111)-oriented large-grain (50 um) Ge crystals directly formed on flexible plastic substrate by gold-induced layer-exchange crystallization

Orientation-controlled large-grain Ge crystals grown on plastic substrates (softening temperature: 300 °C) are desired to realize advanced flexible electronics, where various functional devices are integrated on flexible substrates. To achieve this, gold-induced crystallization (annealing temperature: 250 °C) using a-Ge/Au stacked structures is developed on plastic substrates, where thin-Al2O3 layers (thickness: 7 nm) are introduced at a-Ge/Au interfaces. Interestingly, (111)-oriented nucleation at the Au/plastic interface dominates over random bulk nucleation in Au layers. As a result, the formation of (111)-oriented large-grain (;50 um) Ge crystals directly on flexible plastic substrates is realized. This technique will pave the way for advanced flexible electronics.