Growth and Characterization of (103)-and (110)-YBa2Cu3O7 Superconducting Thin Films
|Keywords:||釔鋇銅氧;超導-常態-超導;台階邊緣結構;雙晶晶界接面;相干長度;銅氧平面;約瑟芬遜弱連;反射式高能量電子繞射儀;(YBa2Cu3O7-x);(Superconductor-Normal metal-Superconductor, SNS);(step-edge junction);(grain boundary junction);(coherence length);(CuO2 plane);(Josephson Weak-links);(RHEED)|
ABSTRACT This thesis is aimed to describe systematic studies of correlating the growth mechanisms and final grain morphologies of YBCO superconducting thin films with various processing parameters, such as the initial state of the substrate and growth temperatures. With the aid of the unique technology developed in our laboratory for depositing large area YBCO thin films by laser ablation, a detailed monitoring of grain evolution right from the first few monolayers is realized. It was found that even on the same substrate, the detailed substrate surface morphology as well as the growth temperatures are the keys in ultimately determining the final orientation and grain morphology of the laser ablated YBCO thin films. Based on the fundamental knowledge gained from the investigations, the growth and characterizations of (110)-and (103) oriented YBCO thin films growth on the same (110) SrTiO3 substrates are emphasized. Both the scanning electron microscopy (SEM) and atomic force microscopy (AFM) were employed to study the growth mechanism and evolution of grain morphology in films grown in different orientations. In addition to the detailed morphology characterizations, we proposed, for the first time, a novel method, combining conventional X-ray diffraction with film oxygen content controls, to distinguish the YBCO (110) and YBCO (103) films, which was virtually undiscernable by various techniques. The firm identification of exact film orientation has proven to be very important for supporting the interpretations of X-ray absorption spectra in determining the hole concentrations allocated in various bands associated with different Cu-O bonding. Furthermore, it also provides some new insights on the studies of femtosecond pump-probe measurements on carrier relaxation anisotropies in this material system. The prominent features of these studies together with the transport property and high-frequency (mm-wave band) microwave surface resistance anisotropies displayed by these films will also be described and briefly discussed.
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