2D Si island nucleation on the Si(111) surface at initial and late growth stages: On the role of step permeability in pyramidlike growth

Initial and late stages of 2D Si island nucleation and growth (2DNG) on extra-large (~100 μm) and medium size (1–10 μm) atomically flat Si(111)-(7×7) terraces bordered by step bunches have been studied by in situ REM at T=600–750 °С. At first, the layer-by-layer 2DNG takes place on whole terraces and 2D island concentration dependence on deposition rate R corresponds to critical nucleus size i=1. Continuous 2DNG triggers morphological instabilities: elongated pyramidlike waves and separate pyramids emerge on all terraces at T≤720 °С and T=750 °С, respectively. Both instabilities arise due to the imbalance of uphill/downhill adatom currents related with large Ehrlich-Schwöbel (ES) barriers and permeability of straight [11¯2]-type step edges. However, the first one is initiated by dominant downhill adatom current to distant sinks: bunches, wave's step edges, and “vacancy” islands emerging on terraces due to 2D island coalescence. As a result, top layer size decreases to the critical terrace width λ where 2DNG takes place. From the analysis of λ∝R^−χ/2 scaling at T=650 °C, we have found that i increases from i=2 on a three-layer wave to i=6–8 on a six-layer wave. This authenticates the significance of downhill adatom sink to distant steps related to the step permeability. The second instability type at T>720 °C is related to the raising of uphill adatom current due to slightly larger ES barrier for step-up attachment comparing to the step-down one (E_ES ⁻~0.9 eV [Phys. Rev. Lett. 111 (2013) 036105]). This leads to “second layer” 2D nucleation on top layers, which triggers the growth of separate pyramids. Because of small difference between ES barriers, net uphill/downhill adatom currents are nearly equivalent, and therefore layer coverage distributions of both instabilities display similar linear slopes.