Dislocation dynamics during thin film deposition

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Dislocation dynamics during thin film deposition


Author: Liu, Wing-chung Averil
Title: Dislocation dynamics during thin film deposition
Degree: Ph.D.
Year: 2003
Subject: Hong Kong Polytechnic University -- Dissertations
Thin films
Thin film circuits
Dislocations in metals
Department: Dept. of Mechanical Engineering
Pages: xviii, 221 leaves : ill. ; 30 cm.
InnoPac Record: http://library.polyu.edu.hk/record=b1733026
URI: http://theses.lib.polyu.edu.hk/handle/200/5877
Abstract: The nucleation of dislocations and their subsequent propagation during thin film deposition are studied. Aiming to reveal the generic mechanisms, the case of tungsten on a substrate of the same material is investigated. Three-dimensional (3D) molecular dynamics (MD) method is used. The substrate is under either uni-axial compression or tension along the [111] direction, with the thermodynamically favoured (011) surface being horizontal. Studies are carried out when the strain (either tensile or compressive) is applied along either [111] or [211] direction. In this thesis, we first focus on the kinetic processes of newly deposited adatoms on the film substrate. These serve as the guidelines of designing our MD simulations. Then, we focus on the growth mechanisms of deposited film under stressed substrate. In particular, we focus on the mechanisms of dislocation nucleation and propagation. These mechanisms affect or dictate the microstructure evolution processes of thin films, and thereby their performance. In the first part of my study, diffusion of adatoms on the tungsten (011) surface is investigated using the MD method. The formation energy of each defect cluster, consisting of a few adatoms, is calculated using a combination of annealing and quenching techniques. Diffusion mechanisms of clusters are elucidated by the analysis of atomic trajectories, and are confirmed by calculating energy states along the diffusion path and by fitting the diffusion coefficients to one or more Arrhenius functions. Our dynamic results show that a W adatom diffuses much slower, with diffusion coefficient 1.3x10⁻³e⁻⁰·⁵⁴ᵉᵛ/ᵏᵀcm²/s. The W dimmer and trimmer are probably the critical nucleus in three-dimensional growth. Then, we simulate the nucleation and propagation of dislocations during deposition of tungsten thin films on an uni-axially and bi-axially stressed tungsten substrate. This mimics the dislocation dynamics processes in a polycrystalline thin film, in which one grain is surrounded and thereby strained by its neighbors. At the same time, this simulated condition resembles that of heteroepitaxy, except that chemical effects are absent. Our results show that a dislocation nucleates near a surface step/groove through the ejection/insertion of an atom from a surface layer. Following the ejection / insertion, atoms on the surface layer relax along one of the <111> directions to take up / fill in the extra space of the resulting vacancy. When the relaxed space is small, glissile dislocations are formed. They propagate by gliding along one of the <111> directions and align on the plane with the largest Schmid factor. The obtained Burgers vectors align on the glide plane with the largest shear stress. Therefore, movement of atoms along the slip direction, which results in the nucleation dislocation half loop, is attributed to the large resolved shear stress. However, when the relaxed space is large, a sessile dislocation half loop is formed by climbing. In this case, the Burgers vectors align in one of the <111> directions on the surface (011) plane. This results in the formation of a sessile dislocation half loop. Finally, the effect of surface steps on the nucleation of dislocations is investigated. Samples of thin films of critical thickness are annealed under the uni-axial stressing conditions, with surface steps along various directions. The results confirm that the dislocation nucleation during thin film deposition is dictated by the surface steps. Further, propagation is found to be aided by the presence of surface steps, but is possible beyond the stepped region.

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