Semiconductor quantum devices

The operation of devices with small enough dimensions for electrons to exhibit wavelike behavior is explained. Two types of device are examined: vertical quantum devices, which include the resonant tunneling structure and the single-electron transistor, and lateral, which include the quantum interference transistor and the spin precession device. The advantages and drawbacks of the two types are identified     

Classical and quantum mechanical transport simulations in open quantum dots

Simulations of open dot arrays are performed where the electron motion is considered in a harmonic potential and in a static magnetic field. Results concern electron trajectories and Poincaré sections from the classical calculation. The analysis of the Poincaré sections reveals a strong influence of the length of the constriction on the chaotic regions in phase space. Density probabilities from the quantum mechanical calculation show a significant correspondence to classically calculated trajectories.     

Application of the R-matrix method in quantum transport simulations

The paper gives an overview of recently developed method for effective quantum transport simulations in nanoscale electronic devices. In the present formulation the device is treated as a set of independent close subsystems with appropriate low-dimensional basis representations. In continuous transport models, the local R-matrix basis makes it possible to avoid discretization of the device area and achieve a much higher numerical accuracy with a lower computational burden compared to common grid schemes. Furthermore, the local basis representation provides a suitable framework for studying ionized impurity scattering by adjusting the shape of the device elements and their internal coordinate representation. Non-equilibrium current carrying electronic states are constructed by a recursive propagation scheme such that the major portion of the computation time scales linearly with the device volume. As an illustration, we apply the method to study ionized impurity scattering in a short Si channel.     

Application of the R-matrix method in quantum transport simulations

The R-matrix method based on a continuous model is generalized as to become applicable to the atomistic transport simulations with a tight-binding device Hamiltonian. The device elements are introduced as arbitrary atomic clusters in the device area and freedom in the device fragmentation can be used to reduce computations. In the ballistic regime, the best computer performance is achieved by taking individual atoms as the device elements. Non-equilibrium current carrying electronic states are constructed by the forward-backward R-matrix propagation scheme which does not require large computer operations and mass storage. The method is applied to quantum transport in p-Si nanowire with random vacancies and surface roughness.     

The Rashba effect and non-Abelian phases in quantum wire devices

We present a 2D method for studying quantum transport through arbitrary geometries under the influence of both external magnetic fields and Rashba spin-orbit coupling. We apply this method to two geometries—a straight wire and a ring structure. Our results reveal the precession of spin-polarized modes in the straight wire, and conductance resonances in the ring due to phase shifts from the magnetic field and from the Rashba effect. The conductance resonances arising from the Rashba effect are due to the non-Abelian nature of the phase shifts acquired by electrons propagating around the ring.     

Monte Carlo simulations of nanometric devices beyond the “mean-field” approximation

For nanoscale electron devices, the role of a single-electron (or a single-impurity) can have a large impact on their electrical characteristics. A new method for introducing the long-range and short-range Coulomb interaction in semiconductor semi-classical Monte Carlo simulations is presented. The method is based on directly dealing with a many-particle system by solving a different Poisson equation for each electron. The present work shows the numerical viability of this alternative approach for nanoscale devices with few (<100) electrons. The method is compared with the traditional “mean-field” Monte Carlo simulations. It is shown, numerically, that the “mean-field” approximation produces important errors for aggressively-scaled devices.     

Self-consistent time-dependent boundary conditions for static and dynamic simulations of small electron devices

A quasi-analytical self-consistent and time-dependent boundary conditions algorithm for micro- and nanoscale electron transport simulators is presented and discussed. The algorithm is the result of imposing overall charge neutrality and current conservation along the reservoirs, the leads and the active region. Only an explicit numerical simulation of the active region is required. By means of analytical solutions of the spatial distribution for the charge density, electric field and potential energy along the leads, the algorithm is able to self-consistently translate standard “metallic” boundary conditions in the reservoirs into intricate constraints at the borders of the active region. The algorithm is robust, requires small computational effort and it is suitable for any (classical or quantum) electronic device simulator for stationary (DC) and dynamic (transients, noise and AC) regimes up to several THz. The algorithm is specially welcomed for dynamical regime simulations, where the predictions of the time-evolution of the electrostatic potential, electric field and charge density at the borders of the active region are rather complicated.     

