Nanocrystalline and amorphous silicon thin film transistors deposited by microwave plasma electron cyclotron resonance chemical vapor deposition: Material analysis, device fabrication and characterization.

摘要

Bottom-gated thin film transistors, employing ECR-CVD deposited nc-Si films as active channel layers, were fabricated, characterized and studied. These TFT's were fabricated at a low temperature range, 150°C–400°C and on different substrates, such as c-Si, quartz, glass and plastic. The effects of substrate temperatures and H₂ dilutions during the Si film deposition on the film quality and TFT performance were studied. It was found that the TFT performance was improved as the substrate temperature and the H₂ amount increased, including a higher mobility, a lower threshold voltage and a smaller subthreshold swing. This was due to the interaction of the H atom and nc-Si/a-Si network and the formation of strong Si-H and Si-Si bonds, which produced stable and high-performance TFT's.; Device-quality oxide was deposited at 150°C–300°C by PECVD using the decomposition of TEOS as the TFT gate dielectrics, which showed a high breakdown field of >8 MV/cm. The MOS capacitor C-V curves showed little stretch-out in the transition region and small horizontal shift compared with the theoretical C-V curve, which implied a low surface state density (∼10 11cm−2eV−1) at the Si/SiO ₂ interface and a low density of fixed charges in the oxide.; The TFT performance is closely related to the active Si deposition conditions. For the TFT's with Si film deposited at 400°C with 10 mTorr H₂, a mobility of 16.1 cm2/V-s, a threshold voltage of 2.6 V and a subthreshold swing of 0.45 V/decade were obtained for the TFT on a quartz substrate and a mobility of 13.3 cm2/V-s, a threshold voltage of 3.0V and a subthreshold swing of 0.48 V/decade were obtained for the TFT on a glass substrate. These TFT's had W/L of 200 μm/25 μm, which gave the best result among the several gate dimension designs. The incorporation of H₂ during the deposition enhanced the crystallinity of the Si film, which was verified by the TEM measurements. For TFT's, the H₂ incorporation not only passivated the dangling bonds in the active Si film but also filled the traps in the gate oxide near the Si/SiO₂ interface, resulting in much-improved TFT characteristics, such as high on-current and steep subthreshold behavior. No post-hydrogenation process was carried out for these TFT's. The TFT's on plastic substrates were fabricated with a maximum temperature of 200°C, showing a mobility of 4.5 cm 2/V-s, a threshold voltage of 3.7 V and a subthreshold swing of 0.69 V/decade. The TFT's on plastic foil make the flexible, rugged and low-cost LCD applications possible. (Abstract shortened by UMI.)