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Quantum cascade lasers are unipolar semiconductor lasers covering a wide range of the infrared and terahertz spectrum. Lasing action is achieved by using optical intersubband transitions between quantized states in specifically designed multiple-quantum-well heterostructures. A systematic improvement of quantum cascade lasers with respect to operating temperature, efficiency, and spectral range requires detailed modeling of the underlying physical processes in these structures. Moreover, the quantum cascade laser constitutes a versatile model device for the development and improvement of simulation techniques in nano- and optoelectronics. This review provides a comprehensive survey and discussion of the modeling techniques used for the simulation of quantum cascade lasers. The main focus is on the modeling of carrier transport in the nanostructured gain medium, while the simulation of the optical cavity is covered at a more basic level. Specifically, the transfer matrix and finite difference methods for solving the one-dimensional Schrodinger equation and Schrodinger-Poisson system are discussed, providing the quantized states in the multiple-quantum-well active region. The modeling of the optical cavity is covered with a focus on basic waveguide resonator structures. Furthermore, various carrier transport simulation methods are discussed, ranging from basic empirical approaches to advanced self-consistent techniques. The methods include empirical rate equation and related Maxwell-Bloch equation approaches, self-consistent rate equation and ensemble Monte Carlo methods, as well as quantum transport approaches, in particular the density matrix and non-equilibrium Green's function formalism. The derived scattering rates and self-energies are generally valid for n-type devices based on one-dimensional quantum confinement, such as quantum well structures. (C) 2014 AIP Publishing LLC.

As semiconductor devices scale to new dimensions, the materials and designs become more dependent on atomic details. NEMO5 is a nanoelectronics modeling package designed for comprehending the critical multi-scale, multi-physics phenomena through efficient computational approaches and quantitatively modeling new generations of nanoelectronic devices as well as predicting novel device architectures and phenomena. This article seeks to provide updates on the current status of the tool and new functionality, including advances in quantum transport simulations and with materials such as metals, topological insulators, and piezoelectrics.

The Empirical Tight Binding (ETB) method is widely used in atomistic device simulations. The reliability of such simulations depends very strongly on the choice of basis sets and the ETB parameters. The traditional way of obtaining the ETB parameters is by fitting to experiment data, or critical theoretical bandedges and symmetries rather than a foundational mapping. A further shortcoming of traditional ETB is the lack of an explicit basis. In this work, a DFT mapping process which constructs TB parameters and explicit basis from DFT calculations is developed. The method is applied to two materials: GaAs and MgO. Compared with the existing TB parameters, the GaAs parameters by DFT mapping show better agreement with the DFT results in bulk band structure calculations and lead to different indirect valleys when applied to nanowire calculations. The MgO TB parameters and TB basis functions are also obtained through the DFT mapping process.

Transistor designs based on using mixed Gamma-L valleys for electron transport are proposed to overcome the density of states bottleneck while maintaining high injection velocities. Using a self-consistent top-of-the-barrier transport model, improved current density over Si is demonstrated in GaAs/AlAsSb, GaSb/AlAsSb, and Ge-on-insulator-based single-gate thin-body n-channel metal-oxide-semiconductor field-effect transistors. All the proposed designs successively begin to outperform strained-Si-on-insulator and InAs-on-insulator (InAs-OI) in terms of ON-state currents as the effective oxide thickness is reduced below 0.7 nm. InAs-OI still exhibits the lowest intrinsic delay (tau) due to its single Gamma valley.

A characteristic feature of quantum cascade lasers is their unipolar carrier transport. We exploit this feature and realize nominally symmetric active regions for terahertz quantum cascade lasers, which should yield equal performance with either bias polarity. However, symmetric devices exhibit a strongly bias polarity dependent performance due to growth direction asymmetries, making them an ideal tool to study the related scattering mechanisms. In the case of an InGaAs/GaAsSb heterostructure, the pronounced interface asymmetry leads to a significantly better performance with negative bias polarity and can even lead to unidirectionally working devices, although the nominal band structure is symmetric. The results are a direct experimental proof that interface roughness scattering has a major impact on transport/lasing performance. (C) 2013 Optical Society of America

In this work, the low rank approximation concept is extended to the non-equilibrium Green’s function (NEGF) method to achieve a very efficient approximated algorithm for coherent and incoherent electron transport. This new method is applied to inelastic transport in various semiconductor nanodevices. Detailed benchmarks with exact NEGF solutions show (1) a very good agreement between approximated and exact NEGF results, (2) a significant reduction of the required memory, and (3) a large reduction of the computational time (a factor of speed up as high as 150 times is observed). A non-recursive solution of the inelastic NEGF transport equations of a 1000 nm long resistor on standard hardware illustrates nicely the capability of this new method.

