Research

Active Versus Passive Flow Control in UVC FILTERs for COVID-19 Containment

This paper presents a study of the effectiveness of ultraviolet (UVC) filters for indoor air purification in response to the COVID-19 pandemic. The paper compares active and passive flow control systems in low-cost portable UVC filtration units designed to inactivate pathogens, including the SARS-CoV-2 virus responsible for COVID-19. The aim was to improve air quality and pathogen deactivation rates in indoor environments, which are critical during pandemic situations.

The authors designed, simulated, and constructed two different devices—one using active flow control with electronic components and another using passive mechanical elements to regulate airflow. Both systems were tested, and the passive device demonstrated greater performance in terms of energy efficiency, production costs, and long-term maintenance, while still achieving similar pathogen inactivation rates.

This technology, developed during the COVID-19 pandemic, offers significant benefits for public health, not only for COVID-19 containment but also for other airborne pathogens. The passive device, in particular, was shown to provide a highly efficient solution for environments such as hospitals and public spaces, where clean air is critical to reducing the spread of infectious diseases.

Main Contributions

• Development of a passive UVC air filtration device that performs pathogen inactivation at a lower cost and with higher energy efficiency than active electronic systems.
• Design and simulation of active and passive UVC filtration devices, with experimental validation of pathogen inactivation rates, showing that both systems achieve more than 99% pathogen deactivation.
• Application of UVC filtration technology for COVID-19 containment and broader public health needs during the pandemic.

Key Aspects

• UVC filters, both active and passive, demonstrate effectiveness in indoor air purification by inactivating pathogens, including viruses and bacteria.
• Passive flow control systems offer lower energy consumption and reduced production costs, making them suitable for large-scale deployment in public spaces during health emergencies.
• The study contributes to the development of air purification technologies that help mitigate the spread of airborne diseases, especially in high-risk indoor environments.

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An Analytical Model for Square Gate-All-Around (GAA) MOSFETs Including Quantum Effects

The main contribution of this work is the development of an analytical model for square gate-all-around (GAA) MOSFETs, which includes quantum effects. This model allows for the precise description of the inversion charge distribution in these devices, which is crucial for compact modeling. Additionally, the inversion charge distribution functions are used to calculate important parameters such as the centroid of the inversion charge and the gate-to-channel capacitance, making this model fundamental for optimizing these devices in future sub-22 nm integrated circuit technologies.

The following equation is presented:

\[ \rho(y, z) = q N_{inv} \left| A_0 \sin\left(\frac{\pi y}{t_{Si}}\right)^{\frac{1}{2}} \sin\left(\frac{\pi z}{t_{Si}}\right)^{\frac{1}{2}} \left(e^{-\frac{b(t_{Si} - y)}{t_{Si}}} + e^{-\frac{by}{t_{Si}}}\right) \left(e^{-\frac{b(t_{Si} - z)}{t_{Si}}} + e^{-\frac{bz}{t_{Si}}}\right) \right|^2 \]

This equation is the result of a creative process in which an analytical representation of the charge density \(\rho(y, z)\) is constructed based on the gate voltage and the geometry of the device. This equation describes how the charge carriers (electrons or holes) are distributed within the structure of the multi-gate MOSFET device, depending on the geometric and electrical parameters. The way the carriers are distributed is deeply influenced by the electric potential generated, which is a solution to Poisson's equation, governing the relationship between the charge density and the electrostatic potential. At nanometric scales, the situation is complicated by quantum effects. Here, carriers cannot occupy any position within the device, but must obey the restrictions imposed by the Schrödinger equation, which governs the probability of finding a carrier at a particular location within the confined geometry of the device.

This process leads to the quantization of the available states for the carriers, meaning that carriers can only occupy certain energy levels and specific positions within the device. The presented equation reflects this quantization, as it describes how carriers are distributed based on probabilities that depend on both the silicon geometry (\(t_{Si}\)) and the applied potential, through the sinusoidal and exponential functions in \(y\) and \(z\).

The main contribution of this equation is its ability to accurately integrate quantum effects into the modeling of nanoscale devices, representing how charge carriers respond to the interaction between the classical Poisson solution and the quantum dictates of Schrödinger. It is a significant advance in understanding multi-gate MOSFET devices in sub-nanometric technologies.

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An Analytical Model for Square Gate-All-Around (GAA) MOSFETs Solving the 2D Poisson Equation

This work is, to date, the best contribution I have made in the field of research. This is mainly because we managed to solve an estimated non-linear partial differential equation (PDE), which represents a significant advance in modeling Gate-All-Around (GAA) MOSFET devices.

The key differential equation we solved, the 2D Poisson equation, includes the inversion charge density, which allowed us to accurately model the electric potential inside these advanced devices. Below is the main equation presented in the article:

\[ \frac{\partial^2 \psi(x, y)}{\partial x^2} + \frac{\partial^2 \psi(x, y)}{\partial y^2} = \frac{q}{\epsilon_{Si}} n_i e^{\frac{q \psi(x, y)}{kT}} \]

Where \( q \) is the electron charge, \( \epsilon_{Si} \) is the permittivity of silicon, \( k \) is the Boltzmann constant, \( T \) the temperature, and \( n_i \) the intrinsic electron density. This equation is fundamental for calculating the electric potential and inversion charge density in square geometry GAA MOSFETs, which had not been modeled with such precision before.

This advance allowed for the development of a compact model that facilitates the design and optimization of MOSFET devices at nanometric scales, particularly in low-power and high-frequency applications. The integration of quantum effects and the precise solution of this equation are a significant contribution to the scientific community, especially in the modeling of sub-22 nm devices.

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Difficulties in Solving Non-linear Partial Differential Equations

Non-linear partial differential equations (PDEs) are considerably more difficult to solve than linear equations for several fundamental reasons:

• Superposition not applicable: In linear PDEs, the principle of superposition can be applied, allowing complex solutions to be constructed by adding simpler solutions. This is not possible with non-linear PDEs, where non-linear terms introduce complex interactions between variables.
• Unpredictable behavior: Non-linear equations often exhibit chaotic behavior or are highly sensitive to initial and boundary conditions. Small differences in input data can generate completely different results, making it very difficult to find exact or approximate solutions.
• No general methods: While many linear PDEs have well-established analytical methods (such as separation of variables, Fourier or Laplace transforms), there is no general method for solving non-linear PDEs. Each type of equation may require specific techniques that are usually more complex, and in many cases, only approximate or numerical solutions are possible.
• Complex phenomena: Non-linear PDEs often represent complex phenomena in nature, such as turbulence in fluids, the propagation of non-linear waves, or the dynamics of biological systems. These phenomena do not have simple or intuitive solutions, as interactions between variables tend to be non-trivial.

