Stephanie Law

University of Delaware

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Zhengtianye Wang

University of Delaware

Daniel Wasserman

The University of Texas at Austin

Matthew F. Doty

University of Delaware

Joshua M. O. Zide

University of Delaware

Sivakumar Vishnuvardhan Mambakkam

University of Delaware

John Q. Xiao

University of Delaware

P. C. Canfield

Iowa State University

Theresa P. Ginley

University of Delaware

Yong Wang

Zhejiang University

Patrick Sohr

Microsoft (United States)

Cooperative Institutions

University of Delaware

University of Illinois Urbana-Champaign

Pennsylvania State University

Iowa State University

Ames National Laboratory

Massachusetts Institute of Technology

Chinese Academy of Sciences

University of Chinese Academy of Sciences

DuPont (United States)

University of California, Santa Barbara

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Semiconductors for infrared plasmonics and hyperbolic metamaterials

Stephanie Law

Transformation Optics

10.1117/12.2649321

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

ACS Applied Nano Materials (2024)

Mingyu Yu Maria Hilse Qihua Zhang Yongchen Liu Zhengtianye Wang

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.

10.1021/acsanm.3c05984

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Coupled Dirac Plasmons in Topological Insulators

Advanced Optical Materials (2018)

Theresa P. Ginley Stephanie Law

Abstract Topological insulators are predicted to house spin‐polarized 2D Dirac plasmons. In topological insulator thin films, Dirac plasmons on the top and bottom surfaces are coupled, giving rise to an unusual dispersion relationship. These plasmons are of interest both for fundamental science as well as applications in terahertz sensing and waveguiding. In this article, conclusive evidence is presented for coupled Dirac plasmons excited in the surface states of topological insulator thin film stripe arrays by investigating how the plasmon frequency depends on both film thickness and stripe width. Bi 2 Se 3 films of thickness 50–200 nm are patterned into periodic stripes of width 1–4 µm to excite plasmons. The plasmon frequency dependence on both film thickness and stripe width is found to be in good agreement with theoretical models for coupling 2D Dirac plasmons. Alternative explanations such as 2D massive plasmons are shown to be inconsistent with data. Finally, effective mode indexes of up to 211 are reported, demonstrating incredible confinement of light in the terahertz frequency range.

Topological insulator

10.1002/adom.201800113

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Citations (41)

Topological Insulator Film Growth by Molecular Beam Epitaxy: A Review

Crystals (2016)

Theresa P. Ginley Yong Wang Stephanie Law

In this article, we will review recent progress in the growth of topological insulator (TI) thin films by molecular beam epitaxy (MBE). The materials we focus on are the V2-VI3 family of TIs. These materials are ideally bulk insulating with surface states housing Dirac excitations which are spin-momentum locked. These surface states are interesting for fundamental physics studies (such as the search for Majorana fermions) as well as applications in spintronics and other fields. However, the majority of TI films and bulk crystals exhibit significant bulk conductivity, which obscures these states. In addition, many TI films have a high defect density. This review will discuss progress in reducing the bulk conductivity while increasing the crystal quality. We will describe in detail how growth parameters, substrate choice, and growth technique influence the resulting TI film properties for binary and ternary TIs. We then give an overview of progress in the growth of TI heterostructures. We close by discussing the bright future for TI film growth by MBE.

Topological insulator

Surface States

10.3390/cryst6110154

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Citations (87)

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

ACS Applied Materials & Interfaces (2024)

Qihua Zhang Maria Hilse Wesley Auker Jennifer L. Gray Stephanie Law

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 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 °C, a Te/Sn flux ratio of 3, and a growth rate of 0.48 Å/s, fully coalesced and single crystalline SnTe (111) epitaxial layers with X-ray rocking curve full-width-at-half-maxima of 0.09° 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.

Mosaicity

10.1021/acsami.4c10296

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Polariton design and modulation via van der Waals / doped semiconductor heterostructures

Research Square (Research Square) (2023)

