Polycrystalline thin-film transistors fabricated on high-mobility solid-phase-crystallized Ge on glass

Low-temperature formation of Ge thin-film transistors (TFTs) on insulators has been widely investigated to improve the performance of Si large-scale integrated circuits and mobile terminals. Here, we studied the relationship between the electrical properties of polycrystalline Ge and its TFT performance using high-mobility Ge formed on glass using our recently developed solid-phase crystallization technique. The field-effect mobility μ_FE and on/off currents of the accumulation-mode TFTs directly reflected the Hall hole mobility μ_Hall, hole concentration, and film thickness of Ge. By thinning the 100-nm thick Ge layer with a large grain size (3.7 μm), we achieved a high μ_Hall (190 cm²/Vs) in a 55-nm thick film that was almost thin enough to fully deplete the channel. The TFT using this Ge layer exhibited both high μ_FE (170 cm²/Vs) and on/off current ratios (∼10²). This is the highest μ_FE among low-temperature (<500 °C) polycrystalline Ge TFTs without minimizing the channel region (<1 μm).