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

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

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

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