The nonequilibrium Green's function approach is applied to the design of three-well indirect pumping terahertz (THz) quantum cascade lasers (QCLs) based on a resonant phonon depopulation scheme. The effects of the anticrossing of the injector states and the dipole matrix element of the laser levels on the optical gain of THz QCLs are studied. The results show that a design that results in a more pronounced anticrossing of the injector states will achieve a higher optical gain in the indirect pumping scheme compared to the traditional resonant-tunneling injection scheme. This offers in general a more efficient coherent resonant-tunneling transport of electrons in the indirect pumping scheme. It is also shown that, for operating temperatures below 200 K and low lasing frequencies, larger dipole matrix elements, i.e., vertical optical transitions, offer a higher optical gain. In contrast, in the case of high lasing frequencies, smaller dipole matrix elements, i.e., diagonal optical transitions are better for achieving a higher optical gain. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.3697674]

Modeling and simulation take an important role in the exploration and design optimization of novel devices. As the downscaling of electronic devices continues, the description of interfaces, randomness, and disorder on an atomistic level gains importance and continuum descriptions lose their validity. Often a full-band description of the electronic structure is needed to model the interaction of different valleys and nonparabolicity effects. NEMO 5 [1] is a modeling tool that addresses these issues and is able to provide insight into a broad range of devices. It unifies the capabilities of prior projects: multiscale approaches to quantum transport in planar structures in NEMO-IO [2], multimillion-atom simulations of strain and electronic structure in NEMO-30 [3] and NEMO-30-Peta [4], and quantum transport in nonplanar structures in OMEN [5]. NEMO 5 aims at becoming a community code whose structure, implementation, resource requirements and license allow experimental and theoretical researchers in academia and industry alike to use and extend the tool.

An enhanced valence force field model for zinc-blende crystals is developed to provide a unified description of the isothermal static and dynamical lattice properties of gallium arsenide. The expression for the lattice energy includes a second-nearest-neighbor coplanar interaction term, the Coulomb interaction between partially charged ions, and anharmonicity corrections. General relations are derived between the microscopic force constants and the macroscopic elastic constants in zinc-blende crystals. Applying the model to gallium arsenide, parameter sets are presented that yield quantitative agreement with experimental results for the phonon dispersion, elastic constants, sound velocities, and Gru ̈neisen mode parameters.

Novel device concepts and better channel materials than Si are required to improve the performance of conventional metal-oxide-semiconductor field-effect transistors (MOSFETs). The exploration of III-V semiconductors is mainly driven by the extremely high electron mobility of the materials. Recently, several researches have demonstrated that III-V high electron mobility transistors (HEMTs) can achieve high-speed operation at low supply voltage for applications beyond Si-CMOS technology. While the intrinsic device performance looks promising, current prototypes are dramatically influenced by high contact resistances. From a modeling point of view the understanding of the intrinsic device performance is now quite advanced, while the understanding of the contacts remains quite limited. Hence, a precise theoretical approach is required to model the contact characteristics. This work investigates the contact resistance physics of InAs HEMT transistors. The Nano-Electronic Modeling Tool (NEMO5) is used to solve the non-equilibrium Green’s function (NEGF) formalism which embeds Schrödinger and Poisson equations self-consistently. For this study a real-space effective mass approximation with a simple phonon scattering is utilized.

The development of a new nanoelectronics modeling tool, NEMO5, is reported. The tool computes strain, phonon spectra, electronic band structure, charge density, charge current, and other properties of nanoelectronic devices. The modular layout enables a mix and match of physical models with different length scales and varying numerical complexity. NEMO5 features multilevel parallelization and is based on open-source packages. Its versatility is demonstrated with selected application examples: a multimillion-atom strain calculation, bulk electron and phonon band structures, a 1-D Schrodinger-Poisson simulation, a multiphysics simulation of a resonant tunneling diode, and quantum transport through a nanowire transistor.

The impact of interface roughness scattering on the device performance of state-of-the-art THz quantum cascade lasers is theoretically analyzed using the non-equilibrium Green's function formalism. It is shown that for a particular direct transition QCL design style rough interfaces hardly change the electronic energy spectrum, but dramatically reduce the occupation inversion and the opltical gain. A spatial separation leading to a diagonal transition laser, leads in contrast to a limited sensitivity to interface roughness. Diagonal transition QCL designs result in a better device performance and an improved reliability with respect to interface roughness induced by growth quality variations.

A new adaptive quadrature algorithm that places a greater emphasis on cost reduction while still maintaining an acceptable accuracy is demonstrated. The different needs of science and engineering applications are highlighted as the existing algorithms are shown to be inadequate. The performance of the new algorithm is compared with the well known adaptive Simpson, Gauss-Lobatto and Gauss- Kronrod methods. Finally, scenarios where the proposed al- gorithm outperforms the existing ones are discussed.