RANDOM-WALK MECHANISM FOR STEP RETRACTION ON HYDROGEN-ETCHED SI(111)

We report on step retraction on hydrogen-etched Si(lll) surfaces; The study was performed by kinetic Monte Carlo simulations and in situ hig h-temperature scanning tunneling microscopy. The origin of the step re traction is the random walk of surface monovacancies. They are caused by desorption of silicon hydrides from the hydrogen-exposed surface, w hich causes a weak etching effect. The vacancies diffuse until they re ach a step or another surface vacancy, where they are annihilated. Thi s results in bilayer step retraction or vacancy cluster coarsening. Fo r sufficiently high temperatures and slow enough etching, all created vacancies reach the terrace steps, which results in maximal step retra ction. For sufficiently low temperatures and fast enough etching the s tep retraction is effectively suppressed by the creation of vacancy cl usters in the terraces. For intermediate temperatures and etching, a t ransition regime is found, where initially all surface vacancies diffu se to the terrace step edges and annihilate. However, the probability for the creation of vacancy clusters in the terraces is not neglectabl e, so after a widely distributed time a sufficiently large number of m onovacancies meet to form a stable vacancy cluster, which effectively slows down The step retraction rate.