Mingze He Joseph R. Matson Mingyu Yu Angela Cleri Sai Sunku

Abstract Hyperbolic phonon polaritons (HPhPs) can be supported in highly anisotropic materials, where the real parts of their permittivities along different directions are opposite in sign as a result of spectrally offset optical phonons. Compared to surface polaritons, HPhPs offer further confinement of long-wavelength light to deeply subdiffractional scales, and volume propagation that enables control of the polariton wavevector by changing the underlying medium. This allows for greater control of polaritonic resonators and near-field polariton propagation without deleterious etching of hyperbolic materials. Yet, conventionally used noble metal and dielectric substrates restrict the tunability of this approach, leaving most of the wavevector inaccessible. To overcome this challenge, we demonstrate that using doped semiconductors, e.g., InAs and CdO, can enable near-continuous tuning and access to both the maximum and minimum wavevectors (~8.3 times experimentally demonstrated). We further elucidate HPhP tuning with the plasma frequency of an InAs substrate, which features a significant wavevector discontinuity and modal order transition when the substrate permittivity crosses -1 in the Reststrahlen band. Around the transition point, the HPhP system is sensitive to perturbations, e.g., the working frequency, InAs plasma frequency and superstrate, thus it is suitable for sensing and modulation applications. We also illustrate that the hBN/InAs platform allows for active modulation at picosecond timescales by photo-injecting carriers into the InAs substrate, demonstrating a dynamic wavevector change of ~20%. Overall, the demonstrated hBN/doped semiconductor platform offers significant improvements towards manipulating HPhPs, and enormous potential for engineered and modulated polaritonic systems for applications in on-chip photonics and planar metasurface optics.

Wave vector

10.21203/rs.3.rs-2744039/v1

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Making the Mid-IR nano with epitaxial plasmonic devices

Stephanie Law Christopher Roberts Sandeep Inampudi W. Streyer Aaron Rosenberg

We have extensively investigated heavily doped semiconductors as potential plasmonic metals at long wavelengths. The ability to control the doping level in a semiconductor material, both III-V's (InAs/InSb) and Silicon, allows for control of the metal's optical properties, and adds an intriguing additional controllable parameter to the design of plasmonic structures. These materials can be quite accurately modeled using the Drude formalism, even for energies larger than the band gap, and have a number of attractive qualities, including control of carrier concentration (and thus plasma frequency, ωρ), as well as single-crystal material quality, atomic-layer control of thicknesses, and the potential for integration with epitaxially-grown mid-IR optoelectronic devices. In this presentation, I will discuss recent developments in epitaxial plasmonic devices for mid-IR applications. First, the growth and characterization of our materials will be discussed, as well as the material limitations. Subsequently, I will demonstrate the doped semiconductors potential as epsilon-near-zero (ENZ) materials. At ENZ frequencies, we have demonstrated enhanced coupling to sub-wavelength waveguides, offering a potential route towards overcoming the mismatch between the micron-scale light of the mid-IR and the nano-scale. In addition, we have shown that near ENZ, these materials thin (d ≪ λo) loss-less dielectric films to serve as perfect absorbing layers, by controlling the metal/dielectric interface phase shift in thin film interference structures.

Nanophotonics

10.1109/drc.2014.6872391

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Citations (1)

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

arXiv (Cornell University) (2024)

Mingyu Yu Jiayang Wang Sahani Amaya Iddawela Molly M. McDonough Jessica L. Thompson

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.

Dangling bond

Passivation

10.48550/arxiv.2401.10425

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Epitaxial growth of engineered metals for mid-infrared plasmonics

Journal of Vacuum Science & Technology B Nanotechnology and Microelectronics Materials Processing Measurement and Phenomena (2013)

Stephanie Law Yu Lan Daniel Wasserman

The authors demonstrate the ability of high-quality epitaxial InAs films to be used as wavelength-flexible, low-loss, engineered plasmonic metals across the mid-infrared spectral range. Films are grown by molecular beam epitaxy and characterized by Hall effect measurements, atomic force microscopy, and infrared reflection and transmission spectroscopy. The losses of our plasmonic material are studied as a function of InAs doping density, growth rate, buffer layer type, and substrate type. High growth rates are shown to be integral to obtaining films with low losses and doping densities approaching 1×1020 cm−3.

10.1116/1.4797487

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Citations (54)

Materials Handling for Electrical Modification of a Complex Target Surface: Analysis and Feasibility

David Giles Stephanie Law Joseph W. Tringe

This project effort, conducted as feasibility investigations addresses the transport and deposition of particulates using traveling field, electrohydrodynamic atomization and gas carrier methods. The method of investigation was experimental, using existing experimental and pre-commercial apparatus. All methods were found to be successful to varying degrees. Preliminary results were presented at LLNL in a project review meeting. The most promising methods for particle delivery were electrodynamic atomization/spraying and gas-carrier propulsion. Traveling field delivery is limited by scale up considerations and the requirement for transport through close tolerances. Electrodynamic atomization requires use of low electrical conductivity liquid carrier phases but is scalable by ganging multiple orifices and atomizing tips. Gas carrier delivery is attractive because no liquid carrier is needed and momentum can higher than the other traveling field or electrodynamic processes. Subsequent phases of the project will address electrodynamic and gas-carrier delivery.

Electrohydrodynamics

Deposition

10.2172/945885

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Citations (2)