arXiv subjects

Stephanie Law

Publications and source records attributed to Stephanie Law.

At least 19 recordsLinked to original sources

arXiv2026

Transport Evidence of Magnetic Polarization in the Altermagnetic Candidate MnTe

The ability to precisely control magnetic properties is central to the development of future spin-based electronics. In this work, we report the successful growth of epitaxial {\alpha}-MnTe thin films on InP(111) substrates using molecular beam epitaxy. Magneto-transport measurements at low temperatures reveal a distinct, hysteretic butterfly longitudinal magnetoresistance alongside a nonlinear transverse magneto-resistance response, suggesting the presence of a finite net magnetic polarization in the films. To understand the origin of this behavior, density functional theory (DFT) calculations were performed. While pristine bulk MnTe is a compensated antiferromagnet, our computational results suggest multiple pathways through which a finite magnetization can emerge in thin-film geometries, including interface-induced symmetry breaking and point defects. These findings demonstrate an epitaxial route for engineering magnetic responses in thin films.

cond-mat.mtrl-sci↗

arXiv2026

Molecular Beam Epitaxy of Mn2In2Se5 van der Waals Layers Using Mn Intercalation

The weak van der Waals (vdW) force in layered chalcogenide materials has enabled the growth of ternary chalcogenide layers using unconventional approaches. Here, we report the molecular beam epitaxy (MBE) growth of Mn2In2Se5, a spin glass material with high level of magnetic frustration, through the heterointegration of MnSe on In2Se3. Directly depositing {\alpha}-MnSe on the vdW In2Se3 layers results in Mn intercalation, transforming the In2Se3 layer into Mn2In2Se5. Large growth windows, including substrate temperatures from 250-450 {\deg}C and Se:Mn flux ratio of 1.1-3.1, have been identified for the intercalation process. With an optimized MnSe deposition time, smooth, single-crystalline, and (0001)-oriented Mn2In2Se5 layers with a root-mean-square (RMS) roughness of 1.5 nm can be synthesized. Further extending the MnSe deposition time results in the growth of uniform rock-salt structured {\alpha}-MnSe (111) layers with a thickness of up to 8 nm and a narrow full-width-at-half-maximum (FWHM) of 0.35{\deg} in MnSe (222) XRD rocking curves. This report presents a unique approach for the growths of uniform and single-crystalline Mn2In2Se5 vdW layers using MBE, and potentially opens a pathway for synthesis of ternary vdW chalcogenides by intercalation of new atomic species in binary vdW chalcogenides.

cond-mat.mtrl-sci↗

arXiv2026

Multi-modal machine learning analysis of GaSe molecular beam epitaxy growth conditions

Autonomous synthesis platforms integrating machine learning with in situ diagnostics have the potential to revolutionize thin-film growth by enabling real-time process optimization and reducing the need for manual tuning. However, their application to molecular beam epitaxy (MBE) remains underdeveloped. Here, we present a machine learning-guided framework for MBE growth of GaSe films, leveraging reflection high-energy electron diffraction (RHEED) as an in situ diagnostic alongside ex situ characterization via X-ray diffraction and atomic force microscopy. Unsupervised learning on RHEED patterns reveals a well-defined boundary between high- and low-quality samples, capturing physically meaningful features. Mutual information analysis shows a strong correlation between RHEED embeddings and rocking curve full-width at half-maximum (fwhm), while the correlation with AFM root-mean-square (RMS) roughness is weak. Among key growth conditions, growth rate most strongly influences fwhm, whereas the Se/Ga flux ratio primarily affects RMS roughness and the RHEED embeddings. Supervised learning models trained to predict fwhm and RMS roughness demonstrate moderate accuracy, with significant improvement achieved by incorporating RHEED embeddings. Furthermore, anomaly detection via residual analysis in supervised learning aligns well with unsupervised classification from RHEED, reinforcing the reliability of the predictive models. This study establishes a data-driven framework for machine learning-assisted MBE, paving the way for real-time process control and accelerated optimization of thin-film synthesis.

