N. Biunno

Formation of thin superconducting films by the laser processing method

We have prepared thin films of Y‐Ba‐Cu‐O superconductors using a pulsed laser evaporation technique. Thin films were formed on (100) Si, (100) MgO, (1102) sapphire, (100) SrTiO3, and amorphous SiO2 substrates using a XeCl excimer laser (λ=0.308 μm, τ=45×10−9 s). The depositions were done in an ultrahigh vacuum chamber with pressure of about 10−6 Torr during thin‐film formation. The deposition by pulsed nanosecond laser irradiation results in stoichiometry close to that of the target. The thickness of the film was controlled by varying the pulse energy density and the number of pulses. The substrate temperature was kept at 470 °C during deposition. Subsequent annealing treatments were carried out at 900 and 650 °C in oxygen atmosphere to recover the superconducting properties of these thin films. The resistance of these films was measured as a function of temperature using the four‐point probe method. These thin films were analyzed using cross‐section transmission electron microscopy, Rutherford backscatt...

Spectroscopic analysis of electronically excited species in XeCl excimer laser‐induced plasmas from the ablated high‐temperature superconductor YBa2Cu3O7

Optical spectroscopic analyses have been performed to study luminescence from plasmas produced by ablation of YBa2Cu3O7 single‐phase high Tc bulk superconductors exposed to XeCl excimer laser (308 nm) pulses. Only excited atomic neutral and single ionized species (Cu/Cu+, Ba/Ba+, Y/Y+) were observed within the experimental resolution of an optical multichannel analyzer detection system, when irradiating the targets in vacuum (∼10−5–10−4 Torr). Conspicuously absent in the spectra (300–800 nm range) are molecular emission bands that would appear if large excited molecules or fragments were present. Implications of the present results are discussed which relate to an early hypothesis about the laser ablation mechanism and their influence on high Tc film characteristics.

Ion-assisted laser processing of CNx films

We have produced CNx thin films with x = 0–0.70 by using an ion-assisted pulsed laser deposition method. In this method a graphite target was ablated while simultaneously bombarding the substrate with nitrogen ions. The deposition rate, ion current, substrate temperature and substrate bias were varied to enhance nitrogen incorporation and to optimize the properties of the thin films. The films were characterized using Rutherford backscattering-channelling. Auger electron spectroscopy, electron energy loss spectroscopy, transmission and scanning electron microscopy, Raman spectroscopy and nano-indentation hardness measurements. The average nitrogen concentration was found to vary (N/C atomic ratio) in the range 0–0.70. The transmission electron microscopy studies showed that these films were polycrystalline with a fine-grain equiaxed structure (average size 10 nm) above a substrate temperature of 200°C. The sp3/sp2 ratio was determined to be 25%–30% using electron energy loss spectroscopy. The Raman studies showed two peaks at 1285 and 1575 cm−1 corresponding to sp3 and sp2 bonding respectively. The films exhibited qualitatively very high hardness and thermal annealing resistance.

Low‐temperature processing of titanium nitride films by laser physical vapor deposition

We have investigated the formation of polycrystalline TiN films on (100) Si substrates using a low‐temperature laser processing method. The films were deposited by laser ablation of a TiN hot‐pressed pellet in the presence of neutral or ionized nitrogen using a XeCl excimer laser (wavelength 308 nm, pulse duration 45×10−9 s, and energy density of 4–5 J cm−2). The substrate temperature ranged from 25 to 550 °C. Plan‐view and cross‐section transmission electron microscopy studies show that the films are polycrystalline (average grain size ∼100 A) with face‐centered‐cubic structure and lattice constant of 4.25 A. It is interesting to note that the average grain size remained approximately constant with substrate temperature up to 550 °C. Chemical composition was analyzed by Rutherford backscattering and Auger electron spectroscopy as a function of film depth. The results show that the films reproduced closely the chemical composition of the TiN target which contained some oxygen, and that the oxygen content ...

Microstructural and compositional variations in laser-deposited superconducting thin films

We have investigated thickness and composition variations in laser‐deposited superconducting thin films and their effects on the microstructure development after subsequent annealing to recover the superconducting properties. Superconducting thin films were deposited on magnesium oxide substrates using a pulsed excimer laser (λ=308 nm, τ=45×10−9 s). These films were annealed at about 900 °C in helium and oxygen atmospheres followed by slow cooling to room temperature. The forward directed nature of the laser‐induced deposition process in controlling the spatial variation in thickness of the film was examined using gas dynamics of the laser generated plasma. The compositional variation as a function of spatial distance was determined by Rutherford backscattering spectrometry. Correlation of the presence of different phases with the average composition in the film was done using transmission electron microscopy combined with x‐ray microanalysis techniques. The slight enrichment of yttrium in the film result...

