Low-temperature (∼250°C) gold-induced lateral growth of Sn-doped Ge on insulator enhanced by layer-exchange reaction

Low-temperature (≤250°C) formation of Sn-doped Ge (GeSn) (substitutional Sn concentration: 2-3%) on insulator is desired to realize flexible electronics, where high-speed thin-film transistors are integrated on flexible plastic-substrates (softening temperature: ∼300°C). To achieve this, gold-induced lateral crystallization of amorphous GeSn (a-GeSn) is investigated. Here, a-GeSn (Sn concentration: 0%-20%) on insulator structures, having Au-island patterns, were annealed. For initial Sn concentrations of 0%-5%, high-speed lateral crystallization proceeds around Au-patterns, and large grown regions (∼5-20 μm) are obtained by low-temperature short-time annealing (150°C-250°C, 10 min). However, bottom regions of GeSn are not crystallized at 150°C-200°C, while the whole films including bottom regions are crystallized at 250°C. These phenomena are caused by the following temperature-dependent growth mechanisms. At 150°C-200°C, Au atoms are supplied into surface-regions of a-GeSn films from Au-islands by surface-diffusion, resulting in crystallization of only surface-regions. On the other hand, at 250°C, layer-exchange of Au/GeSn occurs, which increases supply channel of Au for diffusion. This results in complete crystallization of GeSn to the bottom. Substitutional Sn concentrations in grown layers are increased with increasing annealing temperature. As a result, GeSn films (substitutional Sn concentration: ∼3%) are obtained at ∼250°C.