cond-mat.mtrl-sci↗

arXiv2026

Molecular Beam Epitaxy Growth of Wafer-scale SnSe van der Waals Ultrathin Layers

Tin selenide (SnSe) is a van der Waals (vdW) layered post-transition metal monochalcogenide compound which is promising for a wide range of device applications when its thickness is reduced to a few layers. Hence, developing a mature synthesis technique to obtain wafer-scale, high-quality ultrathin SnSe layers is crucial. In this work, we present a comprehensive study on the effect of growth parameters on the material quality of ultrathin SnSe thin films grown by molecular beam epitaxy. A growth window including substrate temperature of 210-270{\deg}C and low Se/Sn flux ratio with Se valve position of 10-30 mils has been identified which results in SnSe films with root-mean-square (RMS) roughness as low as 0.6 nm and full-width-at-half-maximum (FWHM) of 0.1{\deg} in SnSe (400) x-ray diffraction (XRD) rocking curve. Finally, using a three-step growth approach, we demonstrate wafer-scale coalesced ultrathin SnSe layers with thicknesses from 20 nm down to 5 nm, with good crystallinity, structural quality, and surface morphology. This work establishes a growth condition framework for MBE-grown SnSe and presents a viable route for developing wafer-scale single-layer films, unlocking the potential of this highly promising material for advanced device integration.

cond-mat.mtrl-sci↗

arXiv2026

Machine Learning Guided Polymorph Selection in Molecular Beam Epitaxy of In2Se3

Indium selenide (In2Se3), a layered chalcogenide with multiple polymorphs, is a promising material for optoelectronic and ferroelectric applications. However, achieving polymorph-pure thin films remains a major challenge due to the complex growth space. In this work, Bayesian optimization (BO) is successfully leveraged to guide the molecular beam epitaxy growth of In2Se3 on Al2O3 substrates. By training a predictive Gaussian process regressor with sequential learning, we efficiently explored substrate temperature, indium flux, selenium flux, and cracker temperature, reducing experimental trials required for successful synthesis. A {\gamma}-In2Se3 film with 91% phase purity was achieved in fewer than 10 BO run samples. Attempts to isolate {\alpha}-In2Se3 were limited by amorphous film formation at low temperatures, indicating that single-step codeposition is unsuitable for crystalline {\alpha}-In2Se3 on Al2O3. Overall, this study validates BO as a powerful approach for phase-selective growth in complex material systems.

cond-mat.mtrl-sci↗

arXiv2025

Examining the Spin Structure of Altermagnetic Candidate MnTe Grown with Near Ideal Stoichiometry

Altermagnets are a recently-discovered class of materials with magnetic ordering that have a zero net magnetization and a momentum-dependent spin splitting in their band structure, arising from a collinear spin arrangement with alternating polarizations in the crystal lattice. The nickeline-structured manganese telluride ({\alpha}-MnTe) is an attractive altermagnet candidate due to its predicted large spin splitting energy and a transition temperature near 300K. In this work, we present a thorough investigation of the spin structure of {\alpha}-MnTe thin films grown by molecular beam epitaxy with very high crystal quality and low residual magnetization. The epitaxial {\alpha}-MnTe films have a full-width-at-half-maximum of 0.1{\deg} as measured by x-ray-diffraction rocking curves and a root-mean-square roughness below 1 nm. Neutron diffraction measurements confirm the antiferromagnetic order in the {\alpha}-MnTe film and show a N\'eel temperature of 307 K. Polarized neutron reflectometry detects a vanishingly small net magnetization which may be confined to the MnTe/InP interface, highlighting the near-ideal stoichiometry in the sample. In vacuo angle resolved photoemission spectroscopy reveals that the bulk band spectrum of the MnTe films is consistent with the weak altermagnetic order as theoretically predicted and observed for the high symmetry nodal plane in the center of the Brillouin zone. This study establishes optimized growth conditions for the synthesis of stoichiometric {\alpha}-MnTe thin films which exhibit exceptional structural and magnetic ordering, thereby providing a robust platform for the precise characterization of their altermagnetic properties.

