SPONTANEOUS AND STM-INDUCED REACTION OF XEF2 WITH SI(111)-7X7 AT LOW-COVERAGE

Chemical reactivity on Si(111)-7 x 7 at the atomic scale as it occurs spontaneously, or as it is induced by phenomena occurring between the tip and the Si substrate in a scanning tunneling microscope (STM), is investigated in the presence of XeF2 as a reactant gas. The STM has be en modified for this purpose to operate in the corrosive environment a nd in temperatures in the range of 250-300 K. Reactant molecules are a dsorbed on the surface at a lower temperature which prevents desorptio n, diffusion, or spontaneous reactions during the time span necessary for STM-induced experiments. A voltage pulse is applied to the STM tip located above the adsorbed molecule to generate a highly localized an d strong electric field. The adsorbed molecule undergoes a chemical re action to form volatile fluoride resulting in removal of a silicon ato m from the surface. The volatile product species is identified as SiF2 . The Si atoms neighboring the reaction site undergo minimum displacem ent as a result of the process. The spontaneous reaction kinetics of X eF2 on the Si surface at low temperatures, after low coverage depositi on, is also studied. Low temperatures provide the necessary time to ac quire STM information on the number of adsorbed molecules before any s ubstantial reaction takes place. in time, the adsorbed molecules eithe r desorb or react with Si surface atoms to produce volatile products a nd therefore leave vacancies behind. Once the surface is free of adsor bed fluorinated molecules, excess vacancies resulting from the reactio n are counted and the true removal probability is calculated.