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.