Structural changes and alloying of annealed iron layers on GaAs(0 0 1) and GaAs(1 1 0)

Abstract

The nucleation and morphological changes during thermal processing of iron layers on Ga-rich GaAs(0 0 1)-(2 × 6) and on GaAs(1 1 0) were studied with scanning tunnelling microscopy. At room temperature the initial nucleation is strongly influenced by the rows of the surface reconstruction on GaAs(0 0 1), whereas on GaAs(1 1 0) larger 3D-islands are formed right away. Further growth above 1 ML is nearly identical for both surfaces showing a granular layer structure with a hindered coalescence of the grains. This behaviour is discussed in terms of the intrinsic growth behaviour of Fe(0 0 1) and Fe(1 1 0). After an initial tendency to form a continuous layer, the Fe-films on both substrates break up around 300 °C during thermal processing. On GaAs(1 1 0) the out-diffusion of substrate material is already seen at 250 °C. On GaAs(0 0 1) the annealed film shows two types of islands: a mesa-like type with a flat surface, which can be identified as a ternary Fe 3Ga 2− x As x alloy, and a second one, whose surface is in-plane with the substrate level which is identified as Fe 2As. On the substrate which has been laid open again in the annealing process the initially Ga-rich reconstruction changes to an As-rich (2 × 4). On GaAs(1 1 0) only one type of islands is present after annealing at 450 °C with a roof shaped form which is tentatively also assigned to a ternary Fe–Ga–As alloy.