Surface diffusion and surface atomic roughness on Ir(001) surface and terraces

Diffusion of iridium and rhenium on Ir(001) surfaces have been studied by field ion microscopy. Self-adsorbed iridium adatoms have been found to diffuse favorably along 〈100〉 directions by an atomic exchange-replacement mechanism. Rhenium adatoms force out one of four nearest neighboring iridium substrate atoms to form ReIr dimer vacancy complexes at ∼ 240 K, which can change orientation by 90° in the temperature range from 210 to 280 K. The configuration of the ReIr dimer vacancy complex is considered to be the intermediate step of the atomic exchange-replacement diffusion mechanism. Unlike the diffusion of adatoms on the Ir(001) surface, ledge atoms on Ir(001) terraces diffuse only along 〈110〉 steps at low temperatures by an ordinary atomic hopping diffusion mechanism. The diffusing ledge atom is trapped at one end of the step. The barrier potential that a trapped ledge atom needs to overcome for diffusing away from the trap is found to be 0.73 eV, which is about 0.11 eV higher than the barrier for a diffusing ledge atom migrating along the step (0.62 eV). Surface step atoms start to dissociate from various sites randomly at ∼ 16 of the melting temperature of iridium so that one-dimensional step roughening occurs. By measuring the mean square deviation of the step heights at various temperatures, the kink energy for the Ir(001) steps is found to be 1.36 eV.