Crystallization and decomposition of co-sputtered amorphous silicon-aluminum thin films

The crystallization of co-sputtered Si-Al amorphous alloys was investigated by transmission electron microscopy (TEM), differential scanning calorimetry (DSC), optical microscopy and X-ray diffraction. The Si₆₅Al₃₅ and Si₆₁Al₃₉ films crystallize at 270 and 240 °C, respectively, into crystalline Si (c-Si) and crystalline Al (c-Al) phases by nucleation and growth of 'spherulites', which are three-dimensional colonies of fine c-Si and c-Al grains. When homogeneous precrystallization of Al phase is absent in the amorphous matrix, each spherulite possesses strong in-plane Al {111} texture. The activation energy for the growth rate of the spherulites is 1.2 ± 0.1 eV, while that for the nucleation rate is 2.1 ± 0.1 eV for Si₆₅Al₃₅ and 1.9 ± 0.1 eV for Si₆₁Al₃₉. A simulation of the reaction based on the classical transformation theory was able to reproduce the DSC profiles of these samples. During the growth of spherulites, in situ high-resolution TEM revealed a several-nanometer-thick Al layer between the amorphous matrix and the growing crystalline Si phase. This finding and the activation energy for the growth rate support a mechanism whereby the growth of the spherulites is limited by the diffusion of Si within the cAl phase.