Search

Ion beam assisted deposition (IBAD) is used to biaxially texture magnesium oxide (MgO), which is useful as a template for the heteroepitaxial growth of various thin film devices and most notably as a template layer for high temperature superconductors. Improvements in the quality of IBAD MgO films have been largely empirical and there is uncertainty as to the exact mechanism by which this biaxial texture is developed. Using a specially built quartz crystal microbalance (QCM) as both a substrate and monitor in conjunction with reflected high-energy electron diffraction (RHEED) acting on the same surface, we have probed the initial stages of IBAD MgO growth in-situ. We have correlated corresponding RHEED images with real-time mass accumulation QCM data during the film growth. During IBAD growth, the mass accumulation exhibits a sharp change in slope corresponding to a sudden decrease in growth rate. Corresponding RHEED images show an abrupt onset of crystallographic texture at this point. A simple model incorporating differential etch rates of the MgO film and silicon nitride substrate can be used to fit the data but is inconsistent with the behavior during ion etching with no growth. It is, therefore, postulated that a more complex mechanism is responsible for the observed behavior.

Epitaxial ferromagnetic SrRuO3 thin films with a room-temperature resistivity of 300 microOmega.cm have been successfully grown on LaAlO3(001) substrates at a processing temperature in the range of 550-750 degrees C by a polymer-assisted deposition technique. X-ray diffraction analysis shows good epitaxial quality of SrRuO3 thin films, giving values of the full width at half-maximum (FWHM) of 0.42 degrees from the rocking curve for the (002) reflection and 1.1 degrees from the in-plane phi scan for the (204) reflection. Both the resistivity and the magnetization versus temperature measurements show that the SrRuO3 films are ferromagnetic with a transition temperature of 160 K. The spontaneous magnetization near the ferromagnetic transition follows the scaling law, and the low-temperature magnetization follows the Bloch law.

Ion Beam Assisted Deposition (IBAD) of Magnesium Oxide (MgO) has been proven to be a viable route for producing template films used to deposit high quality YBCO coated conductors on flexible polycrystalline metal substrates. Here we will discuss improvements in this process using a dual ion assist beam configuration. Dual ion assist beam deposition of MgO reduces the requirements for substrate surface finishing while maintaining comparable film quality (phi scan full‐width at half‐maximum values between 7 and 8 degrees). Furthermore, this adaptation of the IBAD process eliminates the degradation of MgO texture observed in thick IBAD MgO films deposited on silicon nitride. We have deposited films up to 50 nanometers thick without degradation of in‐plane texture. Increasing the MgO thickness increases the chemical stability of the template layer and can eliminate the necessity for subsequent buffer layers or the application of the homoepitaxial MgO layer needed to stabilize the thin, conventional IBAD MgO layer. Initial results of subsequently deposited YBCO on these dual assist ion beam MgO templates are quite promising.

We have studied the growth of magnesium oxide using ion-beam assisted deposition (IBAD) to achieve (100) oriented, bi-axially textured films with low mosaic spread, for film thicknesses of 10 nm on silicon substrates. We have refined the process by using reflected high-energy electron diffraction (RHEED) to monitor the growth of IBAD MgO films and found that the diffracted intensity can be used to determine (and ultimately control) final in-plane texture of the film. Here we present results on our work to develop the use of real-time RHEED monitoring to deposit well-oriented IBAD MgO films. The results have been corroborated with extensive grazing-incidence X-ray diffraction (GID). Results of these analyses have allowed us to deposit films on metallic substrates with in-plane mosaic spread less than 7°.

Two conducting oxides, La0.5Sr0.5CoO3(LSCO) and SrRuO3, were deposited by pulsed laser ablation onto silicon substrates coated with biaxially textured MgO on an amorphous silicon nitride isolation layer. Comparison is made between templates using just 10 nm of ion-beam assisted deposited (IBAD) MgO and substrates with an additional 100 nm of homoepitaxial MgO. Both of these conducting oxide layers exhibited in-plane and out-of-plane texture, on the order of that obtained by the underlying MgO. The SrRuO3 was c-axis oriented on both substrates, but exhibited a slightly sharper out-of- plane texture when the homoepitaxial MgO layer was included. On the other hand, the LSCO showed only (100) orientation when deposited directly on the IBAD-MgO templates, whereas a significant (110) peak was observed for films on the homoepitaxial MgO. A simple calculation of the distribution of grain boundary angles, assuming a normal distribution of grains, is also presented.

Ion-beam assisted deposition was used to fabricate biaxially aligned cubic zirconia on flexible metal substrates. These films were used as templates for heteroepitaxially deposited YBa2Cu3O7-δ (YBCO) films. The quality of the crystalline texture of the template films has a direct influence on the superconducting properties of the final YBCO films. We describe our efforts to fabricate high-quality cubic zirconia templates on short-length substrates processed in stationary mode and meter-long substrates processed in continuous mode. Our best phi scan FWHM values for the films on the meter-long tapes were 12.6 degrees for the template and 6.1 degrees for the overcoated YBCO. This meter-long tape had a self-field, 75 K, superconducting critical current of 122 amps.

Continuous coaters capable of producing 1.1 m long x 1 cm wide tapes of YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}} (YBCO) on biaxially oriented yttria-stabilized zirconia (YSZ) on flexible Ni-alloy substrates have been developed at this laboratory. Using a 1 {micro}V/cm criterion, the authors have achieved transport critical current (I{sub c}) values of 29 A (75 K, self field) between voltage taps spaced 1 m apart. The corresponding critical current density (J{sub c}) value for this tape is 290 kA/cm{sup 2}. For shorter tapes, (12 cm voltage tap separation) they have attained J{sub c} values of 0.67 MA/cm{sup 2}. Individual 1 x 1 cm sections within these shorter tapes have attained J{sub c} values of 1 MA/cm{sup 2}.

