Ultra-thin films of low damping ferromagnetic insulators with perpendicular magnetic anisotropy have been identified as critical to advancing spin-based electronics by significantly reducing the threshold for current-induced magnetization switching while enabling new types of hybrid structures or devices. Here, we have developed a new class of ultra-thin spinel structure Li0.5Al1.0Fe1.5O4 (LAFO) films on MgGa2O4 (MGO) substrates with: 1) perpendicular magnetic anisotropy; 2) low magnetic damping and 3) the absence of degraded or magnetic dead layers. These films have been integrated with epitaxial Pt spin source layers to demonstrate record low magnetization switching currents and high spin-orbit torque efficiencies. These LAFO films on MGO thus combine all of the desirable properties of ferromagnetic insulators with perpendicular magnetic anisotropy, opening new possibilities for spin based electronics.

Electrically tunable TiN/SiO2/TiN epsilon-near-zero photonic structures were fabricated using DC magnetron sputtering. Reflectance spectra in visible/near-IR for bulk and multilayered TiN/SiO2/TiN structures with optimal parameters exhibit spectral shift at the epsilon-near-zero spectral point up to ~10 nm due to applied voltage (12 V).

Electrically tunable TiN/SiO 2 /TiN epsilon-near-zero photonic structures with various parameters were fabricated using the reactive DC magnetron sputtering approach. Effective medium approximation was used to predict the optical permittivity of a multilayered TiN/SiO 2 metamaterial and guide the design/fabrication. Experimental reflectance measurements for tunable TiN/SiO 2 /TiN structures were obtained using the ellipsometer technique in the visible and near-infrared spectral ranges. Results show that reflectance for biased (12 V) and un-biased bulk TiN/SiO 2 /TiN structure changes up to ∼ 2% with the spectral shift at the ENZ spectral point ∼ 10 nm for samples with an optimal SiO 2 dielectric layer (thickness d=10 nm). Reflectance measurements for multilayered tunable TiN/SiO 2 /TiN structures show strong variation in reflectance change for s- polarized light at epsilon-near-zero wavelengths due to applied voltage (12 V). We expect that the results of this research study of the tunable TiN/SiO 2 /TiN epsilon-near-zero photonic structures will potentially be useful for the photonic density of states engineering, surface sensing, and metamaterial-based super-resolution imaging.

Journal Article In Operando Transmission Electron Microscopy Studies on Diffusion-Induced Phenomena at Dielectric-Electrode Interfaces in Ge2Te3-Based Memristor Devices Get access Krishnamurthy Mahalingam, Krishnamurthy Mahalingam Air Force Research Laboratory, Materials and Manufacturing Directorate, Wright-Patterson AFB, OHUnited States Corresponding author: krishnamurthy.mahalingam.ctr@afrl.af.mil Search for other works by this author on: Oxford Academic Google Scholar Austin Shallcross, Austin Shallcross Department of Electrical and Computer Engineering, University of Dayton, Dayton, OH, United States Search for other works by this author on: Oxford Academic Google Scholar Cynthia T Bowers, Cynthia T Bowers Air Force Research Laboratory, Materials and Manufacturing Directorate, Wright-Patterson AFB, OHUnited States Search for other works by this author on: Oxford Academic Google Scholar Derek Winner, Derek Winner Air Force Research Laboratory, Materials and Manufacturing Directorate, Wright-Patterson AFB, OHUnited States Search for other works by this author on: Oxford Academic Google Scholar Albert Hilton, Albert Hilton Air Force Research Laboratory, Materials and Manufacturing Directorate, Wright-Patterson AFB, OHUnited States Search for other works by this author on: Oxford Academic Google Scholar Sabyasachi Ganguli, Sabyasachi Ganguli Air Force Research Laboratory, Materials and Manufacturing Directorate, Wright-Patterson AFB, OHUnited States Search for other works by this author on: Oxford Academic Google Scholar Eunsung Shin, Eunsung Shin Department of Electrical and Computer Engineering, University of Dayton, Dayton, OH, United States Search for other works by this author on: Oxford Academic Google Scholar Guru Subramanyam Guru Subramanyam Department of Electrical and Computer Engineering, University of Dayton, Dayton, OH, United States Search for other works by this author on: Oxford Academic Google Scholar Microscopy and Microanalysis, Volume 29, Issue Supplement_1, 1 August 2023, Pages 1611–1612, https://doi.org/10.1093/micmic/ozad067.827 Published: 22 July 2023

