Length-scale dependent variation of the first nucleated phase in nickel-silicon multilayers

We have studied the first phase nucleation behavior of a series of equiatomic Ni-Si multilayers with ultrathin repeating subunits (lambda=1.7-5.8 nm) using x-ray reflectivity and x-ray diffraction. The variation in first nucleated phase is understood in terms of the model of Gosele and Tu, who predicted three length-scale dependent reaction routes in thin film diffusion couples: (i) For multilayers with lambda>5.0 nm, interfacial nucleation of Ni2Si is observed, consistent with studies of bulk diffusion couples. (ii) For multilayers with 3.8<lambda<5.0 nm, interfacial nucleation tends towards phases richer in Si due to a waning supply of Ni within the multilayer. (iii) For multilayers with lambda<2.0 nm complete mixing of the Ni-Si multilayers prefigures first phase nucleation of NiSi. In this regime the composition of the first nucleation phase is determined by the amorphous precursor, which in turn is dictated by the relative ratios of Ni and Si in the initial multilayer. A fourth scenario, between (ii) and (iii), in which Ni2Si is the first nucleated phase is also observed. A possible mechanism for this unexpected result is discussed. (C) 2003 American Institute of Physics.