimization.

cond-mat.mtrl-sci↗

arXiv2023

Resistive Switching Conducting Filament Electroformation with an Electrothermal Phase Field Method

A phase field method self-consistently coupled to continuum heat transport and charge conservation is used to simulate conducting filament dynamical evolution and nanostructure of electroformed resistive switching thin films. Our method does not require a pre-defined idealized conducting filament, as previous methods do, instead treating its dynamical evolution as a stochastic diffuse interface problem subject to a variational principle. Our simulation results agree well with available experimental observations, correctly reproducing electroformed conducting filament nanostructure exhibited by a variety of resistive switching thin films.

physics.app-ph↗

arXiv2023

Atomic Layer Deposited Protective Coating of Aluminum Oxide on Silver-based Telescope Mirror A Comparison Between a Pure Ozone and H2O Precursor

Although silver-based telescope mirrors excel over other materials such as gold and aluminum in the visible-infrared spectral range, they require robust protective coatings to overcome their inherent low durability. Our research shows that a single-layer of aluminum oxide (AlOx) deposited through thermal atomic layer deposition (ALD) using trimethylaluminum (TMA) and water (H2O) at low temperatures (~60°C) serves as an acceptable protective coating without adversely impacting the optical performance of the mirrors. While silver-based mirrors protected with a single-layer of AlOx perform decently in the field, in environmental tests under high-humidity at high-temperature conditions that accelerate underlying failure mechanisms, they degrade quickly, suggesting that there is room for improvement. This paper describes a study that compares the performance and endurance of two sets of silver-based mirrors protected by a single-layer of AlOx prepared by thermal ALD with two types of oxygen precursors: H2O and pure ozone (PO). The study shows that while the two types of samples, regardless of their oxygen precursors, initially have comparable spectral reflectance, the reflectance of the samples with AlOx protective coatings prepared with PO remain nearly constant 1.6 times longer than those with AlOx protective coatings prepared with H2O in the environmental test, suggesting promising characteristics of AlOx protective coatings prepared with PO.

astro-ph.IM↗

arXiv2022

Semiconductor-like Optical Properties Unveiled by Modeling of Short-Period Aluminum Oxide-Copper Multi-Layered Nanocomposites Deposited by Sputtering Atomic Layer Augmented Deposition (SALAD)

In our previous publication, we discussed the optical properties of multi-layered nanocomposite samples consisting of 300 pairs of an AlOx layer and a Cu layer produced from sputtering atomic layer augmented deposition (SALAD). These samples displayed unusual spectral reflectance that could not be well described via conventional means and could not be explained as an interpolation of the constituent materials. Computational models were then needed to both describe the results that were obtained and to predict the samples' other material properties. This process was approached by using the transfer-matrix method to describe the spectral reflectance based on the constituent materials, and then the dispersion method was used to find the composite index of refraction for the samples. The models successfully described pure Cu but encountered minor difficulties when it came to AlOx, as such, more research is needed to either properly model non-holomorphic materials like AlOx with the current algorithm or to find a better model that can adequately describe both Cu and AlOx. The dispersion model implemented predicts that the samples, composed of many thin film layers of metal and dielectric, would display semiconductor-like behavior. Therefore, this indicates that SALAD's combination of sputtering and ALD permits the generation of novel metal-dielectric based semiconductors.

cond-mat.mtrl-sci↗

arXiv2022

Laser-induced crystallization of copper oxide thin films: A comparison made between Gaussian and chevron-beam profiles provides a clue for the failure of Gaussian-beam profile

