Dislocation Dynamics in Semiconductors: Nucleation and Propagation - Essay Example

Dislocation Dynamics in Semiconductors: Nucleation and Propagation The plasti exhibited bysemiconductors has been the subject in several research and studies. Despite the major steps taken towards explaining the mechanisms involve in simple and nanostructured semiconductors, the mechanisms of dislocations are still unclear. Thus, various models have been developed to explain the mechanisms involved in the dislocation dynamics of semiconductors, particularly nucleation and propagation. The models range from thin-film simulations, kink dislocations, double kink mechanism, Shuffle-set Dislocation Nucleation, Molecular dynamics mechanism, and Misfit dislocations. The models apply varied scientific principles in exploring the dislocation dynamics of semiconductors. In all the models, nucleation and propagation occur in successions. Dislocation in semiconductors is affected by factors like changes in temperatures and the presence of impurities. Dislocation dynamics is responsible for the plastic and ductile nature of most semiconductors. Dislocation is an important aspect in the physics of materials and requires extensive research. Table of Contents Abstract 2 1.Introduction 4 1.Thin-film Simulations 4 2.Simulation Method 6 3.Kink Dislocations 6 4.1 Technical Details of the Calculations 8 4.2 The double kink mechanism 8 4.3 Factors affecting the kink mechanism 9 4.3.1 Localized obstacles 9 4.3.1.2 Influence of impurities on dislocation mobility 9 4.Molecular dynamics mechanism 9 5.Misfit dislocations 11 6.Shuffle-set Dislocation Nucleation 11 7.Conclusion 12 Bibliography 12 1. Introduction Dislocations dynamics is significant in the physics (mechanical and electrical) of semiconductors. Dislocations carry plasticity along crystalline elements. Moreover, the dislocations form centers that trap and scatter electronic carriers. Numerous experimental information regarding the dislocation dynamics of semiconductors that are tetrahedral bonded are available in various research articlesi. The principal slip systems for silicon are the 60°, while the orientation of the screw dislocations falls along the slip planeii. The two systems form a slithered configuration and dissociate to form pairs of fractional dislocations that bound ribbons of inherent stacking faultiii. Dissociations reduce the energy utilized in strain. Moreover, the lower the energy of the stacking faults, the more favorable the dislocations occur, energetically. The theory holds for semi-conductors (III-V and II-VI) and germanium. 1. Thin-film Simulations Thin films semiconductors are an engineering marvel in the contemporary science world. For instance, the polycrystalline thin films that have reduced defect concentration are omnipresent to contemporary engineeringiv. The reduced defect concentration facilitates the processingv. In conjunction with curiosity in scientific practices, many applications trigger intensive research of the procedures involved in the processing of polycrystalline (semi-conductor) thin films. Typical examples of compounds with polycrystalline thin films include copper and aluminum thin films. The thin films take the structure of metal lines, for instance, the films are suitable for inter-connection of transistors in many kinds of integrated circuitsvi. It is a common knowledge in science dislocations and grain boundaries are open conduits for speedy diffusion. The fast diffusion is responsible failures in electromigration. The speedy diffusion can be minimized by controlling the microstructure, specifically the texture, of the thin films of the aluminum. Modelings of multiscale materials comprehensively demonstrate the grain-evolution of grains and the mechanisms of dislocation in semi-conductors. The process by which dislocations nucleate at surfaces and the ensuing propagation into the thin films of the polycrystalline semiconductors is similar to epitaxial thin filmsvii. For the thin films of the polycrystalline semiconductors, the nucleation of the dislocations occurs during the development of the film. The process is representative of the deposition of the films on stressed substrates, made of similar materials. On the other hand, during epitaxy, the nucleation of dislocations occurs when films deposit on substrates of either different or similar materials. In general, stress occurs in the hetero-epitaxy. The theoretical research of the dynamics of dislocation in epitaxial thin films of semiconductors entails two phases: atomistic formulation and analytical formulation. The theory of analytical formulation of the dynamics of dislocations in thin semiconductor films was originally the idea of Van der Merwe and Frank and in the year 1949viii. In the theory, simple models of springs on a one-dimensional scales and substrates on periodic modulation were utilizedix. The two conflicting features are the straining energy of thin semiconductor films caused by mismatching of the film-substrate and the additional energy arising from the field of strain of the dislocations. Dislocations are nucleated when their presence causes a decrease in the total energy. For any particular system of film and substrate, the critical condition leads to critical depth of thin semiconductor filmsx. At this point, the resulting strain energy caused by the mismatching of the film and substrate is so great. Thus, the reduction of the total energy occurs with the generation of dislocations. The basis of the formulation lies on the minimization of energy among probable configurations of thin semiconductor filmsxi. Nevertheless, the nucleation and propagation of dislocations still elopes the thinking spectrum of many scientists. In an effort to discern the mechanisms behind nucleation and propagation of dislocations, various assumptions can be considered. The semi-circular loops occurring on the film surface are representative of the nuclei of dislocationsxii. In the model, the loops hold critical radii that determine the mechanisms of dislocations. When the radii of the loops exceed the critical radii, propagation of the dislocations occur into the semiconductor filmxiii. When the loop radii fall below the critical radii, the film surface absorbs dislocations. 