EQUIPMENT SIMULATION OF SIGE HETEROEPITAXY - MODEL VALIDATION BY AB-INITIO CALCULATIONS OF SURFACE-DIFFUSION PROCESSES

Incorporation of very little Ge into a Si surface significantly increa ses deposition during chemical vapor deposition. This is due to the fa ct that hydrogen and chlorine desorb faster from the SiGe surface maki ng available additional surface sites for adsorption. Two mechanisms a re discussed to explain the observed catalytic effect: (i) the diffusi on model where surface diffusion of H and Cl atoms from Si to Ge sites opens up an energetically more favorable path for H and Cl desorption via Ge surface sites and (ii) the collective model where incorporatio n of Ge into Si stimulates an overall change of the electronic structu re of the surface, thus leading to increased desorption. Ab initio clu ster calculations are used in this work to evaluate both models. Bindi ng energies of H and Cl atoms on Si, Ge, and SiGe surfaces are calcula ted. It is observed that Si-H, Ge-H and Si-Cl, Ge-Cl binding energies do not change whether their neighboring surface atoms are Si atoms or Ge atoms. An overall change of the electronic structure of the surface due to Ge incorporation cannot be observed, making the collective mod el highly unprobable. To evaluate the diffusion model transition state s for migration between different surface sites need to be located and the activation barriers need to be calculated. Surface diffusion of H and C1 atoms from Si to Ge is found to be energetically more favorabl e than desorption of H-2, HCl, or SiCl2 from Si. Surface diffusion on mixed SiGe surfaces leads to enhanced desorption via Ge surface sites. Thus the diffusion model is considered a valid description. Macroscop ic reactor simulations prove that the diffusion model can accurately d escribe enhanced deposition to explain the observed catalytic effect e ncountered during growth of SiGe heterolayers. (C) 1997 American Vacuu m Society.