Component mode synthesis approaches for quantum mechanical electrostatic analysis of nanoscale devices

In this paper, two component mode synthesis (CMS) approaches, namely, the fixed interface CMS approach and the free interface CMS approach, are presented and compared for an efficient solution of 2-D Schr?dinger-Poisson equations for quantum-mechanical electrostatic analyses of nanostructures and devices with arbitrary geometries. In the CMS approaches, a nanostructure is divided into a set of substructures or components and the eigenvalues (energy levels) and eigenvectors (wave functions) are computed first for all the substructures. The computed wave functions are then combined with constraint or attachment modes to construct a transformation matrix. By using the transformation matrix, a reduced-order system of the Schr?dinger equation is obtained for the entire nanostructure. The global energy levels and wave functions can be obtained with the reduced-order system. Through an iteration procedure between the Schr?dinger and Poisson equations, a self-consistent solution for charge concentration and potential profile can be obtained. Numerical calculations show that both CMS approaches can largely reduce the computational cost. The free interface CMS approach can provide significantly more accurate results than the fixed interface CMS approach with the same number of retained wave functions in each component. However, the fixed interface CMS approach is more efficient than the free interface CMS approach when large degrees of freedom are included in the simulation.     

The importance of thermal conductivity modeling for simulations of self-heating effects in FD SOI devices

In this review paper we want to emphasize the importance of having accurate thermal conductivity models for modeling self-heating effects on the device level. For that purpose, we first consider thin silicon films and calculate (using Sondheimer’s approach) their thermal conductivity that incorporates boundary scattering. We then compare the obtained thermal conductivity data with experimental measurements to prove the excellent model agreement with the experimental trends. The parameterized thermal conductivity data are then used in the higher level modeling of self-heating effects in fully-depleted (FD) SOI devices from different technology generations. We find that temperature and thickness dependent modeling of the thermal conductivity is essential for the 25 nanometers technology node. We have also taken into account the anisotropy of the thermal conductivity and modeled devices with (100) and (110) crystallographic orientation. We found out that from thermal point of view the (110) device behaves better, but the (100) device has higher on-current.     

Scattering and space-charge effects in Wigner Monte Carlo simulations of single and double barrier devices

Transport in single and double barrier devices is studied using a Monte Carlo solver for the Wigner transport equation. This approach allows the effects of tunneling and scattering to be included. Several numerical methods have been improved to render the Wigner Monte Carlo technique more robust, including a newly developed particle annihilation algorithm. A self-consistent iteration scheme with the Poisson equation was introduced. The role of scattering and space charge effects on the electrical characteristics of n-i-n nanostructures, ultra-scaled double gate MOSFETs, and GaAs resonant tunneling diodes is demonstrated. An erratum to this article can be found at     

3D Monte-Carlo device simulations using an effective quantum potential including electron-electron interactions

Effective quantum potentials describe the physics of quantum-mechanical electron transport in semiconductors more than the classical Coulomb potential. An effective quantum potential was derived previously for the interaction of an electron with a barrier for use in particle-based Monte Carlo semiconductor device simulators. The method is based on a perturbation theory around thermodynamic equilibrium and leads to an effective potential scheme in which the size of the electron depends upon its energy and which is parameter-free. Here we extend the method to electron-electron interactions and show how the effective quantum potential can be evaluated efficiently in the context of many-body problems. The effective quantum potential was used in a three-dimensional Monte-Carlo device simulator for calculating the electron-electron and electron-barrier interactions. Simulation results for an SOI transistor are presented and illustrate how the effective quantum potential changes the characteristics compared to the classical potential.     

3D Monte Carlo simulation of FinFET and FDSOI devices with accurate quantum correction

The performance of FinFET and FDSOI devices is compared by 3D Monte Carlo simulation using an enhanced quantum correction scheme. This scheme has two new features: (i) the quantum correction is extracted from a 2D cross-section of the 3D device and (ii) in addition to using a modified oxide permittivity and a modified work function in subthreshold, the work function is ramped above threshold to a different value in the on-state. This approach improves the accuracy of the quantum-correction for multi-gate devices and is shown to accurately reproduce 3D density-gradient simulation also at short channel lengths. 15 nm FDSOI device performance with thin box and back-gate bias is found to be competitive: compared to a FinFET with (110)/〈110〉 sidewall/channel orientation, the on-current for N-type devices is 25 % higher and the off-current is only increased by a factor of 2.5.     

A numerical method for a transient quantum drift-diffusion model arising in semiconductor devices

This paper describes a numerical method for a transient quantum drift-diffusion model arising in semiconductor devices. The discretization method is presented with emphasis on adaptive time discretization. An adaptive time step algorithm is constructed by introducing the derivative of the free energy of the system, which has an essential property to understand the carrier behavior of the time-dependent problems. The algorithm is verified with switching characteristics of one-dimensional n⁺–n–n⁺ silicon diodes. It is shown that the time step is adapted to the switching characteristics. The new algorithm significantly reduces the total number of time steps.     