cond-mat.mtrl-sci↗

arXiv2025

Perspective: Mitigation of structural defects during the growth of two-dimensional van der Waals chalcogenides by molecular beam epitaxy

The growth of wafer-scale van der Waals (vdW) thin films and heterostructures by molecular beam epitaxy (MBE) is important for future applications in quantum technologies, next generation optoelectronic devices, and fundamental physics investigations. When grown using co-deposition methods that are typically used for compound semiconductor MBE, vdW materials typically show a high density of structural defects including twin or antiphase domains, spiral growth, and pyramidal growth. These defects are caused by the relatively weak film/substrate interaction and/or the poor wettability of typical substrates by many vdW materials. These difficulties can be mitigated using a multi-step growth procedure in which growth stages including nucleation and coalescence can be rigorously controlled, resulting in high-quality deposition of vdW thin films. This article will describe a general recipe for the growth of highly-crystalline wafer-scale vdW thin films by MBE.

cond-mat.mtrl-sci↗

arXiv2025

Structural phase transitions between layered Indium Selenide for inte-grated photonic memory

The primary mechanism of optical memristive devices relies on the phase transitions between amorphous-crystalline states. The slow or energy hungry amorphous-crystalline transitions in optical phase-change materials are detrimental to the devices scalability and performance. Leveraging the integrated photonic platform, we demonstrate a single nanosecond pulse triggered nonvolatile and reversible switching between two layered structures of indium selenide (In2Se3). High resolution pair distribution function reveals the detailed atomistic transition pathways between the layered structures. With inter-layer shear glide and isosymmetric phase transition, the switching between alpha and beta structural states contain low re-configurational entropy, allowing reversible switching between layered structures. Broadband refractive index contrast, optical transparency, and volumetric effect in the crystalline-crystalline phase transition are experimentally characterized in molecular beam epitaxy-grown thin films and compared to ab initials calculations. The nonlinear resonator transmission spectra measure an incremental linear loss rate of 3.3 GHz introduced by 1.5 micrometer long In2Se3 covered lay-er, resulting from the combinations of material absorption and scattering.

physics.optics↗

arXiv2025

The explanation dialogues: an expert focus study to understand requirements towards explanations within the GDPR

Explainable AI (XAI) provides methods to understand non-interpretable machine learning models. However, we have little knowledge about what legal experts expect from these explanations, including their legal compliance with, and value against European Union legislation. To close this gap, we present the Explanation Dialogues, an expert focus study to uncover the expectations, reasoning, and understanding of legal experts and practitioners towards XAI, with a specific focus on the European General Data Protection Regulation. The study consists of an online questionnaire and follow-up interviews, and is centered around a use-case in the credit domain. We extract both a set of hierarchical and interconnected codes using grounded theory, and present the standpoints of the participating experts towards XAI. We find that the presented explanations are hard to understand and lack information, and discuss issues that can arise from the different interests of the data controller and subject. Finally, we present a set of recommendations for developers of XAI methods, and indications of legal areas of discussion. Among others, recommendations address the presentation, choice, and content of an explanation, technical risks as well as the end-user, while we provide legal pointers to the contestability of explanations, transparency thresholds, intellectual property rights as well as the relationship between involved parties.

cs.CY↗

arXiv2024

Growth Conditions and Interfacial Misfit Array in SnTe (111) films Grown on InP (111)A Substrates by Molecular Beam Epitaxy

