Erich Wimmer

Computational band-structure engineering of III–V semiconductor alloys

Accurate band structures of binary semiconductors AB (A=Al, Ga, In and B=P, As, Sb) and selected ternary III–V semiconductors were calculated using an all-electron screened exchange approach within the full potential linearized augmented plane-wave method. Fundamental band gaps and Γ–L and Γ–X separations in higher-lying conduction bands are predicted with an accuracy of a few tenths of 1 eV. Screened exchange also performs better than the local density approximation for calculating conduction-band effective masses. Highly n-doped InPAs materials with compositions near InP0.2As0.8 offer lower effective masses, greater optical band-gap shifts, and potentially higher electron mobility than n-doped InGaAs materials with comparable band gaps.

Interfacial oxygen and nitrogen induced dipole formation and vacancy passivation for increased effective work functions in TiN/HfO2 gate stacks

Effective work function (EWF) changes of TiN/HfO2 annealed at low temperatures in different ambient environments are correlated with the atomic concentration of oxygen in the TiN near the metal/dielectric interface. EWF increases of 550 meV are achieved with anneals that incorporate oxygen throughout the TiN with [O]=2.8×1021 cm−3 near the TiN/HfO2 interface. However, further increasing the oxygen concentration via more aggressive anneals results in a relative decrease of the EWF and increase in electrical thickness. First-principles calculations indicate the exchange of O and N atoms near the TiN/HfO2 interface cause the formation of dipoles that increase the EWF.

Ab initio calculations for industrial materials engineering: successes and challenges

Computational materials science based on ab initio calculations has become an important partner to experiment. This is demonstrated here for the effect of impurities and alloying elements on the strength of a Zr twist grain boundary, the dissociative adsorption and diffusion of iodine on a zirconium surface, the diffusion of oxygen atoms in a Ni twist grain boundary and in bulk Ni, and the dependence of the work function of a TiN-HfO(2) junction on the replacement of N by O atoms. In all of these cases, computations provide atomic-scale understanding as well as quantitative materials property data of value to industrial research and development. There are two key challenges in applying ab initio calculations, namely a higher accuracy in the electronic energy and the efficient exploration of large parts of the configurational space. While progress in these areas is fueled by advances in computer hardware, innovative theoretical concepts combined with systematic large-scale computations will be needed to realize the full potential of ab initio calculations for industrial applications.

Gate-last TiN/HfO2 band edge effective work functions using low-temperature anneals and selective cladding to control interface composition

Silicon N-metal-oxide-semiconductor (NMOS) and P-metal-oxide-semiconductor (PMOS) band edge effective work functions and the correspondingly low threshold voltages (Vt) are demonstrated using standard fab materials and processes in a gate-last scheme employing low-temperature anneals and selective cladding layers. Al diffusion from the cladding to the TiN/HfO2 interface during forming gas anneal together with low O concentration in the TiN enables low NMOS Vt. The use of non-migrating W cladding along with experimentally detected N-induced dipoles, produced by increased oxygen in the TiN, facilitates low PMOS Vt.

Benchmark and testing of the local density functional method for molecular systems

The prediction of molecular properties from computer simulations is playing an increasingly important role in the chemical and pharmaceutical industry (Dixon, 1987; Dixon et al., 1988a; 1988b). Although molecular modeling groups were already popular in the 1970’s with the promise of solving the rational drug design problem in a quantitative way, the available computational resources and software were not adequate for the task. With the more ready access by chemists to supercomputers in the 1980’s and the revolutions in theoretical developments, algorithm design and software implementation, it is now possible for computational science to have a quantitative impact on molecular design. The primary areas that have been responsible for this renaissance have been the development of force field methods including molecular dynamics and the more routine application of quantum chemical methods (Hehre et al., 1986) to increasingly more complex molecular systems of interest to the bench chemist. This chapter will focus on the latter application area.

Dipole controlled metal gate with hybrid low resistivity cladding for gate-last CMOS with low Vt

In this contribution, NMOS and PMOS band edge effective work function (EWF) and correspondingly low Vt are demonstrated using standard fab materials and processes in a gate-last scheme. For NMOS, the use of an Al cladding layer results in Vt = 0.08 V consistent with NMOS EWF = 4.15 eV. Migration of the Al cladding into the TiN and a relatively low oxygen concentration near the TiN/HfO2 interface are responsible for the low EWF. For PMOS, employing a W cladding layer along with a post-TiN anneal in an oxidizing ambient results in elevated oxygen concentration near the TiN/HfO2 interface and Vt = −0.20 V consistent with a PMOS EWF =5.05 eV. First-principles calculations indicate N atoms displaced from the TiN during the oxidizing anneal form dipoles at the TiN/HfO2 interface that play a critical role in determining the PMOS EWF.

