The Unexpected Role of As in Driving the Selective Growth of InAs Quantum Dots on GaAs

Here we show a new effect due to the Arsenic flux in the Molecular Beam Epitaxy growth of InAs Quantum Dots on GaAs(001) at temperatures higher than 500 °C and an high As/In flux ratio. We show that, by changing and tuning the direction of the As flux on a rippled substrate, a selective growth can be obtained where the dots form only on some appropriately orientated slopes of a sequence of mounds elongated along the[-110] surface direction. Since the relative As flux intensity difference over the two opposite mound slopes is very small (2-5%) the observed large effect cannot be explained simply as a pure shadowing effect and reveals instead that As, whose contribution to the modeling of growth has often been ignored or underestimated, probably for a lack of knowledge, plays a fundamental role at these growth conditions. To explain our experiment we have developed a kinetic model which explicitly takes into account the coupling between cations (In) and anions (As), and find that the very small surface gradient in the anion flux, due to the oblique evaporation on the mounded surface, is responsible for a massive drain of cations toward the surfaces anion-rich areas, thus generating the selective growth of Quantum Dots. We expect a comparable behavior for the anions of other III-V and II-VI compound semiconductors.