Fabrication and Optoelectronic Properties of Y-Ba-Cu-0 Thin Films with Different Oxygen Contents

Applications of high-temperature superconductors (HTS) demand not only high quality thin films with excellent electrical properties, but also appropriate patterning techniques. Conventional fabrication methods usually lead to a degradation of patterned HTS thin films, as a consequence of contamination, acidlwater reaction, and physical damage of the film surface. In this work, we developed a non-destructive laser-writing technique for patterning YBa2Cu30x (YBCO) thin films. We demonstrate that laser writing allows reproducible patterning of fine YBCO structures without the need for expensive photomasks and vacuum systems. Further, we investigated mechanisms of the optical response in YBCO for potential optoelectronic applications. We have demonstrated that selective laser writing is capable of creating either a superconducting or insulating phase of YBCO. The patterned oxygen-rich lines exhibit the same superconducting properties as that of the as-deposited films, whereas the oxygen-depleted patterns are characterized by a semiconducting transport which is described by Mott's variable-range hopping. Since our laser-patterning is free of foreign contamination, patterns with the highest quality can be obtained. Moreover, the ability of erasing and rewriting patterns in this technique provides great flexibility for circuit restructuring and error correction. Laser-written structures are found to be stable for a period of over 2 years. Microwave testing confirmed the very low attenuation of our laser-written transmission lines. Femtosecond pump-probe experiments have been performed on YBCO thin films with several oxygen contents. In semiconducting YBCO, band filling across the charge-transfer (CT) gap, as well as free-carrier absorption inside the 0-2p band have been observed. Optical response in both white-light and low-temperature measurements was found to follow a stretched-exponential decay with a temperatureindependent dispersion factor and a characteristic time which decreases with temperature. By separating the contributions from photo-induced bleaching and freecarrier absorption, we obtained the bandwidth of the 0-2p band to be 1.9 eV. Whereas the semiconductor-like CT transition still dominated the response of a 30-K YBCO sample, Fermi smearing governed the signals in films with higher oxygen doping levels. The Fermi level position was monitored as a function of the oxygen content and the observed signal sign dependence on the pump intensity was explained by a photoinduced shift in the Ferrni level. At temperatures below Tc, fast nonbolometric response was observed at low pump intensities while the slow bolometric term dominated at high intensities.