GAS-PHASE ETCHING OF SI(111)-(7X7) SURFACES BY OXYGEN OBSERVED BY SCANNING-TUNNELING-MICROSCOPY

Structural and mechanistic aspects of the initial stages of gas phase etching of Si(111) - (7 x 7) surfaces, by reaction with 02 at temperat ures between 910 and 1070 K, were investigated by scanning tunneling m icroscopy. On smaller terraces, the reaction is found to proceed via a step flow mechanism, where Si is effectively removed at the edges of retracting terraces. On very large terraces, nucleation and growth of vacancy islands contribute in addition. The data lead to a reaction me chanism involving the formation of volatile SiO on the terraces, by re action of 0 adatoms and Si surface atoms, subsequent vacancy diffusion , and finally the annihilation of vacancies at step edges or in vacanc y islands. The temperature dependence of the resulting step structures , ranging from irregular and nearly dendritic forms at approximately 9 10 K via faceted steps at approximately 970 K to rounded shapes above 1000 K, results from step pinning by oxide nuclei and annealing effect s at higher temperatures.