Sung Hun Wee

Strain-driven oxygen deficiency in self-assembled, nanostructured, composite oxide films.

Oxide self-assembly is a promising bottom-up approach for fabricating new composite materials at the nanometer length scale. Tailoring the properties of such systems for a wide range of electronic applications depends on the fundamental understanding of the interfaces between the constituent phases. We show that the nanoscale strain modulation in self-assembled systems made of high-T(c) superconducting films containing nanocolumns of BaZrO(3) strongly affects the oxygen composition of the superconductor. Our findings explain the observed reduction of the superconducting critical temperature.

Formation of Self-Assembled, Double-Perovskite, Ba2YNbO6 Nanocolumns and Their Contribution to Flux-Pinning and Jc in Nb-Doped YBa2Cu3O7-? Films

Ba2RENbO6 (RE = rare earth elements including Y) compounds are considered new additives for superior flux-pinning in YBa2Cu3O7-δ (YBCO) films due to their excellent chemical inertness to and large lattice mismatches with YBCO. Simultaneous laser ablation of a YBCO target and a Nb metal foil attached to the surface of the target resulted in epitaxial growth of YBCO films having columnar defects comprised of self-aligned Ba2YNbO6 (BYNO) nanorods parallel to the c-axis of the film. Compared to pure YBCO, YBCO+BYNO films exhibited no Tc reduction as well as superior Jc performance with higher self- and in-field Jc by a factor of 1.5–7 and also exhibited a strong Jc peak for H∥c indicative of strong c-axis correlated flux-pinning.

Heteroepitaxial film silicon solar cell grown on Ni-W foils

Heteroepitaxial semiconductor films on low-cost, flexible metal foil templates are a potential route to inexpensive, high-efficiency solar cells. Here, we report epitaxial growth of Si films on low-cost, flexible, biaxially-textured Ni-W substrates. A robust buffer architecture comprised of multiple epitaxial oxide layers has been developed to grow high quality, heteroepitaxial Si films without any undesired reaction between the Si film and the metal substrate and with a single biaxial texture. XRD analysis including ω-scans, φ-scans, and pole figures confirms that the buffers and silicon are all epitaxial, with excellent cube-on-cube epitaxy. A photo-conversion efficiency of 1.1% is demonstrated from a proof-of-concept heteroepitaxial film Si solar cell.

Engineering nanocolumnar defect configurations for optimized vortex pinning in high temperature superconducting nanocomposite wires

We report microstructural design via control of BaZrO3 (BZO) defect density in high temperature superconducting (HTS) wires based on epitaxial YBa2Cu3O7-δ (YBCO) films to achieve the highest critical current density, Jc, at different fields, H. We find the occurrence of Jc(H) cross-over between the films with 1–4 vol% BZO, indicating that optimal BZO doping is strongly field-dependent. The matching fields, Bφ, estimated by the number density of BZO nanocolumns are matched to the field ranges for which 1–4 vol% BZO-doped films exhibit the highest Jc(H). With incorporation of BZO defects with the controlled density, we fabricate 4-μm-thick single layer, YBCO + BZO nanocomposite film having the critical current (Ic) of ~1000 A cm−1 at 77 K, self-field and the record minimum Ic, Ic(min), of 455 A cm−1 at 65 K and 3 T for all field angles. This Ic(min) is the largest value ever reported from HTS films fabricated on metallic templates.

Strong enhancement of flux pinning in thick NdBa2Cu3O7- δ films grown on ion-beam assisted deposition-MgO templates via three-dimensional self-assembled stacks of BaZrO3 nanodots

Thick, epitaxial NdBa2Cu3O7-δ (NdBCO) films (up to 2.1 μm) with a high density of nanoscale columnar defects parallel to the c-axis of the film were deposited on ion-beam assisted deposition-MgO templates via pulsed laser deposition. The columnar defects were composed of self-assembled BaZrO3 (BZO) nanodots. A significant enhancement in Jc for H∥c is found for these films. In addition, an overall improvement in the in-field Jc at all field orientations is observed. Compared to pure NdBCO of similar thickness, the in-field Jc at H∥c for 0.7 μm thick NdBCO+BZO film is improved by a factor of 2–4 in the field range of 0.1–4 T and 10–20 at 7–8 T at 77 K. In addition, a smaller α∼0.17 is found in the field regime where Jc∼H−α. Also, a higher maximum pinning force, Fpmax∼14 GN∕m3, is found at 3 T and a larger Hirr over 8 T is found for the NdBCO films with columnar defects.

Heteroepitaxial Cu2O thin film solar cell on metallic substrates.

