Interfacial Nucleation Control in Amorphous GeSn Thin Films Using Bilayer Structure

Remarkable progress has been made in germanium-based thin-film transistors in recent years. However, achieving both high field-effect mobility and a high on–off ratio is difficult because the crystallinity of polycrystalline Ge degrades as it becomes thinner. In this study, we investigated the interfacial nucleation control and grain size enlargement in the solid-phase crystallization of amorphous Ge thin films (≤50 nm). A bilayer structure consisting of top nucleation and bottom nucleation suppression layers promoted nucleation at the surface rather than at the substrate interface, resulting in a significant enlargement of the grain size. In addition, Sn doping in Ge increased the grain size to 12 μm, which was larger than that of most polycrystalline Ge thin films and contributed to the reduction in acceptor defects and improvement in hole mobility. The resulting hole mobility (260 cm2 V–1 s–1) and hole concentration (3 × 1017 cm–3) in the GeSn layer were among the highest for polycrystalline Ge-based thin films. These results will contribute to the realization of low-temperature Ge-based thin-film transistors that could be superior to single-crystal Si transistors, leading to innovations in display devices.