We have developed Y2O3:Al2O3 (YAlO) composites to simplify the architecture of superconducting YBa2Cu3O7?x (YBCO) thick films on polycrystalline metal substrates. By implementing the use of YAlO, we have reduced the total number of non-superconducting layers between the polycrystalline metal substrate and the YBCO film from five (as in the standard architecture used by industry) to three. The YBCO films grown on this simplified platform exhibited an in-plane mosaic spread of less than 4? in full width at half-maximum, correlated pinning centered at , and an ? value (the proportionality factor of the critical current density H ? ?) of around 0.38 over the field range of 0.1?1.0?T. We believe that the excellent structural stability at high temperatures and the exceptional chemical inertness in an oxidizing environment make YAlO a good choice for use in the growth of biaxially oriented MgO and subsequent buffer and superconducting layers.

We have used an ion beam sputtered Y2O3?Al2O3 (YALO) composite to simplify the architecture of high temperature superconducting (HTS) coated conductors (CCs) based on a IBAD MgO template. By implementing YALO, we have reduced the total non-superconducting layers between the polycrystalline metal substrate and the superconducting film from five (the standard architecture) to four. Well textured (Y,Gd)Ba2Cu3O7?x ((Y, Gd)BCO) films have been successfully grown by MOCVD on this simplified template. The microstructural characterization revealed that all layers are continuous and uniform with sharp and clean interfaces. Additionally, the YALO maintained its amorphous nature after the deposition of the superconductive layer, which is a plus in terms of its efficiency as a diffusion barrier. The achievement of a self-field critical current of 230? A?cm?1 at 75.5?K is another proof of the effectiveness of YALO as a diffusion barrier and nucleation seed for the MgO. The transport properties under an applied magnetic field of MOCVD grown (Y, Gd)BCO on LMO buffered MgO/YALO/Ni-alloy are comparable with those of (Y, Gd)BCO on a standard architecture, thus demonstrating good compatibility between the simplified template with the MOCVD grown (Y, Gd)BCO. The use of a single composite YALO layer instead of individual layers of Y2O3 and Al2O3 for the large scale fabrication of HTS CCs based on IBAD MgO provides advantages such as potentially reduced cost due to the reduced number of fabrication steps.

Nanocomposite (BiFeO3)0.5:(Sm2O3)0.5 films were deposited on (001) oriented Nb-doped SrTiO3 substrates by pulsed laser deposition. The leakage current density versus electric field characteristics were investigated and compared with those of as-deposited and annealed pure BiFeO3 (BFO) thin films. The dominant leakage mechanisms of nanocomposite films were space-charge-limited current and Poole–Frenkle emission for positive and negative biases, respectively. The leakage current density of nanocomposite films was reduced three orders of magnitude in comparison with the as-deposited pure BFO films. The less oxygen vacancies in the BFO phase in the nanocomposite is believed to contribute to the leakage reduction.

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.

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.

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.

Epitaxial c-axis oriented BiFeO3 (BFO) thin films were deposited on conductive SrRuO3 (SRO) on (001) SrTiO3 substrates by pulsed laser deposition. A Pt/BFO/SRO capacitor was constructed by depositing a top Pt electrode. The leakage current density versus. electric field characteristics were investigated from 80to350K. It was found that the leakage mechanisms were a strong function of temperature and voltage polarity. At temperatures between 80 and 150K, space-charge-limited current was the dominant leakage mechanism for both negative and positive biases. On the other hand, at temperatures between 200 and 350K the dominant leakage mechanisms were Poole-Frenkle emission and Fowler-Nordheim tunneling for negative and positive biases, respectively.

Epitaxial c-axis-oriented Bi3.15Nd0.85Ti3O12 (BNT) thin films with thickness ranging from 150to350nm were deposited on conductive SrRuO3 (SRO) on (001) SrTiO3 substrates by pulsed laser deposition. The top Pt electrode was deposited by sputtering to construct a capacitor Pt/BNT/SRO. The authors have evaluated the effective thickness (ti) and dielectric constant (ei) of interfacial layers at the Pt/BNT and BNT/SRO interfaces based on the optical refractive index of the BNT layer and the capacitance frequency as well as the current-voltage characteristics of the capacitors. Using a series capacitor model, they have found that the dielectric constant of bulk BNT and the ti∕ei ratio are 586 and 1.46nm, respectively. Knowing the optical dielectric constant (eopt) and the product of eoptti of BNT thin films, the authors have estimated that the effective thickness and dielectric constant of the interfacial layers are 20.1nm and 13.7, respectively.

We have deposited epitaxial SrRuO3 (SRO) thin films on SiO2∕Si substrates using biaxially oriented MgO templates produced by ion-beam-assisted deposition. The strain states of the SRO films are strongly affected by the crystallinity of the templates: the better the in-plane texture of the template is, the more in-plane tensile stress the films are subjected to. On the other hand, SRO films are relatively loosely strained with lattice parameters closer to bulk property if the templates have a broad in-plane alignment. The magnetization of the SRO films is well described by Bloch’s T3∕2 law at low temperatures, which is well known as the result of spin wave excitation: M(T)∕M(0)=1−AT3∕2. The strained films show larger spin wave parameter A values, reflecting larger magnetization suppression. The suppression of magnetization in the strained films is attributed to the change in the magnetic coupling since this coupling is very sensitive to the interatomic distance.