The effect of boundary conditions in ion mixing of multilayered NiSi samples

Abstract

In ion mixing of metal-semiconductor systems, bilayer samples initially lead to the formation of equilibrium phases. However, in multilayered samples where the average composition is fixed, ion irradiation at high doses leads to metastable (or amorphous) phase formation. The metastable phase appears to form at a critical ion dose, apparently without the formation of any compounds at subcritical doses (at least not reported in the literature). The objective of this study is to investigate the effect of individual layer thickness of NiSi multilayered samples on ion induced reactions. Four types of NiSi samples were used: (i) Ni( ~ 700 Å)/Si〈100〉, (ii) two layers of Ni( ∼350 Å) interposed with two layers of Si( ∼ 360 Å) on SiO 2, with an average composition of Ni 1.65 Si (this composition is in the two phase region of the phase diagram), (iii) three layers of Ni( ∼ 240 Å) interposed with three layers of Si( ∼ 260 Å) on SiO 2, and (iv) five layers of Ni( ∼140 Å) interposed with five layers of Si( ∼145 Å) on SiO 2. It was found that the mixing efficiency increases with the number of layers in the samples. Amorphous phase formation was observed only in samples with six and ten individual layers at high doses. An equilibrium phase of Ni 2 Si does appear to be detectable by X-ray diffraction in all types of samples at low doses. Based on this observation, and to a first order approximation, the boundary condition imposed by multilayered samples is similar to that imposed by bilayer samples for ion mixing at low doses. As mixing continues under ion irradiation the boundary conditions start to differ for these two types of samples. For bilayer samples, Ni 2Si grows with dose, as reported in the literature. For multilayer samples, amorphous phase formation is possible when uniform mixing is achieved since the average composition of these samples is in a two phase region of the phase diagram. The relationship between layer thickness, mixing efficiency and amorphous phase formation is discussed.