Origin of A- or B-type ${\mathrm{NiSi}}_{\mathrm{i}}$2 determined by in in situ transmission electron microscopy and diffraction during growth

In situ transmission electron microscopy during the growth of ${\mathrm{NiSi}}_2$ on Si by molecular beam epitaxy shows that the metastable phase θ-${\mathrm{Ni}}_2$Si grows at ∼300 °C upon the reaction of certain deposited Ni films on (111) but never on (100) Si. Its existence correlates with the subsequent growth of type-A ${\mathrm{NiSi}}_2$ at 450 °C. The low energy of epitaxial interfaces can influence phase formation in very thin films, leading to unusual thickness-dependent behavior such as the type-B–type-A ${\mathrm{NiSi}}_2$ enigma.