Random-walk mechanism for step retraction on hydrogen-etched Si(111)

Abstract

We report on step retraction on hydrogen-etched Si(111) surfaces. The study was performed by kinetic Monte Carlo simulations and in situ high-temperature scanning tunneling microscopy. The origin of the step retraction is the random walk of surface monovacancies. They are caused by desorption of silicon hydrides from the hydrogen-exposed surface, which causes a weak etching effect. The vacancies diffuse until they reach a step or another surface vacancy, where they are annihilated. This results in bilayer step retraction or vacancy cluster coarsening. For sufficiently high temperatures and slow enough etching, all created vacancies reach the terrace steps, which results in maximal step retraction. For sufficiently low temperatures and fast enough etching the step retraction is effectively suppressed by the creation of vacancy clusters in the terraces. For intermediate temperatures and etching, a transition regime is found, where initially all surface vacancies diffuse to the terrace step edges and annihilate. However, the probability for the creation of vacancy clusters in the terraces is not neglectable, so after a widely distributed time a sufficiently large number of monovacancies meet to form a stable vacancy cluster, which effectively slows down the step retraction rate.