Superdiffusive heat conduction in semiconductor alloys -- II. Truncated L\'evy formalism for experimental analysis

Nearly all experimental observations of quasi-ballistic heat flow are interpreted using Fourier theory with modified thermal conductivity. Detailed Boltzmann transport equation (BTE) analysis, however, reveals that the quasi-ballistic motion of thermal energy in semiconductor alloys is no longer Brownian but instead exhibits L\'evy dynamics with fractal dimension $\alpha < 2$. Here, we present a framework that enables full 3D experimental analysis by retaining all essential physics of the quasi-ballistic BTE dynamics phenomenologically. A stochastic process with just two fitting parameters describes the transition from pure L\'evy superdiffusion as short length and time scales to regular Fourier diffusion. The model provides accurate fits to time domain thermoreflectance raw experimental data over the full modulation frequency range without requiring any `effective' thermal parameters and without any a priori knowledge of microscopic phonon scattering mechanisms. Identified $\alpha$ values for InGaAs and SiGe match ab initio BTE predictions within a few percent. Our results provide experimental evidence of fractal L\'evy heat conduction in semiconductor alloys. The formalism additionally indicates that the transient temperature inside the material differs significantly from Fourier theory and can lead to improved thermal characterization of nanoscale devices and material interfaces

Non Integer Identification of Rotor Skin Effect in Induction Machines

Fractional identification of rotor skin effect in induction machines is presented in this paper. Park‘s transformation is used to obtain a system of differential equations which allows to include the skin effect in the rotor bars of asynchronous machines. A transfer function with a fractional derivative order has been selected to represent the admittance of the bar by the help of a non integer integrator which is approximated by a J+1 dimensional modal system. The machine parameters are estimated by an output-error technique using a non linear iterative optimization algorithm. Experimental results show the performance of the modal approach for modeling and identification.DOI:http://dx.doi.org/10.11591/ijece.v3i3.228

On microscopic derivation of a fractional stochastic Burgers equation

We derive from a class of microscopic asymmetric interacting particle systems on ${\mathbb Z}$, with long range jump rates of order $|\cdot|^{-(1+\alpha)}$ for $0<\alpha<2$, different continuum fractional SPDEs. More specifically, we show the equilibrium fluctuations of the hydrodynamics mass density field of zero-range processes, depending on the stucture of the asymmetry, and whether the field is translated with process characteristics velocity, is governed in various senses by types of fractional stochastic heat or Burgers equations. The main result: Suppose the jump rate is such that its symmetrization is long range but its (weak) asymmetry is nearest-neighbor. Then, when $\alpha<3/2$, the fluctuation field in space-time scale $1/\alpha:1$, translated with process characteristic velocity, irrespective of the strength of the asymmetry, converges to a fractional stochastic heat equation, the limit also for the symmetric process. However, when $\alpha\geq 3/2$ and the strength of the weak asymmetry is tuned in scale $1-3/2\alpha$, the associated limit points satisfy a martingale formulation of a fractional stochastic Burgers equation.Comment: 24 page

A challenge to the Delta G~0 interpretation of hydrogen evolution

Platinum is a nearly perfect catalyst for the hydrogen evolution reaction, and its high activity has conventionally been explained by its close-to-thermoneutral hydrogen binding energy (G~0). However, many candidate non-precious metal catalysts bind hydrogen with similar strengths, but exhibit orders-of-magnitude lower activity for this reaction. In this study, we employ electronic structure methods that allow fully potential-dependent reaction barriers to be calculated, in order to develop a complete working picture of hydrogen evolution on platinum. Through the resulting ab initio microkinetic models, we assess the mechanistic origins of Pt's high activity. Surprisingly, we find that the G~0 hydrogen atoms are kinetically inert, and that the kinetically active hydrogen atoms have G's much weaker, similar to that of gold. These on-top hydrogens have particularly low barriers, which we compare to those of gold, explaining the high reaction rates, and the exponential variations in coverages can uniquely explain Pt's strong kinetic response to the applied potential. This explains the unique reactivity of Pt that is missed by conventional Sabatier analyses, and suggests true design criteria for non-precious alternatives

