Scanning tunneling microscopy was used to characterize the developing surface morphology found during typical temperature programmed desorption experiments for halogen-GaAs. Surfaces exposed to Br 2 at 300 K were heated to temperatures between 450 and 675 K, followed by scanning at room temperature. This made it possible to relate the temperature-dependent gas phase etch product distribution to the surface structure and thereby examine atomic-level surface processes associated with the evolution of volatile products. We associate the desorption of GaBr 3 around 500 K with the initiation of single-layer-deep terrace pits. Desorption of GaBr and As 2 above 600 K accounts for the lateral enlargement of the pits.

Surface morphologies associated with thermal desorption: Scanning tunneling microscopy studies of Scanning tunneling microscopy was used to characterize Br-exposed GaAs͑110͒ surfaces that were heated to 700 K to induce surface etching. Areal analysis of etched surfaces showed that the etch yield ͑number of substrate atoms removed per adsorbed Br atom͒ decreased as the initial coverage increased. This reflects competition between reaction channels involving GaBr and GaBr 3 evolution which are determined by the local surface Br concentration. A kinetic model demonstrates that the percentage removed by GaBr 3 increases with initial coverage but that most of the Ga atoms are removed as GaBr. Arsenic desorbs spontaneously.