Ge/Si(001)c(4×2)interface formation studied by high-resolution Ge3dand Si2pcore-level spectroscopy

Abstract

Ge epitaxy on $\mathrm{Si}(001)c(4\ifmmode\times\else\texttimes\fi{}2)$ was monitored by high-resolution core-level spectroscopy up to the deposition of 20 ML. Ge $3d$ and Si $2p$ core-level spectra were acquired at normal- and grazing-emission angles. Taking into account the whole evolution of the Ge $3d$ core-level line shape during growth, in addition to the three surface components previously reported for thin Ge layers, a fourth component was identified and attributed to bulklike Ge atoms. For submonolayer coverage, the difference between the number of up- and down-dimer sites occupied by Ge unequivocally indicated the formation of mixed Si-Ge dimers. The presence in the Ge $3d$ core level of the component related to second-layer atoms demonstrated that Ge starts to diffuse below the surface before all the dimer sites are occupied. During deposition of the second 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 Ge coverage of 20 ML, the persistence of surface components in the Si $2p$ core level is related to segregation of Si in the Ge layer or, more appropriately, to Stranski-Krastanov growth, which determines an inhomogeneous Ge coverage, locally as low as 1\char21{}2 ML.