Sigma-Delta control of charge trapping in heterogeneous devices

Dielectric charging represents a major reliability issue in a variety of semiconductor devices. The accumulation of charge in dielectric layers of a device often alters its performance, affecting its circuital features and even reducing its effective lifetime. Although several contributions have been made in order to mitigate the undesired effects of charge trapping on circuit performance, dielectric charge trapping still remains an open reliability issue in several applications. The research work underlying this Thesis mainly focuses on the design, analysis and experimental validation of control strategies to compensate dielectric charging in heterogeneous devices. These control methods are based on the application of specifically designed voltage waveforms that produce complementary effects on the charge dynamics. Using sigma-delta loops, these controls allow to set and maintain, within some limits, the net trapped charge in the dielectric to desired levels that can be changed with time. This allows mitigating long-term reliability issues such as capacitance-voltage (C-V) shifts in MOS and MIM capacitors. Additionally, the bit streams generated by the control loops provide real-time information on the evolution of the trapped charge. The proposed controls also allow compensating the effects of the charge trapping due to external disturbances such as radiation. This has been demonstrated experimentally with MOS capacitors subjected to various types of ionizing radiation (X-rays and gamma rays) while a charge control is being applied. This approach opens up the possibility of establishing techniques for active compensation of radiation-induced charge in MOS structures as well as a new strategy for radiation sensing. A modeling strategy to characterize the dynamics of the dielectric charge in MOS capacitors is also presented. The diffusive nature of the charge trapping phenomena allows their behavioral characterization using Diffusive Representation tools. The experiments carried out demonstrate a very good matching between the predictions of the model and the experimental results obtained. The time variations in the charge dynamics due to changes in the volatges applied and/or due to external disturbances have been also investigated and modeled. Moreover, the charge dynamics of MOS capacitors under sigma-delta control is analyzed using the tools of Sliding Mode Controllers for an infinite sampling frequency approximation. A phenomenological analytical model is obtained which allows to predict and analyze the sequence of control signals. This model has been successfully validated with experimental data. Finally, the above control strategies are extended to other devices such as eMIM capacitors and perovskite solar cells. Preliminary results including open loop and closed loop control experiments are presented. These results demonstrate that the application of the controls allows to set and stabilize both the C-V characteristic of an eMIM capacitor and the current-voltage characteristic (J-V) of a perovskite solar cell.La carga atrapada en dieléctricos suele implicar un problema importante de fiabilidad en muchos dispositivos semiconductores. La acumulación de dicha carga, normalmente provocada por las tensiones aplicadas durante el uso del dispositivo, suele alterar el rendimiento de éste con el tiempo, afectar sus prestaciones a nivel de circuital e, incluso, reducir su vida útil. Aunque durante años se han realizado muchos trabajos para mitigar sus efectos no deseados, sobre todo a nivel circuital, la carga atrapada en dieléctricos sigue siendo un problema abierto que frena la aplicabilidad práctica de algunos dispositivos. El trabajo de investigación realizado en esta Tesis se centra principalmente en el diseño, análisis y validación experimental de estrategias de control para compensar la carga atrapada en dieléctricos de diversos tipos de dispositivos, incluyendo condensadores MOS, condensadores MIM fabricados con nanotecnología y dispositivos basados en perovskitas. Los controles propuestos se basan en utilizar formas de onda de tensión, específicamente diseñadas, que producen efectos complementarios en la dinámica de la carga. Mediante el uso de lazos sigma-delta, estos controles permiten establecer y mantener, dentro de unos límites, la carga neta atrapada en el dieléctrico a valores prefijados, que pueden cambiarse con el tiempo. Esto permite mitigar problemas de fiabilidad a largo plazo como por ejemplo las derivas de la curva capacidad-tensión (C-V) en condensadores MOS y MIM. Adicionalmente, las tramas de bits generadas por los lazos de control proporcionan información en tiempo real sobre la evolución de la carga. Los controles propuestos permiten también compensar los efectos de la carga atrapada en dieléctricos debida a perturbaciones externas como la radiación. Esto se ha demostrado experimentalmente con condesadores MOS sometidos a diversos tipos de radiación ionizante (rayos X y gamma) mientras se les aplicaba un control de carga. Este resultado abre la posibilidad tanto de establecer técnicas de compensación activa de carga inducida por radiación en estructuras MOS, como una nueva estrategia de sensado de radiación. Se presenta también una estrategia de modelado para caracterizar la dinámica de la carga dieléctrica en condensadores MOS. La naturaleza difusiva de los fenómenos de captura y eliminación de carga en dieléctricos permite caracterizar dichos fenómenos empleando herramientas de Representación Difusiva. Los experimentos realizados demuestran una muy buena correspondencia entre las predicciones del modelo y los resultados experimentales obtenidos. Se muestra también como las variaciones temporales de los modelos son debidas a cambios en las formas de onda de actuación del dispositivo y/o a perturbaciones externas. Además, la dinámica de carga en condensadores MOS bajo control sigma-delta se analiza utilizando herramientas de control en modo deslizante (SMC), considerando la aproximación de frecuencia de muestreo infinita. Con ello se obtiene un modelo analítico simplificado que permite predecir y analizar con éxito la secuencia de señales de control. Este modelo se ha validado satisfactoriamente con datos experimentales. Finalmente, las estrategias de control anteriores se han extendido a otros dispositivos susceptibles de sufrir efectos de carga atrapada que pueden afectar su fiabilidad. Así, se han llevado a cabo experimentos preliminares cuyos resultados demuestran que la aplicación de controles de carga permite controlar y estabilizar la característica C-V de un condensador eMIM y la característica corriente-tensión (J-V) de una célula solar basada en perovskitas.Postprint (published version

