Cooperative nucleation leading to ripple formation in InGaAs/GaAs films

In0.25Ga0.75As epilayers were grown on GaAs (001) substrates (1.8% misfit strain) by molecular beam epitaxy to investigate the two-dimensional to three-dimensional transition as a function of thickness (t less than or equal to 30 MLs). Tapping-mode atomic force micrographs show the evolution of the morphology as a function of thickness. As the film is deposited, the nucleation of 3D islands followed by cooperative nucleation of pits is observed. As the thickness increases, both islands and pits continue to nucleate and grow until they coalesce, resulting in a fully formed ripple morphology running along the [1 (1) over bar 0]. The ripples also exhibit a secondary alignment roughly along the [310] which is attributed to the nucleation of islands with {136} faces. (C) 2000 American Institute of Physics. [S0003-6951(00)00917-7].