The Work of Mathematician I. K. Sabitov

I. K. Sabitov, a prominent Russian mathematician, is known for his contributions to the solution of non-linear partial differential equations, as demonstrated in his influential work "Solutions of \[\Delta u = f(x, y)e^{cu}\] in some special cases" "https://istina.msu.ru/publications/article/6764725/". Sabitov explored advanced methods for solving complex partial differential equations, applying techniques that have significantly advanced the understanding of non-linear systems.

His work on these equations, in particular, has provided the scientific community with valuable tools to address non-trivial problems in various fields. Sabitov's ability to apply special and holomorphic functions in the complex plane was essential for better understanding how to solve non-linear partial differential equations under specific conditions.

Thanks to I. K. Sabitov's work on holomorphic functions in the complex plane, it was possible to apply his methods to my research, which allowed me to successfully solve the non-linear partial differential equation presented in this work.

On the other hand, the work of the Russian mathematician S. Yu. Savitóv should not be confused with I. K. Sabitov.

S. Yu. Savitóv was a Russian mathematician and physicist known for his contributions to the theory of non-linear waves and solitons, phenomena that are strongly associated with non-linear partial differential equations.

One of the most important problems he addressed was the study of solitary waves, which are stable solutions of certain non-linear PDEs, such as the Korteweg-de Vries (KdV) equation. These waves travel without dispersing, despite the presence of non-linearities, and have found applications in fields such as optics, fluid theory, and plasma physics.

Savitóv's work particularly focused on the development of mathematical methods to analyze the stability and formation of solitons in non-linear systems. Among Savitóv's key advances is his use of analytical and numerical methods to demonstrate how soliton-like solutions emerge in systems where non-linear equations describe wave interaction. His research not only provided a better understanding of the properties of these systems but was also crucial for applying solitons in modern technology, such as data transmission over fiber optics.

Time-domain Numerical Modeling of THz Photoconductive Antennas

This article presents an innovative computational procedure to simulate the time-domain behavior of photoconductive antennas (PCAs) made of semiconductor and metallic materials. The study addresses one of the key challenges in THz technology: accurately modeling the interaction between charge carriers and electromagnetic fields in the terahertz (THz) regime. The importance of this model lies in its ability to precisely represent the electromagnetic radiation from these devices, a crucial aspect for various applications, such as spectroscopy in the terahertz range.

One of the most notable contributions is the development of a detailed set of explicit numerical equations, derived using finite-difference time-domain (FDTD) techniques, which couple Poisson's and Maxwell's equations with the charge carrier drift-diffusion model. Through this approach, the distribution of carriers in both transient and steady-state conditions is modeled, enabling the evaluation of the electromagnetic fields generated by the acceleration of these carriers within the PCAs. This procedure shows excellent correlation with previously reported experimental data, underscoring the model's accuracy.

Main Contributions

• Development of a robust numerical simulator that couples Maxwell's equations and the drift-diffusion model, allowing for the simulation of both generation and radiation processes in THz photoconductive antennas. This approach is particularly useful for time-domain spectroscopy applications in the terahertz range.
• Introduction of a detailed procedure to resolve the steady-state condition of charge carriers in PCAs, providing a precise foundation for calculating radiated fields. This steady-state condition is crucial as it directly affects radiation efficiency and antenna behavior.
• Validation of the numerical model through comparison with experimental measurements, demonstrating the simulator's ability to accurately predict the real behavior of PCAs under various operating conditions.
• The study also delves into the importance of considering non-homogeneous, field-dependent mobilities in semiconductors, which significantly improves the accuracy of the models compared to more simplified approaches that do not include these spatial variations.

Key Insights

• The model describes how charge carriers, induced by a laser pulse, interact with the electromagnetic field generated in the photoconductive antenna, allowing the analysis of PCA behavior in the THz regime. The generation and recombination of electron-hole pairs play a crucial role in the antenna dynamics and the emission of electromagnetic waves.
• The study highlights how the choice of different geometric configurations of the electrodes (such as face-to-face dipoles and transmission line dipoles) affects PCA performance, providing valuable insights for improving antenna design.
• The article also introduces a novel methodology for analyzing the transient and steady-state phases of electric and magnetic fields, using a double-update procedure for carrier distributions. This approach ensures numerical stability and model accuracy throughout the time-domain simulations.
• One of the most interesting features of the model is its ability to reproduce the hysteresis effect due to preconditioning in the semiconductor, which allows for more accurate simulation of transient and steady-state phenomena in PCAs. This consideration had not been previously included in other models.

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On the Numerical Modeling of Terahertz Photoconductive Antennas

This paper explores the significant impact of mobility models on the description of carrier dynamics for the analysis of radiative semiconductor photoconductive devices in the terahertz (THz) regime. The authors developed a simulator that self-consistently solves both the semiconductor device physics and Maxwell's equations to study the radiated electromagnetic fields. A key focus of this work is on the importance of accurately modeling the steady-state regime of the semiconductor device, which is crucial for the precise calculation of radiated fields, particularly in the broadside direction.

One of the primary contributions of this paper is the demonstration of how an accurate steady-state description of the electric potential, field distributions, and local mobility is essential for achieving realistic results in terahertz photoconductive antenna (PCA) simulations. The study shows that previous models, which did not consider detailed steady-state regimes, failed to capture the full complexity of the carrier interactions and their effects on the radiated electromagnetic fields.

Main Contributions

• The paper introduces a comprehensive numerical model that integrates both Poisson's and Maxwell's equations with a drift-diffusion model, providing a highly accurate description of semiconductor carrier dynamics in the THz regime.
• It emphasizes the importance of accurate mobility models in steady-state conditions, showing that even slight inaccuracies in the steady-state regime can lead to significant errors in the predicted radiated fields.
• The study demonstrates, through numerical simulations, the need for a detailed consideration of steady-state mobility distributions, electric fields, and the influence of doping concentrations on device behavior.
• It is the first time that a detailed steady-state consideration has been shown to significantly influence the outcomes of PCA simulations, marking a shift in how such devices are modeled in the terahertz range.

Key Insights

• The simulation results are validated against experimental data, confirming the superior accuracy of the numerical models presented in the study compared to simpler, previous models.
• The authors highlight the importance of accurately modeling the carrier mobility in the semiconductor. In particular, the field-dependent mobility model used in this work allows for a much more precise description of the device behavior under different operational conditions.
• This paper also presents a novel approach to considering the preconditions (steady-state regime) before the laser pulse excitation, introducing the concept of hysteresis in THz devices for the first time.
• This hysteresis effect is shown to play a critical role in the transient behavior of the devices, influencing the final radiated electromagnetic fields and leading to a better understanding of the device performance.