LHPG method is used to grow high optical quality single crystal fibers. For LHPG, polycrystalline ceramics can play important roles as feedstocks and cladding on the single crystal fibers.

Combining topological insulators (TIs) and magnetic materials in heterostructures is crucial for advancing spin-based electronics. Magnetic insulators (MIs) can be deposited on TIs using the spin-spray process, which is a unique non-vacuum, low-temperature growth process. TIs have highly reactive surfaces that oxidize upon exposure to atmosphere, making it challenging to grow spin-spray ferrites on TIs. In this work, it is demonstrated that a thin titanium capping layer on TI, followed by oxidation in atmosphere to produce a thin TiOx interfacial layer, protects the TI surface, without significantly compromising spin transport from the magnetic material across the TiOx to the TI surface states. First, it was demonstrated that in Bi2Te3/TiOx/Ni80Fe20 heterostructures that TiOx provided an excellent barrier against diffusion of magnetic species, yet maintained a large spin-pumping effect. Second, the TiOx was also used as a protective capping layer on Bi2Te3, followed by the spin-spray growth of the MI, NixZnyFe2O4 (NZFO). For the thinnest TiOx barriers, Bi2Te3/TiOx/NZFO samples had AFM disordered interfacial layer because of diffusion. With increasing TiOx barrier thickness, the diffusion was reduced, but still maintained strong interfacial spin-pumping interaction. These experimental results demonstrate a novel method of low-temperature growth of magnetic insulators on TIs enabled by interface engineering.

Chemical and electrical measurements of Ti/(010) beta-Ga2O3 and Ti/(001) beta-Ga2O3 interfaces were conducted as a function of annealing temperature using x-ray photoelectron spectroscopy (XPS), current density-voltage (J-V), and capacitance-voltage (C-V) measurements. XPS revealed partial Ti oxidation at both interfaces in the as-deposited condition, with more Ti oxidation on the (001) beta-Ga2O3 epilayer surface than the (010) beta-Ga2O3 substrate surface. The amount of oxidized Ti increased with annealing temperature. The Schottky barrier heights for as-deposited (unannealed) Au/Ti/(010) beta-Ga2O3 and Au/Ti/(001) beta-Ga2O3 contacts as determined from J-V and C-V measurements were between 0.64 and 0.83 eV. Shifts in XPS core level peaks for Ti/(010) beta-Ga2O3 suggest that the Schottky barrier height decreases with temperature up to 350 degrees C for 10-min anneals and increases for 10-min anneals >= 460 degrees C. Taken together, the results suggest a strong dependence of Ti reactivity on the beta-Ga2O3 surface, which can affect the electrical performance and stability of Ti/beta-Ga2O3 ohmic contacts at elevated temperatures.

The standard method for growth rate determination in semiconductor thin films, by Molecular Beam Epitaxy (MBE), is through RHEED intensity oscillations prior to device layer epitaxy. High quality III-Nitride epitaxy occurs with metal-rich surfaces and under step-flow growth conditions, which do not produce RHEED oscillations. This article demonstrates the capability to monitor the growth rate of gallium nitride (GaN), at any point during film growth with high fidelity, under step-flow growth conditions. RHEED intensity vs. time measurements determine the growth rate by Metal Modulated Epitaxy (MME). Utilizing differential analysis, a factor of 2x improvement in accuracy is demonstrated, with a Standard Error less than 4%. Complementary analysis with X-Ray Diffraction and RHEED identify the Ga bilayer thickness as 2.34 ML +/- 0.08 ML, representing the first time RHEED analysis has been used to characterize the thickness of the Ga bilayer on GaN.

