The development of new electronic devices based on High critical Temperature (HTc) superconductors is determined by the capability of growing high quality thin films. In fact, the very high theoretical critical currents of many ceramic superconductors with Tc above liquid nitrogen temperature can only be approached in defect-free single crystals; unfortunately, it is difficult, or even impossible, to grow crystals big enough to be used in technological applications. On the other hand, it is relatively simple to deposit by PVD techniques oriented thin films on appropriate single-crystal substrates, so that the transport properties of the superconducting layer are close to those exhibited by small crystals. Furthermore, thin films are almost ideal objects to fabricate electronic devices.
XRD is one of the best, and certainly most complete techniques to study thin films; in fact, besides the traditional phase identification, XRD may be used to obtain in a non-disruptive way a lot of microstructural information, like for instance residual stresses, texture, crystallite size and microstructural distortions. XRD residual stress and texture analyses are well- known techniques, and the second one in particular is routinely employed to check the degree of epitaxy of thin films, which is probably the most relevant structural property of interest to HTc thin films applications. On the contrary, XRD Line Profile Analysis (LPA) of highly textured thin films is much less frequent. Despite the little use, LPA can be used with success to have a detailed vision of the degree of crystalline order inside thin film grains, specially when they are highly textured [1,2].
[00l] oriented thin films of YBa2Cu3O${7-\delta}$ (YBCO) were deposited on NdGaO$_{3}$ [00l] single crystal substrates, by using Channel Spark Ablation (CSA) [3], a recently developed and inexpensive technique. Even if the degree of epitaxy among thin films produced under various process conditions is comparable, superconducting properties are quite scattered; in the present work we showed that differences in Tc$_{o}$ and DTc (width of the superconducting resistive transition) may be attributed to lattice disorder inside grains.
Highly oriented thin films produced under epitaxial conditions on single-crystal substrates are typically made of columnar grains ([00l] in this case), which can extend significantly from the interface toward the surface. Under these conditions LPA of XRD patterns collected in conventional Bragg-Brentano (BB) geometry may be conducted on several diffraction orders from the {00l} planes, even for very thin films (less than 100 nm), leading to a very accurate crystallite size and microstrain analysis. Figure 1 shows column length and microstrain distributions of three YBCO films with different Tc$_{o}$ and DTc; considering the specific highly oriented microstructure, in this case crystallite size identifies with the length of coherently diffracting domains along the growth direction, which is a measure of the degree of perfection of crystalline superconducting domains. In fact, better superconducting properties are associated to lower microstrain and longer crystallites. This behaviour has been recently confirmed by an analogous study extended to a number of MBa$_{2}$Cu$_{3}$O$_{7-\delta}$ thin films (were M is Y or a rare earth) deposited by various PVD techniques on different substrates, with or without buffer layers [4]
1, P. Scardi, L. Lutterotti, L. Correra and S. Nicoletti: J. Mat.
Research, 8 [11] (1993) 2780-2784
2. P. Scardi: Science and Technology of Thin Films, edited
by F.C. Matacotta and G. Ottaviani. World Scientific publish.
(1995). In press.
3. V.A. Dediu, Q.D. Jiang, F.C. Matacotta, P. Scardi, M.
Lazzarino, G. Nieva, L. Civale: Supercond. Sci. Technol 8
(1995) 160-164.
4. P. Scardi, V.A. Dediu, F.C. Matacotta, L. Correra. (1995).
In preparation.