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Performance Comparison of THz Photoconductive Antennas

This document is one of the most important in my thesis, as it provides, from a relatively simple model, a description of the non-linear behaviors necessary to adequately model photoconductive antennas in the terahertz (THz) regime. At the time, methods such as Monte Carlo or finite elements were suggested as essential for this type of modeling due to the intrinsic complexity of the phenomena involved. However, this study demonstrates that using a simplification based on the dependence of the carrier mobility on the electric field can yield precise and highly efficient experimental results.

The article explores the influence of bias electrode geometry on the performance of photoconductive antennas. It presents a methodology to numerically calculate the operating bandwidth and radiation efficiency of PCAs (photoconductive antennas). The numerical results are validated through comparisons with experimental measurements, lending credibility to the simulations presented.

Main Contributions

• A numerical model based on the finite-difference time-domain (FDTD) method is introduced, which self-consistently solves the drift-diffusion and Maxwell equations.
• The study highlights how the choice of electrode geometry can double the efficiency of the antennas, with a bandwidth penalty generally less than 10%.
• The influence of factors such as bias voltage, semiconductor doping, and incident optical power on the overall performance of the antennas is presented.
• For the first time, it is demonstrated how the use of optimal electrode configurations can significantly increase radiation efficiency in PCAs.

Key Insights

• Numerical results are validated through comparisons with experimental measurements, confirming the accuracy of the simulations and their ability to predict the real performance of PCAs under different operating conditions.
• The study also shows how increasing optical power can cause a saturation effect in the antennas due to the accumulation of photogenerated carriers near the electrodes.
• Additionally, it analyzes how the mobility of the carriers is affected by the applied electric field, which directly impacts radiation efficiency.

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Time-domain Numerical Modeling of Terahertz Receivers Based on Photoconductive Antennas

This paper marked a turning point in my research, published after my PhD defense, and played a significant role in advancing the understanding of photoconductive antennas (PCAs) in the terahertz (THz) domain. It laid the foundation for the process of both emission and reception in near-field PCAs, which would later be presented at a prominent conference in Tucson, Arizona https://doi.org/10.1109/IRMMW-THz.2014.6956333.

The paper focuses on developing a simulator that couples semiconductor charge transport equations with Maxwell's equations to study the performance of terahertz receivers based on PCAs. This model allowed for an accurate characterization of PCAs, confirming experimental results through simulations. The key breakthrough was the detailed analysis of how a photoconductive receiver antenna detects THz radiation by convolving the photoconductivity of the receiver with the electric field generated by an emitter PCA. This simulation tool was critical for understanding and optimizing THz time-domain spectroscopy (THz-TDS) systems.

Main Contributions

• The development of a robust simulator to model photoconductive antennas, essential for THz-TDS applications.
• The introduction of a simple yet highly accurate numerical approach to model the reception processes in PCAs, balancing the trade-off between simplicity and accuracy.
• Numerical results were validated against experimental data, showcasing the simulator's precision in reproducing real-world measurements.
• The simulation tool allowed for the study of key factors such as carrier lifetime, mobility, and noise levels, which are crucial for the signal-to-noise ratio (SNR) in THz detectors.

Key Insights

• The model accurately simulated the current generated in a PCA when exposed to an incident electric field, highlighting how factors such as photoconductivity and incident field strength influence the detected signal.
• The study demonstrated how omitting certain components, like the PCA lens and semi-insulating substrate, could still yield reliable results in specific scenarios, reducing computational complexity.
• This paper established the drift-diffusion model as a viable approach for simulating PCA behavior, offering faster computation times compared to Monte Carlo methods while maintaining accuracy.
• The paper also introduced a new method to model the PCA reception process coherently, providing both amplitude and phase information without requiring additional complex techniques like Kramers-Kronig.

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A precise analytical model for drift velocity and mobility in InGaAs

This article introduces an analytical model to accurately describe the drift velocity and mobility of electrons and holes in In0.53Ga0.47As under different electric field conditions and dopant concentrations. Using data simulated by Monte Carlo methods, the model combines mathematical simplicity with precision, making it ideal for implementation in compact and efficient simulations.

The following equation (Equation 1) models the drift velocity of electrons as a function of the electric field:

\[ v_e(|\vec{E}|) = \left\{ \begin{array}{ll} \frac{A_e\left(\sin\left( \frac{\pi|\vec{E}|}{10}\right)\right)^{b_e}}{e^{c_e|\vec{E}|^2}} & \text{if } |\vec{E}| < E_{c,e} \\ \frac{D_e|\vec{E}|}{\left(1+\frac{|\vec{E}|-E_{c,e}}{3}\right)^f_e} & \text{if } |\vec{E}| \geq E_{c,e} \end{array} \right. \]

This equation describes how the electron velocity increases with the electric field until it reaches a saturation point, effectively capturing the transitions between different transport regimes.

Similarly, Equation 3 is used to describe the drift velocity of holes as a function of the electric field:

\[ v_h(|\vec{E}|) = \left\{ \begin{array}{ll} \frac{A_h\ln(|\vec{E}|+1)}{(|\vec{E}|+1)^{b_h}} & \text{if } |\vec{E}| < E_{c,h} \\ \frac{C_h\tanh(|\vec{E}|)}{\ln(|\vec{E}|+d_n)^{f_h}} & \text{if } |\vec{E}| \geq E_{c,h} \end{array} \right. \]

Both equations accurately capture the carrier dynamics in the InGaAs material, facilitating fast and precise simulations of electronic devices at the macroscopic level.

Main Contributions

• Development of a simple and precise analytical model for mobility in In0.53Ga0.47As, applicable to compact simulations.
• The model accounts for both electron and hole drift velocity, offering a comprehensive solution for charge transport in this semiconductor material.
• Model validation through comparisons with Monte Carlo-based simulations and experimental data.

Key Aspects

• The model is easy to implement in electronic device simulations, providing quick and accurate results on charge carrier behavior.
• This model captures both low and high electric field transport regimes, offering a more realistic description of transport in InGaAs.
• The model's simplicity ensures low computational cost, allowing its use in intensive simulations of circuits and devices.

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Modeling of Axicon Lenses with Chiral Focusing Properties through Analytical Functions

This article discusses the modeling of axicon lenses with chiral focusing properties using analytical functions. Axicon lenses have the ability to focus incident parallel light over a large depth of field, producing non-diffractive Bessel beams. These beams are widely used in optical signal processing, transparent material processing, and metal cutting. The study explores how asymmetric and twisted axicon lenses can create complex structured light fields with specifically designed intensity, phase, and polarization distributions.

The authors provide a set of analytical functions that allow for the definition of such lenses, ensuring freedom in designing the lens profile. Before manufacturing these lenses, simulations are conducted using the Finite Difference Time Domain (FDTD) method to predict their electromagnetic output, optimizing lens designs to maximize resource efficiency.

