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Sieun and Nguyen give talks at EMA!

January 26, 2021 By John Heron

Sieun and Nguyen both gave virtual talks at ACerS Electronic Materials and Applications (EMA) last week. Congratulations! Their abstracts are included below.

Sieun’s Abstract: Rutile GeO2: an ultra-wide-band-gap semiconductor for power electronics. Ultra-wide-band-gap (UWBG) semiconductors have tantalizing advantages for power electronics as their wider band gaps enable higher breakdown voltages. A handful of materials such as AlN/AlGaN, -Ga2O3, and diamond have been developed for UWBG semiconducting devices, however, they are still facing numerous challenges, such as doping asymmetry and/or inefficient thermal conduction. In our work, we have identified rutile GeO2 (r-GeO2) to be a promising, yet unexplored UWBG (4.68 eV) semiconductor. Our first-principles calculations predict shallow ionization energies for donors such as Sb, As, and F, a phonon-limited electron mobility of 289 cm2 V-1s-1, and a breakdown electric field of 7.0 MV cm-1, which lead to a higher Baliga figure of merit than -Ga2O3. We also predicted that p-type doping is promising in r-GeO2: the calculated ionization energy for Al acceptors is 0.45 eV and the calculated phonon-limited hole mobility is 27 cm2 V-1s-1. r-GeO2 also has superior thermal conductivity (45 W m–1 K–1 (calculated) and 51 W m–1 K–1 (experiment)) relative to -Ga2O3. Though the thin-film synthesis of r-GeO2 has remained challenging due to its highly metastable amorphous phase, we demonstrate the first synthesis of single crystalline epitaxial thin films of r-GeO2 on a sapphire substrate using ozone-assisted molecular beam epitaxy. Our work motivates further exploration of r-GeO2 as an alternative UWBG semiconductor that can overcome the limitations of the current state-of-the-art UWBG materials.

Nguyen’s abstract: Magnetic properties of thin single crystal Cr2O3 films. Magnetoelectric materials show potential for low-power spintronics via the electric field control of magnetization. Antiferromagnet Cr2O3 is a room temperature magnetoelectric yet the existence of twin domains in thin films grown on metallic electrodes leads to high leakage current and low dielectric breakdown fields. By using an isostructural epitaxial oxide electrode, V2O3, recent studies have shown the possible elimination of these twin domains. Dielectric properties of 200 nm thick films show improved performance, however, for next generation logic and memory the films must be scaled down. Here we present the electrical endurance and magnetic properties of very thin (30-60 nm) single crystal Cr2O3 films grown by pulsed laser deposition onto V2O3 buffered (0001) oriented Al2O3 substrates. A 60 nm single crystal thin film has bulk-like resistivity (1012 Ωcm) and significantly improved breakdown voltage (150-300 MV/m). From magnetometry of a Cr2O3/ferromagnet heterostructure, the blocking temperature is found to be at 285 K, higher than twinned films with similar or greater thickness in literature. Further, Second Harmonic Generation confirms bulk magnetoelectric order of our single crystal thin film at room temperature. These results indicate the importance of crystallinity to realize bulk like properties in very thin films at room temperature.

Filed Under: Conferences

New Publication! Multiferroic heterostructures for spintronics

January 4, 2021 By John Heron

Abstract: For next-generation technology, magnetic systems are of interest due to the natural ability to store information and, through spin transport, propagate this information for logic functions. Controlling the magnetization state through currents has proven energy inefficient. Multiferroic thin-film heterostructures, combining ferroelectric and ferromagnetic orders, hold promise for energy efficient electronics. The electric field control of magnetic order is expected to reduce energy dissipation by 2–3 orders of magnitude relative to the current state-of-the-art. The coupling between electrical and magnetic orders in multiferroic and magnetoelectric thin-film heterostructures relies on interfacial coupling though magnetic exchange or mechanical strain and the correlation between domains in adjacent functional ferroic layers. We review the recent developments in electrical control of magnetism through artificial magnetoelectric heterostructures, domain imprint, emergent physics and device paradigms for magnetoelectric logic, neuromorphic devices, and hybrid magnetoelectric/spin-current-based applications. Finally, we conclude with a discussion of experiments that probe the crucial dynamics of the magnetoelectric switching and optical tuning of ferroelectric states towards all-optical control of magnetoelectric switching events.

