GaAs/AlGaAs quantum dots shape control by droplet epitaxy

Self-assembled quantum dots (QDs) are object of extensive research and application in optoelectronics because of the possibility to tune their optical and electronic performances by a proper design of their morphological properties, like size and shape.Morphological control is therefore of primary importance to obtain fine-tuning of the electron states and of the emission of the QDs. Size and shape determine the confinement potential of electrons and holes, the transitions between different energy levels and the consequent optical emission. The available design degrees of freedom remain limited due to energetic driven evolution of the case of Stranski-Krasyanov (SK) QD self-assembling, thus reducing the possibilities of a real on demand design of their electronic properties. To overcome the SK growth limitations, a kinetic limited growth procedure, the Droplet Epitaxy (DE) was introduced [1-2]. Unlike the SK self-assembly technique, DE does not rely on strain for the formation of three-dimensional (3D) crystals. DE is based on the subsequent deposition of III and V column elements at controlled temperatures and fluxes. In this work, we show that it is possible to control the QD aspect ratio (height to base ratio) and QD exposed facets by choosing proper As pressure and substrate temperature during the crystallization step of the growth (figure 1). The DE-QD aspect ratio can be changed between 0.5 and 0.1 while the exposed facets continuously change from planes compatible with {111} to {511} orientation. In order to explain the observed phenomenology we developed a model, considering the relevant phenomena taking place during the cristallization, that is As impinging flux. As diffusion in the droplet, Ga diffusion etc. The results of the numerical simulation of QD profiles at different As fluxed and substrate temperatures are shown in figure 2. The model predictions are good agreement with the experimental results. In conclusion, we show that it is possible to control faceting and aspect ratio in DE-QDs thus allowing for a complete fine-tuning of electron states and electron-phonon interaction in self-assembled DE QDs, of great importance for many optoelectronic applications. [1] N. Koguchi and K. Ishige, Japanese Journal of Applied Physics32, 2052–2058 (1993). [2] C. Somaschini, S. Bietti, N. Koguchi, and S. Sanguinetti, Nano Letters9, 3419–24 (2009).