Comprehensive study of Al-induced layer-exchange growth for orientation-controlled Si crystals on SiO₂ substrates

Orientation-controlled crystalline Si films on insulating substrates are strongly required to achieve high-performance thin-film devices for next-generation electronics. We have comprehensively investigated the layer-exchange kinetics of Al-induced crystallization (AIC) in stacked structures, i.e., amorphous-Si/Al-oxide/Al/SiO₂-substrates, as a function of the air-exposure time of Al surfaces (t_air: 0-24 h) to form Al-oxide interface-layers, the thickness of Al and Si layers (d_Al, d_Si: 50-200 nm), the annealing temperature (450-500°C), and the annealing time (0-50 h). It has been clarified that longer t_air (>60 min) and/or thinner d_Al and d_Si (<50 nm) lead to the (111) oriented growth; in contrast, shorter t_air (<60 min) and/or thicker d_Al and d_Si (>100 nm) lead to the (100) oriented growth. No correlation between the annealing temperature and the crystal orientation is observed. Detailed analysis reveals that the layer-exchange kinetics are dominated by "supply-limited" processing, i.e., diffusion of Si atoms into Al layers through Al-oxide layer. Based on the growth rate dependent Si concentration profiles in Al layers, and the free-energy of Si at Al-oxide/Al or Al/SiO₂ interfaces, a comprehensive model for layer-exchange growth is proposed. This well explains the experimental results of not only Si-AIC but also another material system such as gold-induced crystallization of Ge. In this way, a growth technique achieving the orientation-controlled Si crystals on insulating substrates is established from both technological and scientific points of view.