The work also demonstrates the modeling of the electromagnetic fields generated by axicon lenses through simulations. Analytical verification related to conservative magnitudes, such as the Poynting vector and chirality flux, is also included. Numerical results show 3D representations of these fields, confirming the accuracy of the designed lenses.

In particular, Equation 19 describes the relationship between the chirality flux and the Poynting vector. This equation is key in demonstrating the conservation of magnitudes in a source-free system, where the Poynting vector can be assumed as a partial source of the chirality flux. This is important for validating numerical implementations, as fulfilling this relationship ensures the accuracy of the electromagnetic field simulation generated by the axicon lenses. \[ \nabla \times \Omega(\mathbf{r}, t) = (\mathbf{E} \times \nabla) \times \mathbf{E}(\mathbf{r}, t) + (\mathbf{H} \times \nabla) \times \mathbf{H}(\mathbf{r}, t) \]

Additionally, Equations 11, 12, and 13 provide analytical examples of how to sculpt the structure of a lens to generate illumination with chirality. These equations describe asymmetric and twisted profiles of axicon lenses, allowing precise control over the distribution of the generated light's intensity, phase, and polarization, which is fundamental for advanced optical manipulation applications. \[ x(\theta, z) = \begin{cases} f_r(z) \cos(\theta) e^{-c\theta} + x_{\text{ref}} & \text{if } \theta < \theta_{\text{limit}} \\ f_r(z) \cos(\theta) e^{-c\theta_{\text{limit}}} + x_{\text{ref}} & \text{if } \theta \geq \theta_{\text{limit}} \end{cases} \] \[ y(\theta, z) = \begin{cases} f_r(z) \sin(\theta) e^{-c\theta} + y_{\text{ref}} & \text{if } \theta < \theta_{\text{limit}} \\ f_r(z) \sin(\theta) e^{-c\theta_{\text{limit}}} + y_{\text{ref}} & \text{if } \theta \geq \theta_{\text{limit}} \end{cases} \] \[ z(\theta, z) = \begin{cases} \sqrt{\theta z_{\text{jump}}^2} + z_{\text{ref}} & \text{if } \theta < \theta_{\text{limit}} \\ \sqrt{\theta_{\text{limit}} z_{\text{jump}}^2} + z_{\text{ref}} & \text{if } \theta \geq \theta_{\text{limit}} \end{cases} \]

Main Contributions

• Introduction of analytical functions for designing axicon lenses with freedom in profile design.
• Simulation of axicon lenses using the FDTD method to predict electromagnetic output and optimize designs.
• Analytical verification of conservative quantities like the Poynting vector and chirality flux in modeled lenses.
• Demonstration of non-diffractive Bessel beam generation for applications in optical and material processing.

Key Aspects

• Axicon lenses provide a large depth of field and generate non-diffractive Bessel beams for various applications.
• The study highlights the importance of simulations in optimizing lens designs before production.
• Analytical modeling ensures accuracy and efficiency in lens designs, validated through numerical results.

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Full Explicit Numerical Modeling in Time-Domain for Nonlinear Electromagnetics Simulations in Ultrafast Laser Nanostructuring

This paper introduces a fully explicit finite-difference time-domain (FDTD) method for modeling nonlinear electromagnetics, focusing on its application to ultrafast laser nanostructuring. Here we developed a stable algorithm capable of handling complex nonlinear phenomena such as Kerr and Raman effects, plasma generation, and light interactions at metal-dielectric interfaces. The algorithm’s accuracy and stability were theoretically proven, making it a powerful tool for simulating laser-material interactions at the nanometric scale.

One of the most significant contributions of this work is the detailed study of numerical stability, particularly the identification of stability conditions that ensure convergence. The research highlights how to optimize the framework to maintain convergence when dealing with highly nonlinear effects in nanostructured materials.

The nonlinear effects considered include multiphoton ionization, free-electron plasma generation, and metal dispersion. Additionally, this paper also addresses the stability conditions for the FDTD algorithm. Appendix A of the article provides a detailed derivation of the Von Neumann stability criteria and the Routh-Hurwitz criterion, ensuring the robustness of the numerical scheme across various nonlinear scenarios.

Main Contributions

• Development of a fully explicit FDTD method capable of handling nonlinear electromagnetic effects in ultrafast laser nanostructuring.
• Accurate simulation of complex phenomena such as Kerr effect, Raman effect, and plasma generation at nanometric metal-dielectric interfaces.
• Introduction of a stable algorithm with theoretical proofs of energy conservation and numerical stability.

Key Insights

• The research establishes clear stability frameworks for nonlinear simulations, ensuring the convergence of results under various physical conditions.
• The method efficiently models free-electron plasma generation and Kerr-induced nonlinearities in nanostructured dielectrics.
• This approach significantly enhances the accuracy of simulations involving light-matter interactions, especially at the nanoscale.

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Comparative study of grids based on the cubic crystal system for the FDTD solution of the wave equation

This article introduces a comparative analysis of different grids based on the cubic crystal system for the explicit solution of the wave equation using the finite difference time domain (FDTD) method. The grids studied include the simple cubic (SC), body-centered cubic (BCC), face-centered cubic (FCC), and compact packing cubic (CPC).

One of the most important contributions of this work is the detailed study of numerical stability conditions and the identification of frameworks where the grids are most efficient and accurate. Special emphasis is placed on how each grid offers advantages in terms of physical dispersion error and relative anisotropy depending on the characteristics of the problem being solved.

The article also addresses the study of computational complexity, evaluating the cost associated with implementing each grid in terms of simulation time and computational load. The results show that the BCC grid, though complex, offers the best trade-off between accuracy and computational costs for specific applications.

Main Contributions

• Development of a comprehensive comparative analysis of cubic grids (SC, BCC, FCC, and CPC) for the solution of the wave equation using FDTD.
• Detailed study of numerical stability and convergence conditions, based on the analysis of the Hessian matrix.
• Analysis of the minimum physical dispersion error and relative anisotropy in the different grids under the same number of points per wavelength.
• Evaluation of the computational cost associated with each grid, demonstrating that the BCC grid offers an optimal balance between accuracy and efficiency.

Key Insights

• The work establishes clear frameworks for selecting the appropriate grid based on the characteristics of the problem, highlighting the importance of balancing accuracy and computational cost.
• The BCC grid proved to be the most efficient in terms of physical dispersion error under the same conditions, even surpassing the FCC grid in specific situations.
• The FCC grid remains the best choice in cases where low relative anisotropy must be maintained, while the SC grid is preferred for problems with a dominant single propagation direction.

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Illumination study of a quantum MIM diode for the mid-infrared

This article explores the illumination effects on quantum metal-insulator-metal (MIM) diodes in the mid-infrared range, focusing on improving the harvested tunneling current through optimized illumination techniques. The study investigates the use of a distributed illumination method combined with a Kretschmann and Reather prism-based configuration. This technique extends the tunneling event across the entire diode junction, significantly increasing both the quantum tunneling current and the diode's responsivity.

