An investigation of ultra-thin InAs layers deposited on GaAs is carried out combining theoretical calculations with results of high-resolution transmission electron microscopy, atomic force microscopy and photoluminescence. Five period InAs/GaAs epilayers were grown by metalorganic vapor phase epitaxy at a very low growth rate, with different InAs deposition times, in order to investigate the morphological and optical evolution from extended 2D InAs flat areas of various thicknesses, starting at 1 monolayer, to the nucleation of 3D InAs islands. The coexistence of extended monolayer-flat 2D terraces of different thicknesses and 3D islands is demonstrated. Optically active InAs 2D terraces with a thickness beyond the critical value are detected. For longer deposition times, quantum dots are nucleated and their size increases at the expense of the 3 monolayer thick 2D layers.