Sieun gave a virtual talk at the American Physical Society (APS) March Meeting last week. Congratulations! Her abstract is included below.

Epitaxial stabilization of rutile germanium oxide thin film by molecular beam epitaxy

Ultrawide-band-gap (UWBG) semiconductors have tantalizing advantages for power electronics. Materials such as AlN/AlGaN, β-Ga₂O₃, and diamond have been developed for UWBG semiconducting devices, however, they are still facing challenges, such as doping asymmetry and/or inefficient thermal conduction. Rutile GeO₂ (r-GeO₂) has been theoretically established to have an UWBG (4.68 eV), high electron and hole mobility (289 cm² V^-1s^-1 and 28 cm² V^-1s^-1), high thermal conductivity (51 W m^–1 K^–1) and ambipolar dopability. The synthesis of r-GeO₂ thin films has not been reported but is critical to enable microelectronics applications. Here, we report the growth of single-crystalline r-GeO₂ thin films on R-plane sapphire substrates using molecular beam epitaxy. We control the competing reactions between the deeply metastable glass phase formation and rutile phase formation as well as absorption and desorption by utilizing (1) a buffer layer with reduced lattice misfit, and (2) the growth condition that allows the condensation of the preoxidized molecular precursor yet provides sufficient adatom mobility. The findings advance the synthesis of single-crystalline films of materials prone to glass formation and provide opportunities to realize promising UWBG semiconductors.

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.

Sieun and Peter both gave contributed talks on entropy-stabilized oxides at ACerS EMA 2020 in Orlando last week.

Tunability of native defect density through local configuration-controlled disorder in entropy-stabilized oxides
S. Chae
Abstract: Entropic stabilization has become a strategy to create new oxide materials and novel properties, however, achieving an atomistic understanding of these properties has been challenged by the local compositional and structural disorder that underlies their fundamental structure-property relationships. Here, we combine high-throughput atomistic calculations, machine-learning algorithms, and experimental characterization to investigate the role of local configurational and structural disorder on the thermodynamics of intrinsic point defects in (MgCoNiCuZn)O-based entropy-stabilized oxides (ESOs) and their influence on the electrical properties. From theory, we find that the cation-vacancy formation energy decreases with increasing local tensile strain, while oxygen-vacancy formation depends on the local structural distortion associated with the local configuration of chemical species. Through relatively small changes in the mole fraction of cations, the equilibrium defect density can be tuned by over two orders of magnitude. Vacancies in ESOs exhibit deep thermodynamic transition levels leading to transport via variable range hopping. Our results motivate tuning local structural distortions by local alloy composition as an engineering principle to enable controlled defect formation in multi-component oxides.

Electronic and magnetic interplay in entropy stabilized oxide thin films
P. Meisenheimer
Abstract: A unique benefit to entropic stabilization is the increased solubility of elements, which opens a broad compositional space with subsequent local A unique benefit to entropic stabilization is the increased solubility of elements, which opens a broad compositional space with subsequent local chemical and structural disorder resulting from different atomic sizes and preferred coordinations of the constituents. In the antiferromagnetic entropy-stabilized oxides studied here, we see that by tuning the chemistry, and thus the concentration of local structural distortions, we can either induce or reclaim a large degree of frustration in the magnetic lattice of the material. As the large dielectric response of these materials may be strongly linked to their structure, here we study the interplay of electronic and magnetic functional responses in entropy-stabilized oxides. Our results reveal that the unique characteristics of entropy stabilized materials can be utilized to engineer and enhance magnetic functional phenomena in oxide thin films, as well as offer a powerful platform for the study of defects and functional properties.

Last week at MRS fall meeting, Peter gave a talk on composite multiferroic materials and Nguyen presented a poster on thin film Cr₂O₃.

Nguyen’s poster was titled: Electrical and Magnetic Properties of Thin Single Crystal Cr2O3 Films 
Abstract: Magnetoelectric materials have been of great interest due to their potential for low-power spintronic devices via the electric field switching of magnetization. Antiferromagnet Cr2O3 is one of a very few room temperature magnetoelectrics and possesses unique properties such as uncompensated surface spins and perpendicular magnetic anisotropy. [1] Since the first demonstration of the electric field control of exchange bias in bulk single crystal Cr2O3 heterostructures [2], intense effort has focused the demonstration of magnetoelectric switching using Cr2O3 thin films at room temperature. [3,4] The existence of twin domains in thin films grown on metallic electrodes, however, leads to high leakage current and dielectric breakdown fields that can only be circumvented by growing rather thick films (250-500 nm). [4,5] By using an isostructural epitaxial oxide electrode, V2O3, recent studies have shown the reduction and even possible elimination of twin domains in Cr2O3 films. [3] Dielectric and magnetoelectric switching studies of 200 nm thick films show bulk like performance, however, for next generation logic and memory the films must be scaled down. [6] Here we present an investigation of 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. Our results show that 60 nm single crystal thin film has bulk-like resistivity ( 10^12 cm) and significantly improved breakdown voltage (150-300 MV/m). Using magnetometry, we investigate exchange bias of thin film Cr2O3/ferromagnet heterostructure. The blocking temperature is found to be at 285 K which is higher compared to twinned films with similar or greater thickness in literature. [7] 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.

