Droplet-confined alternate pulsed epitaxy of GaAs nanowires on Si substrates down to CMOS-compatible temperatures

The self-catalyzed epitaxial growth of free-standing GaAs nanowires on Si substrates by molecular beam epitaxy typically requires high enough temperatures, where the Ga adatoms can diffuse efficiently along the surface of the substrate and the nanowire sidewalls before their incorporation into the liquid Ga droplets at the top of the nanowires. On the other hand, the use of high temperatures imposes several limitations concerning the interruption and resumption of the growth in an accurate and defect-free way, the realization of well-defined composition or doping profiles in axial heterostructures and, finally, the compliance with the thermal budget restrictions of fully-processed Si-CMOS circuits that is necessary for the integration of the two material technologies. Here, we introduce the droplet-confined alternate pulsed epitaxy for the self-catalyzed growth of GaAs nanowires on Si(111) substrates in the temperature range from 550 °C down to 450 °C. This unconventional growth mode is a modification of the migration-enhanced epitaxy, where alternating pulses of Ga and As4 are employed instead of a continuous supply. The enhancement of the diffusion length of Ga adatoms on the {1-10} nanowire sidewalls allows for their targeted delivery to the Ga droplets at the top of the nanowires and, thus, for a highly directional growth along the nanowire axis even at temperatures as low as 450 °C. We demonstrate that the axial growth can be controlled with the ultimate accuracy of one monolayer, while it can be simply and abruptly interrupted at any time without the formation of any defects. Taking advantage of these unique possibilities, we were able to probe the growth mechanisms in specially designed experiments and describe quantitatively the population dynamics of As inside the Ga droplets.