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Title:Non-adiabatic electron-ion dynamics in semiconductors under ionizing particle irradiation
Author(s):Lee, Cheng-Wei
Director of Research:Schleife, Andre
Doctoral Committee Chair(s):Schleife, Andre
Doctoral Committee Member(s):Trinkle, Dallas; Krogstad, Jessica; Bellon, Pascal
Department / Program:Materials Science & Engineerng
Discipline:Materials Science & Engr
Degree Granting Institution:University of Illinois at Urbana-Champaign
Ehrenfest MD
First-principles calculation
non-adiabatic electron-ion dynamics
mixed quantum-classical dynamics
electronic stopping
particle radiation
Abstract:Highly energetic ions have broad applications from semiconductor industry to medicine. However, as they impact materials, they inevitably cause damage, which fundamentally determines the materials' properties. Hence, the success of these applications relies on quantitative understanding of how energetic ions interact with target materials. Studying highly energetic ions is challenging since they trigger multiple length- and time-scale processes. Generally speaking, energetic ions are slowed down differently at different velocities: Electronic stopping dominates at high velocity while nuclear stopping prevails at low velocity. The elastic collisions between slow ions and target materials were well-studied in the past. However, how energetic ions interact with target materials, especially when defects exist, remains unclear. Furthermore, excited electrons generated during the impact inevitably affect the atoms in a target material but the quantitative relation is unknown. To address these challenges, we first use the first-principles method, which goes beyond the commonly used analytic models, to study the electronic stopping of proton in ideal bulk materials and their results set the bases for following studies. Since point defects and interfaces play a critical role in semiconductors, we investigate their effects on electronic stopping and identify the underlying mechanisms. We also investigate how the electronic excitations near the defect affect the dynamics of atoms in the target material. To address the limit imposed by computational cost, we devise a new approach that bridges time scales from ultrafast electron dynamics directly after impact, to ion diffusion. Our results on neutral oxygen vacancy in MgO show that the enhancement in atomic diffusion can be three orders of magnitude, given large enough concentration of oxygen vacancy and ion beam flux. Such significant enhancement happens at the low velocity regime that no effect of electronic excitation is expected. Lastly, driven by actual application, we extend the description to heavy ions, which can have multiple electron shells with different energy depths. As a result, their equilibrium charges significantly affect the electronic stopping. Our results for self-irradiated Si show that channeling heavy ions can have larger electronic stopping than off-channeling ones, which is different from known results for light ions. To better understand this, we identify the underlying relation between core electrons, charge, impact parameter, and electronic stopping. Specifically for this phenomenon, larger electronic stopping for channeling heavy ions is attributed to larger equilibrium charge. Overall, we discover several mechanisms that challenge the current understandings of interaction between energetic ions and target materials. Therefore, our discoveries have the potential to change the way ion beam experiments are designed and applied.
Issue Date:2019-10-11
Rights Information:Copyright 2019 Cheng-Wei Lee
Date Available in IDEALS:2020-03-02
Date Deposited:2019-12

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