BR-2 AND CL-2 ADSORPTION AND ETCHING OF GAAS(110) STUDIED BY USE OF SCANNING-TUNNELING-MICROSCOPY

Scanning tunneling microscope images obtained after GaAs(110) exposure to Br-2 and Cl-2 reveal dissociative adsorption and etching at steps and terraces, depending-on temperature, fluence, and flux. Exposure at 300 K produces a surface structure with halogen features localized within the rectangle formed by four surface As atoms, having (1X1) symmetry, and with halogen features localized on top of the surface As atoms. Adsorption for substrate temperatures between similar to 400 K and similar to 525 K results in linear chains along the substrate [001] direction. These chains coalesce with mixed (2X1) and c(2X2) domains. Exposure at 625 K produces volatile Pr and Cl products and etching is manifest by singlelayer etch pits. Longer exposures at the same flux at 625 K yield a steady-state condition in which five layers are exposed. Etching at 725 K is dominated by single-l;eight-step retreat and triangular double-layer etch pit formation on extended terraces. The nucleation density of double-layer etch pits can be increased and single-height-step retreat can be inhibited at 725 K by increasing the Br-2 or Cl-2 flux.