I.O. Usov

Rectifying current-voltage characteristics of BiFeO3∕Nb-doped SrTiO3 heterojunction

Epitaxial c-axis oriented BiFeO3 (BFO) thin films were deposited on (001) Nb-doped SrTiO3 (Nb-STO) substrates by pulsed laser deposition. Introducing Bi vacancies caused the BFO thin film to evolve to a p-type semiconductor and formed a p-n heterojunction with an n-type semiconductor Nb-STO. The current density versus voltage (J-V) and capacitance versus voltage (C-V) characteristics of the heterojunction were investigated. A typical rectifying J-V effect was observed with a large rectifying ratio of 5×104. Reverse C-V characteristics exhibited a linear 1∕C2 versus V plot, from which a built-in potential of 0.6V was deduced. The results show a potential application of BFO/Nb-STO heterojunction for oxide electronics.

The effect of excess atomic volume on He bubble formation at fcc-bcc interfaces

Atomistic modeling shows that Cu–Nb and Cu–V interfaces contain high excess atomic volume due to constitutional vacancy concentrations of ∼5 at. % and ∼0.8 at. %., respectively. This finding is supported by experiments demonstrating that an approximately fivefold higher He concentration is required to observe He bubbles via through-focus transmission electron microscopy at Cu–Nb interfaces than in Cu–V interfaces. Interfaces with structures tailored to minimize precipitation and growth of He bubbles may be used to design damage-resistant composites for fusion reactors.

Structural and optical properties of ZnO thin films by rf magnetron sputtering with rapid thermal annealing

Epitaxial ZnO thin films were grown on c-plane sapphire substrates by rf magnetron sputtering at room temperature followed by a rapid thermal annealing process. We found that crystallinity of the films was strongly affected by the partial oxygen pressure during deposition. Both x-ray diffraction and transmission electron microscopy studies revealed that the ZnO films grow epitaxially predominantly with aligned ZnO domains. An unresolved excitonic resonance was observed in the optical absorption spectrum. Nevertheless, refractive index and absorption edge of the ZnO films are similar to that of single crystal ZnO.

Transient enhanced diffusion of aluminum in SiC during high temperature ion implantation

6H–SiC wafers were implanted at room temperature (RT) and at 1700 °C high temperature (HT) with 50 keVAl+ ions to doses from 1.4×1014 to 1.4×1016 cm−2. Compared to samples implanted at RT, the samples implanted at high temperature display considerable aluminum redistribution. The diffusion of Al is shown to be a transient effect with different decay times in the near-surface region and in the bulk. Investigation of the crystalline structure indicated that in the near-surface region dislocation loops grow in size and Al precipitates are formed as the dose of Al implanted at HT is increased. Changes in the structure of the implanted layer may have a strong effect on the redistribution of Al. The observed redistribution can be explained by a dissociative diffusion mechanism during the high-temperature implantation.

High tunability of lead strontium titanate thin films using a conductive LaNiO3 as electrodes

Highly tunable films of dielectric PbxSr1−xTiO3 (with x=0.3 and 0.4) have been deposited on conductive LaNiO3 coated LaAlO3 substrates using a sol-gel technique. The processing condition was found to greatly influence the microstructure as well as the dielectric and electrical properties of the films. At room temperature, dielectric tunability values as high as 70% and 78.6% at an applied electric field of 223kV∕cm were achieved for the Pb0.3Sr0.7TiO3 and Pb0.4Sr0.6TiO3 films, respectively.

Spherical nanoindentation study of the deformation micromechanisms of LiTaO3 single crystals

Herein, spherical nanoindentation (NI) was used to investigate the room temperature deformation behavior of C-plane LiTaO3 single crystals loaded along the [0001] direction as a function of ion irradiation. When the NI load-displacement curves of 3 different nanoindenter radii (1.4 μm, 5 μm, and 21 μm) were converted to NI stress-strain curves, good agreement between them was found. The surface first deforms elastically – with a Young’s modulus of 205 ± 5 GPa, calculated from the stiffness versus contact radii curves and 207 ± 3 GPa measured using a Berkovich tip – and then plastically deforms at ≈ 6 GPa. Repeated loading into the same location results in large, reproducible, fully reversible, nested hysteresis loops attributed to the formation of incipient kink bands (IKBs). The latter are coaxial fully reversible dislocation loops that spontaneously shrink when the load is removed. The IKBs most probably nucleate within the (101−2) twins that form near the surface. The sharper radii resulted in twin nuc...

