Trevor L. Goodrich

Oxygen-defect-induced magnetism to 880 K in semiconducting anatase TiO2−δ films

We demonstrate a semiconducting material, TiO2??, with ferromagnetism up to 880?K, without the introduction of magnetic ions. The magnetism in these films stems from the controlled introduction of anion defects from both the film?substrate interface as well as processing under an oxygen-deficient atmosphere. The room-temperature carriers are n-type with n~3 ? 1017?cm?3. The density of spins is ~1021?cm?3. Magnetism scales with conductivity, suggesting that a double exchange interaction is active. This represents a new approach in the design and refinement of magnetic semiconductor materials for spintronics device applications.

Synthesis of ordered arrays of multiferroic NiFe2O4-Pb(Zr0.52Ti0.48)O3 core-shell nanowires

A synthesis method was developed for producing core-shell nanowire arrays, which involved a combination of a modified sol-gel process, electrochemical deposition, and subsequent oxidization in anodized nanoporous alumina membranes. This method was applied to generate ordered arrays of one dimensional multiferroic NiFe2O4 core and Pb(Zr0.52Ti0.48)O3 (PZT) shell nanostructures. Extensive microstructural, magnetic, and ferroelectric characterizations confirmed that the regular arrays of core-shell multiferroic nanostructures were composed of a spinel NiFe2O4 core and perovskite PZT shell. This synthesis method can be readily extended to prepare different core-shell nanowire arrays and is expected to pave the way for one dimensional core-shell nanowire arrays.

Epitaxial growth of M-type Ba-hexaferrite films on MgO (111)‖SiC (0001) with low ferromagnetic resonance linewidths

Barium hexaferrite (BaM) films were deposited on 10nm MgO (111) films on 6H silicon carbide (0001) substrates by pulsed laser deposition from a homogeneous BaFe12O19 target. The MgO layer, deposited by molecular beam epitaxy, alleviated lattice mismatch and interdiffusion between film and substrate. X-ray diffraction showed strong crystallographic alignment while pole figures exhibited reflections consistent with epitaxial growth. After optimized annealing, these BaM films have a perpendicular magnetic anisotropy field of 16900Oe, a magnetization (as 4πMs) of 4.4kG, and a ferromagnetic resonance peak-to-peak derivative linewidth at 53GHz of 96Oe, thus demonstrating sufficient properties for microwave device applications.

Epitaxial MgO as an alternative gate dielectric for SiC transistor applications

Power transistor applications require alternative gate dielectrics on SiC that can operate at high fields without breaking down, as well as provide a high quality interface in order to minimize mobility degradation due to interface roughness. We have grown epitaxial MgO (111) crystalline layers on 6H-SiC (0001) substrates and characterized their structural and electrical properties. Measurements of gate leakage, breakdown fields, and dielectric properties make epitaxial MgO a potential candidate gate dielectric for SiC-based transistors.

Low temperature growth of crystalline magnesium oxide on hexagonal silicon carbide (0001) by molecular beam epitaxy

Magnesium oxide (111) was grown epitaxially on hexagonal silicon carbide (6H-SiC) (0001) substrates at low temperatures by molecular beam epitaxy and a remote oxygen plasma source. The films were characterized by reflection high-energy electron diffraction, Auger electron spectroscopy, x-ray photoelectron spectroscopy, and atomic force microscopy. Crystal structure, morphology, and growth rate of the magnesium oxide (MgO) films were found to be dependent on the magnesium flux, indicating a magnesium adsorption controlled growth mechanism. The single crystalline MgO thin films had an epitaxial relationship where MgO (111)‖6H-SiC (0001) and were stable in both air and 10−9Torr up to 1023K.

Epitaxial growth of barium hexaferrite film on wide bandgap semiconductor 6H–SiC by molecular beam epitaxy

Epitaxial barium ferrite (BaM) films have been successfully grown by molecular beam epitaxy (MBE) for the first time on 6H silicon carbide substrates by using a 10 nm single crystalline MgO (1 1 1)//SiC(0 0 0 1) template, also grown by MBE. X-ray photoelectron spectroscopy showed that the thin MgO template in the early stages of film growth prevented the diffusion of Si into the BaM film. Background oxygen pressure (containing both O atoms and O2, but no ionic species) is critical for determining the chemistry and surface structure of BaM. An oxygen deficient or rich environment will cause impurity phases of Fe3O4 or α-BaFe2O4, respectively. For BaM films grown in an optimal oxygen environment, x-ray diffraction showed a strong c-axis perpendicular to the substrate plane while the pole figure exhibited reflections consistent with epitaxial growth. Vibrating sample magnetometry showed a perpendicular magnetic anisotropy field of 16 200 Oe and a magnetization (as 4πMs) of 4.1 kG.