Full Text available from Physical Sciences Reviews

Filed Under: Publications

New Publication! Property and cation valence engineering in entropy-stabilized oxide thin films

October 19, 2020 By John Heron

Abstract: We present data for epitaxial thin films of the prototypical entropy-stabilized oxide (ESO), Mg0.2Ni0.2Co0.2Cu0.2Zn0.2O, that reveals a systematic trend in lattice parameter and properties as a function of substrate temperature during film growth with negligible changes in microstructure. A larger net Co valence in films grown at substrate temperatures below 350 °C results in a smaller lattice parameter, a smaller optical band gap, and stronger magnetic exchange bias. Observation of this phenomena suggests a complex interplay between thermodynamics and kinetics during ESO synthesis; specifically thermal history, oxygen chemical potential, and entropy. In addition to the compositional degrees of freedom available to ESO systems, subtle nuances in atomic structure at constant metallic element proportions can strongly influence properties, simultaneously complicating physical characterization and providing opportunities for property tuning and development.

Full text available from Physical Review Materials

Filed Under: Publications

New Publication! Thermal conductivity of rutile germanium dioxide

September 17, 2020 By John Heron

Abstract: Power electronics seek to improve power conversion of devices by utilizing materials with a wide bandgap, high carrier mobility, and high thermal conductivity. Due to its wide bandgap of 4.5 eV, β-Ga2O3 has received much attention for high-voltage electronic device research. However, it suffers from inefficient thermal conduction that originates from its low-symmetry crystal structure. Rutile germanium oxide (r-GeO2) has been identified as an alternative ultra-wide-bandgap (4.68 eV) semiconductor with a predicted high electron mobility and ambipolar dopability; however, its thermal conductivity is unknown. Here, we characterize the thermal conductivity of r-GeO2 as a function of temperature by first-principles calculations, experimental synthesis, and thermal characterization. The calculations predict an anisotropic phonon-limited thermal conductivity for r-GeO2 of 37 W m−1 K−1 along the a direction and 58 W m−1 K−1 along the c direction at 300 K where the phonon-limited thermal conductivity predominantly occurs via the acoustic modes. Experimentally, we measured a value of 51 W m−1 K−1 at 300 K for hot-pressed, polycrystalline r-GeO2 pellets. The measured value is close to our directionally averaged theoretical value, and the temperature dependence of ∼1/T is also consistent with our theory prediction, indicating that thermal transport in our r-GeO2 samples at room temperature and above is governed by phonon scattering. Our results reveal that high-symmetry UWBG materials, such as r-GeO2, may be the key to efficient power electronics.

Full text available from Applied Physics Letters

Filed Under: Publications

New Publication! Bulk-like dielectric and magnetic properties of sub 100 nm thick single crystal Cr2O3 films on an epitaxial oxide electrode

September 8, 2020 By John Heron

Abstract: The manipulation of antiferromagnetic order in magnetoelectric Cr2O3 using electric field has been of great interest due to its potential in low-power electronics. The substantial leakage and low dielectric breakdown observed in twinned Cr2O3 thin films, however, hinders its development in energy efficient spintronics. To compensate, large film thicknesses (250 nm or greater) have been employed at the expense of device scalability. Recently, epitaxial V2O3 thin film electrodes have been used to eliminate twin boundaries and significantly reduce the leakage of 300 nm thick single crystal films. Here we report the electrical endurance and magnetic properties of thin (less than 100 nm) single crystal Cr2O3 films on epitaxial V2O3 buffered Al2O3 (0001) single crystal substrates. The growth of Cr2O3 on isostructural V2O3 thin film electrodes helps eliminate the existence of twin domains in Cr2O3 films, therefore significantly reducing leakage current and increasing dielectric breakdown. 60 nm thick Cr2O3 films show bulk-like resistivity (~ 1012 Ω cm) with a breakdown voltage in the range of 150–300 MV/m. Exchange bias measurements of 30 nm thick Cr2O3 display a blocking temperature of ~ 285 K while room temperature optical second harmonic generation measurements possess the symmetry consistent with bulk magnetic order.

Full text available from Nature Scientific Reports

Filed Under: Publications

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Our research is at the intersection of multiple disciplines, drawing on principles and methodologies from materials science, chemistry, physics, and electrical engineering. Our mission is to pioneer … Read More

News

New Publication! “Engineering antiferromagnetic magnon bands through interlayer spin pumping”

March 28, 2025 By Avery-Ryan Ansbro

New Publication! “Polydopamine-Assisted Electroless Deposition of Magnetic Functional Coatings for 3D-Printed Microrobots”

January 31, 2025 By Avery-Ryan Ansbro

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