Nucleation and growth of the first phase in sputter-deposited Nb/Al multilayer thin films

The formation of the first phase in the reaction of sputter-deposited Nb/Al multilayer thin films has been studied by power-compensated and heat-flux differential scanning calorimetry, x-ray diffraction and transmission electron microscopy. The modulation periods of the films are in the range of 10--500 nm. Both types of calorimetric measurements, performed at a constant heating rate, show the presence of two peaks (A and B) for the formation of the single product phase, NbAl3. Isothermal calorimetric scans show that peak A is associated with a nucleation and growth type transformation. The formation of NbAl3 is thus interpreted as a two-stage process of nucleation and lateral growth to coalescence (peak A) followed by normal growth until the consumption of one or both reactants (peak B). Transmission electron microscopy investigations of samples annealed into the first stage of NbAl3 formation show the presence of this phase at the Nb/Al interface and its preferential growth along the grain boundaries of the Al layer. The latter highlights the role of reactant phase grain structure in product phase formation