The growth and annealing behavior of thin Iridium films on the Ir(111) surface was studied with respect to stacking fault and twin formation by means of scanning tunneling microscopy (STM), low energy electron diffraction (LEED) and surface X-ray diffraction (SXRD). It was found that by heterogeneous nucleation of new faults at phase boundaries between areas of regular and faulted stacking, initial faulted areas spread into their surrounding (proliferation effect). In the process, the phase boundary is stabilized and evolves to a persistent fault structure, and a transition from layer-by-layer growth to a defect dominated growth with a fixed length scale takes place. During this transition, the majority of the surface area becomes twinned. A step-influenced enhancement of the stacking fault probability initially supports the effect. By the supply of additional energy in the order of 50 eV per deposited atom using ion beam assisted deposition (IBAD), stacking fault and twin formation can be effectively lifted. Various strategies of IBAD have been tested. The strongly twinned Ir/Ir(111) films exhibit a high stability against thermal annealing. Annealing is found to take place in a two step process: At 800 K-1000 K the boundaries between areas of different stacking are diminished. Only beyond 1200 K also the twins themselve heal. Structure models of the boundaries involved are presented. The boundaries between different stacking areas are identified to consist of {111}/{115} boundaries dissociated into coherent {111}/{111} and {112}/{552} boundaries. The influence of adsorbates on the growth of Ir/Ir(111) was studied for CO and O. It was found that exposure of the sample to CO or O during deposition prevents coalescence, leads to columnar growth and multiple twinning. For both adsorbates, the island number density is increased, indicating a reduced mobility on the surface
