Fluence dependent oscillatory amorphization and recrystallization in ion irradiation

Abstract

Ion-beam-induced amorphization and recrystallization are well-known phenomena. At a constant ion flux, there is a substrate temperature TR such that, for T<TR the irradiation produces amorphization, whereas for T>TR, it produces recrystallization. However, both the processes do not happen at a given substrate temperature. Here we present a novel phenomenon of ion fluence dependent oscillatory amorphization and recrystallization in silicon at room temperature in a Si(5nm)/Ni(15nm)/Si system. Before the deposition of Ni on Si, a buffer Si layer was deposited on the Si substrate. Ion irradiation was carried out with 1MeV Si+ ions in the fluence range 1×1014–3×1017ions/cm2. At a fluence of 5×1016ions/cm2, Si is amorphous up to a depth of ∼1.4μm. However, at a fluence of 1×1017ions/cm2 two narrow bands of recrystallized (rc) Si are formed within the amorphized Si. At 2×1017ions/cm2 these rc-Si bands become amorphous again. At 3×1017ions/cm2 the broader rc band reappears. This oscillatory amorphization/recrystallization behaviour is Ni-mediated. Energy dispersive X-ray map of Ni shows Ni accumulation at the top of the rc-Si bands. The top of the broader rc band is actually the buffer-Si/substrate-Si interface. TRIM simulation shows the appearance of a Ni-displacement-induced vacancy peak at the position of the narrow rc-Si band. Accumulation of Ni in these regions is associated with the recrystallization process. The Ni layer, with incorporation of Si, also passes through amorphization and recrystallization. At the highest fluence η-NiSi nanocrystals are formed in the Ni layer.