Tin telluride (SnTe) is an IV-VI semiconductor with a topological crystalline insulator band structure, high thermoelectric performance, and in-plane ferroelectricity. Despite its many applications, there has been little work focused on understanding the growth mechanisms of SnTe thin films. In this manuscript, we investigate the molecular beam epitaxy (MBE) synthesis of SnTe (111) thin films on InP (111)A substrates. We explore the effect of substrate temperature, Te:Sn flux ratio, and growth rate on the film quality. Using a substrate temperature of 340 {\deg}C, a Te:Sn flux ratio of 3, and a growth rate of 0.48 {\AA}/s, fully coalesced and single crystalline SnTe (111) epitaxial layers with x-ray rocking curve full-width-at-half-maxima (FWHM) of 0.09{\deg} and root-mean-square surface roughness as low as 0.2 nm have been obtained. Despite the 7.5% lattice mismatch between the SnTe (111) film and the InP (111)A substrate, reciprocal space mapping indicates that the 15 nm SnTe layer is fully relaxed. We show that a periodic interfacial misfit (IMF) dislocation array forms at the SnTe/InP heterointerface, where each IMF dislocation is separated by 14 InP lattice sites/13 SnTe lattice sites, providing rapid strain relaxation and yielding the high quality SnTe layer. This is the first report of an IMF array forming in a rock-salt on zinc-blende material system and at an IV-VI on III-V heterointerface, and highlights the potential for SnTe as a buffer layer for epitaxial telluride film growth. This work represents an important milestone in enabling the heterointegration between IV-VI and III-V semiconductors to create multifunctional devices.

cond-mat.mtrl-sci↗

arXiv2024

Quasi-van der Waals Epitaxial Growth of {\gamma}'-GaSe Thin Films on GaAs(111)B Substrates

GaSe is an important member of the post-transition metal chalcogenide family and is an emerging two-dimensional (2D) semiconductor material. Because it is a van der Waals material, it can be fabricated into atomic-scale ultrathin films, making it suitable for the preparation of compact, heterostructure devices. In addition, GaSe possesses unusual optical and electronic properties, such as a shift from an indirect-bandgap single-layer film to a direct-bandgap bulk material, rare intrinsic p-type conduction, and nonlinear optical behaviors. These properties make GaSe an appealing candidate for the fabrication of field-effect transistors, photodetectors, and photovoltaics. However, the wafer-scale production of pure GaSe single crystal thin films remains challenging. This study develops an approach for the direct growth of nanometer-thick GaSe films on GaAs substrates using molecular beam epitaxy. It yields smooth thin GaSe films with the rare {\gamma}'-polymorph. We analyze the formation mechanism of {\gamma}'-GaSe using density functional theory and speculate that it is stabilized by Ga vacancies since the formation enthalpy of {\gamma}'-GaSe tends to become lower than that of other polymorphs when the Ga vacancy concentration increases. Finally, we investigate the growth conditions of GaSe, providing valuable insights for exploring 2D/3D quasi-van der Waals epitaxial growth.

cond-mat.mtrl-sci↗

arXiv2024

Review of Nanolayered Post-transition Metal Monochalcogenides: Synthesis, Properties, and Applications

Nanolayered post-transition metal monochalcogenides (PTMMCs) stand out as promising advanced two-dimensional (2D) materials. Beyond inheriting the general advantages associated with traditional 2D materials, they exhibit unique properties, including a wide bandgap range covering the ultraviolet to the mid-infrared spectral ranges, thickness-dependent bandgap behaviors, good nonlinear optical performance, high thermoelectric coefficients, and ferroelectricity. Consequently, these materials hold significant potential in diverse applications such as photodetectors, field effect transistors, thermoelectrics, ferroelectrics, photovoltaics, and electrochemical devices, especially in the manufacturing of nanoscale devices. However, there is still a lack of systematic understanding of the PTMMC family. This study provides a broad overview of the crystal structures, bandgap structures, synthesis methods, physical properties, and state-of-the-art applications of PTMMC materials with a motif of X-M-M-X (M=Ga, In, Ge, Sn; X=S, Se, Te). An outlook for the development trends is emphasized at the end, underscoring the critical importance of this work to the future exploration of nanolayered PTMMCs.