One of the most notable contributions of this work is the exploration of numerical stability and optimization in diode responsivity. The research highlights how distributed illumination leads to a more uniform electric field across the junction, improving the quantum rectification process compared to traditional methods. The study of nonlinear effects is also emphasized, particularly in the context of how quantum tunneling is enhanced by the applied illumination configuration.

Additionally, this study examines the computational modeling using the ADE-FDTD method to solve Maxwell's equations with quantum tunneling incorporated. This numerical approach helps accurately simulate the interaction between the electromagnetic fields and the MIM diode, providing insights into how different illumination angles and metal thicknesses affect performance.

Main Contributions

• Development of a new distributed illumination technique using a Kretschmann and Reather prism to enhance quantum tunneling across the MIM diode.
• Improvement of the quantum diode's responsivity by increasing the uniformity of the electric field along the junction.
• Use of the ADE-FDTD method to model the interaction of electromagnetic fields with the MIM structure, improving the accuracy of numerical simulations.

Key Insights

• Distributed illumination increases the harvested current significantly, with a responsivity of 0.24 A/W, which is about 12 times higher than previous techniques.
• The Kretschmann prism configuration facilitates stronger electromagnetic field penetration in the MIM junction, leading to more effective quantum rectification.
• Uniform field distribution across the diode junction enhances performance compared to conventional focused illumination techniques.

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BCC-Grid versus SC-Grid in the modeling of a sheet of graphene as a surface boundary condition in the context of ADE-FDTD

This article presents a comparative study between the simple cubic grid (SC-Grid) and body-centered cubic grid (BCC-Grid) for modeling a graphene sheet as a surface boundary condition using the Auxiliary Differential Equation Finite-Difference Time-Domain (ADE-FDTD) method. The study focuses on the intraband and interband contributions of graphene’s conductivity while considering the metal in contact as a dispersive medium.

One of the key contributions of this work is the use of the **BCC-Grid**, which avoids discontinuities in the normal components of the electric and magnetic fields on the graphene surface, proving superior to the traditional SC-Grid for these applications. The BCC-Grid offers computational advantages by reducing complexity while maintaining accuracy, especially in mid-infrared and far-infrared frequencies where volumetric models of graphene become impractical.

Main Contributions

• Development of a surface boundary condition for graphene using ADE-FDTD, incorporating both intraband and interband conductivity contributions.
• Introduction of the BCC-Grid, which ensures continuity of electric and magnetic field components on the graphene surface, improving upon the SC-Grid approach.
• Significant reduction in computational burden with BCC-Grid, achieving the same accuracy as SC-Grid with eight times fewer grid points.
• The ability to represent a three-dimensional object like graphene as a two-dimensional mathematical boundary condition is crucial for computational efficiency and simulation accuracy. This approach reduces processing requirements by avoiding the need to model the entire volume, allowing faster and more manageable simulations.

Key Insights

• The BCC-Grid eliminates the need to handle discontinuities in the electric field at the graphene interface, which are problematic in the SC-Grid.
• The method allows for accurate simulation of metal-graphene interactions, useful for a wide range of electronic and photonic applications.
• The computational efficiency gained by using BCC-Grid makes it ideal for high-performance computing simulations, significantly reducing memory and time requirements.
• Modeling graphene as a mathematic two-dimensional boundary condition is essential for accurately simulating surface phenomena while avoiding the complexity of a full three-dimensional object. This approach enhances precision without sacrificing simulation efficiency.

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Pulsed Photoconductive Antennas for Emission in the Terahertz Regime

This article presents a detailed analysis of the use of pulsed photoconductive antennas for emission in the terahertz (THz) range. Using simulations with commercial software, the study addresses aspects such as efficiency, antenna geometry, and substrate doping distribution. Additionally, it explores the key role of plasmonic effects in enhancing laser absorption in the semiconductor.

Plasmons, which are collectives of synchronized electrons under the influence of an electromagnetic field, enable the injection of an electromagnetic field into structures smaller than the wavelength, achieving greater efficiency in generating photoconductive current in the antennas. This phenomenon allows illuminating areas in devices that would otherwise not be reachable.

Furthermore, through an appropriate doping distribution, the transport across the semiconductor is primarily carried out by electrons, which have much higher mobility than holes, contributing to greater efficiency in the devices. This approach is applied in cases where maximum efficiency is achieved, allowing electrons to be the main charge carriers.

The photodember effect is also relevant and can be exploited in vertical devices when the laser illumination is applied in the same direction as the current flow, that is, in the same spatial direction. This alignment maximizes the generation of photocurrent and improves the overall efficiency of the device.

Main Contributions

• Improvement of laser absorption in the semiconductor through the use of plasmonic nanoantennas, increasing the generation of photocurrent.
• Comparison between vertical and planar photoconductive antennas, demonstrating that optimized vertical antennas achieve greater efficiency due to the alignment of the photogenerated current with the laser field.
• Incorporation of a three-dimensional plasmonic structure that maximizes efficiency at terahertz frequencies, allowing higher resolution in nanoscale applications.
• Optimization of the impedance matching between the semiconductor and the laser wavelength, significantly improving terahertz generation.

Key Aspects

• Plasmonic effects allow the injection of electromagnetic fields into sub-wavelength structures, expanding the capabilities of photoconductive antennas.
• Vertical antennas outperform planar ones in terms of efficiency, especially when combined with three-dimensional plasmonic structures and impedance optimization.
• Simulations demonstrate the importance of doping distribution in the semiconductor and the alignment of current with the laser illumination direction.

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Super-Efficient Metal Drilling with Ultrafast Non-Diffractive Laser Beams

This paper presents a highly efficient drilling process in non-transparent metallic materials, enabled by the use of ultrafast non-diffractive Bessel-type laser beams. Applied for deep drilling through a 200 μm thick steel plate, the Bessel beam proves to be twice as efficient compared to a Gaussian beam with similar fluence and spot size. While surface ablation occurs with the same efficiency for both beams, the increase in drilling efficiency results from the beam’s self-replication and reconstruction along the axis, driven by internal reflections within the crater at near-grazing incidence, avoiding potential obstructions.

The mechanism arises from a geometry of oblique wave vectors with low angular dispersion, generating a propagation length beyond the range allowed by the geometric projection of the channel. With only the main lobe selected by the channel entrance, reflection on the sidewalls leads to the re-folding of the lobe onto the axis, enhancing and replicating the beam multiple times within the channel. This process is critically assisted by the reduction of particle shielding, facilitated by the intrinsic self-healing ability of the Bessel beam. Therefore, the drilling process is sustained in a way that is unique compared to the conventional Gaussian beam.

