Small and large angle X-ray diffraction in metallic multilayers

Grzegorz Gladyszewski

Thin Films Laboratory, Lublin University of Technology, ul. Nadbystrzycka 38, 20-618 Lublin, Poland

Many physical properties of artificially layered structures are strongly dependent on the atomic arrangement at the interface. Indeed, the interfacial roughness affects the electrical and optical properties of semiconductor superlattices, plays an important role in the long-range magnetic coupling in magnetic multilayers, or strongly influences the specular reflectivity from multilayer x-ray mirrors. A detailed characterization of the interface is therefore essential. The most widely used ex-situ structure and interface characterization technique for multilayers is x-ray diffraction (XRD). The nondestructive XRD technique is commonly used in q-2q geometry, in which the scattering vector is perpendicular to the sample surface and only –so called – “one dimensional” information in the growth direction is obtained. Nevertheless, a specific multilayer x-ray diffraction profile allows one to deduce some structural parameters by fitting the measured intensity profiles with model calculations using the computer programs SUPREX [1] and SLERF [2]. In this way the quantitative values for the rms roughness of the interfaces may be determined, but the method cannot provide information about the lateral character of the roughness. Indeed, both models are only valid for specular x-ray scattering. An interfacial roughness may be laterally as well as vertically correlated. Both kinds of correlation influence strongly their small- as well as large- angle nonspecular x-ray diffraction profiles. It becomes clear that the most valuable information about a real multilayer structure can be obtained from nonspecular x-ray scattering measurements [3]. Unfortunately, the most of reported in the literature data is based on simple specular q-2q scans. This fact is apparently caused by the fact that interpretation of nonspecular scans cannot be performed in the straightforward way, as it is nowadays possible for specular scans. An example of the usefulness of nonspecular x-ray scattering measurements is provided by the stress determination that often is carried out on the basis of so called sin²y method. This method has been well developed for polycrystalline thin films. However, for multilayered epitaxial systems the method may lead to a false result. Therefore, 3D models must be used to interpret the performed scans in a correct way. In this work, a review of 1D, 2D and 3D models necessary for x-ray diffraction profiles interpretation will be done. Both specular and nonspecular measurements will be described. Application of analytical as well as simulation models will be discussed.

1. E.E.Fullerton, I.K.Schuller, H.Vanderstraten, Y.Brunseraede, Phys. Rev. B 45 (1992) 9292.

2. G.Gladyszewski, Thin Solid Films 204 (1991) 473.

3. G.Gladyszewski and Y.Bruynseraede, Phys. Rev. B 54 (1996) 11672, and references therein.