Impact of hydrogen and oxygen defects on the lattice parameter of chemical vapor deposited zinc sulfide

The lattice parameter of cubic chemical vapor deposited (CVD) ZnS with measured oxygen concentrations <0.6 at. % and hydrogen impurities of <0.015 at. % has been measured and found to vary between −0.10% and +0.09% relative to the reference lattice parameter (5.4093 A) of oxygen-free cubic ZnS as reported in the literature. Defects other than substitutional O must be invoked to explain these observed volume changes. The structure and thermodynamic stability of a wide range of native and impurity induced defects in ZnS have been determined by ab initio calculations. Lattice contraction is caused by S-vacancies, substitutional O on S sites, Zn vacancies, H in S vacancies, peroxy defects, and dissociated water in S-vacancies. The lattice is expanded by interstitial H, H in Zn vacancies, dihydroxy defects, interstitial oxygen, Zn and [ZnHn] complexes (n = 1,…,4), interstitial Zn, and S2 dumbbells. Oxygen, though present, likely forms substitutional defects for sulfur resulting in lattice contraction rather th...

Formation of nickel-platinum silicides on a silicon substrate: Structure, phase stability, and diffusion from ab initio computations

The formation of Ni(Pt)silicides on a Si(001) surface is investigated using an ab initio approach. After deposition of a Ni overlayer alloyed with Pt, the calculations reveal fast diffusion of Ni atoms into the Si lattice, which leads initially to the formation of Ni2Si. At the same time, Si atoms are found to diffuse into the metallic overlayer. The transformation of Ni2Si into NiSi is likely to proceed via a vacancy-assisted diffusion mechanism. Silicon atoms are the main diffusing species in this transformation, migrating from the Si substrate through the growing NiSi layer into the Ni2Si. Pt atoms have a low solubility in Ni2Si and prefer Si-sites in the NiSi lattice, thereby stabilizing the NiSi phase. The diffusivity of Pt is lower than that of Ni. Furthermore, Pt atoms have a tendency to segregate to interfaces, thereby acting as diffusion barriers.

Physical modeling — A new paradigm in device simulation

We go far beyond classical TCAD in and create a simulation framework that is ready for devices based on contemporary and future technology nodes. We do so by extending the common drift-diffusion-type device simulation framework with additional tools: (i) a k p-based subband structure tool, (ii) a deterministic subband Boltzmann transport solver, and (iii) a TCAD-compatible quantum transport solver, to capture every important aspect of device operation at the nano-scale. An atomistic ab-initio tool suite complements the framework providing material properties that would be hard to obtain otherwise. The capabilities of the approach are demonstrated on two different devices featuring non-planar geometry and alternative channel materials.

Future in biomolecular computation

SummaryLarge-scale computations for biomolecules are dominated by three levels of theory: rigorous quantum mechanical calculations for molecules with up to about 30 atoms, semi-empirical quantum mechanical calculations for systems with up to several hundred atoms, and force-field molecular dynamics studies of biomacromolecules with 10,000 atoms and more including surrounding solvent molecules. It can be anticipated that increased computational power will allow the treatment of larger systems of ever growing complexity. Due to the scaling of the computational requirements with increasing number of atoms, the force-field approaches will benefit the most from increased computational power. On the other hand, progress in methodologies such as density functional theory will enable us to treat larger systems on a fully quantum mechanical level and a combination of molecular dynamics and quantum mechanics can be envisioned. One of the greatest challenges in biomolecular computation is the protein folding problem. It is unclear at this point, if an approach with current methodologies will lead to a satisfactory answer or if unconventional, new approaches will be necessary. In any event, due to the complexity of biomolecular systems, a hierarchy of approaches will have to be established and used in order to capture the wide ranges of length-scales and time-scales involved in biological processes. In terms of hardware development, speed and power of computers will increase while the price/performance ratio will become more and more favorable. Parallelism can be anticipated to become an integral architectural feature in a range of computers. It is unclear at this point, how fast massively parallel systems will become easy enough to use so that new methodological developments can be pursued on such computers. Current trends show that distributed processing such as the combination of convenient graphics workstations and powerful general-purpose supercomputers will lead to a new style of computing in which the calculations are monitored and manipulated as they proceed. The combination of a numeric approach with artificial-intelligence approaches can be expected to open up entirely new possibilities. Ultimately, the most exciding aspect of the future in biomolecular computing will be the unexpected discoveries.