S. P. Pai

High quality zinc oxide films by pulsed laser ablation

Abstract We have investigated the structural, electrical and optical properties of thin films of zinc oxide deposited by laser ablation. We find that even at room temperature the films are c-axis oriented on both crystalline and amorphous substrates. The electrical dc resistivity is independent of substrate temperature and is about 2–3 mΩ cm−1. The films have low absorption coefficient in the visible part of the spectrum and represent some of the best ZnO films deposited at low temperature.

Improved microwave performance of Ag‐doped Y1Ba2Cu3O7−δ thin film microstrip resonators

Microwave transmission properties of undoped and Ag‐doped laser ablated thin film Y1Ba2Cu3O7−δ microstrip resonators have been studied both on <100≳ MgO and <100≳ LaAlO3 substrates at X‐band frequencies. While the Q factor and microwave surface resistance, Rs, of undoped films showed better performance on LaAlO3 as compared to that on MgO, Ag‐doped films on LaAlO3 showed far greater improvement as reflected not only by a decrease in Rs but also by a total absence of its microwave power dependence up to 13 dBm at 77 K. These results are explained as due to the influence of Ag in increasing the grain size and grain alignment and thus, significantly decreasing the density of grain boundary weak links which are known to affect the microwave transmission in high temperature superconductor films.

Temperature dependence of magnetoresistance and nonlinear conductance of the bicrystal grain boundary in epitaxial La0.67Ba0.33MnO3 thin films

We investigated conduction through an artificial grain-boundary junction made in La0.67Ba0.33MnO3 thin films, deposited on a 36.7° SrTiO3 bicrystal substrate using a laser ablation technique. The grain boundary exhibits substantial magnetoresistance at low temperatures and also shows nonlinear I–V characteristics. Analysis of temperature dependence of the dynamic conductance allows us to identify three carrier transport mechanisms across the grain boundary. These mechanisms exist in parallel, and at a given temperature one mechanism may dominate. Particularly, at higher temperatures (T>175 K) the transport across the grain boundary involves spin–flip scattering, which we establish leads to decrease of the bicrystal grain-boundary contribution in magnetoresistance. At lower temperature (4.2–45 K), tunneling through a disordered oxide at the grain boundary dominates, whereas in the temperature range from 100 to 175 K, carrier transport is dominated by inelastic tunneling via pairs of manganese atoms.

Dielectric properties of oriented thin films of PbZrO3 on Si produced by pulsed laser ablation

PbZrO3 is an antiferroelectric perovskite with TC≈230 °C. We have deposited single phase, perfectly c-axis oriented thin films of PbZrO3 on Si(100) substrates by pulsed laser ablation at 700 °C. The growth conditions (substrate temperature, ambient oxygen pressure, and laser energy density) have been optimized and the morphology of the films studied by scanning electron microscopy and atomic force microscopy. From a study of the dielectric hysteresis of the films and a measurement of the temperature dependence of their capacitance, we find that films thicker than ≈300 nm are antiferroelectric, while thinner films (<300 nm) appear to exhibit ferroelectric behavior.

c‐axis oriented ferroelectric thin films of PbTiO3 on Si by pulsed laser ablation

We have deposited single phase c‐axis oriented ferroelectric thin films of PbTiO3 on Si(100) by pulsed laser ablation technique in situ. It has been shown that the formation of a nonferroelectric, Pb2Ti2O6 pyrochlore phase at the interface could be avoided by raising the substrate temperature.

SILVER DOPING AND ITS INFLUENCE ON THE OXYGENATION DURING IN SITU GROWTH OF YBA2CU3O7-X THIN FILMS

The influence of Ag doping on both oxygenation and deoxygenation during growth of YBa2Cu3O7−x films at 700 °C by pulsed laser deposition has been studied. Experiments show that undoped and Ag‐doped films grown at 200 mTorr oxygen pressure and quenched immediately after termination of growth have a superconducting transition temperature Tc of 61 and 86 K, respectively. The high Tc of 86 K obtained with quenched Ag‐doped films is thought to be due to both enhanced oxygen incorporation and reduced deoxygenation associated with Ag doping. These results show that Ag‐doped YBa2Cu3O7−x films could be crucial for realizing devices based on multilayer structures.

