K. Scheerschmidt

Molecular dynamics simulations of silicon wafer bonding

Molecular dynamics simulations based on a modified Stillinger-Weber potential are used to investigate the elementary steps of bonding two Si(001) wafers. The energy dissipation and thus the dynamic bonding behaviour are controlled by the transfer rates for the kinetic energy. The applicability of the method is demonstrated by studying the interaction of perfect wafer surfaces (UHV conditions). First calculations covering the influence of surface steps, rotational misorientations and adsorbates are presented.

Electron holography: II. First steps of high resolution electron holography into materials science

Abstract By means of electron holography, the complex electron wave is transferred from the electron microscope to a computer. Consequently, all desirable wave-optical procedures can be numerically applied in a very flexible way to extract and to analyze quantitatively the amplitude and the phase of the object exit wave.

Dissociation of screw dislocations in (001) low-angle twist boundaries: A source of the 30 o partial dislocations in silicon

The first experimental evidence for dissociation of grain boundary screw dislocations is presented for (001) low-angle twist boundaries in silicon. Using a combination of high-resolution electron microscopy and the weak-beam technique of transmission electron microscopy, it is found that the grainboundary screw dislocations (b = 1/2 ) can dissociate in the (111) plane into two 30o partials, forming an intrinsic stacking fault, as do lattice screw dislocations of the glide set. On dissociation one partial dislocation stands off the grain-boundary plane. Some segments of the grain-boundary screw dislocations, however, may remain undissociated. An atomic model for the undissociated screw dislocation core, as well as a mechanism of its transformation into cores of individual 30o partials upon dissociation, are proposed on the basis of classical molecular dynamics simulations with an empirical interatomic potential. The model enables an understanding of the results of electron microscopy investigations.

Silver clustering in sodium silicate glasses: a molecular dynamics study

Abstract Molecular dynamics (MD) computer simulations with empirical potentials are applied to model the structure of sodium silicate glasses and the mobility of metallic particles enclosed. The particles are assumed to be generated by the Na–Ag ion-exchange, reducing the Ag ions and subsequent annealing. The theoretical investigations consider the modification of the glass structure owing to the ion exchange as well as the migration and clustering of the silver particles. Moreover, from the study of the migration processes we refine the empirical potentials applied to MD simulations and subsequent high resolution electron microscope (HREM) image calculations.

Electron microscope characterization of CdSe/ZnSe quantum dots based on molecular dynamics structure relaxations

Molecular dynamics simulations using empirical potentials are applied to characterize the structure, the energy relaxation and the stability of pyramidal-shaped quantum dots in the CdSe/ZnSe system. The relaxed structure models are used for a reliable interpretation of electron microscope investigations to analyze the size, the shape and the strain fields of the quantum dots. Though the elastic strains modify the electron microsope image contrast by creating virtual truncations of the pyramids or additional black-white lobes, optimum imaging conditions chosen will reveal the shape and the size of the dots.

Relativistic effects in elastic scattering of electrons in TEM.

Transmission electron microscopy typically works with highly accelerated thus relativistic electrons. Consequently the scattering process is described within a relativistic formalism. In the following, we will examine three different relativistic formalisms for elastic electron scattering: Dirac, Klein-Gordon and approximated Klein-Gordon, the standard approach. This corresponds to a different consideration of spin effects and a different coupling to electromagnetic potentials. A detailed comparison is conducted by means of explicit numerical calculations. For this purpose two different formalisms have been applied to the approaches above: a numerical integration with predefined boundary conditions and the multislice algorithm, a standard procedure for such simulations. The results show a negligibly small difference between the different relativistic equations in the vicinity of electromagnetic potentials, prevailing in the electron microscope. The differences between the two numeric approaches are found to be small for small-angle scattering but eventually grow large for large-angle scattering, recorded for instance in high-angle annular dark field.

Molecular dynamics studies of silica wafer bonding

Molecular dynamics simulations are performed to investigate the atomic processes initiated by the adhesion of two silica surfaces, which are covered with adsorbates of oxygen, hydrogen or water molecules. The calculations describe the mechanism of hydrophilic silicon wafer bonding in terms of empirical potentials assumed. The challenge of the macroscopically relevant computations is to understand and to predict the formation of covalent bonds as a function of initial silica structures, external forces, adsorbates, and annealing temperatures applied.

Empirical molecular dynamic study of SiC(0001) surface reconstructions and bonded interfaces

Empirical molecular dynamics simulations based on the Tersoff potential are carried out for SiC(0001) surfaces and bonded interfaces. It is demonstrated that such a classical interatomic potential is able to correctly describe SiC-4H (0001)3×3 and 3×3R30° surface reconstructions. The surprising accuracy of the empirical simulations compared to results of density functional methods as well as experiments is demonstrated not only by obtaining reasonable structural parameters, but also by the correct prediction of such intricate effects like buckling in the topmost carbon layer of the 3×3 surface and polymerization in the silicon wetting layer of the 3×3 reconstruction. Because of the established good applicability of the Tersoff potential the simulations are used to predict the formation of SiC interfaces to be generated by wafer bonding and so far experimentally unobserved. It is shown that the bond energy crucially depends on the local atomic structure at the interface. The resulting bond energies range f...

Quantum Dot Structures in the InGaAs System Investigated by TEM Techniques

Quantum dot structures have gained increasing interest in materials science due to their special electrical and optical behavior. A combination of electron-opti cal techniques is applied to correlate such properties with the morphology and structure of quantum dots in the InGaAs system. TEM techniques, e.g. imaging by conventional diffraction contrast, by high-resolution TEM and by energy filtering (EFTEM) are focused on the determination of parameters, like shape and size of islands, their chemical composition and the complex lattice strain fields. An image contrast analysis in terms of shape and strain demands the application of image simulation techniques based on the dynamical theory and on structure models refined by molecular dynamics or molecular static energy minimization.

Atomistic study of the (001), 90° twist boundary in silicon

A new type of a structural unit (the 42m dreidl) is proposed on the basis of molecular dynamics simulations for the core model of the (001), 90 e twist grain boundary in silicon. The structural unit resembles a polyhedron in which some edges, not corresponding to bonds between atoms, are absent. The dreidl has the 42m (D2d) point-group symmetry and consists of 14 atoms which form eight ® vemembered rings maintaining tetrahedral bonding in the boundary core. Molecular dynamics simulations with the empirical TersoA potential were performed to evaluate the energy of the (001), 90 e twist boundary at T = 0K. The eA ect of both rigid-body translations parallel to the grain boundary plane and alternative reconstructions involving conventional structural units was investigated. Despite the high degree of dimerization the twist boundary was found to have a low energy compared with structural models of twist grain boundaries in silicon previously studied.