2. Simulation Method The method of molecular dynamics (MD) is used in the research of the process by which dislocations in thin semiconductor film deposits are nucleated and propagated. In the method, tungsten forms the representative material. Tungsten portrays elastic isotropy. Moreover, tungsten does not have partial dislocations (Shockley partial dislocations) that would otherwise affect the results of the experimental procedure. 3. Kink Dislocations The resultant 30° and 90° partials are perceived to undergo vital reconstructions. The reconstructions eliminate bonds that might be unsaturated. Thus, the reconstruction ensures restoration of the fourfold atomic synchronization at the cores. The perception coincides with the low density with EPR suggested measurements present in dangling bonds. The dynamics of the dislocation encompass two major processes: nucleation and propagation. During dislocation, the kinks that lie along the lines of dislocation are nucleated and propagated. Thermal fluxes or the effect of applied stress results in the nucleation of double kinks in the line of dislocation. Dissociation results upon attaining a phase of critical separationxiv. Consequently, discrete kinks spread in opposite directions. The resulting effect is the generation of displacements that are representative of a dislocation segment. An in-depth comprehension of the characteristic atomic-scale composition and the dynamics of kinks are of huge significance. Similarly, the various barricades to the dislocation motion are vital in the study of dislocations in semi-conductorsxv. The theoretical model developed by Hirth and Lothe (HL) double-kink nucleating and the enormous probable barriers to kink motion influence the speed of propagation in the dynamics of dislocation. The theory holds for semiconductor materials. By contrast, the kinks encounter minimal barricades to motion and nucleation forms the sole controller of the speed of dislocation. Table of kink dislocations Dislocation Kink type Formation energy Migration barrier 30˚ LK 0.35 1.53 30˚ RK 1.24 2.10 90˚ LR 0.12 1.62 90˚ RL 0.12 1.62 Experiments 0.4-0.7 1.2-1.8 The HL model is normally utilized in interpreting experiments involving dislocation mobility. Moreover, another theoretical model suggests that the process by which obstacles pin kinks along the lines of dislocation affects the motionxvi. Empirical studies suggest that the potential barriers are huge, and thus choosing between the two models is problematic. The experimental procedures lack an inclusive microscopic picturexvii. Correlated issues, for instance, photo-plastic effects in semiconductors and reliance of dislocation dynamics on doping would benefit from an in-depth comprehension of structures of dislocation on atomic scales. 4.1 Technical Details of the Calculations The TBTE parameters are utilized in calculations of dislocation dynamics. The parameters give a clear description of the acoustic-phonon modes and Silicon’s constants of elasticityxviii. The parameters are adequate in providing descriptions of the strain fields related to dislocation centers and other relevant defects. 4.2 The double kink mechanism Virgin crystals The motions of dislocation motion exhibited in intrinsic crystals that have no interior fields of stress occur by the process of thermal activationxix. Thermal activation causes the sharp double kinks to nucleate and propagate over Peierls barriers (primary and secondary). The primary barriers arise from the high levels of Peierls potential of the semiconducting materialsxx. The dislocation motion occurring from one point to another requires the destruction of covalent bonds that hold the constituent molecules together. The secondary barriers arise from reconstruction of the cores. For the demonstration of the dislocation dynamics in silicon, the Hirth and Lothe (HL) is the most suitable. The model illustrates the dislocation dynamics related to the high Peierls potential exhibited by siliconxxi. Other models only rely on the existence of weak obstacles along the faults of dislocation. Thus, the models have been inappropriate for many years. Nevertheless, the effects of point defects that occur in the core and on the secondary barrier’s matrix remain unclear. 4.3 Factors affecting the kink mechanism 4.3.1 Localized obstacles The occurrence of localized obstacles on the planes of slip, for instance, impurities or minute precipitates and forest dislocations, can hinder the movement of dislocations in the semiconductorsxxii. Consequently, the hindrance delays movement of the systems of slip plane, controlled by the double-kink mechanismxxiii. Existing models for demonstrating dislocation velocities in crystals that are covalently bonded can be improved to cater for localized obstacles. 