Main Contributions

• Super-efficient drilling of non-transparent metals using non-diffractive Bessel beams compared to Gaussian beams.
• Demonstration of the self-replication and reconstruction of the Bessel beam within the drilling channel, increasing its efficiency at depth.
• Simulations using the finite-difference time-domain (FDTD) method to quantify beam propagation and its interaction with obstacles along the path.

Key Aspects

• Bessel beams offer higher drilling efficiency in deep drilling applications, overcoming the limitations of Gaussian beams in terms of dispersion and particle shielding.
• The self-healing property of the Bessel beam ensures continuous energy delivery even in the presence of obstacles, contributing to its superior performance.
• The beam’s geometric design is key to ensuring enhanced performance in advanced laser processing applications.

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Implementation of Open Boundary Problems in Photoconductive Antennas Using Convolutional Perfectly Matched Layers

This paper presents a method for simulating an open boundary problem using the Finite Differences Time Domain (FDTD) method in the context of photoconductive antennas emission. To achieve this, the simulation employs Convolutional Perfectly Matched Layers (CPML). The semiconductor region, where transient currents are generated during the simulation, is treated as an "active" medium, extended virtually beyond its physical boundaries or the computational domain.

The use of CPML enables efficient simulation of transient electromagnetic fields within semiconductors, helping to avoid field discontinuities. This approach is particularly relevant for modeling multiscale features, which are present in the photoconductive antenna’s structure and response. The paper also highlights how the current densities inside the CPML region are modeled using drift-diffusion equations, ensuring accuracy even when the electric field varies spatially.

Additionally, an appropriate doping distribution ensures that most of the transport in the semiconductor is carried out by electrons, which have much higher mobility than holes. This optimization is crucial for achieving higher efficiency in devices. By designing the doping profile carefully, electrons become the primary charge carriers, leading to improved performance.

The photodember effect is also important and can be exploited in vertical devices when the laser illumination is applied in the same direction as the current flow, maximizing photocurrent generation and enhancing overall device efficiency.

Main Contributions

• Development of a method to simulate open boundary problems in photoconductive antennas using CPML, improving the accuracy of the FDTD simulations.
• Introduction of a multiscale modeling approach that accounts for the transient behavior of the semiconductor region, enhancing simulation results.
• Validation of the simulation method by comparing results from both large and truncated computational domains, showing accurate and reliable output.
• Use of drift-diffusion models for current density simulation in the CPML region, maintaining a stable solution for electric field variations.

Key Aspects

• The CPML method avoids electric field discontinuities and helps accurately simulate transient electromagnetic phenomena in photoconductive antennas.
• Through careful doping distribution, electron mobility is maximized, leading to more efficient semiconductor devices.
• The photodember effect is effectively utilized in vertical devices where the laser and current flow are aligned, improving device performance.

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Total Field and Scattered Field Formulation in a Non-Uniform Grid Using FDTD-DPW

This paper presents the extension of the discrete plane wave technique in finite-difference time-domain (FDTD-DPW) to a non-uniform grid. Using the total field/scattered field scheme, this technique allows for the propagation of a plane wave almost perfectly, isolated in the total field domain without significant reflections in the scattered field domain (with a margin of -300 dB). The method is applicable to any propagation angle and grid cell aspect ratio.

The use of FDTD-DPW facilitates the simulation of plane waves in non-uniform grids, making it suitable for multiscale electromagnetic problems. Unlike the standard formulation, this technique does not require interpolation and maintains the phase velocity between the one-dimensional incident wave and the generated three-dimensional wave, eliminating non-physical reflections in the scattered field region.

Additionally, this technique allows for the propagation of broadband waves or multiple plane waves in a non-uniform grid without the need for artificial media to correct phase velocity errors. Although there is a loss of central difference, compromising second-order accuracy, the reduction in computational load compensates for this effect, and the results show a good agreement between the incident and propagated fields.

This technique is validated in both uniform and non-uniform computational domains, showing a significant reduction in computational requirements while maintaining sufficient accuracy for many practical applications in multiscale domains.

Main Contributions

• Extension of the FDTD-DPW technique from uniform grids to non-uniform grids for the propagation of broadband plane waves.
• Elimination of the need for interpolations by maintaining the phase velocity of the incident and generated plane waves in non-uniform grids.
• Significant reduction in computational load, especially in multiscale problems where a non-uniform grid is used to capture detailed features in certain areas of the computational domain.
• Validation of the technique in simulations showing good accuracy and scattered field isolation, even in non-uniform grids.

Key Aspects

• The FDTD-DPW method avoids non-physical reflections in the scattered field domain by eliminating the need for interpolations in plane wave propagation.
• The technique is applicable to a wide range of electromagnetic problems, particularly those with multiscale characteristics.
• Although there is a slight loss of accuracy due to the lack of central difference, the reduction in computational load compensates for this effect in practical applications.

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Numerical study of graphene heat spreaders for a THz quantum diode based on a G-MGIM junction

This article presents a numerical study on graphene heat spreaders for a terahertz (THz) receiver based on a metal-insulator-metal (MIM) junction. The study explores how a multilayer graphene top sheet can dissipate heat from a thin metal layer impacted by the laser, thanks to graphene’s high thermal conductivity. This heat is transferred to the receiver's metal side pads, which act as thermal dissipators.

The use of surface plasmon polaritons (SPP-TW) enables the propagation of electromagnetic waves in structures whose size is below the diffraction limit, introducing the electromagnetic field into the MIM junction and improving the diode's responsivity.

Additionally, two graphene layers, one over the top metal contact and another in the region where the SPP-TW is induced, improve the device’s performance. Graphene helps increase the asymmetry in the current-voltage curve through a Seebeck effect, which is induced by the SPP-TW waves.

This work also evaluates the thermal behavior of the graphene layers as heat spreaders and how they affect the electrical conductivity of the top metal contact, following the Wiedemann-Franz law and considering the reduction in conductivity due to increased temperature.

Main Contributions

• Numerical study of the use of graphene as a heat spreader in a quantum receiver based on an MIM junction.
• Improvement of asymmetry in the current-voltage curve through a Seebeck effect induced by SPP-TW in the G-MGIM junction.
• Evaluation of the thermal and electrical conductivity of graphene in contact with aluminum, analyzing the temperature's influence on conductivity.
• Simulation of the thermal dissipation process in the MIM junction with graphene layers, demonstrating a significant reduction of heat in the top metal contact.

Key Aspects

• Graphene acts as an efficient heat spreader due to its high thermal conductivity, transferring heat to the metal side pads.
• The graphene layers also contribute to improving the diode’s electrical performance through the Seebeck effect and SPP-TW.
• The use of graphene reduces thermal degradation of the top metal contact, ensuring better stability in the quantum device’s operation.

