Morphology and strain - induced defect structure of ultrathin epitaxial Fe films on Mo „ 110 ...

Fe films in a coverage range of 0.4<u<4.7 ML were deposited on a Mo~110! substrate in the 300<T <700 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-bylayer to Stranski-Krastanov growth is observed in films grown in the 300<T<345 K range at around a 1.8 ML coverage. Strain-relieving dislocation defects appear along the @001̄# 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 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.