Thin, crystalline MgO on hexagonal 6H-SiC(0001) by molecular beam epitaxy for functional oxide integration

MgO thin films are proposed as a template for the effective integration of three and four element oxides on wide band gap SiC for next generation multifunctional devices. Oriented, crystalline MgO(111) of 20–380A is grown on 6H-SiC(0001) by molecular beam epitaxy at a substrate temperature of 140°C using a magnesium effusion cell and a remote oxygen plasma source with ion deflection plates located at the end of the plasma discharge tube and approximately 7in. from the sample surface. Films are conformal to the steps of the cleaned SiC surface with a rms roughness of 0.45±0.05nm. Magnesium adsorption controls the growth rate in an excess oxygen environment with Mg:O flux ratios of 1:99–1:20, where the oxygen flux is the equivalent molecular oxygen. The oxygen plasma, which was determined to be free of ions when the ion deflection plates are energized, does impact nucleation and initial stages of the MgO film formation, and there may be evidence of etching mechanisms involved in the thicker film growth. Che...

Improved epitaxy of barium titanate by molecular beam epitaxy through a single crystalline magnesium oxide template for integration on hexagonal silicon carbide

Crystalline MgO(111) has the potential to be an effective template for the heteroepitaxial integration of BTO(111) and other functional oxides on 6H-SiC(0001). Deposition of MgO on 6H-SiC(0001) at 140°C resulted in a twinned structure with only (111) orientation. By heating the MgO(111) after deposition to 650°C at a background pressure of 1.0×10−9Torr or depositing the MgO at 650°C, the twinned structure can be minimized, resulting in a reflection high energy electron diffraction (RHEED) pattern characteristic of random epitaxial islands. The use of a 2nm MgO(111) template layer proved optimal for deposition of crystalline BTO(111) by molecular beam epitaxy on 6H-SiC(0001). The BTO was found to be twinned with a 60° in-plane rotation. The grain formation of the BTO resulted in a transmission dominated RHEED pattern. Deposition of BTO at a higher substrate temperature resulted in large grain formation, 50nm in size, but an increased surface roughness of 1.4±0.1nm over a 1μm2 area. In order to integrate BT...

The catalytic activity of space versus terrestrial synthesized zeolite Beta catalysts in the Meerwein Ponndorf Verley Reactions: Support for PFAL as the Lewis active site for cis-alcohol selectivity

The Lewis activity of the Meerwein-Ponndorf-Verley (MPV) reactions is hypothesized to be due to partial framework aluminum (PFAl) that is either octahedrally or tri-coordinated. Crystals grown in the free-fall environment of low earth orbit (LEO) are more uniform; that is, have fewer lattice “defects” compared to those grown in a gravity field (i.e., on earth). Therefore, crystals grown in orbit should be less catalytically active relative to their earth grown counterparts. The catalytic activity towards the MPV reaction, and the associated IR and XPS spectrum for zeolite Beta that was synthesized on earth (1g) and aboard the International Space Station (10−3–10−5g) were compared in their as-synthesized forms, and after applying heat treatment protocols designed to stress the crystal structure to generate Lewis acid sites (i.e., tri and octahedrally coordinated PFAl). The activity of the MPV reaction and cis-alcohol selectivity over the heat-treated flight samples was observed to be lower than the identically heat-treated terrestrial zeolite Beta samples. Higher MPV activity as well as cis-alcohol selectivity is related to both a higher number of partial framework Al atoms (PFAl), and a constrained pore volume. As PFAl are created by the destruction of the framework upon heat treatment, flight samples were shown to be thermally more stable with fewer lattice defects and less associated stress in zeolite Beta crystals. The changes observed in the IR spectra, as well as the XPS Al Auger and 2p peaks, of the terrestrial samples support this conclusion. Additionally, the flight samples showed higher tr-alcohol selectivity, which implies more pore volume and less channel blockage. This is consistent with the fact that crystals grown in space have less stress, fewer lattice defects, and thus there are fewer channel obstructions.