Microstructure and properties of YBa2Cu3O9−δ superconductors with transitions at 90 and near 290 K

We have investigated the microstructure and properties of YBa2Cu3O9−δ that contain resistive transitions at 90 and near 290 K using high‐resolution and analytical transmission electron microscopy and Raman scattering techniques. The specimens contain normal orthorhombic (Pmmm space group) phase associated with Tc=90 K, and another phase which we tentatively assign it to be associated with the resistive transitions near 290 K. The new phase has a c axis larger by about 16% and it grows epitaxially on the {100} planes of the orthorhombic (tripled perovskite unit cell) phase having Tc=90 K.

Laser deposition of epitaxial titanium nitride films on (100) MgO

We have investigated the formation of epitaxial (100) oriented TiN films grown on (100) MgO substrates using an excimer laser (wavelength 308 nm, pulse duration 45 ns, and energy density (5–6 J cm−2) physical vapor deposition method. The films were deposited by laser ablation of a TiN target pellet in high vacuum (∼3.0×10−7 Torr), with the substrate temperature ranging from 450 to 750 °C. The epitaxial films were obtained at relatively low substrate temperatures (∼450 °C). The deposited films were analyzed using cross‐section and plan‐view transmission electron microscopy, x‐ray diffraction, Rutherford backscattering/channeling, Auger electron spectroscopy, and electron channeling patterns. The results indicate epitaxial film growth (〈100〉 TiN parallel to 〈100〉MgO) near the stoichiometric TiN composition, and low oxygen content. The minimum channeling yield χmin was found to be ≤10%. The room‐temperature resistivity was as low as 50 μΩ cm and the lattice parameter was found to be 4.218 A.

Orientation dependence of twinning characteristics in Y-Ba-Cu-O superconducting thin films

The twinning characteristics of epitaxial and textured superconducting thin films grown on (100) SrTiO3 and (100) MgO substrates have been found to be dependent on the film orientation. Microstructural details of thin films deposited by the pulsed‐laser evaporation technique followed by annealing in oxygen and helium atmospheres at 900 °C for short time intervals were examined using cross‐section and planar transmission electron microscopy. The number density of twins in textured thin films having their c axis normal to the substrate were found to be significantly higher than those in textured films having their c axis parallel to the substrate. Calculations of stresses induced in thin films as a result of the tetragonal to orthorhombic phase transition indicated that large shear stresses and strain energies were introduced in films with their c axis perpendicular to the substrate, thereby resulting in a high number density of twins in this orientation.

Twinning characteristics in YBaCuO superconducting thin films

We have analyzed the nature, distribution and number density of twins as a function of orientation of superconducting YBa2Cu3O7 thin films deposited on strontium titanate (100), magnesium oxide (100) and ytria-stabilized zirconia substrates. The twinning characteristics in these high Tc superconducting thin films have been found to be different from those in bulk materials as a results of texturing, interfacial stresses and constraints during crystal growth. The free surface has been found to play an important role in the relaxation of stresses introduced in the film during tetragonal-to-orthorhombic phase transformation. Detailed electron microscopy studies were performed on superconducting thin films deposited by evaporation of bulk YBa2Cu3O7 targets using a pulsed excimer laser (λ = 308 nm; τp = 45 × 10−9 s). Calculations of stresses in thin films indicated that higher shear stresses and strain energies were introduced in films with the c axis normal to subtrate than in those parallel to the substrate, thereby resulting in a higher number density of twins in the first case. The electron diffraction patterns of twinned crystals were simulated to understand the crystallography of the twinning process, and high resolution transmission electron microscopy studies were performed to determine the bending of planes across the twin boundary. Good agreement between experimental observations and theoretical models was obtained.

Low Temperature Processing of TiN Epitaxial and Polycrystalline Films by Laser Physical Vapor Deposition

We have investigated the formation of polycrystalline TiN films on (100) Si substrates and epitaxial (100)-oriented TiN films on (100) MgO substrates by laser physical vapor deposition. The films were deposited by excimer laser (wavelength 308 nm, pulse duration 45 nanosecond, and energy density 4-6 J cm-2 ) ablation of a TiN target pellet in high vacuum (~ 3.0 x 10-7 torn) with the substrate temperature ranging from 25 to 750°C. The epitaxial films on Mg° were obtained at relatively low substrate temperatures ( 450°C ) and polycrystalline films on Si were obtained at substrate temperatures ranging from 25 to 550°C. The deposited films were analyzed using cross-section and plan-view transmission electron microscopy, X-Ray diffraction, Rutherford backscattering / channeling patterns, Auger electron spectroscopy, and electron channeling patterns. Results for epitaxial film growth indicate <100> TiN parallel to <100> MgO with the minimum channeling yield( xmin ) < 10% and room temperature resistivity of 501.112-cm. Typical resistivity values of the polycrystalline films were 150 I. -cm and microhardness values were found to be as high as 17 GPa. The polycrystalline films are very dense with an average grain size of 100 A which remained approximately constant with substrate temperature up to 550°C. The polycrystalline films were of equiaxed nature with no preferred growth orientation. These microstructural features couple with low temperature deposition and dense nature make these films suitable for many advanced applications.