Effect of laser fluence on outgrowth formation and interfacial reaction in YBCO/CeO 2/Al 2O 3 films

Abstract

High laser fluence values (∼4–5 J/cm 2) for laser deposition of YBCO films on CeO 2 buffered sapphire (Al 2O 3) substrates resulted in a strong decrease of the surface outgrowth density as compared to deposition at low laser fluence (∼1.6 J/cm 2). Cross-sectional transmission electron microscopy (TEM) revealed that the surface outgrowths emanated from interfacial reactions. At low laser fluence, the interfacial reactions of YBCO with the CeO 2 film and Al 2O 3 substrate producing BaCeO 3 and BaAl 2O 4, respectively, were more extensive and led to a high density of surface outgrowths on YBCO. The surface outgrowths are mostly YCuO 2 or CuO precipitated on top of BaCeO 3 and BaAl 2O 4. Moreover, the BaAl 2O 4 phase is also found as a continuous layer of ∼10–20 nm thickness along the CeO 2–Al 2O 3 interface. At high laser fluence (>4.0 J/cm 2), the interfacial reactions were suppressed and led to fewer surface outgrowths and improved film smoothness. The few outgrowths were mostly comprised of Y 2O 3 or YCuO 2 formed on top of localized BaCeO 3 interfacial reactions. BaAl 2O 4 was confined along the CeO 2–Al 2O 3 interface as a very thin layer of ∼2.5 nm. TEM observations also revealed that the twin structure is more homogeneous, possibly due to the absence of surface outgrowths which block the regular twin structure formation. High critical current density ( J c) values in excess of 1.0×10 6 A/cm 2 were obtained in all films, and there was no significant decrease in J c even with decreased outgrowth density.