Growth Mechanisms and Surface Morphology of Ybco Based High-T c Thin Films and Heterostructures for sns Junctions
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ABSTRACT We have controlled the nanoscale growth mechanism and surface morphology of YBa 2Cu 307 (YBCO) based high-Tc thin films and heterostructures, using miscut SrTiO 3 substrates. On exact (001) SrTiO 3 substrates, the YBCO films grow in a screw dislocation growth mode. The barrier layers (La 64Sr 1.6Cu 8 O 20 and PrBa 2Cu 3O 7) grown on top of such a YBCO film also show spiral growth features, indicating pseudomorphic growth. On miscut substrates (with miscut angle >_4' toward [010]) the YBCO films grow by step-flow. However, the La 64Sr1 6Cu 80 20 layers grown on such YBCO bottom electrodes, show a high degree of step bunching with rough surface. In contrast, the PrBa 2Cu 30 7 layers show clear step-terrace surface morphology similar to the underlying YBCO bottom electrode, suggesting the existence of periodic nanoscale steps at the S-N interface. These heterostructures can be used for the fabrication of SNS Josephson junctions to take advantage of the proximity effect coupling at the nanoscale steps at the interface.. INTRODUCTION Most of the applications of high-Tc superconductors (HTS) to analog and digital electronic devices involve superconductor-normal metal-superconductor (SNS) Josephson junctions. For instance, superconducting quantum interference devices (SQUIDs)' used for high sensitivity magnetic field detection require two SNS Josephson junctions in a loop. High-speed low power A/D converters and switching devices based on the rapid single flux quantum (RSFQ) 2 logic circuits need an array of hundreds of SNS junctions on a single device. However, for such applications it is essential that the spread in crucial junction properties such as critical current, 1c, normal junction resistance, RnA, and IcR,, product be less than 10% over the entire device. Programmable Josephson voltage standards that consist of a series array of hundreds of nonhysteretic resistively shunted junctions require minimal spread in Ic, Rn and IcRn.product. 3',4 Sensitivity and switching speed are severely degraded in mismatched junctions. Thus, a critical issue in SNS Josephson junction technology is the development of a reproducible and reliable fabrication process to ensure a uniform junction properties within each junction and over the entire device. As the coherence length of high-Tc superconductors is small (-10A), the control of superconductor-normal metal interfaces is a paramount issue in SNS Josephson junction based devices. The magnitude and reproducibility of the junction properties are strongly influenced by the interfaces at the superconductor-normal metal barrier boundaries. The interface resistance and the IcRn product which determines the signal to noise ratio (higher the IcRn product, higher the signal/noise ratio) depend among other things on the nature of the interface between the S and N layers and the nanoscale defects in the S and N layers. Therefore, in order to control the magnitude and spread of the junction properties and obtain a smooth and defect-free interface, it is essential to achieve nanoscale
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