A. Inam

Observation of two distinct components during pulsed laser deposition of high Tc superconducting films

Using Rutherford backscattering technique, we have measured the angular distribution of the composition and thickness of the Y‐Ba‐Cu oxide film deposited by firing excimer laser (30 ns, 248 nm) pulses at a stoichiometric Y1Ba2Cu3O7−x pellet. The angular distribution consisted of two distinct components: one a cos θ component, a result of evaporation, and the other a highly forward directed component, a result of a secondary ejection process. The evaporated component is nonstoichiometric, as one would expect, whereas the forward‐directed component has a composition close to that of the pellet. Further, the forward‐directed stoichiometric component increases with the laser energy density in comparison with the evaporated component. These observations are discussed in the context of current models of laser‐induced material ejection at surfaces.The laser energy dependence of the deposition is of critical importance in controlling the film stoichiometry.

Pulsed laser etching of high Tc superconducting films

Etching of Y‐Ba‐Cu‐O superconducting thin films has been accomplished using a pulsed excimer laser (248 nm, 30 ns). Etch depth as a function of the number of laser pulses was linear over a wide range of incident laser energy densities. An etch threshold energy density of 0.11 J/cm2 was observed and etch rate per pulse scaled linearly with the logarithm of the incident energy density. The dependence is adequately explained by a linear absorption model with an inverse absorption length of 2.3×105 cm−1.

Application of a near coincidence site lattice theory to the orientations of YBa2Cu3O7-x grains on (001) MgO substrates

Various orientations of YBa2Cu3O7−x grains in polycrystalline films prepared on (001)MgO substrates by in situ laser deposition were determined using electron diffraction. Eight different types of in‐plane orientations have been observed. These orientations agree well with the prediction of a simplified theory of near coincidence site lattice between YBa2Cu3O7−x and MgO. The YBa2Cu3O7−x grains were found to have a high probability of forming low angle or low Σ boundaries among themselves. These grain boundaries are of low energy and should exhibit a high connectivity of Cu‐O‐Cu chains. Therefore, YBa2Cu3O7−x thin films on MgO can attain a Tc of ∼90 K and a Jc of ∼106 A/cm2 at 77 K.

Nature of the pulsed laser process for the deposition of high Tc superconducting thin films

The pulsed laser thin‐film deposition process can enable preparation of thin films of complex composition with good control over the film stoichiometry. The film compositions are similar to that of the target pellet and as a consequence this technique appears to be an ideal method for preparing high Tc thin films on a variety of substrates.The factors which contribute to this beneficial phenomenon have been explored by a laser ionization mass spectrometry (LIMS) and a post ablation ionization (PAI) neutral velocity analysis technique in order to determine the mass and velocities of the laser ejected material. In addition, x‐ray absorption measurements on films deposited onto substrates at room temperature were performed in order to identify the presence of short‐range crystalline order in the films. Both of these studies rule out the ejection of stoichiometric clusters of material from the pellet during the laser ablation/deposition process. Instead, binary and ternary suboxides are emitted from the targe...

Microwave properties of highly oriented YBa2Cu3O7−x thin films

We have performed intra‐ and extra‐cavity microwave frequency (1–100 GHz) measurements on high quality Y1Ba2Cu3O7−x superconducting thin films on (100) LaAlO3 substrates. The ∼0.3 μm thin films fabricated by the pulsed laser deposition technique exhibit superconducting transition temperatures >90 K, as determined by resistivity and ac susceptibility measurements, and critical current densities of 5×106 A/cm2 at 77 K. Moreover, ion beam channeling minimum yields of ∼3% were measured, indicating the extremely high crystalline quality of films grown on the LaAlO3 substrate. Microwave surface resistance values at 77 K for these films are found to be more than one to two orders of magnitude lower than for copper at 77 K for almost the entire frequency range explored. We postulate that the reason we observe such low surface resistances in these films is the virtual absence of grain and phase boundaries coupled with the high degree of crystallinity. Furthermore, we believe that the residual resistance measured b...

