Amorphous-crystalline phase transition during the growth of thin films:  The case of microcrystalline silicon

Thin silicon films of varying thickness were deposited on foreign substrates by electron-cyclotron resonance chemical vapor deposition from ${\mathrm{SiH}}_4{-\mathrm{H}}_2$ mixtures at 600 K. Optical thickness measurements, Rutherford backscattering, and transmission electron microscopy reveal that a thin amorphous interlayer of some 10 nm thickness has formed upon the substrate, before the growth of a microcrystalline layer begins. The amorphous layer is found to be deposited with a higher rate than the crystalline phase. Since similar effects have been observed for a large variety of deposition techniques, the amorphous-crystalline phase transition is considered as an inherent property of the growth of thin silicon films on foreign substrates at low homologous temperatures. The change in growth mode is interpreted in terms of Ostwald’s rule of stages, which predicts the evolution of film growth to proceed via a set of phases of descending metastability and nucleation rate. In applying capillarity theory a criterion is derived from the ratio of amorphous-phase and crystalline-phase nucleation rates $J_a{/J}_c.$ This ratio is developed into basic thermodynamic functions and is shown to govern the formation of either the stable or metastable phase. The approach is of general validity for thin-film deposition processes. In the case of microcrystalline silicon, experimental measures can be derived from the developed model to directly design the evolution of film structure.