NONSPECULAR X-RAY REFLECTION FROM SELFORGANIZED INTERFACES IN A SIGE/SI MULTILAYER
J. Grim1, V. Holŭ2, J. Kubena3, J. Stangl2, A. A. Darhuber2, S. Zerlauth2, F. Schäffler2, and G. Bauer2
1 Laboratory of Thin
Films and Nanostructures, Faculty of Science, Masaryk University,
Kotlárská 2, 611 37
Brno, Czech Republic
2 Institute of Semiconductor Physics, Kepler
University, Altenbergerstr. 69, A-4040 Linz, Austria.
Keywords: Selforganization, epitaxial growth,
SiGe, x-ray reflection
Nonspecular (diffuse) X-ray reflectivity was used for the investigation of selforganized interfaces in a SiGe/Si multilayer grown by MBE. Compressive deformation in the growing layer leads to a roughening of its surface (Stranski-Krastanow growth mode) and gives rise to a nearly periodic distribution of homogeneously sized surface islands. Under suitable growth conditions, the epitaxial growth can also be modeled as a motion of monolayer steps (step-flow growth). In this growth mode, the strain in the growing layer causes bunching of the moving monolayer steps. This growth process occurs in strained multilayers grown on substrates misoriented from (001) planes. The resulting surface consists of a nearly periodic sequence of mesoscopic step bunches and atomically flat terraces between them. Such a structure resembles a random staircase, its mean slope equals the miscut angle of the surface with respect to (001). Both island structure and a random interface staircase were observed in the studied sample.
The step-flow growth creates strongly anizotropic interfaces. In order to study this anizotropy, we have measured the reciprocal space distributions of the diffusely reflected intensity in various azimuths of the scattering plane. In reciprocal space, the intensity was concentrated in periodical sheets, which indicated that the interface profiles were strongly replicated from interface to interface. The replication angle was determined from the tilt of these sheets. Further, we have found distinct dependence of the scattered intensity on the azimuth of the scattering plane. In the azimuths parallel ("upstairs") and antiparallel ("downstairs") to the miscut direction the intensity distributions were strongly asymmetrical with respect to the surface normal and an exaggerated space frequency was present in the power spectrum of the interface roughness. We have ascribed this frequency to the mean width of the staircase terraces. The intensity distributions perpendicular to the miscut ("along the stair steps") were symmetrical, but another typical space frequency was present in the power spectrum. This effect was caused by the scattering on periodically distributed selforganized islands lying on the staircase terraces and in two previous cases ("upstairs" and "downstairs") it was overlapped by the stronger effect of scattering on the random staircase.
The distribution of the scattered intensity in the reciprocal space has been simulated using a structure model of selfsimilar interface patterns and the DWBA approach. Good correspondence to measured data has been achieved. From the fit we determined the mean shapes of the staircase steps and the dots as well as the mean distance between them. These values were also supported by x-ray diffraction measurements and TEM observations.