Ge/Si(001)c(4X2) interface formation studied by high-resolution Ge 3d and Si 2p core-level spectroscopy

Ge epitaxy on Si(001)c(4 x 2) was monitored by high-resolution core-level s pectroscopy up to the deposition of 20 ML. Ge 3d and Si 2p core-level spect ra were acquired at normal- and grazing-emission angles. Taking into accoun t the whole evolution of the Ge 3d core-level line shape during growth, in addition to the three surface components previously reported for thin Ce la yers, a fourth component was identified and attributed to bulklike Ge atoms . For submonolayer coverage, the difference between the number of up- and d own-dimer sites occupied by Ge unequivocally indicated the formation of mix ed Si-Ge dimers. The presence in the Ge 3d core level of the component rela ted to second-layer atoms demonstrated that Ge starts to diffuse below the surface before all the dimer sites are occupied. During deposition of the s econd ML, the behavior of the bulklike component indicated that Ge diffuses to layers deeper than the second, probably in order to occupy sites under tensile stress. At a Ce coverage of 20 ML, the persistence of surface compo nents in the Si 2p core level is related to segregation of Si in the Ge lay er or, more appropriately, to Stranski-Krastanov growth, which determines a n inhomogeneous Ge coverage, locally as low as 1-2 ML.