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Modeling cation disorder in ZnGeN₂ and its impact on structural and electronic properties
Cordell, Jacob J.
Cordell, Jacob J.
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2022
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Abstract
Materials discovery is driven by a constant need for improved functionality in the building blocks of technology. The ordering of atoms in crystalline materials provides a means of realizing new functionalities to meet these challenges. Low efficiency in green emission of inorganic light emitting diodes is one challenge that new understanding and control of site ordering could solve. For this purpose, cation ordering in ZnGeN$_2$ is studied to uncover the underlying physics of site ordering in semiconductors and inform efficient device design. ZnGeN$_2$ exhibits a wide range of long-range order that could be used to tune the band gap and other properties of the system, but faces other challenges such as carrier localization, which could hinder device performance.
This thesis examines the role of point defects in ZnGeN$_2$ and their impact on properties. From a defect perspective, the $Zn_{Ge}+Ge_{Zn}$ pair appears in strong concentrations justifying the study of non-dilute fractions of this point defect pair. The concentration of this pair, however, is constrained by the interplay of enthalpy, which favors the ground state structure, and entropy, which stabilizes specific degrees of order at elevated temperatures. This thermodynamic relationship creates ranges of order parameters and thereby properties that are more likely than others.
The possibility of controlling the electronic band structure of ZnGeN$_2$ by tuning the site order of the material while changing the structure to a much lesser degree makes ZnGeN$_2$ a promising candidate for optoelectronics. The most significant adaptations of ZnGeN$_2$ with long-range order occur in the band edge energies and localization of states near the valence band edge. These changes present an opportunity to design ZnGeN$_2$/GaN heterostructures for use in light emitting diodes based on multiple architectures depending on the order parameter of ZnGeN$_2$.
Through the lens of the site ordering of chemical species, this thesis provides a framework for efficiently investigating a material with a single changing order parameter. The method created in this work outlines a means of better understanding the relationship among order, structure and properties to expedite materials research and better understand the interplay of physical mechanisms for pairing materials with applications.
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