Matthew Brahlek

Understanding the fundamental properties of materials is necessary for all modern electronic technologies. Toward this end, the fabrication of new ultrapure thin film materials is critical to discover and understand novel properties that can allow further development of technology. Oxides are a vast class of materials that abound with diverse properties, and, therefore, harnessing such phases is essential for realizing emerging technologies. Pushing forward, however, requires understanding basic properties of insulating, semiconducting, and metallic oxides, as well as the more complex phases that arise out of strong electronic correlations unique to this class of materials. In this review, we will focus on one of the unique aspects of oxides: the ultra-high-conductivity metallic state, which can be an important component for future all-oxide microelectronics such as low-loss interconnects and gate-metals, spintronics, as well as future quantum technologies that employ emergent magnetic or superconducting ground states. Like most oxides, a major challenge to understanding and ultimately integrating high-conductivity metals into new technologies is the ability to synthesize high-quality materials. Therefore, we will frame the discussions in the context of epitaxial film growth via molecular beam epitaxy (MBE), which has provided insights into the electronic behavior of specific materials while providing samples with unprecedented quality. We will highlight and underscore how MBE has enabled developments and deeper understanding of their properties and how it plays a critical role in the future of this unique class of materials.

This paper shows how data‐driven machine learning approaches can improve growth control, reproducibility, and physical insight in the pulsed laser deposition (PLD) growth of correlated oxides. Despite well‐known relationships between growth conditions and material properties, consistently producing high‐quality films of complex materials like remains difficult due to the highly non‐equilibrium nature of PLD and the defects and competing phases that accumulate during growth. Here, we use an active learning framework based on Gaussian process Bayesian optimization that incorporates measured bulk and surface lattice properties along with impurity phase information to efficiently map the multidimensional growth space of by PLD. By tuning the relative weighting of these properties, the model identifies an optimized region where phase‐pure films of exhibit two‐dimensional surfaces, near‐ideal lattice parameters, and minimal sub‐bandgap optical absorption. The trained model reveals clear competition among different defect formation mechanisms that are connected to unseen parameters like supersaturation and surface mobility, thus giving insight into the highly non‐equilibrium process of PLD growth. Together, this demonstrates that property‐guided machine learning can accelerate materials optimization while providing a new way to address fundamental growth mechanisms in PLD that enable understanding and utilization of quantum phenomena found in complex oxides.

Phase selecting nearly degenerate crystalline polymorphs during epitaxial growth can be challenging yet critical to targeting physical properties for specific applications. Here, we establish how phase selectivity of altermagnetic and noncentrosymmetric polymorphs of MnTe can be programmed by subtle changes to the surface of lattice-matched InP substrates in molecular beam epitaxy growth. Bulk altermagnetic MnTe is thermodynamically stable in the hexagonal NiAs-structure and is synthesized here on the polar (111)A surface (In-terminated) of InP, while the noncentrosymmetric, cubic ZnS-structure with wide band gap (>3 eV), which epitaxially matches III-V materials, is stabilized on the (111)B surface (P-terminated). Electron microscopy, X-ray photoemission spectroscopy, and reflection high-energy electron diffraction indicate that phase selection is triggered at the interface and proceeds along the growing surface. First-principles calculations suggest that interfacial termination and strain have a significant effect on the interfacial energy; stabilizing the NiAs polymorph on the In-terminated surface and the ZnS structure on the P-terminated surface. Selectively grown, high-quality, phase pure films of both MnTe polymorphs will enable our understanding of the novel properties of these materials, thereby facilitating their use in new applications ranging from spintronics to microelectronic devices.

The epitaxial synthesis of high‐quality 2D layered materials is an essential driver of both fundamental physics studies and technological applications. Bi ₂ Se ₃ , a prototypical 2D layered topological insulator, is sensitive to defects imparted during the growth, either thermodynamically or due to the film‐substrate interaction. In this study, it is shown that step‐terminated Al ₂ O ₃ substrates with a high miscut angle (3°) can effectively suppress a particular hard‐to‐mitigate defect, the antiphase twin. Systematic investigations across a range of growth temperatures and substrate miscut angles confirm that atomic step edges act as preferential nucleation sites, stabilizing a single twin domain. First‐principles calculations suggest that there is a significant energy barrier for twin boundary formation at step edges, supporting the experimental observations. Detailed structural characterization indicates that this twin‐selectivity is lost through the mechanism of the 2D layers overgrowing the step edges, leading to higher twin density as the thickness increases. These findings highlight the complex energy landscape unique to 2D materials that is driven by the interplay between substrate properties, nucleation dynamics, and defect formation, and overcoming and controlling these are critical to improve material quality for quantum and electronic applications.

Quantum materials with novel spin textures from strong spin‐orbit coupling (SOC) are essential components for a wide array of proposed spintronic devices. Topological insulators have a necessary strong SOC that imposes a unique spin texture on topological states and Rashba states that arise on the boundary, but there is no established methodology to control the spin texture reversibly. Here, it is demonstrated that functionalizing Bi ₂ Se ₃ films by altering the step‐edge termination directly changes the strength of SOC and thereby modifies the Rashba strength of 1D edge states. Scanning tunneling microscopy/spectroscopy shows that these Rashba edge states arise and subsequently vanish through the Se functionalization and reduction process of the step edges. The observations are corroborated by density functional theory calculations, which show that a subtle chemical change of edge termination fundamentally alters the underlying electronic structure. Importantly, fully reversible and repeatable switching of Rashba edge states across multiple cycles at room temperature is experimentally demonstrated. The results imply Se functionalization as a practical method to control SOC and spin texture of quantum states in topological insulators.

Understanding fundamental properties of materials is necessary for all modern electronic technologies. Toward this end, the fabrication of new ultrapure thin film materials is critical to discover and understand novel properties that can allow further development of technology. Oxide materials are a vast material class abound with diverse properties, and, therefore, harnessing such phases is critical for realizing emerging technologies. Pushing forward, however, requires understanding basic properties of insulating, semiconducting and metallic oxides, as well as the more complex phases that arise out of strong electronic correlations unique to this class of materials. In this review, we will focus on one of the unique aspects of oxides: the ultra-high conductivity metallic state, which can be a critical component for future all-oxide microelectronics such as low-loss interconnects and gate-metals, spintronics, as well as future quantum technologies that employ emergent magnetic or superconducting ground states. Like most oxides, a critical challenge to understanding and ultimately integrating high-conductivity metals into new technologies is the ability to synthesize high-quality materials. Therefore, we will frame the discussions in the context of epitaxial film growth via molecular beam epitaxy (MBE), which has provided insights into the electronic behavior of specific materials while providing samples with unprecedented quality. We will highlight and underscore how MBE has enabled developments and deeper understanding of their properties and how it plays a critical role in the future of this unique class of materials.