Microcrystalline silicon: An emerging material for stable thin-film transistors

Top-gate and bottom-gate microcrystalline-silicon thin-film transistors (TFTs) have been produced at low temperature (150-250°C) by the standard radio-frequency glow-discharge technique using three preparation methods: the hydrogen dilution of silane in hydrogen, the layer-by-layer technique, and the use of SiF₄-Ar-H₂ feedstock. In all cases, a stable top-gate TFT with mobility values around 1 cm²/V-sec have been achieved, making them suitable for basic circuit on glass applications. Moreover, the use of SiF₄ gas combined with specific plasma treatments of the a-SiN:H dielectric produces large columns, even at the interface with the dielectric. This leads to stable bottom-gate TFTs, fully compatible with today's a-Si:H production facilities, reaching mobility values up to 3 cm²/V-sec. These devices are an interesting alternative to laser-crystallized polysilicon thin films in a growing number of applications.