A Preliminary Study For Fabricating High Quality Superconducting YBa₂Cu₃O_6.9 Films By Photo-assisted MOCVD

In this thesis, the growth of purely c-axis oriented YBa2Cu3O6.9 (YBCO) films by photo-assisted MOCVD was discussed. Thin YBCO films (less than 1μm) grown on LaAlO3(100) [LAO(100)] with high crystalline quality was successfully fabricated by a photo-assisted MOCVD system disigned by ourselves. Tests of one thin film sample with thickness of about 0.4μm indicated it is c-axis oriented, with no visible grain boundary in its sectional SEM mcrograph. This means this YBCO film is single-crystal like. On the otherhand, this film has high superconducting quality also. Its superconducting critical temperature (Tc) is 90K, and its superconducting critical current density (Jc) is 1.33MA/cm2 (77K, 0T). Then, thick YBCO films with thickness above 2μm were also succesfully prepared. As shown by both SEM micrographs and XRD 2θ-scanning plots. These thick YBCO-films have similar high crystslline quality as that for the thin YBCO films. As high quality, large area YBCO films are practically needed for the application of these films to the fabrication of superconducting microwave filters, optimization of the processing conditions for the growth of thick YBCO films was done in this thesis.Firstly, substrate temperature window (Ts window) for the growth of purely c-axis YBCO films with high crystalline qulity was more precisely examined by using the photo-assisted MOCVD system on our lab. Secondly, an appropriate Ts of 820℃was chosen for the gowth of YBCO thick films. During the growth of these YBCO films, the influence of changing airflow and reactor pressure to film growth rate was discovered. From our primary study Finally, the optimized growth conditions indicated above were chosen for the preparation of YBCO thick films. The fastest growth rate of films with high crystalline quality was obtained by changing the evaporation rate of the Y and Cu precursors. The best respective evaporation rate along with the best proportion of Y and Cu precursors concusumed in the films growth were then found for the growth of YBCO thick films with high crystalline quality.Besides, high crystalline quality, c-axis oriented YBCO thick films on 1-inch single sided LAO(100) wafers have also been successfully fabricated by the same system. XRD and SEM tests for evenly cut small pieces of a 1 inch YBCO film fabricated indicates. The YBCO film prepared over the entire 1" are as of high crystalline quality, c-axis oriented YBCO thick films. However, as now there is only gas inlet (indined above the surface of the substrate), and the susceptor was not rotated yet, it would be understanble that there would be a large variation in thickness over the 1" area of the YBCO thick films prepared. The thickness here ranges from 1.2 to 2.6μm. There is only piece among the 13 cut pieces from this 1" YBCO film which showes there are few grain boundaries in it. The cause of this phenomenon is discussed. In this thesis, the advantages of the photo-assited MOCD techniques employed for the deposition of YBCO films was also been discussed.