Interface Roughness in Strained Si/SiGe Multilayers
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1Institut fidr Halbleiterphysik, Johannes Kepler Universitdit, A-4040 Linz, Austria Walter Schottky Institut, TU Mtinchen, Am Coulombwall 2, D-85748 Garching, Germany
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ABSTRACT Diffuse x-ray reflection from a SiGe/Si multilayer grown pseudomorphically on slightly miscut Si(001) substrates has been studied theoretically and experimentally. In the framework of the Distorted-Wave Born Approximation (DWBA), we demonstrated that the distribution of the diffusely scattered intensity gives conclusive information on both the amount and the in-plane and inter-plane correlation properties of the interface roughness. The best model for the description of the interface-morphology was found to be a combination of a two-level model and a staircase model. INTRODUCTION The assessment and control of the interface roughness is important for the electrical [1,2] and optical properties of semiconductor epi-layers. In multilayers, the evolution of the roughness with increasing number of periods is of interest. For thin layers, uncorrelated roughness of the two hetero-interfaces leads to substantial well width fluctuations, whereas correlated roughness does not. Therefore a comprehensive picture of the interface morphology and its correlation parallel and perpendicular to the growth direction is desirable. The only non-destructive method for the characterization of the roughness of the surface and buried interfaces is x-ray reflectivity, in particular two-dimensionally resolved measurements. X-ray scattering from rough interfaces in SiGe multilayers has been studied by several authors [3-6]. Their experimental results were that the interface profiles of different interfaces are strongly correlated, and, moreover, the direction of the highest correlation does not coincide with the growth direction. In the case of multilayers grown on miscut [00 1]-substrates, the distribution of the diffusely scattered intensity in reciprocal space reflects the terraced structure of the interfaces. The mean width and height of the terraces are correlated via the miscut angle (between the growth direction n and [001]) and the direction along the steps is always perpendicular to the plane determined by n and [001]. In [5], the shape of the terraces has been studied. The authors found that steps are created by bunching of existing substrate steps which results in a wavy shape of the interfaces. However, this finding was supported only by qualitative considerations. In this paper, we use the concepts introduced in [10] to simulate the reciprocal space maps of diffuse x-ray reflection from a SiGe multilayer. The simulation of the scattering is based on the Distorted-Wave Born Approximation (DWBA, [7]) and the roughness replication model described in [13]. From the analysis the most suitable model for the interface-morphology is found. THEORETICAL CONSIDERATIONS Theoretical works on diffuse x-ray reflectivity from multilayers [7-9] allow the simulation the reciprocal space maps, if the correlation function of the roughness profiles of permanent address:
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