Atomic structure and composition of the 2 x N reconstruction of the Ge wetting layer on Si(001) investigated by surface x-ray diffraction

The 2 x N reconstruction of the Ge/Si(001) wetting layer has been investigated by surface x-ray diffraction. At a substrate temperature of 670 degrees C, the average N periodicity decreases from N = 11.5 to 8 with an increasing Ge coverage from one to three monolayers (ML). The top layer consists of asymmetric dimers with a bond length in the range of 2.50-2.60 angstrom and a buckling angle in the range of 9.4 degrees-15.6 degrees, depending on the Ge coverage. The obtained dimer bond lengths are similar to those calculated for alternating asymmetric mixed dimers. Intermixing of Ge with Si is found down to the sixth (eighth) layer for 2 (from 3 to 5) ML coverage. For 2 ML coverage, a quantitative surface x-ray diffraction data set has been measured. It is analyzed using a model describing the atomic structure and Ge occupation probability with a limited set of parameters to bypass the intrinsic lack of appreciable reflections of the 2 x N (N = 9) reconstruction. The Ge occupation probability varies periodically along the N direction, having its minimum value below the dimer vacancy lines. In addition, a more direct calculation of the Lorentz and detector acceptance corrections is given for rocking and radial scans.