| Since from 1987, the year YBCO was synthesized, the Cu oxide based high-temperture superconducting (HTc) materials had been discovered with various series of materials , as that of the Y, Bi, Tl-series, and Hg-series. Among them, YBCO has been being distingwished as a representative Y-series HTc material, and has been being intensively studied. This is resulted from three magor characteristics of YCBO. At first, YBCO is a HTc material being easyly prepared with high purity, in concern with both chemical composition and crystalline phase. Besides, YBCO has high capability to resist high applied magnetic filed. Thirdly, YBCO (composed with Y, Ba, Cu, and O) has no poisonous elements such as Tl or Hg. Thus, in the field of large current application, YBCO is now the major HTc material for fabricating the so-called 2nd generation HTc conduction type. In the field of low current application, high quality, large-area, and double-sided YBCO epitaxial films are now the major material being used to fabricate superconducting microwave filters (SMF) for preliminary commercial usage in wireless communication. At liquid N2 temperature (77K) and 10 GHz, c-axis oriented epitaxial films of YBCO have their microwave surface resistance (Rs) being about two-orders lower than that of the traditional materials such as Cu. Thus, with microwave filters being made from YBCO, they have low in-band insertion lose and high out-band suppresstion, So, this kind of SMF are called"ideal filters". Now, the techniques for designing and fabricating SMF devices themselves are pretly matured worldly. However, the ways to prepare high quality, large area, double-sided YBCO films are in need still. Presently, there are a number of ways to prepare this kind of YBCO films, such as MOCVD, PLD, MOD etc. Inside China, there are some research institutions dedicated to the studies of fabaricating this kind of YBCO films. However, none of them could provide products for commercial usage with their own proprietary rights yet. Thus, the main purpose of this study is to make some preliminary investigation for preparing high quality, large-area, double-sided YBCO films with purely c-axis orientation by the film growth technique of photo-assisted MOCVD. By using this technique, YBCO films had been successfully grown on 1 inch LAO(100) substrates, with Jc > 1MA/cm2, Rs < 0.5mΩ, as required for being capable to be used for SMF fabrication. At the sametime, this kind of YBCO films had also been successfully grown on 2 inch LAO (100) substrates, with only film uniformity needs to be improved yet. This uniformity problem comes essentially from the size of the MOCVD reactor being used, the reactor which is large enough to grow 1 inch YBCO films uniformly, but not for 2 inch YBCO films yet. Besides, we had made certain comparative studies of growing c-axis oriented YBCO films on other susbstrates such as MgO and polycrystalline Ag.The film growth technique of MOCVD has certain obvious advantages over others. At first, it has large area deposition, and then has high film productivity. Secondly, it is not limited by line-of-sight deposition. In addition, layer-by-layer film growth is feasible by MOCVD. Overall, there are plenty of films growth factors could be manipulated by MOCVD (this involes more technical difficulties too). Thus, MOCVD is a film growth technique mostly being appropriete for commercial usage. On the otherhand, for processing electronic materials, photo-assisted MOCVD have some obvious advantage over that of the traditional thermal-based MOCVD techniqucs. These advanges are essentially caused from the fact that photo-actication, either chemically or physically, is much more efficient than the traditional thermal-activation. From our studies, it is found that the crystalline quality of YBCO films grown by photo-assisted MOCVD is exceptionally good. This means the YBCO films grown are single-crystal like, with no SEM visiable grain boundaries. Besides, the YBCO film growth rate is very high, up to 0.2μm/min, with film thickness as thick as about 4.5μm for the timebing.For using the photo-assisted MOCVD system installed in our lab to grow YBCO films. At first, we have to identify the c-axis oriented YBCO film growth parameters, such as film growth temperature, reactor pressure, mass flow rates for various gases involved, etc. Thus small-size film samples were prepared for searching and identifying these YBCO film growth parameters. As compared with similar works done before by others, it was found that at appropriate reactor pressure and mass flow rates for various gases involved, the"c-axis YBCO film growth window(on LAO (100))"at film growth temperature(Ts) of about 800℃has a window width of about 30℃. If Ts would be lower than this window width, a-axis oriented YBCO films were appeared. If Ts being too high, the YBCO film would be melted irreversibly. Besides, works were done also for trying to improve the usage efficiency of the metalorganic sources for growing YBCO films by reducing mass flow rates of the gases involved, along with decreasing source evaporation temperatures.Next, double-sided, epitaxial YBCO films were grown on 1 inch LAO(100) substrates by photo-assisted MOCVD. For film growth temperature could be distributed more uniformly on a 1 inch LAO substrate, a more appropriate, stainless steel based and rotatable susceptor was designed. In concern with avoiding possible damages would be caused on the backside of a LAO substrate with its front side being deposited, a hollow-center, stainless steel based susceptor was used for the intended YBCO film growths. However, due to light transparency of the LAO subtrates being used for the film growths and the reality of reduced physical contact area between substrate and the center-hollowed susceptor, the problem of nonuniformity in temperature distribution was appeared at film growths still. Thus, we had to adapt a new processing procedure to grow this kind of double-sided YBCO films by using a"three-step"growth technique. After testing, satisfied results were found both in crystalline quality and electrical characteristics. With this technique after this kind of YBCO film growths on 1 inch LAO substratis, similar growths were also performed on 2 inch LAO substrates. However, though high crystalline quality could also be found for the YBCO-films grown on 2 inch LAO substrates, film uniformity was found a problem there. Obviously, the size of the photo-assisted MOCVD reactor though be large enough to grow YBCO-films on 1 inch substrates, but not for 2 inch substrates. A high quality, double-sided epitaxial YBCO film with 2 inch in diameter is generally the minimum size required for being used in SMF fabrication.At last, a comparative study of c-axis oriented YBCO films grown on various substrates such as LAO (100), MgO(100) and polycrystalline Ag was also done. It was found, hygroscopicity was found a series problem for substrates of MgO. Besides, the c-axis YBCO film growth temperature window for HTc films being grown on polycrystalline Ag was about 5℃only. Results would be better if this kind of YBCO film could be grown on single-crystal substrates of Ag.
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