An effective quantum potential for particle–particle interactions in three-dimensional semiconductor device simulations

The classical Coulomb potential and force can be calculated efficiently using fast multi-pole methods. Effective quantum potentials, however, describe the physics of electron transport in semiconductors more precisely. Such an effective quantum potential was derived previously for the interaction of an electron with a barrier for use in particle-based Monte Carlo semiconductor device simulators. The method is based on a perturbation theory around thermodynamic equilibrium and leads to an effective potential scheme in which the size of the electron depends upon its energy and which is parameter-free. Here we extend the method to electron-electron interactions and show how the effective quantum potential can be evaluated efficiently in the context of many-body problems. Finally several examples illustrate how the momentum of the electrons changes the classical potential.     

Towards realistic modeling of plasmonic nanostructures: a comparative study to determine the impact of optical effects on solar cell improvement

Journal of Computational Electronics - Plasmonic structures may improve cell performance in a variety of ways. More accurate determining of the optical influence, unlike ideal simulations, requires...     

Density-gradient theory: a macroscopic approach to quantum confinement and tunneling in semiconductor devices

Density-gradient theory provides a macroscopic approach to modeling quantum transport that is particularly well adapted to semiconductor device analysis and engineering. After some introductory observations, the basis of the theory in macroscopic and microscopic physics is summarized, and its scattering-dominated and scattering-free versions are introduced. Remarks are also given about the underlying mathematics and numerics. A variety of applications of the theory to both quantum confinement and quantum tunneling situations are then reviewed. In doing so, particular emphasis is put on understanding the range of validity of the theory and on its unexpected power as a phenomenology. The article closes with a few comments about the future.     

Electronic and excitonic processes in multilayer organic light emitting devices incorporating PbSe quantum dots

In this work we investigate the operation mechanism of hybrid organic/inorganic quantum dot light emitting devices (QD-LEDs). We employ a numerical method previously established to describe current-voltage characteristics, spatial distributions of charge, electric field, and recombination rate in organic light emitting devices (OLEDs). The numerical solution of the continuity and Poisson equations have been extended to treat internal organic/Quantum Dot (QD) interfaces, recombination processes in the polymer matrix and in the QDs, and the charge acquired by the QDs. The contact boundary condition is taken to be Schottky contact boundary condition. Also, we consider the exciton formation and diffusion processes. The simulation results trend and experimental data are in good agreement.     

Effects of Mg dopant on the degradation of InGaN multiple quantum wells in AlInGaN-based light emitting devices

We investigated the effects of Mg dopant on the degradation of AlInGaN-based light emitting diodes (LEDs) and laser diodes (LDs) with InGaN multi-quantum wells (MQWs). Photoluminescence (PL) intensity of InGaN MQWs was significantly decreased with increasing the Mg intentional doping process in InGaN active region, indicating that Mg dopant could degrade the optical quality of InGaN MQWs. From secondary ion mass spectroscopy (SIMS) analysis of AlInGaN-based LDs grown on GaN/sapphire and GaN substrate with different dislocation densities, we found that Mg concentration of LD on GaN/sapphire was higher than that of LD on GaN substrate at the InGaN MQWs regions. Additionally, we observed that Mg atoms were significantly diffused from p-type layer to InGaN MQWs region in the LD structure after aging evaluation. From these results, we could conclude that Mg diffusion along threading dislocations is one of the major gradual degradation mechanisms of AlInGaN-based LD/LEDs during the device operation under high voltage condition.     

Reliability of asic devices

Abstract

The use of application-specific integrated circuits (ASICs) has become widespread; from use in sophisticated telecommunications equipment through to children's toys. However, aspects of design, fabrication and testing may not be as rigorous as for the standard components that they are replacing.

ASICs by their nature are usually low volume components and tend to be fabricated using the latest technology. However, the additional substantial costs involved with comprehensive reliability and quality assessment mean that testing is often less exhaustive, resulting in a product of uncertain reliability. Conventional wisdom states that fewer components will mean greater reliability; however if the devices are replaced by one of uncertain reliability, this will not necessarily be the case.

This paper presents some considerations that should to be taken into account when contemplating the use of ASICs in place of conventional components. These are highlighted with specific examples seen at Telecom Australia Research Laboratories (TRL).