Direct fabrication of poly-SiGe thin films on amorphous substrates and its application to bottom-gate TFTs

We have developed a new thermal CVD technique for poly-SiGe thin films that meets the requirements for low-cost fabrication of post-amorphous silicon (a-Si:H) TFTs, i.e., Reactive Thermal CVD featuring a set of reactive source materials of disilane (Si₂H₆) and germanium tetrafluoride (GeF₄). We succeeded in deposition of poly-SiGe thin films at 450°C or higher on glass substrates by this technique. And we succeeded in depositing uniform films in a thickness variation less than 5% for the entire area of 6-inch substrate. Thanks to high crystallinity of the present films at the initial stage of the film growth on the substrates, the films exhibited such high crystallinity that we could fabricate bottom-gate TFTs with very thin poly-SiGe films of 30 nm deposited directly on SiO₂/Si substrates, which showed high mobility of 8 cm²/Vs.