Surface segregation and growth-mode transitions during the initial stages of Si growth on Ge(001)2[times]1 by cyclic gas-source molecular beam epitaxy from Si[sub 2]H[sub 6]

Surface morphological and compositional evolution during the initial stages of Si growth on Ge(001)2[times]1 by cyclic gas-source molecular beam epitaxy from Si[sub 2]H[sub 6] has been investigated using [ital in] [ital situ] reflection high-energy electron diffraction (RHEED), Auger electron spectroscopy, electron-energy-loss spectroscopy, and scanning tunneling microscopy, combined with post-deposition high-resolution cross-sectional transmission electron microscopy. The layers were deposited using repetitive cycles consisting of saturation Si[sub 2]H[sub 6] dosing at room temperature, followed by annealing for 1 min at 550 [degree]C. Film growth was observed to proceed via a mixed Stranski--Krastanov mode. Single-step-height two-dimensional growth was obtained for nominal Si deposition thicknesses [ital t][sub Si] up to [congruent]1.5 monolayers (ML). However, the upper layer remained essentially pure Ge which segregated to the surface through site exchange with deposited Si as H was desorbed. At higher [ital t][sub Si], the Ge coverage decreased slowly, the surface roughened, and two-dimensional multilayer island growth was observed for [ital t][sub Si] up to [congruent]7.5 ML, where bulk reflections in RHEED patterns provided evidence for the evolution of three-dimensional island formula.