The use of laser with a Gaussian-beam profile is frequently adopted in attempts of crystallizing non-single-crystal thin films; however, it merely results in the formation of poly-crystal thin films. In this paper, selective area crystallization of non-single-crystal copper(II) oxide (CuO) is described. The crystallization is induced by laser, laser-induced crystallization, with a beam profile in the shape of chevron. The crystallization is verified by the exhibition of a transition from a non-single-crystal phase consisting of small 100 nm x 100 nm grains of CuO to a single-crystal phase of copper(I) oxide (Cu2O). The transition is identified by electron back scattering diffraction and Raman spectroscopy, which clearly suggests that a single-crystal domain of Cu2O with size as large as 5 μm x 1 mm develops. Provided these experimental findings, a theoretical assessment based on a cellular automaton model, with the behaviors of localized recrystallization and stochastic nucleation, is developed. The theoretical assessment can qualitatively describe the laser beam geometry-dependence of vital observable features (e.g., size and gross geometry of grains) in the laser-induced crystallization. The theoretical assessment predicts that differences in resulting crystallinity, either single-crystal or poly-crystal, primarily depend on a geometrical profile with which melting of non-single-crystal regions takes place along the laser scan direction; concave-trailing profiles yield larger grains which lead to single-crystal while convex-trailing profiles results in smaller grains which lead to poly-crystal, casting light on the fundamental question Why does a chevron-beam profile succeed in producing single-crystal while a Gaussian-beam profile fails?

physics.app-ph↗

arXiv2021

Impact of the nucleation of charge clusters on the retention of memristors: a self-consistent phase field computational study

In recent years, resistive RAM often referred to as memristor is actively pursued as a replacement for nonvolatile-flash memory due to its superior characteristics such as high density, scalability, low power operation, high endurance, and fast operating speed. However, one of the challenges that need to be overcome is the loss of retention for both ON- and OFF-states; the retention loss. While various models are proposed to explain the retention loss in memristors consisting of a switching layer, in this paper, we propose that the nucleation of clusters made of electrical charges, charge-clusters, in the switching layer acts as a potential root cause for the retention loss. The nucleation results from localized electric-field produced intermittently during cyclic switching operations. We use the phase-field method to illustrate how the nucleation of charge-clusters gives rise to the retention loss. Our results suggest that the degree at which the retention loss arises is linked to the number of cyclic switching operations since the probability at which nucleation centers form increases with the number of cycle switching operations, which is consistent with a range of experimental findings previously reported.

cond-mat.mes-hall↗

arXiv2021

Critical processing temperature for high performance protected silver thin film mirrors

Silver (Ag) mirrors for astronomical telescopes consist of multiple metallic and dielectric thin films. Furthermore, the topmost surface of such Ag mirrors needs to be covered by a protection coating. While the protection coating is often deposited at room temperature and the entire mirrors are also handled at room temperature, various thin film deposition techniques offer protection coatings with improved characteristics when carried out at elevated temperatures. Thus, in this work, high-performance Ag mirrors were designed and fabricated with a new benchmark. The resulting Ag mirrors were annealed (i.e., post-fabrication annealing) at various temperatures to investigate the viability of introducing thermal processes during and/or after fabrication in improving overall optical performance and durability of protected silver mirrors. In our experiments, Ag mirror samples were deposited by electron-beam evaporation and subsequently annealed at various temperatures in the range from 60 °C to 300 °C, and then the mirror samples underwent an environmental stress test at 80 °C and 80% humidity for 10 days. While all the mirror samples annealed below 200 °C showed negligible corrosion after undergoing the stress testing, those annealed below 160 °C presented spectral reflectivity comparable to or higher than that of as-deposited reference samples. In contrast, the mirror samples annealed above 200 °C exhibited significant degradation after the stress testing. The comprehensive analysis indicated that delamination and voids caused by the growth of Ag grains during the annealing are the primary mechanisms of the degradation.

astro-ph.IM↗

arXiv2020

Visualization and Manipulation of Bilayer Graphene Quantum Dots with Broken Rotational Symmetry and Nontrivial Topology

Electrostatically defined quantum dots (QDs) in Bernal stacked bilayer graphene (BLG) are a promising quantum information platform because of their long spin decoherence times, high sample quality, and tunability. Importantly, the shape of QD states determines the electron energy spectrum, the interactions between electrons, and the coupling of electrons to their environment, all of which are relevant for quantum information processing. Despite its importance, the shape of BLG QD states remains experimentally unexamined. Here we report direct visualization of BLG QD states by using a scanning tunneling microscope. Strikingly, we find these states exhibit a robust broken rotational symmetry. By using a numerical tight-binding model, we determine that the observed broken rotational symmetry can be attributed to low energy anisotropic bands. We then compare confined holes and electrons and demonstrate the influence of BLG's nontrivial band topology. Our study distinguishes BLG QDs from prior QD platforms with trivial band topology.

