Bromine adsorption, reaction, and etching of Cu(100)

The interaction of Br-2 with Cu(100) was characterized using temperature-programmed desorption (TPD) and low-energy electron diffraction (LEED). Initial exposure to Br-2 resulted in the formation of a c(2 x 2) LEED pattern and CuBr desorption peaks at 870 and 1000 K. These desorption peaks saturated at a dose of approximately 5 L and were attributed to Br chemisorption. Continued exposure to Br-2 resulted in the growth of Cu3Br3 desorption peaks associated with the formation of bulk CuBr. The Cu3Br3 desorption peaks exhibited a strong coverage dependence. At low CuBr coverages, desorption peaks at 450 and 485 K were observed, while at intermediate coverages a third peak at 540 K was observed, and at high coverages a single broad peak at 500 K was observed. Similar results were obtained whether the CuBr layer was formed by exposure to Br-2 or by deposition of Cu3Br3 onto a c(2 x 2) layer. The two higher-temperature peaks were shown to be consistent with sublimation of alpha and beta CuBr, while the lowest-temperature peak could not be associated with sublimation of any bulk phase of CuBr. The lowest-temperature peak was attributed to either a grain-size effect or to the interaction of CuBr with chemisorbed Br. At 325 K, growth of CuBr resulted in a hexagonal LEED pattern that disappeared upon annealing to 400 K. Again, the same results were obtained for CuBr formed by reaction with Br-2 and by vapor deposition of Cu3Br3, suggesting that the reaction produces near-equilibrium structures. The hexagonal LEED pattern was attributed to compressed epitaxial CuBr(lll). The reaction of Br-2 with Cu(100) was characterized by a constant sticking coefficient, indicating that the reaction is adsorption-rate limited over the range studied. The sticking coefficient was found to depend on temperature in two distinct ways: (i) annealing the c(2 x 2) layer irreversibly increases the sticking coefficient, and (ii) the sticking coefficient reversibly decreases with increasing reaction temperature. The first effect was attributed to structural changes in the chemisorbed layer.