Using in situ reflection electron microscopy and ex situ atomic force microscopy, we have studied the morphological stability of large-scale (~10–100 μm) Si(1 1 1)-7 × 7 terraces during silicon growth and etching by oxygen and selenium. On the large-scale terraces, silicon growth at substrate temperatures T = 600–770 °C and Si deposition rates R = 0.002–0.2 BL/s proceeds in multilayer mode. Based on RMS surface roughness scaling W ∝ Θβ, we have discerned three modes of morphological instability caused by (I) effective adatom diffusion along step edges at low T and R (β ≈ 0.33), (II) effective diffusion and fast step motion along disordered “1 × 1” regions in 7 × 7 domain boundaries at intermediate T and R (β ≈ 0.2), and (III) accumulation of Si adatoms in high-atom-density “1 × 1” regions on the uppermost terraces at high T and R (β ≈ 0.5). The etching of the singular Si(1 1 1)-7 × 7 surface by oxygen leads to the slow development of multilayer morphology, while selenium-induced etching preserves flat surface morphology with periodic 2D vacancy island nucleation, growth, and coalesce, which is attributed to Se adatom diffusion. On the step-bunched surface, the Si or Se adatom diffusion to the step bunches leads to the self-organization of pyramidlike or valley-like morphology during Si growth or Se-induced etching, respectively.