Etching processes of Si(111) surfaces with bunched steps via atomic hydrogen irradiation intermittently observed by atomic force microscopy

Silicon (Si) (111)-(7 × 7) surfaces with wide terraces and bunched steps were passivated with atomic hydrogen (H) and subsequently etched by irradiation of atomic H. The atomic H can suppress the reactivity of Si surfaces by terminating the dangling bonds of Si surfaces. Meanwhile, atomic H can break the periodic atomic structures such as (7 × 7) on the Si surfaces. In the present study, we intermittently repeated the atomic H irradiation to the H-terminated Si(111) surface and frequency-modulation atomic force microscopy observation in a conventional vacuum chamber. When H₂gas was introduced to the cracker with a gas flow rate of 1 sccm (≈1.7 × 10^-8m³·s^-1), corresponding to an atomic H flux of 2.5 × 10¹⁶cm^-2·s^-1, one hour of the atomic H irradiation increased the roughness of the terrace from 1.4 nm up to 1.9 nm. With increasing the gas flow rate to 10 sccm, pits were formed on the surface and enlarged to 20-40 nm diameters across the bunched step with shallow flat bottoms and non-uniform winding edges. The surface etching probably starts from the adsorption of H on the lower-coordinated Si atoms exposed at the bunched steps. The side walls of pits seemed to consist of {110} and {100} facets that are readily etched due to the smaller number of the back bonds of Si atoms.