Abstract: Ultrawide bandgap (UWBG) semiconductors (E_g >3 eV) have tremendous potential for power-electronic applications. The current state-of-the-art UWBG materials such as β-Ga₂O₃, diamond, and AlN/AlGaN, however, show fundamental doping and thermal conductivity limitations that complicate technological adaption and motivate the search for alternative materials with superior properties. Rutile GeO₂ (r-GeO₂) has been theoretically established to have an ultrawide bandgap (4.64 eV), high electron mobility, high thermal conductivity (51 W m⁻¹ K⁻¹), and ambipolar dopability. While single-crystal r-GeO₂ has been synthesized in bulk, the synthesis of r-GeO₂ thin films has not been previously reported but is critical to enable microelectronics applications. Here, we report the growth of single-crystalline r-GeO₂ thin films on commercially available R-plane sapphire substrates using molecular beam epitaxy. Due to a deeply metastable glass phase and high vapor pressure of GeO, the growth reaction involves the competition between absorption and desorption as well as rutile and amorphous formation. We control the competing reactions and stabilize the rutile-phase growth by utilizing (1) a buffer layer with reduced lattice misfit to reduce epitaxial strain 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 ultra-wide-bandgap semiconductors.

Full text available from Applied Physics Letters

Abstract: Historically, the enthalpy is the criterion for oxide materials discovery and design. In this regime, highly controlled thin film epitaxy can be leveraged to manifest bulk and interfacial phases that are non-existent in bulk equilibrium phase diagrams. With the recent discovery of entropy-stabilized oxides, entropy and disorder engineering has been realized as an orthogonal approach. This has led to the nucleation and rapid growth of research on high-entropy oxides – multicomponent oxides where the configurational entropy is large but its contribution to its stabilization need not be significant or is currently unknown. From current research, it is clear that entropy enhances the chemical solubility of species and can realize new stereochemical configurations which has led to the rapid discovery of new phases and compositions. The research has expanded beyond studies to understand the role of entropy in stabilization and realization of new crystal structures to now include physical properties and the roles of local and global disorder. Here, key observations made regarding the dielectric and magnetic properties are reviewed. These materials have recently been observed to display concerted symmetry breaking, metal-insulator transitions, and magnetism, paving the way for engineering of these and potentially other functional phenomena. Excitingly, the disorder in these oxides allows for new interplay between spin, orbital, charge, and lattice degrees of freedom to design the physical behavior. We also provide a perspective on the state of the field and prospects for entropic oxide materials in applications considering their unique characteristics.

Read the full text open-access in MRS Advances

Abstract: Ferrimagnetic insulators with perpendicular magnetic anisotropy are of particular interest for spintronics due to their ability to mitigate current shunting in spin–orbit torque heterostructures and enable low switching energy, high-density storage magnetic devices. Rare earth iron garnet Tm₃Fe₅O₁₂ (TmIG) is one such material where prior studies have shown that the negative magnetostriction coefficient and isotropic in-plane tensile strain enable the magnetoelastic anisotropy to overcome the demagnetization energy and stabilize perpendicular magnetic anisotropy. However, the investigation of the tunability of the magnetoelastic anisotropy between thin films that possess perpendicular magnetization and quantification of the magnetoelastic constants has not been reported. Here, we quantify the evolution of magnetic anisotropy in (111)-oriented, epitaxial, 17 nm thick thin films of TmIG using a systematic variation of in-plane epitaxial strain (ranging 0.49%–1.83%) imposed by a suite of commercially available garnet substrates. Within the confines of the imposed strain range and deposition condition, the distortion from cubic symmetry is found to be approximately linear within the in-plane strain. The magnetic anisotropy field can be tuned by a factor of 14 in this strain range. The magnetoelastic anisotropy constant, B₂, is found to be approximately constant (∼2500 kJ m⁻³) and more than 2× larger than the reported bulk value (∼1200 kJ m⁻³) for a cubic distortion between 90.17° and 90.71°. B₂ is found to decrease at cubic distortions of 90.74° and larger. Our results highlight strain engineering, and its limitations, for control of perpendicular magnetic anisotropy.

The full text is available as an editor’s choice article from Journal of Applied Physics