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In-situ Observation of Low-Power Nano-Synaptic Response in Graphene Oxide Using Conductive Atomic Force Microscopy

This paper presents an investigation into the low-power synaptic response in graphene oxide (GO)-based nanodevices using conductive atomic force microscopy (CAFM). Electronic synapses with confined sizes of <50 nm² are explored, demonstrating synaptic plasticity at nanometric scales. The GO/metal nano-synaptic devices show phenomena such as potentiation, paired-pulse facilitation (PPF), and excitatory post-synaptic currents (EPSC), with write current levels below 1 μA (≈3 μW power consumption).

The synaptic response is enabled by the resistive switching (RS) behavior in GO devices, where both volatile and non-volatile behaviors are observed depending on the applied voltage conditions. At low currents, volatile RS mimics synaptic phenomena like short-term potentiation (STP) and paired-pulse facilitation (PPF), while higher currents result in non-volatile behaviors like long-term potentiation (LTP).

The use of GO is justified by its low manufacturing cost and its ability to exhibit resistive switching phenomena in very small areas. The research highlights the potential for fabricating ultra-high-density electronic synapses that meet the requirements for neuromorphic systems with over 10¹⁰ devices/mm².

Main Contributions

• Observation of synaptic phenomena in GO/metal devices with areas of <50 nm² using conductive atomic force microscopy.
• Exploration of synaptic plasticity, including potentiation, PPF, and excitatory post-synaptic currents (EPSC) at extremely low current levels.
• Demonstration of both volatile and non-volatile resistive switching, including long-term potentiation (LTP), in low-power electronic synapses.
• Use of graphene oxide for the fabrication of low-cost electronic synapses with high integration for artificial neural networks (ANNs).

Key Aspects

• GO-based synapses exhibit stable and controllable synaptic behavior at low current levels, resulting in low power consumption (≈3 μW).
• The observed synaptic response includes biological synapse-like phenomena, such as PPF and EPSC, in GO/metal devices.
• Resistive switching does not rely on the formation of conductive filaments, allowing its implementation in high-density neuromorphic systems.

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Temperature of Conductive Nanofilaments in Hexagonal Boron Nitride Based Memristors Showing Threshold Resistive Switching

This paper presents a study on the temperature of conductive nanofilaments (CNFs) in memristors based on hexagonal boron nitride (h-BN) exhibiting threshold resistive switching (RS). The devices, consisting of Ag/h-BN/Au structures, operate with ultra-low energy consumption and demonstrate stable switching behaviors at different current levels. At low current (≈1 μA), the temperature of the CNFs is kept low (≈310 K), while higher current levels (≈200 μA) result in non-volatile behavior with temperatures exceeding 500 K, sufficient to form stable conductive nanofilaments.

The study highlights the relationship between CNF temperature and the switching mechanism, with lower temperatures resulting in volatile, threshold-type switching and higher temperatures forming non-volatile CNFs. The role of h-BN’s high in-plane thermal conductivity and its thin structure is emphasized as key factors in achieving stable RS at such low energy levels.

The temperature distribution across the CNF during switching is simulated using both analytical models and 3D heat equation solvers, showing a close correlation between current levels and temperature rise. This study provides valuable insights into the thermal dynamics within memristor devices and the factors enabling reliable operation in threshold-type resistive switching.

Main Contributions

• Comprehensive analysis of CNF temperatures in Ag/h-BN/Au memristors during threshold and non-volatile resistive switching.
• Identification of the critical current level (≈200 μA) at which the device transitions from volatile to non-volatile behavior due to temperature rise in the CNF.
• Thermal simulations demonstrating the impact of current on CNF temperature, revealing the importance of heat dissipation in maintaining device reliability.
• Exploration of the role of h-BN’s high thermal conductivity in controlling CNF temperature, ensuring stable operation at low currents.

Key Aspects

• Lower CNF temperatures (≈310 K) lead to threshold resistive switching, whereas higher temperatures (≈500 K) result in non-volatile behavior.
• h-BN’s in-plane thermal conductivity helps maintain low temperature levels, crucial for efficient memristor operation at ultra-low energy consumption.
• Thermal effects significantly influence the switching dynamics and reliability of memristor devices based on 2D materials like h-BN.

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Thermal Models for Resistive Random Access Memory Circuit Simulation

This paper presents an extensive review of thermal models used for circuit simulations of resistive random access memories (RRAM). The study emphasizes the essential role of temperature in resistive switching (RS) processes, as most RS mechanisms are thermally activated. The review highlights various numerical models based on the heat equation, including simplified compact models for circuit design and physical simulations of RRAM devices.

A key aspect of this study is the detailed analysis of different conductive filament (CF) geometries, lateral heat losses, and the impact of temperature on RS operation. The paper also explores how thermal effects can be incorporated into general memristor models, allowing for accurate circuit-level simulation.

Moreover, the study introduces the concept of thermal crosstalk in crossbar architectures, where the heat generated by one device influences its neighbors. The document also explores the influence of quantum effects on temperature-dependent charge transport in RRAM.

Main Contributions

• Comprehensive review of thermal models in RRAM, emphasizing the heat equation's role in resistive switching mechanisms.
• Introduction of conductive filament geometries and their impact on device temperature, including cylindrical and truncated-cone shaped filaments.
• Presentation of a 3D numerical solution for the heat equation in the context of RRAM circuit simulation, allowing for detailed thermal analysis.
• Analysis of thermal crosstalk in crossbar architectures and the inclusion of temperature as a state variable in general memristor models.

Key Aspects

• Thermal effects are crucial in the operation of RRAM, affecting both the resistive switching behavior and device reliability.
• The study provides methods for modeling thermal dissipation and integrating heat effects into circuit-level simulations for RRAM devices.
• Quantum effects related to temperature are explored, providing insights into the thermal modeling of RRAM charge transport.

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Non-Diffractive Bessel Beams for Ultrafast Laser Scanning Platform and Proof-Of-Concept Side-Wall Polishing of Additively Manufactured Parts

This paper reports the potential use of non-diffractive Bessel beams for ultrafast laser processing in additive manufacturing environments. It showcases the integration of the Bessel beam into a high-speed scanning platform and the proof-of-concept for side-wall polishing of stainless steel-based additively manufactured parts. The Bessel beam demonstrates two significant advantages: its extended non-diffractive length for better sample positioning tolerance and its unique self-reconstruction property, which enables uninterrupted beam access despite obstructions caused by metallic powders in the additive manufacturing environment.

The Bessel beam was integrated into a Galvano scanner platform, which sustained a stable beam profile over a scan field of 35 × 35 mm². The proof-of-concept showcases the advanced capability of this platform by significantly reducing the side-wall surface roughness of an additively manufactured workpiece from Ra 10 µm down to Ra 1 µm.

