Nanowires are expected to become key structures for novel electronic and photonic devices. In particular, GaAs nanowires grown in non-VLS modes are promising for various applications because they have no Ga metal droplet on top of them, in contrast with GaAs nanowires in the self-catalyzed VLS mode. In this paper, we demonstrate that the growth conditions of non-VLS mode GaAs nanowires on silicon substrate can be controlled by changing the pre-irradiation material from arsenic to gallium. Pre-depositing Ga enables the growth of GaAs nanowires in the non-VLS mode at one third the As pressure required for the As pre-irradiation technique. (c) 2013 Elsevier B.V. All rights reserved.