Dimitrios Maroudas

Mechanism of hydrogen-induced crystallization of amorphous silicon

Hydrogenated amorphous and nanocrystalline silicon films manufactured by plasma deposition techniques are used widely in electronic and optoelectronic devices. The crystalline fraction and grain size of these films determines electronic and optical properties; the nanocrystal nucleation mechanism, which dictates the final film structure, is governed by the interactions between the hydrogen atoms of the plasma and the solid silicon matrix. Fundamental understanding of these interactions is important for optimizing the film structure and properties. Here we report the mechanism of hydrogen-induced crystallization of hydrogenated amorphous silicon films during post-deposition treatment with an H2 (or D2) plasma. Using molecular-dynamics simulations and infrared spectroscopy, we show that crystallization is mediated by the insertion of H atoms into strained Si–Si bonds as the atoms diffuse through the film. This chemically driven mechanism may be operative in other covalently bonded materials, where the presence of hydrogen leads to disorder-to-order transitions.

“Coarse” stability and bifurcation analysis using stochastic simulators: Kinetic Monte Carlo examples

We implement a computer-assisted approach that, under appropriate conditions, allows the bifurcation analysis of the “coarse” dynamic behavior of microscopic simulators without requiring the explicit derivation of closed macroscopic equations for this behavior. The approach is inspired by the so-called time-stepper based numerical bifurcation theory. We illustrate the approach through the computation of both stable and unstable coarsely invariant states for kinetic Monte Carlo models of three simple surface reaction schemes. We quantify the linearized stability of these coarsely invariant states, perform pseudoarclength continuation, detect coarse limit point and coarse Hopf bifurcations, and construct two-parameter bifurcation diagrams.

Energetics of formation and migration of self-interstitials and self-interstitial clusters in α-iron

Abstract Energetic primary recoil atoms from fast neutron irradiation generate both isolated point defects and clusters of vacancies and interstitials. Self-interstitial mobility as well as defect cluster stability and mobility play key roles in the subsequent fate of defects and, hence, in the overall microstructural evolution under irradiation. Self-interstitials and two, three and four-member self-interstitial clusters are highly mobile at low temperatures as observed in molecular-dynamics simulations and high mobility probably also extends to larger clusters. In this study, the morphology, energetics and mobility of self-interstitials and small self-interstitial clusters in α-iron are studied by molecular-statics and molecular-dynamics simulations using a Finnis-Sinclair many-body interatomic potential. Self-interstitial migration is found to be a two-step process consisting of a rotation out of the 〈110〉 split-dumbbell configuration into the 〈111〉 split-dumbbell configuration and 〈111〉 translational jumps through the crowdion configuration before returning to the 〈110〉 dumbbell configuration. Self-interstitial clusters of 〈111〉 type split-interstitials assembled on adjacent {110} planes migrate along 〈111〉 directions in an amoeba-like fashion by sequential local dissociation and re-association processes.

Coarse bifurcation analysis of kinetic Monte Carlo simulations: A lattice-gas model with lateral interactions

We present a computer-assisted study of “coarse” stability/bifurcation calculations for kinetic Monte Carlo simulators using the so-called coarse timestepper approach presented in A. G. Makeev, D. Maroudas, and I. G. Kevrekidis, J. Chem. Phys. 116, 10083 (2002). Our illustrative example is a model of a heterogeneous catalytic surface reaction with repulsive adsorbate–adsorbate interactions and fast diffusion. Through numerical continuation and stability analysis, we construct one- and two-parameter coarse bifurcation diagrams. We also discuss several computational issues that arise in the process, the most important of which is the “lifting” of coarse, macroscopic initial conditions (moments of adsorbate distributions) to fine, microscopic initial conditions (distributions conditioned on these moments).

Theoretical analysis of electromigration-induced failure of metallic thin films due to transgranular void propagation

Failure of metallic thin films driven by electromigration is among the most challenging materials reliability problems in microelectronics toward ultra-large-scale integration. One of the most serious failure mechanisms in thin films with bamboo grain structure is the propagation of transgranular voids, which may lead to open-circuit failure. In this article, a comprehensive theoretical analysis is presented of the complex nonlinear dynamics of transgranular voids in metallic thin films as determined by capillarity-driven surface diffusion coupled with drift induced by electromigration. Our analysis is based on self-consistent dynamical simulations of void morphological evolution and it is aided by the conclusions of an approximate linear stability theory. Our simulations emphasize that the strong dependence of surface diffusivity on void surface orientation, the strength of the applied electric field, and the void size play important roles in the dynamics of the voids. The simulations predict void faceti...