cond-mat.mes-hall↗

arXiv2020

An electro-thermal computational study of conducting channels in dielectric thin films using self-consistent phase-field methodology: A view toward the physical origins of resistive switching

A large number of experimental studies suggest two-terminal resistive switching devices made of a dielectric thin film sandwiched by a pair of electrodes exhibit reversible multi-state switching behaviors; however coherent understanding of physical and chemical origins of their electrical properties needs to be further pursued to improve and customize the performance. In this paper, phase-field methodology is used to study the formation and annihilation of conductive channels resulting in reversible resistive switching behaviors that can generally occur in any dielectric thin films. Our focus is on the dynamical evolution of domains made of electrical charges under the influence of spatially varying electric field and temperature resulting in distinctive changes in electrical conductance.

physics.app-ph↗

arXiv2019

Crystallization of Cuprous Oxide Thin Films by Continuous-Wave Laser Diode with Micro-Chevron Laser Beam (μ-CLB)

Crystallization of thin film materials by exploiting laser induced crystallization has been advancing for the past four decades. This unique thin film technique has been predominantly used in processing thin film materials made of a single chemical element; however, harnessing this technique to extend its use for thin film materials containing multiple chemical elements (e.g., metal oxides) unlocks applications currently not accessible. In this study, laser crystallization of a Cu2O strip was demonstrated. A continuous-wave laser diode with a micrometer-scale chevron-shaped beam profile (micro chevron laser beam) was used to crystallize CuO thin films covered with an amorphous carbon (a-C) cap layer, deposited on fused silica substrates. Electron backscatter diffraction, Raman spectroscopy, photoluminescence spectroscopy, and UV-Vis spectroscopy were used to investigate the crystallinity and optical properties of the Cu2O thin films revealing their unique characteristics associated with the crystallization process.

physics.app-ph↗

arXiv2019

Demonstration of Sputtering Atomic Layer Augmented Deposition (SALAD): Aluminum Oxide-Copper Dielectric-Metal Nanocomposite

Designing a thin film structure often begins with choosing a film deposition method that employs a specific primary process by which chemical species are formed and transported. In other words, a film deposition system in which two complementary deposition methods are hybridized should lead to new ways of designing thin film structures with structural complexity tailored at a level that has never been envisioned.This premise inspired us to uniquely combine atomic layer deposition (ALD) and magnetron sputtering (SPU) within a single deposition chamber; the combined film deposition system is referred to as sputtering atomic layer augmented deposition (SALAD). SALAD allows us to take full advantages of both ALDs precise and accurate precursor delivery and SPUs versatility in choices of chemical elements. A SALAD system was designed based on knowledge obtained by computational fluid dynamics with the goal of conceiving a film deposition system that satisfied deposition conditions distinctive for both ALD and SPU, and a prototype SALAD system was assembled by employing off-the-shelf vacuum components.A s a demonstration, the SALAD system was utilized to deposit a unique nanocomposite made of a stack of aluminum oxide thin films by ALD and copper thin films by SPU, AlOx-Cu dielectric-metal (ACDM) nanocomposite thin films, on Si and glass substrates. Spectroscopic reflectivity collected on ACDM nanocomposite thin films shows unique dispersion features to which conventional effective medium theories used for describing optical properties of composites made of a dielectric host that contains metallic inclusions do not seem to simply apply.

physics.app-ph↗

arXiv2019

A Computational Phase Field Study of Conducting Channel Formation in Dielectric Thin Films: A View Towards the Physical Origins of Resistive Switching

A phase field method is used to computationally study conducting channel morphology of resistive switching thin film structures. Our approach successfully predicts the formation of conducting channels in typical dielectric thin film structures, comparable to a range of resistive switches, offering an alternative computational formulation based on metastable states treated at the atomic scale. In contrast to previous resistive switching thin film models, our formulation makes no a priori assumptions on conducting channel morphology and its fundamental transport mechanisms.

physics.app-ph↗