cond-mat.mtrl-sci↗

arXiv2024

Treatment and Aging Studies of GaAs(111)B Substrates for van der Waals Chalcogenide Film Growth

GaAs(111)B is a semiconductor substrate widely used in research and commercial fields due to its low cost, mature synthesis technology, and excellent properties for manufacturing electronic devices. It is not only used to grow three-dimensional (3D) strongly-bonded materials, but has also been used as a substrate for layered, van der Waals (vdW)-bonded chalcogenide film growth. However, GaAs(111)B wafers cannot be directly used for growing epitaxial vdW chalcogenide films for two reasons: (1) the GaAs surface has a substantial number of dangling bonds that need to be passivated for vdW layers growth; (2) the substrate surface is covered with a thin epi-ready oxide layer which must be removed before film growth. In this paper, we optimize the method for deoxidizing GaAs(111)B substrates under a Se overpressure and successfully create a smooth, deoxidized, and passivated substrate for subsequent growth of vdW chalcogenide materials. We demonstrate the benefits of this method for the growth of vdW chalcogenide thin films using GaSe as a representative of vdW chalcogenides. In addition, we find that severely aged substrates have difficulty maintaining a smooth surface during the deoxidation and passivation process and cause GaSe crystals to nucleate in random shapes and orientations. We describe a method using water droplet testing to determine the age of the substrate. Finally, X-ray photoelectron spectroscopy (XPS) characterization reveals that the natural aging of GaAs(111)B in the air results in an increase in surface oxides, Ga2O3 and As2O3, while exposure to ultraviolet (UV)-ozone not only enhances the contents of these two oxides but also generates a new oxide, As2O5. Our research contributes to expanding the compatibility of GaAs(111)B with diverse growth materials and the production of high-quality heterostructure devices.

cond-mat.mtrl-sci↗

arXiv2023

Spectroscopy of van der Waals nanomaterials: Opportunities and Challenges

The study of van der Waals (vdW) materials has seen increased interest in recent years, due to the wide range of uses for these materials because of their unique mechanical, electronic, and optical properties. This area has recently expanded further into studying the behavior of vdW nanomaterials, as decreasing dimensions open up opportunities to interact with these materials in new ways. However, measuring the band structures of nanomaterials, which is key to understanding how confinement affects material properties and interactions, comes with several challenges. In this review, we survey a range of techniques for synthesizing and characterizing vdW nanomaterials, in order to outline the key material and characterization challenges. This includes controlling the Fermi level in vdW nanoparticles, preparing these particles for either ensemble or individual particle measurement, as well as protecting the pristine surface from oxidation.

cond-mat.mes-hall↗

arXiv2023

Epitaxial growth of atomically thin Ga2Se2 films on c-plane sapphire substrates

Broadening the variety of two-dimensional (2D) materials and improving the synthesis of ultrathin films are crucial to the development of the semiconductor industry. As a state-of-the-art 2D material, Ga2Se2 has attractive optoelectronic properties when it reaches the atomically-thin regime. However, its van der Waals epitaxial growth, especially for the atomically-thin films, has seldom been studied. In this paper, we used molecular beam epitaxy to synthesize Ga2Se2 single-crystal films with a surface roughness down to 1.82 nm on c-plane sapphire substrates by optimizing substrate temperature, Se:Ga flux ratio, and growth rate. Then we used a 3-step mode to grow Ga2Se2 films with a thickness as low as 3 tetralayers and a surface roughness as low as 0.61 nm, far exceeding the performance of direct growth. Finally, we found that the surface morphology strongly depends on the Se:Ga flux ratio, and higher growth rates widened the suitable flux ratio window for growing Ga2Se2. Overall, this work advances the understanding of the vdW epitaxy growth mechanism for post-transition metal monochalcogenides on sapphire substrates.

cond-mat.mtrl-sci↗

arXiv2022

Phonon-mediated strong coupling between a three-dimensional topological insulator and a two-dimensional antiferromagnetic material