Atomistic simulation study of the interactions of SiH3 radicals with silicon surfaces

SiH3 radicals created by electron impact dissociation of SiH4 in reactive gas discharges are widely believed to be the dominant precursor for plasma deposition of amorphous and nanocrystalline silicon thin films. In this article, we present a systematic computational analysis of the interactions of SiH3 radicals with a variety of crystalline and amorphous silicon surfaces through atomistic simulations. The hydrogen coverage of the surface and, hence, the availability of surface dangling bonds has the strongest influence on the radical–surface reaction mechanisms and the corresponding reaction probabilities. The SiH3 radical reacts with unit probability on the pristine Si(001)-(2×1) surface which has one dangling bond per Si atom; upon reaction, the Si atom of the radical forms strong Si–Si bonds with either one or two surface Si atoms. On the H-terminated Si(001)-(2×1) surface, the radical is much less reactive; the SiH3 radical was reflected back into the gas phase in all but two of the 16 simulations of...

Absolute densities of N and excited N2 in a N2 plasma

Atomic N and excited N2 (N2*) play important roles in plasma-assisted synthesis of nitride materials, such as GaN. Absolute densities of N and N2* were measured at the substrate plane in an inductively coupled N2 plasma in the pressure range of 10 to 200 mTorr using modulated-beam line-of-sight threshold ionization mass spectrometry. The density of N increased with increasing pressure from 2.9×1018 to 1.8×1019 m−3, while the density of N2* was in the range of 9.7×1017 to 2.4×1018 m−3, with a maximum at 50 mTorr. Based on the appearance potential of N2* at ∼12 eV, we identify this excited molecule as long-lived N2 (A3Σu+) metastable.

Interactions of SiH radicals with silicon surfaces: An atomic-scale simulation study

A comprehensive study is presented of the interactions of SiH radicals originating in silane containing plasmas with crystalline and amorphous silicon surfaces based on a detailed atomic-scale analysis. The hydrogen concentration on the surface is established to be the main factor that controls both the surface reaction mechanism and the reaction probability; other important factors include the location of impingement of the radical on the surface, as well as the molecular orientation of the radical with respect to the surface. On the ordered crystalline surfaces, the radical reacts in such a way as to maximize the number of Si–Si bonds it can form even if such bond formation requires dissociation of the radical and introduction of defects in the crystal structure. The radical is established to be fully reactive with the pristine Si(001)-(2×1) surface. This chemical reactivity is reduced significantly for the corresponding H-terminated surface with a hydrogen coverage of one monolayer. SiH is found to be ...

Modeling of electromechanically-induced failure of passivated metallic thin films used in device interconnections

A theoretical analysis is presented of the failure of metallic thin films used for device interconnections in integrated circuits. Failure is mediated by void dynamics, which is driven by surface electromigration and processing related residual thermal stresses in the films. The analysis is based on surface mass transport modeling coupled strongly with the electrostatic and elastic deformation problems in the metallic films. Special emphasis is placed on the combined effects on void dynamics of anisotropy both in surface diffusivity along a void surface and in the applied stress tensor. A systematic parametric study is carried out based on self-consistent numerical simulations of surface morphological evolution. Void dynamics is analyzed and results are presented for void morphological stability in terms of critical stress levels as a function of stress state and surface mobility anisotropy. Finally, the role of plastic deformation is discussed around crack-like features emanating from void surfaces in ductile metallic films based on results of molecular-dynamics simulations in Cu.

Electromigration-induced failure of metallic thin films due to transgranular void propagation

A theoretical analysis is presented of the failure of metallic thin films due to electromigration-induced morphological evolution of transgranular voids. Self-consistent dynamical simulations emphasize the important roles of the anisotropy of void surface diffusivity, the strength of the applied electric field, and the void size. Our simulations predict formation of stable faceted voids, formation of wedge-shaped voids through a facet selection mechanism, as well as failure due to propagation of slitlike features from void surfaces, in excellent agreement with recent experimental observations.