Formation of artificially-Layered Thin-Film Compounds Using Pulsed-Laser Deposition
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Mat. Res. Soc. Symp. Proc. Vol. 388 01995 Materials Research Society
diffraction peaks attributed to the superlattice structures are observed, even for SrCuO2/(Ca,Sr)CuOZ superlattice structures with SrCuO2 and (Ca,Sr)CuO 2 layer thicknesses of a single unit cell (-3.4 A). The x-ray diffraction data also reveal finite-thickness oscillations in the xray intensity, which is indicative of films with extremely flat surfaces. The growth of superlattice structures by PLD is made possible, in large part, by this surface flatness. In addition, we also report the synthesis of novel artificially-layered HTSc compounds, grown as SrCuO2/BaCuO2 crystalline superlattice structures. 26 In showing that the growth of infinite layer materials in superlattice structures can be controlled on the unit cell scale using conventional PLD with no in situ surface monitoring, this work offers the exciting possibility of greatly broadening the conditions under which new artificially-layered HTSc phases can be formed. EXPERIMENTAL CONDITIONS The films were grown on (100) SrTiO 3 substrates utilizing conventional multi-target PLD. Polycrystalline, orthorhombic (Ca,Sr)CuO2 and cubic BaCuO2 ablation targets were mounted in a multi-target carousel. The (Ca,Sr)CuO2 targets were made by solid state reaction of high-purity SrCO 3 , CaCO 3 , and CuO which were pressed and fired at 1025'C. Powder x-ray diffraction confirmed complete decomposition of the carbonates. The BaCuO2 target was prepared using high-purity BaCuO2 powder. (100) SrTiO 3 substrates were cleaned with solvents prior to being mounted with silver paint on the substrate heater. The KrF excimer laser ablation beam was focused to a 1 cm horizontal line and vertically scanned over the rotating targets to improve film thickness uniformity. The focused laser energy density was approximately 2 J/cm2, and the substrates, heated to 500-625'C, were placed 10 cm from the ablation targets. Film growth was carried out in 200 mTorr of oxygen. Before growing the SrCuO2/BaCuO2 layered structures, a 9.0 nm thick SrCuO2 buffer layer was grown to initiate epitaxial growth of the infinite layer structure on the SrTiO 3 surface. Total film thickness varied from 90 to 120 nm, corresponding to 60 or more superlattice periods. After deposition, the films were cooled at -80°C/min in 200 mTorr of oxygen, with the pressure increased to 760 Torr at 375°C for the SrCuO2/BaCuO2 superlattices. The structure and epitaxy of the films were investigated by x-ray scattering measurements obtained using both two-circle and four-circle diffractometers with monochromated CuKoc x-ray sources. In addition, more sensitive synchrotron x-ray measurements were obtained for one film using beamline X14 at the National Synchrotron Light Source (NSLS) operating with a Si(1 11) monochromator and a Ge(l 11) analyzer set near the CuKoc wavelength. SrCuO2/(Ca,Sr)CuO2 SUPERLATTICES The growth of superlattice structures by PLD requires a well-calibrated and controlled growth rate. Initial estimates of the film thickness per laser shot wer
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