Substrate effects on the properties of Y‐Ba‐Cu‐O superconducting films prepared by laser deposition

Superconducting thin films of Y1Ba2Cu3Ox with superconducting transition temperature (Tc) near 90 K have been prepared by a laser deposition technique. We show that films prepared on sapphire, lithium niobate, and strontium titanate under identical processing conditions exhibit different electrical characteristics. The film surfaces, interfaces, and crystallinity have been studied by a number of analytical techniques. We conclude that the substrate influences the film properties primarily in three ways: the thermal expansion mismatch introduces cracks in the film, the interface reaction changes the film composition, and the substrate lattice influences the crystallographic orientation of the film.

Epitaxial growth of ferroelectric bismuth titanate thin films by pulsed laser deposition

Epitaxial thin films of ferroelectric bismuth titanate Bi4Ti3O12 have been grown by pulsed laser deposition on single‐crystal [100] SrTiO3 substrates. Bismuth titanate has a high Curie temperature (675 °C) and saturation polarization values of 4 and 50 μC/cm2 along the c and b axis, respectively. Its a,b lattice parameters allow thin‐film growth on substrates such as SrTiO3, LaAlO3, MgO, etc. These single crystalline films exhibit good quality as evidenced by x‐ray diffraction, Rutherford backscattering, and transmission electron microscopy. Applications for these films include memory devices and optical displays.

Optical response of nongranular high‐Tc Y1Ba2Cu3O7−x superconducting thin films

We have investigated the optical response of c‐axis oriented crystalline Y1Ba2Cu3O7−x thin films (bridge and meander devices) on strontium titanate and MgO substrates. cw optical response to a He‐Ne laser radiation (wavelength of 0.63 μm) was primarily bolometric. The pulsed optical response was studied with Q‐switched and mode‐locked Q‐switched short pulses from a Nd:YAG laser at the wavelength of 1.06 μm. We identify two distinct components contributing to the pulsed optical response: a nonbolometric (as fast as 1 ns) and a bolometric component (several ns). The bolometric component is strong at temperatures in the vicinity of the transition region to the normal state. The nonbolometric component is dominant at temperatures below the transition region showing weak temperature dependence and a linear dependence on the bias current. These results are discussed using the flux motion model and also electron‐phonon scattering relaxation dynamics of nonequilibrium superconductors based on the theory of Bardee...

Microstructure of in situ epitaxially grown superconducting Y‐Ba‐Cu‐O thin films

The microstructure of in situ epitaxially grown Y‐Ba‐Cu‐O thin films on (001) SrTiO3 substrates was studied using cross‐sectional transmission electron microscopy. The films, prepared by pulsed laser deposition at substrate holder temperature of 650 °C without post‐annealing, exhibit zero resistivity above 90 K and critical currents exceeding 106 A/cm2 at 77 K. The films are of heavily faulted single crystalline structure with the c axis approximately perpendicular to the substrate (001) surface. We suggest that, due to the fast quenching and low substrate temperature, crystalline defects and chemical fluctuations are locked into a faulted structure after each laser pulse. Despite their rather imperfect microstructure, the films are free from macroscopic grain boundaries and secondary phases and possess superb superconducting properties.

Large critical current densities in YBa2Cu3O7−x thin films made at high deposition rates

Critical current densities (Jc) in YBa2Cu3O7−x films made at deposition rates from 0.1 to 14.5 nm/s (∼50 μm/h) were measured using a direct transport method. As the deposition rate was increased by two orders of magnitude, the films exhibited no marked degradation in current carrying capability with Jc of ∼4×106 A/cm2 at 77 K and zero field. Jc for all the films showed similar behavior under a magnetic field up to 8 T, although extra structural defects were found in the films deposited at the higher rates. The results from this experiment indicate the feasibility for coating wires, tapes, and other macroscopic shapes with high Tc superconductors using proper vapor deposition techniques.