Surface morphology evolutions during the first stages of epitaxial growth of Si on Ge(001):: a RHEED study

Solid source molecular beam heteroepitaxial growth of thin Si films on Ge(0 0 1)-2 x 1 pseudo-substrates has been investigated over a large range of substrate temperatures (from room temperature to 400 degrees C) and Si deposition rate (from 1.5 to 25 Angstrom/min) using reflection high-energy electron diffraction (RHEED). Due to the lower surface free energy of Ge with respect to Si, a Volmer-Weber growth mode is expected at equilibrium conditions. Nevertheless, at low temperature, RHEED patterns, in-plane lattice parameter measurements (alpha(parallel to)) and specular beam intensity oscillations reveal prolonged two-dimensional growth imposed by kinetic limitations for this tensile strained system. Actually, the critical thicknesses for which island formation takes place can be significantly increased by lowering the surface diffusion length of the Si adatoms, by either temperature decrease or deposition rate increase. These thicknesses are determined by the passage from streaky to spotty patterns and the onset of concomitant alpha(parallel to) decreases. This indicates varying and delayed 2D-3D transitions with respect to the equilibrium criterion. Such variations are not observed for the Stranski-Krastanov mode of the inverse compressive Ge/Si interface, whose critical thicknesses are only weakly dependent on the kinetic conditions. These differentiated morphology stabilities as a function of the strain sign are discussed in terms of strain dependent adatom diffusivities. (C) 1999 Elsevier Science B.V. All rights reserved.