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

Abstract

Fe films in a coverage range of $0.4<~\ensuremath{\theta}<~4.7\mathrm{ML}$ were deposited on a Mo(110) substrate in the $300<~T<~700\mathrm{K}$ 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 $300<~T<~345\mathrm{K}$ range at around a 1.8 ML coverage. Strain-relieving dislocation defects appear along the $[001\ifmmode\bar\else\textasciimacron\fi{}]$ 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 $(495<~T<~700\mathrm{K}),$ 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.