Honghui Zhou

Temperature-dependent leakage mechanisms of Pt∕BiFeO3∕SrRuO3 thin film capacitors

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.

1000 A cm − 1 in a 2 µm thick YBa2Cu3O7 − x film with BaZrO3 and Y2O3 additions

We report here on significant improvements to the in-field and self-field critical current densities (Jc) of multilayer and single-layer YBa2Cu3O7 − x (YBCO) thick films with BaZrO3 (BZO) and Y2O3 additions. In the former case, the composite film consisted of a five-layer architecture with three YBCO layers and two Y2O3 interlayers in a total thickness of 1.8 µm. The multilayer film produced a Jc (75.6 K, self-field) of 4.3 MA cm − 2 (775 A cm − 1) and a minimum Jc (75.6 K, 1 T) of 1.0 MA cm − 2 (175 A cm − 1) over all field orientations in the maximum Lorentz force configuration. We achieved in a single-layer 2.0 µm thick film a Jc (75.6 K, self-field) of 5.2 MA cm − 2 (1010 A/cm-w) and a minimum Jc (75.6 K, 1 T) of 1.2 MA cm − 2 (234 A/cm-w) in the same measurement configuration. For both kinds of films, scanning transmission electron microscopy and transmission electron microscopy imaging were used to identify a uniformly dispersed second-phase microstructure consisting of short, tilted BZO nanorods and tilted Y2O3 nanoparticle layers. We attribute the enhanced performance of the thick YBCO films to the uniformity of the microstructure and the interaction of two different second-phase materials during film growth.

Ultrathin epitaxial superconducting niobium nitride films grown by a chemical solution technique

Ultrathin epitaxial superconducting NbN (18 nm) films, exhibiting a superconducting transition temperature of 14 K and a critical current density as high as 5.2 MA cm(-2) at 5 K under zero magnetic field, were grown on SrTiO(3) (STO) by a chemical solution technique, polymer assisted deposition (PAD).

Improved microstructure and enhanced low-field Jc in (Y0.67Eu0.33)Ba2Cu3O7−δ films

This paper reports a study on mixed rare-earth barium–copper oxide superconducting films (Y0.67Eu0.33)Ba2Cu3O7−δ of different thicknesses grown on single-crystal SrTiO3 substrates by pulsed laser deposition (PLD). X-ray diffraction (XRD), scanning electron microscopy (SEM) and cross-sectional transmission electron microscopy (TEM) were employed to investigate film crystallinity, surface morphology and microstructure. Critical current density in self-field as a function of film thickness and in an applied magnetic field as a function of field strength and direction were also measured. The results show high quality (Y0.67Eu0.33)Ba2Cu3O7−δ films with a smoother and denser surface morphology compared to that of pure YBa2Cu3O7−δ. A thickness-dependent self-field critical current density quantitatively similar to that of pure YBa2Cu3O7−δ was observed, which suggests that the microstructural evolution in terms of increasing surface roughness and microporosity as well as crystallinity degradation is not likely to be playing a critical role in the form of the dependence. (Y0.67Eu0.33)Ba2Cu3O7−δ films also exhibit improved magnetic field performance at low fields over a wide range of orientations in the intermediate angle region, indicating that probable additional random pinning sites were introduced by the substitution of europium in one-third of yttrium sites.

Effects of the Variable Lorentz Force on the Critical Current in Anisotropic Superconducting Thin Films

When a current is applied perpendicular to the vortex lattice (VL), Lorentz force may cause the VL to drift and flux-flow dissipation is observed. When the current is parallel to the applied magnetic field in a force-free (FF) configuration, a dissipation is also observed but at higher values of applied current. It has been suggested that pinning as well as free surfaces play an important role in the stabilization of the VL in the FF configuration. In thin YBa2Cu3O7 films, FF configurations can be obtained when Hparab with the current flowing parallel to the ab-planes. In this work we study the influence of thickness, growth method and pinning centers on the dissipation mechanism at variable Lorentz force and FF configurations. Comparisons of experiments done at maximum and variable Lorentz force show that there are two pinning regimes when the field is rotated in these configurations; one consistent with only a decrease in the applied force, indicated by the overlap of the power law exponent of the current-voltage curves as the field is rotated toward the ab-planes, and another very close to the ab-planes, where the dissipation characteristics change.

Temperature-dependent leakage mechanisms of BiFeO3 films

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 vs. electric field characteristics were investigated from 80 to 350 K. It was found that the leakage mechanisms were a strong function of temperature and voltage polarity. At temperatures between 80 and 150 K, space-charge-limited current was the dominant leakage mechanism for both negative and positive bias. On the other hand, at temperatures between 200 and 350 K the dominant leakage mechanisms were Poole-Frenkle emission and Fowler-Nordheim tunneling for negative and positive bias, respectively.