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Users Group Meeting 2012
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MedeA® Application Notes for Electronics

Prediction of Schottky Barrier in Electronic Devices The contact resistance between metals and semiconductors in nanoelectronic devices is mainly determined by the Schottky barrier. Controlling the Schottky barrier height (SBH) hence means being able to manipulate the contact resistance. and thereby to reduce the energy consumption as well as the heat production of electronic devices. While so far phenomenological considerations were able to determine the SBH only at a qualitative level, a parameter-free quantitative evaluation is possible via atomistic simulations using the MedeA® software platform.. This application note illustrates the calculation and modification of the SBH for a system of direct technological importance, namely a NiSi/Si contact. Furthermore, the effect of dopant atoms on the SBH, which are needed to tune the SBH for minimal contact resistance of n- and p-doped semiconductors, is investigated. Reduction of the Schottky barrier height is achieved by doping with Ba. This note also demonstrates how the preferred positions of dopant elements can be determined with S as example.	Accurate Band Gaps of Correlated Transition-Metal Oxides from MedeA-VASP with Hybrid-Functionals The electronic structures of the metallic and insulating phases of VO₂ are calculated using density functional theory and modern hybrid functionals as recently implemented in MedeA®-VASP. Strongly contrasting previous calculations as based on local or semilocal approximations, which missed the insulating behavior of the low-temperature phase, these new calculations accurately reproduce the optical band gap and thus bring to an end a fifty-year old controversy on the origin of the metal-insulator transition of VO₂.
Energy Band Structure of InAs with MedeA-VASP The energy band structure of InAs is computed with the HSE06 hybrid functional using MedeA®-VASP. The calculations give a direct band gap of 0.35 eV, which coincides with the experimental value. The Γ-L separation in the lowest conduction band is computed to be 1.11 eV. Screened-exchange FLAPW calculations reported a value of 1.21 eV. The earlier handbook value was 0.74 eV. More recent experiments reported 1.1±0.05 eV, which is perfectly consistent with the present calculations.	Structure and Elastic Properties of (In,Ga)N In the design of substrates for the epitaxial growth of In_xGa_1-xN alloys it is useful to know the elastic properties of the semiconductor as a function of composition. This application note shows the use of MedeA® with VASP 5.2 and the mechanical-thermal (MT) module in computing these properties. Judging by the results for the binaries GaN and InN, the level of accuracy is comparable with that achieved in experiments.
Graphite Electrode Elastic Properties upon Li Intercalation First-principles calculations reveal a three-fold increase in the Young’s modulus of graphite as it is lithiated (C→LiC₆). A linear expression is determined that describes the approximate stiffness of Li intercalated graphite as a function of loading which may lead to greatly improved continuum models of electrode deformation and failure.	Dissociation of SiH₄ on Si(001) Surface A key process in the semiconductor manufacturing is the reactive adsorption of molecules such as silane (SiH₄) and dichlorosilane (SiCl₂H₂) on the surfaces of silicon wafers. This case study demonstrates the calculation of the geometry of a silane molecule on a reconstructed Si(001) surface.
Ferroelectric Properties of BaTiO₃ First-principles computations correctly describe the ferroelectric distortions and the macroscopic polarization of BaTiO₃ in agreement with experiment. Computations of the vibrational properties (phonons) reveal that a cubic perovskite structure of BaTiO₃ becomes stable under compression of the lattice. This demonstrates the usefulness of first-principles calculations in the design and optimization of ferroelectric materials.	Modeling work function changes in CMOS stacks containing HfO₂ high-k dielectrics The work function of the metal gate in a CMOS stack depends on the composition and structure of the interfaces. This is demonstrated here for the case of a Si-HfO₂-W stack by introducing a Hf vacancy at the Si/HfO₂ interface. At a concentration of 1.2 vacancies per nm² the work functions is increased by 500 meV.
Energy band structure of germanium Elemental germanium is a semiconductor with a measured indirect band gap of 0.66 eV. Using a hybrid functional as implemented in VASP 5.2, the computed value is 0.66 eV while standard density functional approaches incorrectly predict Ge to have no band gap. Other features of the band structure such as the direct gap at Γ are also well reproduced by the current level of theory, namely 0.8 eV (measured) and 0.73 eV (computed), thus demonstrating the reliability of this level of approach in predicting energy band structures. This sets the stage for using computations to modify the band structure for example by uniaxial strain to meet specific design criteria.

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