Carrier concentration control by impurity dopants in epitaxial Ga2O3 thin films is progressing to deliver high mobility films for device structures. Si-doped Ga2O3 thin films were fabricated by pulsed laser deposition on (010) β-Ga2O3 substrates from Ga2O3 targets with 0.01–1 wt. % SiO2 yielding films with an electron mobility range consistent with other vapor growth techniques. Single crystal, homoepitaxial growth as determined by high resolution transmission electron microscopy and x-ray diffraction was observed, with a high Si dopant level causing film tensile strain as indicated by both techniques. The influence of oxygen on conductivity using different O2 pressures during deposition and O2/Ar mixtures with a fixed working pressure of 1.33 Pa was determined. With this optimized deposition pressure and atmosphere condition, a carrier concentration and mobility range of 3.25 × 1019 cm−3–1.75 × 1020 cm−3 and 20 cm2/V s–27 cm2/V s was achieved in films from Ga2O3-0.025 wt. % SiO2 and Ga2O3-1 wt. % SiO2 targets, respectively. A highest conductivity of 798 S cm−1 was achieved in films deposited at 550 °C–590 °C with targets of 0.05–1 wt. % SiO2. The electrically active and chemical Si content in films deposited at 550 °C was found to exceed the expected Si ablation target composition in all cases except the highest 1 wt. % SiO2 target attributed to imprecise target manufacturer compositional control at low SiO2 doping levels. Diminished electrical and structural quality films resulted from all targets at a 450 °C deposition temperature.

Layered oxide materials having alternating repeated layer thicknesses of 10 nm or less are difficult to make, especially with sharp interfaces. Nanostructured thin films having repeated layers of two different oxide materials were obtained by using pulsed laser deposition and two independent stationary targets consisting of Al2O3 and BaTiO3. Desired thicknesses were achieved by using a specific number of pulses from a 248-nm KrF excimer laser, at an energy of 450 mJ/pulse, a galvanometer mirror system, and a background pressure of oxygen. Trends in material properties were identified by systematically varying the number of pulses for multiple nanostructured thin films and comparing the resulting properties measured using in-situ spectroscopic ellipsometry and ex-situ capacitance measurements, including relative permittivity and loss. Four films were deposited with a goal of having 0.25-, 1-, 4-, and 10-nm thick layers, and each similar to 220 nm thick. Ellipsometry data were modeled in situ to calculate thickness, n and k. A representative transmission electron microscopy measurement was also collected for the 10-nm sample with corresponding x-ray photoelectron spectroscopy and energy disperive x-ray spectroscopy. Ellipsometry and capacitance measurements were all performed on each of the samples, with one sample having calculated impedance greater than 30 GOhm at 0.001 Hz. (C) 2017 Society of Photo-Optical Instrumentation Engineers (SPIE)

Abstract : Layered oxide materials having alternating repeated layer thicknesses of 10 nm or less are difficult to make, especially with desired morphology and atomic composition. Nanostructured thin-films having repeated layers of two different oxide materials were obtained by using pulsed laser deposition (PLD) and two independent stationary targets consisting of Al2O3, and BaTiO3. Desired thicknesses were achieved by using a specific number of pulses from a 248 nm KrF excimer laser, at an energy of 450 mJ per pulse, a galvanometer mirror system and a background pressure of oxygen. Trends in material properties were identified by systematically varying the number of pulses for multiple nanostructured thin-films and comparing the resulting properties measured using in-situ spectroscopic ellipsometry, and ex-situ capacitance measurements including relative permittivity and loss. Four films were deposited with a goal of having 0.25, 1, 4, and 10 nm thick layers, and each 220 nm thick. Ellipsometry data was modeled in-situ to calculate thickness, n and k. A representative TEM measurement was also collected for the 10 nm sample with corresponding XPS and EDS. Ellipsometry and capacitance measurements were all performed on each of the samples, with one sample having a calculated impedance greater than 30 GOhm at 0.001 Hz.