Arsine Absorption On Si(100) 2x1 - A Photoemission And Scanning-Tunneling-Microscopy Study

We report on a study of the adsorption and decomposition of arsine molecules on clean Si(100) 2 x 1 surfaces using photoemission and scanning tunneling microscopy (STM). Arsine decomposition depends on the substrate temperature and occurs in five regimes: (i) Below 100-degrees-C the arsine dissociatively adsorbs most likely into As-H and Si-H. At these temperatures there is negligible surface diffusion and the saturation arsenic coverage is 20% because the remaining Si sites do not have enough unoccupied near neighbors to provide four bonds for AsH3 adsorption. (ii) Between 100-degrees-C and 400-degrees-C the onset of surface diffusion increases the saturation coverage of As to 25% (i.e., one arsenic per four silicon atoms), As-As dimers form and coalesce into short chains. (iii) Above 400-degrees-C the hydrogen starts to desorb significantly thereby allowing larger As coverages. (iv) This process reaches an extremum at about 575-degrees-C where all hydrogen is desorbed resulting in nearly 100% As coverage. (v) Near 650-degrees-C the As desorption rate becomes significant. The STM images recorded in these regimes reveal a wide variety of surface structures which are due to different relative rates of AsH3 adsorption, H and As desorption, and H, As, and Si surface diffusion. The AsH3 adsorption and H and As desorption processes are discussed in the context of a kinetic model.