PULSED LASER ABLATION : A NEW ROUTE TO SYNTHESIZE NOVEL SUPERCONDUCTING COMPOUNDS AS ORIENTED FILMS

We report here the significance of the pulsed laser ablation technique in stabilizing strained lattices that do not form by the conventional ceramic method and show that the technique offers unique possibilities to probe the structure property relationship in complicated systems. One of such systems is LuBa2Cu3O7−δ; a systematic investigation of structural (in)stability of its superconducting phase is presented here. Our analysis suggests that the system suffers from internal strain due to lower ionic radius of Lu3+; however, the structure can be stabilized only as oriented films on 〈100〉 LaAlO3, 〈100〉 SrTiO3, and 〈100〉 MgO, with excellent superconducting properties (Jc≊5.0×106 A cm−2 at 77 K). We have also investigated similar compounds having their stability close to their crystallographic limit. The important feature of these metastable phases is that they grow only as oriented films. Free energy of epitaxial growth of strained films are investigated and a simple growth model is proposed based on our o...

Giant magnetoresistance studies on La(0.8−x)RxSr0.2MnO3 thin films (R Pr, Nd, Gd, Ho)

Abstract The effect of substituting La by a smaller lanthanide element (R) in La 0.8− x R x Sr 0.2 MnO 3 ( x = 0.1; R  Pr, Nd, Gd, Ho) thin films on the resistivity ( ϱ) and giant magnetoresistance (GMR) behaviour was studied. The magnitude of GMR shows a maximum as a function of temperature. The temperature at which the maximum value of GMR is observed was found to shift systematically towards higher temperatures (from 210 to 300 K) as the size of the substitution ion increases from Ho to Pr. ϱ ( T ) also shows a maximum which shifts to higher temperatures as the size of the substitution ion increases. The zero-field resistivity shows a systematic decrease as the size of R 3+ increases. This result is explained qualitatively by invoking the interrelation between the conduction electron hopping probability and the MnOMn bond angle distortion which, in turn, is governed by the size of the substitution ion. Our results show that the size of the rare earth substitution ion can play an important role in tailoring the resistivity and GMR behaviour of La-R-Sr-Mn-O thin films, particularly for room temperature applications.

GROWTH AND MICROSTRUCTURAL STUDY OF RADIO-FREQUENCY MAGNETRON SPUTTERED MGO FILMS ON SILICON

Microstructure of magnesium oxide films radio frequency (rf) sputtered on 〈100〉 silicon substrates at various argon: oxygen (9:1) gas pressures in the range 1–6.7 Pa and at various substrate temperatures up to 700 °C have been studied using x‐ray diffraction and scanning electron microscopy. The films have shown a tendency for oriented structure with growth orientation perpendicular to the 〈200〉 planes. The tendency for columnar growth has been found to be strong at high argon pressures and has been found to persist to higher substrate temperatures. This observation is consistent with the structure zone model proposed by Movchan and Demchishin [Phys. Met. Mettallogr. 28, 83 (1969)] and later studied by Thornton [J. Vac. Sci. Technol. 11, 666 (1974)]. Annealing of films at 900 °C in oxygen which is needed to reduce oxygen deficiencies and strain generated during growth, and to improve crystallinity by increasing grain size, has been found to cause microcracks in the films depending upon the microstructure ...

Superconductivity and transport behavior of laser ablated Au‐added YBa2Cu3O7−δ thin films

We report here a systematic investigation of superconducting properties of Au‐added YBa2Cu3O7−δ (YBCO) thin films in situ grown by pulsed laser ablation method. Films are highly c‐axis oriented while the corresponding bulk samples contain multiphases. The superconducting transition temperature Tc of films remains close to 90 K even for large concentration of Au (up to 10% by weight) and the critical current denisty Jc is about 5×106 A cm−2 at 77 K (in self‐field) with higher reproducibility when compared with that of undoped films of YBCO on 〈100〉 LaAlO3. Preliminary investigations indicate that Au gets rejected by the grains and dopes the grain boundaries thus improving the stability of the boundaries and hence of the films. Our result also demonstrates that pulsed laser deposition can be used to grow films of compounds that generally do not form as a single phase in bulk by the conventional solid state methods and hence offers unique possibilities in terms of exploring structure‐property relationships i...