Sigma-Delta control of charge trapping in heterogeneous devices

Dielectric charging represents a major reliability issue in a variety of semiconductor devices. The accumulation of charge in dielectric layers of a device often alters its performance, affecting its circuital features and even reducing its effective lifetime. Although several contributions have been made in order to mitigate the undesired effects of charge trapping on circuit performance, dielectric charge trapping still remains an open reliability issue in several applications. The research work underlying this Thesis mainly focuses on the design, analysis and experimental validation of control strategies to compensate dielectric charging in heterogeneous devices. These control methods are based on the application of specifically designed voltage waveforms that produce complementary effects on the charge dynamics. Using sigma-delta loops, these controls allow to set and maintain, within some limits, the net trapped charge in the dielectric to desired levels that can be changed with time. This allows mitigating long-term reliability issues such as capacitance-voltage (C-V) shifts in MOS and MIM capacitors. Additionally, the bit streams generated by the control loops provide real-time information on the evolution of the trapped charge. The proposed controls also allow compensating the effects of the charge trapping due to external disturbances such as radiation. This has been demonstrated experimentally with MOS capacitors subjected to various types of ionizing radiation (X-rays and gamma rays) while a charge control is being applied. This approach opens up the possibility of establishing techniques for active compensation of radiation-induced charge in MOS structures as well as a new strategy for radiation sensing. A modeling strategy to characterize the dynamics of the dielectric charge in MOS capacitors is also presented. The diffusive nature of the charge trapping phenomena allows their behavioral characterization using Diffusive Representation tools. The experiments carried out demonstrate a very good matching between the predictions of the model and the experimental results obtained. The time variations in the charge dynamics due to changes in the volatges applied and/or due to external disturbances have been also investigated and modeled. Moreover, the charge dynamics of MOS capacitors under sigma-delta control is analyzed using the tools of Sliding Mode Controllers for an infinite sampling frequency approximation. A phenomenological analytical model is obtained which allows to predict and analyze the sequence of control signals. This model has been successfully validated with experimental data. Finally, the above control strategies are extended to other devices such as eMIM capacitors and perovskite solar cells. Preliminary results including open loop and closed loop control experiments are presented. These results demonstrate that the application of the controls allows to set and stabilize both the C-V characteristic of an eMIM capacitor and the current-voltage characteristic (J-V) of a perovskite solar cell.La carga atrapada en dieléctricos suele implicar un problema importante de fiabilidad en muchos dispositivos semiconductores. La acumulación de dicha carga, normalmente provocada por las tensiones aplicadas durante el uso del dispositivo, suele alterar el rendimiento de éste con el tiempo, afectar sus prestaciones a nivel de circuital e, incluso, reducir su vida útil. Aunque durante años se han realizado muchos trabajos para mitigar sus efectos no deseados, sobre todo a nivel circuital, la carga atrapada en dieléctricos sigue siendo un problema abierto que frena la aplicabilidad práctica de algunos dispositivos. El trabajo de investigación realizado en esta Tesis se centra principalmente en el diseño, análisis y validación experimental de estrategias de control para compensar la carga atrapada en dieléctricos de diversos tipos de dispositivos, incluyendo