Critical layer thickness for self-assembled InAs islands on GaAs

Using atomic force microscopy (AFM), we have directly observed the progression of surface morphology of InAs deposited by molecular-beam epitaxy on GaAs(100). InAs self-assembled dots (coherent) or relaxed InAs islands (incoherent) are formed depending on the InAs coverage. The InAs coverage was varied continuously and AFM was used to monitor in detail the nucleation and resulting size and shape transition of the InAs self-assembled dots. Dots of uniform size were observed only at the initial stages of this Stranski-Krastanow growth-mode transition. The self-assembled dot density increased very abruptly with total deposited amount of InAs. Treating this InAs growth-mode transition as a first-order phase transition with InAs total coverage as the critical parameter, we extract a critical thickness for surface elastic relaxation of 1.50 ML.