Heteroepitaxial, single-crystal-like Cu2O films on inexpensive, flexible, metallic substrates can potentially be used as absorber layers for fabrication of low-cost, high-performance, non-toxic, earth-abundant solar cells. Here, we report epitaxial growth of Cu2O films on low cost, flexible, textured metallic substrates. Cu2O films were deposited on the metallic templates via pulsed laser deposition under various processing conditions to study the influence of processing parameters on the structural and electronic properties of the films. It is found that pure, epitaxial Cu2O phase without any trace of CuO phase is only formed in a limited deposition window of P(O2) - temperature. The (00l) single-oriented, highly textured, Cu2O films deposited under optimum P(O2) - temperature conditions exhibit excellent electronic properties with carrier mobility in the range of 40–60 cm2 V−1 s−1 and carrier concentration over 1016 cm−3. The power conversion efficiency of 1.65% is demonstrated from a proof-of-concept Cu2O solar cell based on epitaxial Cu2O film prepared on the textured metal substrate.

Multifunctional, phase separated, BaTiO3+CoFe2O4 cap buffer layers for improved flux-pinning in YBa2Cu3O7?? based coated conductors

Phase separated, epitaxial, nanostructured film comprised of BaTiO3 (BTO) and CoFe2O4 (CFO) composite has been developed as a potential multifunctional cap buffer layer for improved flux-pinning in YBa2Cu3O7−δ (YBCO) films. All films were deposited by pulsed laser deposition on SrTiO3 (STO) (100) single crystal substrates. The CFO fraction and growth temperature were identified as key factors for determining the areal number density and mean diameter of the CFO nanocolumns. Compared to the reference sample grown on a pure BTO cap layer, the YBCO films grown on BTO+CFO cap layers show a remarkable improvement in isotropic flux-pinning and, consequently, Jc over the entire field and angle ranges. Transmission electron microscopy analysis confirmed the presence of a very defective YBCO layer containing a high density of randomly distributed defects at the interface area, induced by nanostructural modulation on the surface of the BTO+CFO composite cap layer.

Superconducting properties of YBa2Cu3O7−δ films deposited on commercial tape substrates, decorated with Pd or Ta nano-islands

To obtain an engineered surface for deposition of high-Tc superconductors, nanoscale modulations of the surface of the underlying LaMnO3 (LMO) cap layer is a potential source for generating microstructural defects in YBa2Cu3O7−δ (YBCO) films. These defects may improve the flux pinning and consequently increase the critical current density, Jc. To provide such nanoscale modulation via a practical and scalable process, palladium (Pd) or tantalum (Ta) nano-islands were deposited using dc-magnetron sputtering on the surface of the cap layer of commercial metal tape templates for second-generation wires. The size and density of these nano-islands can be controlled by changing sputtering conditions such as the power and deposition time. Compared to the reference sample grown on an untreated LMO cap layer, the YBCO films grown on the LMO cap layers with Pd or Ta nano-islands exhibited improved in-field Jc performance. Atomic force microscopy and transmission electron microscopy were used to assess the evolving size and density of the nano-islands.

Strong flux-pinning in epitaxial NdBa2Cu3O7−δ films with columnar defects comprised of self-assembled nanodots of BaZrO3

Epitaxial NdBa2Cu3O7−δ (NdBCO) films with and without the incorporation of self-assembled nanodots of BaZrO3 (BZO) were grown on rolling-assisted-biaxially-textured substrates (RABiTS) by pulsed laser deposition. Compared to epitaxial YBa2Cu3O7−δ (YBCO) films on RABiTS, NdBCO films are found to have superior performance in applied fields at all field orientations, suggesting the presence of an increased density of pinning centres. The incorporation of columns of self-assembled nanodots of BZO within the NdBCO films results in further enhancement of the in-field Jc at all field orientations. Jc varies as Jc~H−α at intermediate fields, with α values for NdBCO and BZO-doped NdBCO films being ~0.37 and ~0.43, respectively, which are significantly lower than α~0.52 for YBCO film.

Tuning Flux-pinning in Epitaxial NdBa2Cu3O7-δ Films via Engineered, Hybrid Nanoscale Defect Structures

Epitaxial NdBa{sub 2}Cu{sub 3}O{sub 7-{delta}} films with a hybrid nanoscale defect structure comprised of BaZrO{sub 3} nanodot arrays aligned along the c-axis in one half of the film thickness and aligned perpendicular to the c-axis in the other half thickness of the film were fabricated. Transmission electron microscopy images confirm the orientation of the nanoscale defect structures. The angular dependence of critical current density, J{sub c}, at 77 K, 1 T, shows significantly reduced angular variation of J{sub c}. This study nicely demonstrates how pinning characteristics can be tuned by tuning the nanoscale defect structures within the films.