The work presented in this dissertation is motivated by the need for deeper understanding of phase transformations involving interfaces and defects, to advance the science of materials processing and to address the challenges in current and emerging technology. We take on several independent approaches to the problem, including theory, experiment, and simulation, which allows us to formulate and validate a coherent and comprehensive picture of phase transformation mechanisms. Our model involves recompiling the traditional components of Gibbsian thermodynamics and Classical Nucleation Theory (CNT), to conformably capture the other two classes of phase transformation phenomena, namely phase coexistence (CE) and free growth (FG), and their impact on the nonlinear kinetic phase evolution pathway.
Our work is timely in filling the insufficient coverage of these mechanisms in materials science literature, because they can become especially significant in small and confined material systems, which are becoming progressively more technologically relevant. A particular application that motivated our study is the analysis of microstructure evolution during fabrication of high-uniformity polycrystalline Si (poly-Si) thin films used in backplanes for advanced displays. Nanomaterials, such as transistors with <10 nm features, are also becoming more ubiquitous, and the science of their synthesis and stability can benefit from our work as well.
The experimental data available so far on phase transitions involving interfaces is limited due to the difficulty of distinguishing such localized, transient, and minute quantities of different phases of matter, as well as it is ridden by complications pertaining to each individual material system, all of which obstruct the analysis of physical behavior in terms of a concise and general theoretical description. In the work presented here we aim to bridge this gap between the material processing methods and theory by performing experiments and simulations, specifically designed to avoid excessive complicating factors and facilitate a clear conceptual connection. We explain the observations in the context of a simple phenomenological picture, which we developed from the classical fundamental principles, generalizing them to capture nontrivial phase evolution behaviors. Specifically, we perform (1) a designed laser irradiation experiment, (2) thermodynamic and kinetic analyses, (3) and molecular dynamics (MD) simulation, to capture the interface-involving melting and solidification mechanisms, and quantify their impact on phase initiation and evolution.
In materials processing, only nucleation and growth is typically considered as the governing mechanism of phase initiation and evolution. According to our broader analysis, which is based on the classical principles of Gibbsian thermodynamics, we additionally identify and describe phase coexistence (CE) and free growth (FG) as equally relevant, and possibly dominant, modes of phase transformation behavior encountered in real material systems. These mechanisms are distinct from nucleation, which requires sub-critical clusters to overcome a substantial thermodynamic barrier. CE represents a state where finite quantities of a new phase can spontaneously appear and exist in stable or metastable equilibrium within the parent phase matrix, as encountered for instance in the case of curvature-induced premelting. In contrast, FG is characterized by a critical temperature condition to eliminate the energy barrier for phase transformation, and can be mathematically classified as neither nucleation nor CE. By focusing on CE and FG in this dissertation, we thus capture two out of three mathematically and thermodynamically identifiable initiation modes of phase transformation in condensed systems.
To study how CE and FG can be manifested in systems of our interest, we employ a thermodynamic analysis of phase initiation and evolution that has been recently developed and refined in our group. There, we first recognize the significance of the Gibbs-Thomson Variation (GTV), which determines the thermodynamic driving force per area at a point on the inter-phase interface, based on local interface curvature and temperature. GTV applies everywhere on the inter-phase boundary and identifies the thermodynamically favored interface evolution pathway. When the interface is under morphological equilibrium, as implicitly assumed in Classical Nucleation Theory (CNT) descriptions, GTV can be translated into a global Gibbs-Thomson Function (GTF), which enables us to readily capture the thermodynamic landscape of phase transformation in complex confined systems by simply tracking the morphological-equilibrium curvature evolution function κ^ME (V) of the interface.
When we approach the problem using different methods, we obtain results consistent with our theoretical description, and gain further insight into CE and FG phenomena in systems of our interest. Our experiment involving partial melting of planarized poly-Si thin films indicates that the influence of CE and FG on the melting behavior cannot be dismissed. Applying our thermodynamic analysis to a cuboid grain model, which by design is close to our experimental system, we confirm that CE and FG can indeed be expected or even dominant over a range of realistic material configurations. We pay particular attention to the scenarios where the evolving liquid phase cluster encounters abrupt changes of morphological or chemical boundary conditions, such as connecting with new catalyzing interfaces or other clusters. As a result of such a touch event, sudden and discontinuous change of the solid-liquid interface shape can take place, which we call a transmorphic transition, potentially having a critical impact on the subsequent phase transformation pathway of the entire system.
We further employ discrete cluster kinetics simulation to illustrate a striking example of FG as a phase initiation and evolution mechanism, in which transmorphic transitions are enabled purely by thermal fluctuations, in absence of a thermodynamic driving force. As a completely independent, and therefore meaningful, approach, we also perform molecular dynamics (MD) simulations. This method a priori assumes nothing about the solid-liquid interface curvature, and yet the emergent behavior from thermal fluctuations of individual atoms shows a behavior qualitatively remarkably consistent with our theoretical picture under CE and FG conditions.
In the previous treatments in literature nucleation, CE, and FG have not been as systematically defined and categorized as the only three mechanisms of phase initiation and evolution in condensed and confined systems, and the role of thermal fluctuations in CE and FG has not been studied to the degree presented here. Our work thus has broad implications for the science of phase transformations, as well as applications of contemporary technological relevance, such as melt-mediated synthesis and processing of polycrystalline thin films and nanomaterials.
| Identifer | oai:union.ndltd.org:columbia.edu/oai:academiccommons.columbia.edu:10.7916/gjhy-4s28 |
| Date | January 2024 |
| Creators | Lisenko, Nikita |
| Source Sets | Columbia University |
| Language | English |
| Detected Language | English |
| Type | Theses |
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