Abstract:

Using an ultrahigh-vacuum scanning tunneling microscope, we have studied molecular-beam epitaxy-grown GaAs on vicinal (001)-(2×4) surfaces decorated at 450 °C with submonolayer coverages of InAs. On surfaces misoriented towards the [110] direction (A type), we have observed that the InAs nucleates in terrace defects, at the step edges, and on the terraces. Predominantly, we observe nucleation on the terraces with the formation of islands. The InAs islands are locally ordered into a (4×4) or a c(4×4) In-terminated reconstruction, which easily distinguishes them from the (2×4) GaAs substrate. The InAs growth is predominantly In-rich, although some islands exhibit domains of As-terminated (2×n) reconstruction. With increasing coverage, the anisotropy and density of the islands increases on the terraces while growth at the step edges remains almost constant. Furthermore, the size distribution of the islands in the [110] direction is narrow and independent of coverage for fractional coverages of InAs below θ=0.75 ML, indicating that there is a preferred island width. On surfaces misoriented towards the [1¯10] direction (B type), InAs nucleates predominantly at step edges and in the terrace vacancies. Island growth is observed on terraces larger than 8 nm, with the anisotropy of the islands dependent on the corresponding terrace width. As for the A-type surface, the islands are found to have a preferred width in the [110] direction of 4±1 nm. We suggest that strain effects are limiting the island size in the [110] direction.