Characteristics of Alumina Diffusion Barrier Films on Hastelloy

The diffusion behavior of elements constituting Hastelloy C-276 (C, Si, Mn, Co, W, Fe, Cr, Mo, and Ni) in alumina films was investigated using secondary ion mass spectroscopy. The films were deposited by ion-beam-assisted deposition and annealed in vacuum over a temperature range of 500–1000 °C. Characterization of film microstructure was performed using transmission electron microscopy and selected area diffraction analyses. The films were predominantly amorphous with alumina nanocrystallites nonuniformly dispersed throughout the volume both before and after annealing. A relatively wide interface region between the Hastelloy substrate and alumina film was formed in the as-deposited sample due to ion beam mixing. No diffusion of any of the substrate elements was observed after annealing, except for Mn, Cr, and Ni. The impurity depth distributions consisted of two components, which differed by several orders of magnitude with respect to diffusion coefficient and solubility. Activation energies and temperature dependencies of the diffusion coefficients were determined, and a diffusion mechanism was discussed.

Structural and superconducting properties of (Y,Gd)Ba2Cu3O7−δ grown by MOCVD on samarium zirconate buffered IBAD-MgO

Textured samarium zirconate (SZO) films have been grown by reactive cosputtering directly on an ion beam assisted deposited (IBAD) MgO template, without an intermediate homoepitaxial MgO layer. The subsequent growth of 0.9 µm thick (Y,Gd)Ba2Cu3O7−δ ((Y, Gd)BCO) films by metal organic chemical vapor deposition (MOCVD) yielded well textured films with a full width at half maximum of 1.9° and 3.4° for the out-of-plane and in-plane texture, respectively. Microstructural characterizations of the SZO buffered samples revealed clean interfaces. This indicates that the SZO not only provides a diffusion barrier, but also functions as a buffer for (Y, Gd)BCO grown by MOCVD. The achievement of self-field critical current densities (Jc) of over 2 MA cm−2 at 75.5 K is another proof of the effectiveness of SZO as a buffer on the IBAD-MgO template. The in-field measurements revealed an asymmetric angular dependence of Jc and a shift of the ab-plane maxima due to the tilted nature of the template and (Y,Gd)2O3 particles existing in the (Y, Gd)BCO matrix. The present results are especially important because they demonstrate that high temperature superconducting coated conductors with simpler architecture can be fabricated using commercially viable processes.

Diffusion of boron in 6H and 4H SiC coimplanted with boron and nitrogen ions

The diffusion behavior of boron (B) and nitrogen (N) implanted in 6H and 4H silicon carbide (SiC) samples was investigated using secondary ion mass spectroscopy. The samples were either coimplanted with B and N ions or implanted with each element alone. The annealing was performed at 1700°C for times ranging from 10to1800s in argon ambient or in the vapors of silicon and carbon. Transmission electron microscopy has been used to determine the structural properties of implanted layers after the annealing. The N concentration profiles remained unchanged after the annealing. B atoms showed transient enhanced out- and in-diffusion. The coimplantation reduced the fraction of mobile B atoms participating in out- and in-diffusion processes and resulted in an increase in the density and decrease in size of dislocation loops formed in the implanted layer. The B diffusion coefficients in both SiC polytypes have been determined and a diffusion mechanism has been discussed.

Study of

Biaxially textured SmxZr1-xOy (SZO) thin films have been grown directly on an ion beam assisted deposited (IBAD) MgO template, without using an intermediate homoepitaxial MgO layer. SZO buffer layers were deposited using reactive magnetron sputtering of a Sm (50 at.%)-Zr (50 at.%) alloy target. The SZO texture is similar to that of the MgO template. Epitaxial YBCO films, ~1mum-thick, grown by pulsed laser deposition on SZO buffered coated conductors have high critical current densities. The in-field measurements indicate that superconducting samples with SZO as buffer are as good as STO/homoepitaxial MgO buffered samples. These results demonstrate that SZO is an appropriate buffer for the fabrication of low cost YBCO/IBAD MgO coated conductors with excellent transport properties.