Infrared Spectroscopy of Si(111) and Si(100) Surfaces After HF Treatment: Hydrogen Termination and Surface Morphology

Chemically oxidized and subsequently HF stripped silicon surfaces are extremely clean with all dangling bonds saturated with hydrogen and display remarkably low electronic surface-recombination velocities. EELS and LEED observations have been interpreted as arising from perfect, unreconstructed surfaces with dihydride and monohydride termination on Si(100) and Si(111), respectively. We have used the high resolution, sensitivity and polarization properties of multiple internal reflection infrared spectroscopy to identify hydrogen chemisorption sites (hydrides) and thereby infer surface morphology. We find the HF-prepared surfaces are locally highly ordered but microscopically rough, as evidenced by a number of well-defined sites, such as dihydride on Si(111). Extensive isotopic mixture data combined with ab initio cluster calculations give the first clear evidence for trihydride on Si(111) terraces (between the defect structures), and not for monohydride as previously believed. Similar studies on stepped crystals reveal that the defect structures relieve the strain associated with flat, unreconstructed H-terminated surfaces.