Cooperative nucleation leading to ripple formation in InGaAs/GaAs films

In0.25Ga0.75As epilayers were grown on GaAs (001) substrates (1.8% misfit s train) by molecular beam epitaxy to investigate the two-dimensional to thre e-dimensional transition as a function of thickness (t less than or equal t o 30 MLs). Tapping-mode atomic force micrographs show the evolution of the morphology as a function of thickness. As the film is deposited, the nuclea tion 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 run ning along the [1 (1) over bar 0]. The ripples also exhibit a secondary ali gnment roughly along the [310] which is attributed to the nucleation of isl ands with {136} faces. (C) 2000 American Institute of Physics. [S0003-6951( 00)00917-7].