nextnano(3) is a simulation tool that aims at providing global insight into the basic physical properties of realistic three-dimensional mesoscopic semiconductor structures. It focuses on quantum mechanical properties such as the global electronic structure, optical properties, and the effects of electric and magnetic fields for virtually any geometry and combination of semiconducting materials. For the calculation of the carrier dynamics a drift-diffusion model based on a quantum-mechanically calculated density is employed. In this paper we present an overview of the capabilities of nextnano3 and discuss some of the main equations that are implemented into the code. As examples, we first discuss the strain tensor components and the piezoelectric effect associated with a compressively strained InAs layer for different growth directions, secondly, we calculate self-consistently the quantum mechanical electron density of a Double Gate MOSFET, then we compare the intersubband transitions in a multi-quantum well structure that have been obtained with a single-band effective mass approach and with an 8-band k center dot p model, and finally, we calculate the energy spectrum of a structure in a uniform magnetic field.

A key property of the nitrides is the fact that they possess large spontaneous and piezoelectric polarization fields that allow a significant tailoring of the carrier dynamics and optical properties of nitride devices. In this paper, we review our recent studies of the nitride electronic devices. To assess the potential of this novel material class for modern device applications, we have performed multi-scale calculations that include ab initio density functional calculations, self-consistent multi-band k·p method, and ensamble Monte Carlo simulations. We find that the polarization induced charges at the interfaces produce two-dimensional electron and hole gases of high density up to 1013 cm−2. Our studies also reveal that the nitride based electronic devices have characteristics that predispose them for high power and high frequency applications. We demonstrate also that transistor characteristics are favorably influenced by the internal polarization induced electric fields.

In this work we investigate experimentally and theoretically the feasibility to enhance the nonradiative lifetime of the upper heavy hole state in Si/SiGe quantum cascade (QC) structures within a diagonal transition design. The problem of fast nonradiative optical phonon scattering makes it impossible to achieve population inversion with recently demonstrated Si/SiGe QC mid-infrared emitter structures, which use vertical transitions between the first two heavy hole states in a single quantum well. We will show that it is possible to prolong the upper state lifetime by more than an order of magnitude in a diagonal transition active region design, that uses transitions between heavy hole ground states of spatially separated quantum wells. Our experimental findings derived from electroluminescence measurements are in good agreement with calculated values based on a 6-band k·p model.

We present charge self-consistent 8-band k (.) p calculations of the quantum confined Stark shift of excitons for self-assembled buried InGaAs quantum dots. Many different shapes and alloy profiles of the dots have been systematically studied. We predict the Stark shift of excitons for simultaneously applied vertical and lateral electric fields, taking into account realistic alloy profiles in the quantum dot. The study of the Stark shift of neutral excitons and in both vertical and lateral electric field directions provides a wealth of information about the coupling of the exciton to external perturbations. We show that the combination of lateral and vertical electric fields provides a particularly sensitive probe of the dot shape and alloy composition.

Here, we explore experimentally and theoretically the possibility to prolong the upper hole state nonradiative lifetime of Si/SiGe quantum cascade (QC) structures by using a spatially indirect diagonal transition between two SiGe quantum well ground states. With the recent observation of well resolved midinfrared electroluminescence from heavy hole intersubband transitions in Si/SiGe valence-band QC structures, a Si-based QC laser seems no longer to be out of reach. A long carrier lifetime and maybe population inversion, however, appear to be impossible for structure designs with a vertical intersubband transition studied so far. This is due to the nonresonant behavior of deformation potential scattering dominant in unipolar SiGe. We report on calculations of the band structure using a six-band k⋅p model and of hole deformation potential scattering that predict significantly increased nonradiative lifetimes for large barrier thickness, reaching about 20 ps for 35 Å Si barrier layer width. Electroluminesence measurements of a series of QC structures with varied barrier width reveal comparable efficiencies and the deduced lifetimes confirm our model calculations.

Unipolar intersubband lasers like quantum cascade laser structures might be realized not only in III–V semiconductors but also in Si/SiGe multiple layer structures since no optical transitions across the indirect band gap are involved. We report on well-defined intersubband electroluminescence emission of Si/SiGe quantum cascade structures with different active quantum wells parameters. The complex valence band structure and a nonradiative relaxation rate of about 400 fs were calculated by multiband k⋅p formalism including Si/Ge segregation effects. The observed spectral shift of the electroluminescence peak from 146 to 159 meV is described well by quantum confinement of the two lowest heavy hole subbands. The electroluminescence observed reveals transverse magnetic polarization, a spectral line shape that changes with the direction of the current, and low-energy line broadening with an increase in temperature and current. All these features are described well by the k⋅p model calculation.

We present a simulator for calculating, in a consistent manner, the realistic electronic structure of three-dimensional heterostructure quantum devices under bias and its current density close to equilibrium. The electronic structure is calculated fully quantum mechanically, whereas the current is determined by employing a semiclassical concept of local Fermi levels that are calculated self-consistently. We discuss the numerical techniques employed and present illustrative examples that are compared with quantum transport calculations. In addition, the simulator has been used successfully to study shape-dependent charge localization effects in self-assembled GaAs/InGaAs quantum dots.

A method is developed for calculating, in a consistent manner, the realistic electronic structure of three-dimensional (3-D) heterostructure quantum devices under bias and its current density close to equilibrium. The nonequilibrium electronic structure is characterized by local Fermi levels that are calculated self-consistently. We have applied this scheme to predict asymmetric Stark shifts and tunneling of confined electrons and holes in single-dot GaAs/InGaAs photodiodes.

The generation of high density 2D hole gases is crucial for further progress in the field of electronic and optoelectronic nitride devices. In this paper, we present results of C-V profiling measurements for N-face GaN/AlGaN heterostructures and systematic theoretical studies of Mg- doped GaN/AlGaN gated heterostructures and superlattices. Our calculations are based on a self- consistent solution of the multiband k.p Schrödinger and Poisson equation and reveal that the hole 2D sheet density is mainly determined by the polarization induced interface charges. For an Aluminium concentration of 30%, the induced hole density in the heterostructure can reach values up to 1.5×1013 cm−2. In the GaN/AlGaN superlattices, the hole sheet density increases with the superlattice period and saturates for a period of 40 nm at a value of 1.5×1013 cm−2.

Two dimensional hole and electron gases in wurtzite GaN/AlxGa1-xN/GaN heterostructures are induced by strong polarization induced effects. The sheet carrier concentration and the confinement of the two dimensional carrier gases located close to one of the AlGaN/GaN interfaces are sensitive to a high number of different physical properties such as polarity, alloy composition, strain, thickness and doping. We have investigated the structural quality, the carrier concentration profiles and electrical transport properties by a combination of high resolution x- ray diffraction, Hall effect and C-V profiling measurements. The investigated heterostructures with N- and Ga-face polarity were grown by metalorganic vapor phase or plasma induced molecular beam epitaxy covering a broad range of alloy compositions and barrier thickness. By comparison of theoretical and experimental results we demonstrate that the formation of two dimensional hole and electron gases in GaN/AlGaN/GaN heterostructures both rely on the difference of the polarization between the AlGaN and the GaN layer. In addition the role of polarity on the carrier accumulation at different interfaces in n- and p-doped heterostructures will be discussed in detail