[1] X. He, Y. Wang, N. Wu, A. N. Caruso, E. Vescovo, K. D. Belashchenko, P. A. Dowben, and C. Binek, Nat. Mater. 9, 579 (2010).
[2] P. Borisov, A. Hochstrat, X. Chen, W. Kleemann, and C. Binek, Phys. Rev. Lett. 94, 117203 (2005).
[3] T. Kosub, M. Kopte, R. Hühne, P. Appel, B. Shields, P. Maletinsky, R. Hübner, M. O. Liedke, J. Fassbender, O. G. Schmidt, and D. Makarov, Nat.
Commun. 8, 13985 (2017).
[4] T. Ashida, M. Oida, N. Shimomura, T. Nozaki, T. Shibata, and M. Sahashi, Appl. Phys. Lett. 106, 132407 (2015).
[5] C. Sun, Z. Song, A. Rath, M. Street, W. Echtenkamp, J. Feng, C. Binek, D. Morgan, and P. Voyles, Adv. Mater. Interfaces 4, 1700172 (2017).
[6] A. Mahmood, M. Street, W. Echtenkamp, C. P. Kwan, J. P. Bird, and C. Binek, Phys. Rev. Mater. 2, 044401 (2018).
[7] N. Shimomura, S. P. Pati, T. Nozaki, T. Shibata, and M. Sahashi, AIP Adv. 7, 025212 (2017). 

Peter talk was titled: Epitaxially engineered, enhanced magnetostriction in a strain-driven composite multiferroic
Abstract: Composite multiferroics, composed of a magnetostrictive ferromagnet and a piezoelectric ferroelectric, have widely been targeted for beyond-CMOS logic due to their large coupling coefficients and high operating temperature^1–3. Magnetoelectric multiferroic systems potentially offer the lowest energy dissipation per bit operation in a scalable platform, yet significant materials challenges still exist in the field. For composite multiferroics, this requires finding pathways to enhance piezomagnetic effects and coupling between layers, an effort that has seen relatively little work⁴. Here, we present a means to boost the magnetostriction of Fe_1-xGa_x alloys and magnetoelectric coupling in a Fe_1-xGa_x -(PMN-PT)composite multiferroic heterostructure through epitaxy.
In bulk, the magnetostriction coefficient of Fe_1–xGa_x alloys versus Ga composition peaks near ~18% Ga occurring due to a phase change from the disordered A2 phase to an ordered BCC phase (D0₃), which reduces the magnetostriction coefficient⁵. A distinct advantage of thin film deposition is the potential to access metastable phases through epitaxy, allowing us to promote the chemically disordered BCC (A2) phase in our film at high (22%) Ga concentrations. We demonstrate that thin film epitaxy stabilizes a chemically disordered BCC Fe_0.78Ga_0.22 alloy where the magnetostriction is enhanced by 200-300% relative to the bulk.
Transport-based magnetoelectric characterization shows 90 electrical switch of magnetic anisotropy and one of the largest converse magnetoelectric coefficients ever achieved at room temperature in a composite multiferroic. Energy dissipation per operation scales to 5.9 J cm^-2, making our devices competitive with other state-of-the-art beyond CMOS technologies⁶. This hyperactive performance is achieved through epitaxial stabilization of a disordered, metastable phase of earth-abundant and rare-earth-free magnetostrictor, Fe_0.78Ga_0.22. By epitaxially engineering our ferromagnetic layer to prevent the formation of deleterious intermetallic nanoregions, we provide a pathway to engineering new performance levels in rare-earth free magnetoelastic and magnetoelectric heterostructures.

1. Meisenheimer, P. B., Novakov, S., Vu, N. M. & Heron, J. T. Perspective: Magnetoelectric switching in thin film multiferroic heterostructures. J. Appl. Phys. 123, 240901 (2018).
2. Ramesh, R. & Spaldin, N. A. Multiferroics: progress and prospects in thin films. Nat. Mater. 6, 21–29 (2007).
3. Bibes, M. & Barthélémy, A. Multiferroics: Towards a magnetoelectric memory. Nat. Mater. 7, 425–426 (2008).
4. Shevlin, S. Multiferroics and the path to the market. Nat. Mater. 18, 191 (2019).
5. Du, Y. et al. Relation between Ga ordering and magnetostriction of Fe-Ga alloys studied by x-ray diffuse scattering. Phys. Rev. B 81, 054432 (2010).
6. Manipatruni, S. et al. Scalable energy-efficient magnetoelectric spin–orbit logic. Nature 565, 35–42 (2019).