condensadores MOS, condensadores MIM fabricados con nanotecnología y dispositivos basados en perovskitas. Los controles propuestos se basan en utilizar formas de onda de tensión, específicamente diseñadas, que producen efectos complementarios en la dinámica de la carga. Mediante el uso de lazos sigma-delta, estos controles permiten establecer y mantener, dentro de unos límites, la carga neta atrapada en el dieléctrico a valores prefijados, que pueden cambiarse con el tiempo. Esto permite mitigar problemas de fiabilidad a largo plazo como por ejemplo las derivas de la curva capacidad-tensión (C-V) en condensadores MOS y MIM. Adicionalmente, las tramas de bits generadas por los lazos de control proporcionan información en tiempo real sobre la evolución de la carga. Los controles propuestos permiten también compensar los efectos de la carga atrapada en dieléctricos debida a perturbaciones externas como la radiación. Esto se ha demostrado experimentalmente con condesadores MOS sometidos a diversos tipos de radiación ionizante (rayos X y gamma) mientras se les aplicaba un control de carga. Este resultado abre la posibilidad tanto de establecer técnicas de compensación activa de carga inducida por radiación en estructuras MOS, como una nueva estrategia de sensado de radiación. Se presenta también una estrategia de modelado para caracterizar la dinámica de la carga dieléctrica en condensadores MOS. La naturaleza difusiva de los fenómenos de captura y eliminación de carga en dieléctricos permite caracterizar dichos fenómenos empleando herramientas de Representación Difusiva. Los experimentos realizados demuestran una muy buena correspondencia entre las predicciones del modelo y los resultados experimentales obtenidos. Se muestra también como las variaciones temporales de los modelos son debidas a cambios en las formas de onda de actuación del dispositivo y/o a perturbaciones externas. Además, la dinámica de carga en condensadores MOS bajo control sigma-delta se analiza utilizando herramientas de control en modo deslizante (SMC), considerando la aproximación de frecuencia de muestreo infinita. Con ello se obtiene un modelo analítico simplificado que permite predecir y analizar con éxito la secuencia de señales de control. Este modelo se ha validado satisfactoriamente con datos experimentales. Finalmente, las estrategias de control anteriores se han extendido a otros dispositivos susceptibles de sufrir efectos de carga atrapada que pueden afectar su fiabilidad. Así, se han llevado a cabo experimentos preliminares cuyos resultados demuestran que la aplicación de controles de carga permite controlar y estabilizar la característica C-V de un condensador eMIM y la característica corriente-tensión (J-V) de una célula solar basada en perovskitas

Mathematical model for a radioactive marker in silicide formation

A mathematical model is constructed to interpret the profiles of radioactive (^31)Si tracers in a computer simulation proposed by R. Pretorius and A. P. Botha [Thin Solid Films 91, 99 (1982)]. This model assumes that only Si moves in the silicide, that the Si moves interstitially and convectively, and that the moving Si can exchange sites with the stationary Si in the silicide lattice. An analytical solution of this model is given and confirms the published computer simulation data. However, it is shown that the model is physically inadequate. Solutions of another model which assumes that metal, instead of Si, is the moving species for silicide formation (either interstitially, or substitutionally, or both), with self-diffusion of (^31)Si in the silicide during silicide formation. Almost all the experimental data can be fitted by solutions of both models. These examples demonstrate that radioactive tracer experiments alone are insufficient to determine the moving species when a solid binary compound film forms by reaction of adjacent elemental layers. Both inert marker and tracer data are needed to identify the moving species and the mechanisms