Low-Energy Electron Diffraction Study of Silicon Surface Structures

Low-energy electron diffraction data are presented for superstructures on the clean (100) and (111) surfaces of silicon, and for structures formed by a monolayer of iodine and a monolayer of phosphorus on the (111) surface. Conventionally defined structure factors, modified for atomic structures a few layers thick, have been used to analyze two of these structures with results which are very probably correct. Insufficient data are apt to be the most serious limitation of the technique in structure determination. Observed transitions for the clean surface structures are described and results are presented for transition temperatures and rates of reaction of the surfaces with iodine and with phosphorus as a function of temperature and pressure. Iodine appears to be adsorbed in the ionic state and the calculated Si–I distance is about 3.2 Å. The data show that the physical and chemical properties of a silicon surface at low temperatures depend very much on thermal, chemical, and mechanical history, and the models presented explain this in considerable detail. Multilayer adsorption of iodine and phosphorus, disordered transition structures, and etching effects were also observed. The stable clean silicon surface has silicide properties, and reaction with hydrogen, mercury, and molybdenum is limited to a few active sites under conditions accessible in this work.