Morphology and strain-induced defect structure of ultrathin epitaxial Fe films on Mo(110)

Fe films in a coverage range of (formula presented) were deposited on a Mo(110) substrate in the (formula presented) temperature range. It is found that growth around 300 K is mediated by the step-flow growth mechanism, in contrast with previous studies of the Fe/Mo(110) and Fe/W(110) systems, where growth at 300 K was mediated by two-dimensional island nucleation and coalescence. This difference is attributed to the slightly higher substrate temperature (between 300 and 345 K) during deposition. A transition from layer-by-layer to Stranski-Krastanov growth is observed in films grown in the (formula presented) range at around a 1.8 ML coverage. Strain-relieving dislocation defects appear along the (formula presented) direction in the second Fe layer and develop with increasing film thickness into a dislocation network at around a 2.4 ML coverage. The dislocation defects in the second Fe layer act as preferential nucleation sites for third layer islands. At elevated temperatures (formula presented) the first and second Fe layers are formed by the step-flow growth mechanism. Subsequent coverages are characterized by the formation of distinctive wedge-shaped islands supported on an Fe monolayer. A two-dimensional dislocation network is formed in the fourth Fe layer of these islands, from an array of closely-spaced dislocation lines in the third layer. Similar to the Fe/W(110) system, the magnetic properties of these films are expected to vary significantly on the nanometer scale and they are therefore potential candidates for spin-polarized scanning tunneling microscopy studies.