Pyramid-like Si structures grown on the step bunched Si(111)-(7×7) surface

Pyramid-like Si structures have been observed on large (5 μm) atomically flat terraces of the step bunched Si (111)-(7×7) surface during Si deposition at T = 600°-760°C in the ultrahighvacuum reflection electron microscope (UHV REM). Such structures are the result of sequential two-dimensional island nucleation and growth (2DNG) accompanied by reducing a terrace width after each monolayer formation. The critical terrace width, at which the next 2DNG layer starts, is measured depending on substrate temperature and Si deposition rate. Two different activation energies of 2DNG layer formation (E_2D) are found out: E_2D ≈ 2.4 eV at T <; 700°C and E_2D ≈ 0.5 eV at T >; 700°C. Based on experimental data a critical cluster size for 2D-island nucleation is determined to be 7-10 atoms.