Search

Atomistic quantum mechanical tight-binding (TB) methods that include the full band structure is used to study electron tunneling through three-dimensional models of n(super +)-Si / SiO2 / p-Si capacitors with thicknesses between 0.7 and 4.4 nm. It is found that the microscopic oxide structure influences transmission coefficients and tunnel currents significantly.

We propose a design for a semiconductor structure emitting broadband light in the infrared, based on InAs quantum dots (QDs) embedded into a metamorphic step-graded InxGa1-xAs buffer. We developed a model to calculate the metamorphic QD energy levels based on the realistic QD parameters and on the strain-dependent material properties; we validated the results of simulations by comparison with the experimental values. On this basis, we designed a p-i-n heterostructure with a graded index profile toward the realization of an electrically pumped guided wave device. This has been done by adding layers where QDs are embedded in InxAlyGa1-x-yAs layers, to obtain a symmetric structure from a band profile point of view. To assess the room temperature electroluminescence emission spectrum under realistic electrical injection conditions, we performed device-level simulations based on a coupled drift-diffusion and QD rate equation model. On the basis of the device simulation results, we conclude that the present proposal is a viable option to realize broadband light-emitting devices. [ABSTRACT FROM AUTHOR]

Based on the results of three-dimensional atomistic tight-binding calculations, we argue that the effective tunnel mass of SiO[sub 2] employed as a fitting parameter in standard transfer-matrix multiple-scattering theory calculations increases strongly as the oxide thickness is decreased (we find more than 50% mass enhancement upon reduction of the oxide thickness from 4 to 1 nm). At least five factors, usually neglected in effective-mass-based calculations can contribute to this effect: the nonparabolicity of the complex bands in the gap of SiO[sub 2], the gradual (rather than abrupt) change of the electrostatic potential across the Si/SiO[sub 2] interface, a possible image force correction, the presence of naive defects in the oxide, and the effective-mass approximation itself. Very good quantitative agreement between the theoretical predictions for the thickness dependence of the mass enhancement and corresponding results from transfer matrix fits to experimental currents is obtained if defect densities smaller than 10[sup 10] cm[sup -2] and a small image force correction are assumed. Since the present findings imply significant errors (1-2 orders of magnitude) in tunnel currents through ultrathin oxides calculated with a single thickness-independent tunnel mass, an explicit parametrization of the thickness dependence for use in multiple-scattering calculations is suggested. For 4 nm thin oxides, we obtain a tunnel mass of 0.35 m[sub 0] (0.48 m[sup 0]), if a parabolic (nonparabolic) dispersion of the complex bands in the band gap of the oxide is adopted. Furthermore, the mass at the conduction band bottom of SiO[sub 2] is found to be different from the tunnel mass and estimated to be 0.39 m[sub 0], in good agreement with previous measurements. The calculations also yield an estimate of the errors in oxide thicknesses obtained from current-voltage fitting, which are found to agree well with available experimental data. [ABSTRACT FROM AUTHOR]

In this work, a random distribution of Indium in a quantum well has been considered to study the effect on the energy gap of a GaN/InGaN/GaN LED device. Monte Carlo sampling technique has been used to generate hundreds of atomistic model structures of the device active region. In order to calculate pseudomorphic strain and internal deformations of the alloy, a multiscale method combining continuous media elasticity and atomistic valence force field models has been used. A multiphysic quantum/classical simulation coupling driftdiffusion with empirical tight-binding in order to compute the electron and hole states of the system has been performed. The reliable sp3d5s* parametrization has been used in the calculations of the eigenstates. We have found an energy gap difference of 50.9 meV for x (In) = 0.1 molar fraction between using virtual crystal approximation and a random distribution of In which increase with increasing Indium concentration. Moreover, electrons wave function seems to be more sensitive than holes due to Indium fluctuations [ABSTRACT FROM AUTHOR]