Photoetching of Si(111)-(7x7) studied by STM

A partially chlorinated Si(111)-(7x7) surface was subjected to 193 nm excimer laser radiation. Scanning tunnelling microscopy (STM) was used to count the number of photoetched atoms per unit of illumination time. The adatom etch rate increased linearly with fluence for peak surface temperatures ranging from room temperature up to 1055 degrees C. The slope of this plot implied a constant etch rate of 3.4x10(-8) atoms per photon, i.e. sigma(e) = 1.8x10(-22) cm(2). The surface temperature did not influence the etch rate over this broad temperature range. It follows that the removal of Si adatom was directly due to photolysis, the presumed agents being photoformed electron-hole pairs. The intensity dependence of the etch rate was determined separately for one-, two- and three-atom vacancies, leading to the conclusion that the corresponding photoprocesses involved one, two and three photons, respectively. A statistical analysis of the pair-distribution function for ejected adatoms gave evidence of an increase of greater than or similar to>10(2) in photoetching probability directly adjacent to a single-atom vacancy. Irradiation of the unchlorinated surface also resulted in adatom etching which increased linearly with fluence, but at a lower etch rate corresponding to photoetching with a cross-section of sigma(e) = 0.52x10(-22) cm(2). (C) 1997 Elsevier Science B.V.