Domain Visualization of Molecular Beam Epitaxy-grown Ir(111) Thin Films by Momentum-selected Photoelectron Emission Microscopy

Abstract

We applied a momentum-selected photoelectron emission microscopy (PEEM) approach to image crystallographic domains in metallic thin films, even in the absence of chemical or work-function contrast. The method was employed to epitaxial Ir(111) films (∼60 nm) grown on sapphire(0001) by molecular beam epitaxy (MBE) using a two-step temperature protocol followed by hydrogen annealing, which yielded high crystalline quality. X-ray diffraction confirmed the (111) orientation and the presence of twinning, and rocking-curve analysis revealed a sharp component with a full width at half maximum of 0.06°, reflecting high near-surface crystallinity. Atomic force microscopy showed a low surface roughness of ∼0.3 nm. Local momentum-resolved photoelectron spectroscopy using the photoelectron momentum microscope with a 5-µm field of view revealed large crystal domains with a single crystallographic orientation, extending over area of several hundred µm². In addition, crystallographic symmetry yields mirror-symmetric momentum patterns for twin domains. Placing a contrast aperture at a selected point in momentum space and then switching to PEEM mode, we obtained complementary real-space images that resolve twin domains and their ∼120° linear boundaries—features that have been difficult to resolve with conventional PEEM owing to the negligible work-function contrast. Our results demonstrate that MBE-grown Ir(111) thin films exhibit high quality, approaching that of bulk Ir, and that we have established a growth method to achieve such films. Furthermore, the momentum-selected PEEM we used in this work provides a straightforward route to correlate reciprocal-space symmetry with real-space mapping of twin-domain structures in face-centered cubic metals.