Van der Waals antiferromagnetic and topological insulator materials provide powerful platforms for modern optical, electronic, and spintronic devices applications. The interaction between an antiferromagnet (AFM) and a topological insulator (TI), if sufficiently strong, could offer emergent hybrid material properties that enable new functionality exceeding what is possible in any individual material constituent. In this work, we study strong coupling between THz excitations in a three dimensional (3D) topological insulator and a quasi-two dimensional (2D) antiferromagnetic material resulting in a new hybridized mode, namely a surface Dirac plasmon-phonon-magnon polariton. We find that the interaction between a surface Dirac plasmon polariton in the 3D TI and a magnon polariton in the 2D AFM is mediated by the phonon coupling in the 3D TI material. The coupling of phonons with an electromagnetic wave propagating in the 3D TI enhances the permittivity of the TI thin film in a way that results in a strong correlation between the dispersion of Dirac plasmon polaritons on the surfaces of the TI with the thickness of the TI. As a result, the dispersion of surface Dirac plasmon polaritons in the TI can be tuned toward resonance with the magnon polariton in the AFM material by varying the TI's thickness, thereby enhancing the strength of the coupling between the excitations in the two materials. The strength of this coupling, which results in the surface Dirac plasmon-phonon-magnon polariton, can be parameterized by the amplitude of the avoided-crossing splitting between the two polariton branches at the magnon resonance frequency...

cond-mat.mes-hall↗

arXiv2022

Topological insulator-based Dirac hyperbolic metamaterial with large mode indices

Hyperbolic metamaterials (HMMs) are engineered materials with a hyperbolic isofrequency surface, enabling a range of novel phenomena and applications including negative refraction, enhanced sensing, and subdiffraction imaging, focusing, and waveguiding. Existing HMMs primarily work in the visible and infrared spectral range due to the inherent properties of their constituent materials. Here we demonstrate a THz-range Dirac HMM using topological insulators (TIs) as the building blocks. We find that the structure houses up to three high-wavevector volume plasmon polariton (VPP) modes, consistent with transfer matrix modeling. The VPPs have mode indices ranging from 126 to 531, 10-100x larger than observed for VPP modes in traditional media while maintaining comparable quality factors. We attribute these properties to the two-dimensional Dirac nature of the electrons occupying the topological insulator surface states. Because these are van der Waals materials, these structures can be grown at a wafer-scale on a variety of substrates, allowing them to be integrated with existing THz structures and enabling next-generation THz optical devices.

physics.optics↗

arXiv2022

Growth of Ultrathin Bi$_2$Se$_3$ Films by Molecular Beam Epitaxy

Bi$_2$Se$_3$ is a widely studied 3D topological insulator having potential applications in optics, electronics, and spintronics. When the thickness of these films decrease to less than approximately 6 nm, the top and bottom surface states couple, resulting in the opening of a small gap at the Dirac point. In the 2D limit, Bi$_2$Se$_3$ may exhibit quantum spin Hall states. However, growing coalesced ultra-thin Bi$_2$Se$_3$ films with a controllable thickness and typical triangular domain morphology in the few nanometer range is challenging. Here, we explore the growth of Bi$_2$Se$_3$ films having thickness down to 4 nm on sapphire substrates using molecular beam epitaxy that were then characterized with Hall measurements, atomic force microscopy, and Raman imaging. We find that substrate pre-treatment -- growing and decomposing a few layers of \BiSe before the actual deposition -- is critical to obtaining a completely coalesced film. In addition, higher growth rates and lower substrate temperatures led to improvement in surface roughness, in contrast to what is observed for conventional epitaxy. Overall, coalesced ultra-thin Bi$_2$Se$_3$ films with lower surface roughness enables thickness-dependent studies across the transition from a 3D-topological insulator to one with gapped surface states in the 2D regime.

cond-mat.mtrl-sci↗