4.3.1.2 Influence of impurities on dislocation mobility Atom impurities affect the motion and dynamics of dislocation. The types of such impurities determine the effect on the dynamics of dislocation. The impurities form extra obstacles to nucleation and propagationxxiv. Thus, the impurities trigger the application of extra thermal energy in order to overcome them. Electrically active impurities, for instance, dopants, facilitate the nucleation of kinksxxv. The impurities influence the electric potential that transforms the defect densities at the cores of dislocationxxvi. On the other hand, electrically inactive impurities, for instance, oxygen, exhibit varied effects on the dislocation dynamics. The impurities do not directly affect the activation energy that facilitates the dislocation motionxxvii. The impurities have the effect of incorporating extra components of internal stress that result from the locking effect. 4. Molecular dynamics mechanism Uniaxial mechanism for dislocation dynamics in semiconductors can be demonstrated by the molecular dynamics technique. Uniaxial mechanism can be shown by utilizing the deposition effects of thin semiconductor films (tungsten) when subjected to uniaxial tensionxxviii. Empirical research has shown that the process of dislocation nucleation occurs near the surface of the semiconductors before propagating. According to the models of uniaxial tension that utilize the wave-like dislocation motion, propagation occurs in four major phasesxxix. First, a single atom at the surface squeezes into the film under uniaxial tension under a predetermined direction. The direction may be vertical or horizontal depending on the positioning of the films. The atom located at the uppermost corner of the thin film semiconductor surface then moves downwards. Consequently, the movement results in the elimination of one atomic layer at the surfacexxx. Atoms are then inserted downwards extending along a specific plane on the horizontal surface. The nucleus of dislocation also expands through the process of propagation into the interface of the film and the substratexxxi. The dislocation nucleation on the thin semiconductor film occurs in two planes that have the greatest value of the Schmid factor. However, dislocation may fail to occur on one of the two planes. In such cases, the process of dislocation nucleation fails to reduce the total energy on the relevant planes. Planes of dislocation In the molecular dynamics method, the substrate is subjected to a uniaxial tension in a specified direction. In most experiments, the horizontal surface is the thermodynamically favored planexxxii. The results of the simulation indicate a sequential procedure of nucleation that begins with a step at the surface. The pressing of the surface atom follows a predetermined direction, commonly termed as the 111 direction. The process generates half dislocation loops of Burgers’ vector along the surface that is inclined at an angle of 73˚ to the horizontal planexxxiii. The propagation of the dislocation occurs in a specified direction, commonly termed as the 311 direction. Dislocation nucleation eliminates the sharp surface phase. 5. Misfit dislocations Researchers have developed models for introduction of misfit dislocations in semiconductorsxxxiv. The models are based on the already existing dislocations in the substrates that move up into the epilayer/substrate interfacexxxv. The theory suggests that the critical width occurs when the stresses in the epilayer are adequate to cause bending of the existence dislocations at the interface, forming misfit locations. Misfit dislocations take account of the critical thickness, hc. Representation of misfit dislocation 6. Shuffle-set Dislocation Nucleation Experiments have been performed in the past regarding the shuffle-set dislocation. The device used in the experiments is the semiconductor silicon devicexxxvi. Subjected to extreme thermal processing, nucleation of the shuffle-set dislocation resulted. However, the dislocations changed into detached glide-set dislocations upon hardening at extreme temperatures (500 ºC)xxxvii. Two probable mechanisms exist for the process of nucleating of faultless shuffle-set dislocations under thermal procedures. One mechanism indicates that the dislocation nucleation of the nucleus occurred at low temperatures during the procedures of ion-implantation. The other mechanism indicates that the dislocation nucleation occurred under conditions of high temperatures and high stresses. 7. Conclusion The process of dislocation nucleation and propagation is a kinematic activate mechanism. Thus, the variations in temperature can affect the occurrence of dislocation as well as the propagation and nucleationxxxviii. Furthermore, the dynamics of dislocations are a function of the stoichiometry and atom concentration of the surfacexxxix. The formations of films that are free of strains depend on the introduction of dislocations into a semiconductorxl. Various models attempt to explain the dislocation dynamics of nucleation and propagation that exist in semiconductors. Revision message: references and quality: hi writer, u just explain the articles, u do not explain how these articles are hinder the work of the organizations. Second in paragraph one u did not refer to the number of the article. Third I check article 7 in the original text it does not matching what u have written about it. i go back and read the law, articles 17 and 22 are hinder the work of organizations please explain both and explain how the articles that u mentioned hinder the work of organizations Bibliography Read More