Bi-stability of contact angle and its role in tuning the morphology of self-assisted GaAs nanowires

Abstract

Many applications of semiconductor nanowires (NWs) require small radius NWs on the order of 10 nm. Fabrication of such thin NWs is known to be challenging for many reasons, and hence vaporliquid solid (VLS) III-V NWs with stable radius down to 5 nm have previously been grown by hydride vapor phase epitaxy [1] but with low density and random position. In the specific case of Ga-catalyzed GaAs NWs, which can easily be grown by molecular beam epitaxy (MBE) on patterned SiOx/Si(111) substrates in pure zincblende (ZB) crystal phase and with a high degree of spatial ordering [2], achieving small tip radii is even more challenging because such structures are usually grown under effectively gallium-rich conditions to maintain the gallium droplets on top of NWs. In self-catalyzed VLS growth of III-V NWs, a group III droplet serves as a non-stationary reservoir of gallium and hence changes its volume (size) during growth and is able to either shrink under arsenic-rich conditions, or swell under gallium-rich conditions, or self-equilibrate to a certain stable size under a balanced V/III flux ratio [3]. Therefore, gallium-rich conditions at the beginning of growth necessarily yield the droplet swelling, which is why the tip NW diameter increases with respect to the initial droplet size.