Tkachuk O. Quantum-chemical modeling of surface structures of Ge on the Si(001) surface

Density functional theory (B3LYP, 6-31G**) has been employed for the investigation of the equilibrium spatial and electronic structure of adsorption complexes of one and two Ge atoms as well as diatomic molecule Ge2 with two different clusters: (i) Si96H84 which simulates the Si(001) buckled face and (ii) Si115H80+Si18O30H16 which reproduces an insulating layer on the crystal silicon surface. It is shown that the adsorption of one germanium atom takes place on the down atom of buckled surface >Si=Si< dimers and is accompanied by the energetic effect of 7.8 eV. Such a large adsorption energy indicates for down silicon atoms to be coordinationally unsaturated. Adsorption of the second germanium atom within the same cluster is also exothermic (1.5 eV with respect to the Si96H84•Ge cluster). Later on, as the result of the surface diffusion, a >Ge=Ge< addimer is formed over a number of >Si=Si< dimers in a parallel way. A diffusion of germanium atoms of that >Ge=Ge< surface dimer into the bulk phase is accompanied by the substitution of Si atoms by Ge atom followed by the formation of both mixed >Ge=Si< dimers and pure >Si=Si< ones. Chemical environment of germanium atoms in those adsorption complexes has been determined by the shift of the 3d5/2 component of the Ge3d line with respect to its original position in the Si96H84•Ge2 complex with >Ge=Ge< addimers. Such a shift turns out to be negative for adsorption complexes with the pure >Si=Si< dimer on the surface and two germanium atoms inside the crystalline substrate. 3d5/2 sublevel shift has been interpreted in terms of the “electrostatic potential”, namely, binding energy of core electrons has been considered as a function of the effective charge of a given atom as well as effective charges of neighboring atoms. Transformation of five buckled surface dimers >Ge=Ge< >Ge=Si<, and >Si=Si< on the relaxed Si(001)(4×2) face has been investigated using DFT calculations (B3LYP, 6-31G**) of the Si28H28 cluster. In particular, energetic effects of the vibration of those dimers at fixed bond length and different buckling angles ω have been studied, and corresponding activation barriers have been calculated. For the >Ge=Si< addimer, activation barriers for the change of the orientation (А→В) have been evaluated within the same cell of the Si(001)(4×2) substrate surface, as well as for the diffusion between the adjacent cells. For all the three types of displacements of addimers >Ge=Ge<, >Si=Si<, and >Ge=Si< (in particular, their vibration as a whole, rotation, and diffusion over a number of buckled >Si=Si< dimers of the substrate), the activation energies turned out to be small, less than 1 eV. Such numerical values are in a satisfactory agreement with the STM data available in literature. Calculations performed by the doctoral candidate for the equilibrium structure and X-ray photoelectron spectra of the adsorption complex of one and two germanium atoms as well as diatomic molecule Ge2 on the surface of the SiO2 film covering the Si(001) surface allowed one to get an insight into the charge localization on the adsorbed Ge atoms and Ge2 molecule.