Main Contributions

• Successful integration of the non-diffractive Bessel beam into a high-speed scanning platform for ultrafast laser processing.
• Demonstration of Bessel beam's self-reconstruction property under the obstruction of metallic powders in the additive manufacturing environment.
• Proof-of-concept of side-wall polishing for stainless steel parts manufactured using additive techniques, achieving a surface roughness improvement from Ra 10 µm to Ra 1 µm.

Key Aspects

• The Bessel beam provides extended non-diffractive length, offering significant tolerance for sample positioning.
• The beam's self-reconstruction property allows for uninterrupted processing despite common obstructions in additive manufacturing, such as metallic powder clouds.
• The platform holds great potential for integration into hybrid additive manufacturing systems for inline polishing processes.

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Quantum vs Classical Thermal Model for Resistive Random Access Memories

This paper provides a detailed analysis of the quantum and classical thermal models applied to resistive random access memories (RRAM). It emphasizes the additional computational complexity and time required for quantum thermal transport modeling in RRAM cells, but highlights the significant differences in results between quantum and classical approaches. The quantum model presents more accurate predictions, particularly for the conductive filament (CF) properties during resistive switching, which classical models fail to capture accurately.

The study introduces a methodology that combines simulation results with experimental data to characterize the conductive filaments in nanometric RRAM cells. The key focus is on the differences in maximum filament temperatures between quantum and classical models and how these influence the overall performance of RRAM devices.

The analysis reveals that the quantum approach leads to significantly lower temperature predictions in the conductive filaments during switching processes, affecting the device's longevity and efficiency. These findings are crucial for improving the design of RRAMs used in modern memory technologies and for applications in artificial intelligence, where energy efficiency is critical.

Main Contributions

• Comprehensive comparison between quantum and classical thermal models for RRAM devices, highlighting the advantages of quantum modeling.
• Introduction of a multi-physics system for RRAM cell modeling, combining electrical and thermal properties to optimize device performance.
• Accurate prediction of conductive filament properties, including the temperature profile and its effect on the resistive switching behavior.
• Application of quantum tunneling effects and their impact on the thermal transport within the RRAM cell.

Key Aspects

• The quantum thermal model provides more accurate predictions of filament temperature, leading to improved device reliability and efficiency.
• Classical models underestimate the thermal effects in RRAM devices, particularly in the nanoscale conductive filaments.
• Quantum tunneling plays a crucial role in determining the thermal and electrical properties of RRAM cells, particularly during switching cycles.

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Thermal Compact Modeling and Resistive Switching Analysis in Titanium Oxide-Based Memristors

This article presents an extensive study on thermal compact modeling and the resistive switching behavior of titanium oxide-based memristors. These memristors, which are key components in next-generation non-volatile memory devices, rely on the formation and rupture of conductive filaments (CNFs) within the TiO₂ layer. The study explores the electrical and thermal responses of Au/Ti/TiO₂/Au stack devices, employing both experimental data and numerical simulations to better understand how the CNFs behave under various conditions.

One of the key features of this work is the integration of scanning thermal microscopy (SThM), a highly sensitive technique that allows for precise localization of temperature hotspots on the surface of the memristor. This provides real-time thermal data, which is critical for understanding the relationship between heat generation and resistive switching. Additionally, the study employs COMSOL Multiphysics simulations to model the thermal and electrical behavior of the devices, offering insights into the distribution of heat within the nanostructures.

The thermal resistance values extracted from these experimental and simulated results are integrated into compact models, which are used to improve the accuracy of circuit-level simulations. These compact models allow for more efficient simulation of large-scale memristor arrays, providing engineers with better tools for designing low-power, high-efficiency memory systems.

Main Contributions

• Development of thermal compact models that incorporate experimental and simulated thermal resistance data to improve circuit simulations of memristor devices.
• Use of scanning thermal microscopy (SThM) to map temperature variations across the device surface and correlate them with electrical behavior during resistive switching events.
• Analysis of resistive switching mechanisms and variability by applying different numerical methods to current-voltage (I-V) curves, helping to better understand device reliability and performance under different operating conditions.

Key Insights

• Thermal behavior is a crucial factor in the resistive switching mechanism of memristors, as heat influences the formation and rupture of conductive filaments.
• The combination of accurate thermal and electrical data leads to better compact models, which are essential for optimizing the performance of memristor-based circuits in real-world applications.
• The findings from this study are vital for the future development of low-energy, high-performance memory devices, with potential applications in artificial intelligence and data storage technologies.

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Thermal study of multilayer resistive random access memories based on HfO2 and Al2O3 oxides

This paper presents a comprehensive thermal study of multilayer resistive random access memories (RRAMs) composed of HfO2 and Al2O3 oxide layers. The study includes both simulation and experimental characterization of these devices. Using a 3D heat equation solver created by this website author, the paper analyzes the thermal effects and behavior of the conductive filaments (CF) during the resistive switching process in devices with different oxide stack configurations and electrodes made of Ni and Si-n+.

According to the simulations, the narrow section of the CF is consistently formed in the HfO2 layer, regardless of the stack order. This result is attributed to the higher heat flux in Al2O3 compared to HfO2, which determines the thermal behavior and the overall resistive switching operation. The heat transfer from the CF to the electrodes and the surrounding oxide is thoroughly analyzed, and the lateral heat flux from the CF to the surrounding dielectric is shown to be significant.

The results suggest that, for devices with an HfO2 layer, higher temperatures are reached in the regions surrounding the narrow part of the CF, which lowers the reset voltage. In contrast, devices with an Al2O3-only dielectric layer require higher reset voltages due to the higher thermal conductivity of Al2O3, which dissipates heat more efficiently.

The study emphasizes the importance of considering both the lateral and vertical heat fluxes in RRAM devices, as these thermal effects directly influence the resistive switching characteristics. The authors also highlight the need to include thermal resistances related to the lateral heat flux in compact RRAM models for accurate simulations.

Main Contributions

• Numerical and experimental analysis of thermal effects in HfO2 and Al2O3-based RRAMs during resistive switching.
• Identification of the role of the HfO2 layer in forming the narrow section of the conductive filament, regardless of stack order.
• Detailed study of the heat flux from the CF to the surrounding dielectric and electrodes, emphasizing the lateral heat flux component.
• Simulation of temperature distributions in RRAM devices, demonstrating that devices with Al2O3-only dielectrics require higher reset voltages due to better heat dissipation.

Key Aspects

• Thermal effects play a critical role in the resistive switching behavior of RRAM devices, particularly in the formation and rupture of conductive filaments.
• Lateral heat flux from the conductive filament to the surrounding dielectric is significant and should be considered in RRAM thermal models.
• Devices with HfO2 layers exhibit higher temperatures around the conductive filament, which lowers the reset voltage compared to devices with Al2O3-only dielectrics.

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