High mobility, bottom gate, nanocrystalline silicon thin film transistors incorporating a nitrogenated incubation layer

A novel layer-by-layer growth technique has been developed for the early stages of nanocrystalline silicon (nc-Si) deposition to produce an insulating incubation layer. An electron cyclotron resonance reactor is used to deposit a thin film of nc-Si by plasma enhanced chemical vapour deposition. The deposited layer is then exposed, in situ, to a dense plasma of nitrogen and hydrogen which converts defective, amorphous material into silicon nitride whilst preserving nucleating nano-crystals. This two-stage process of deposition and nitrogenation is repeated four times before a thick nc-Si layer is grown on top. By converting the incubation layer into an insulator, when this material is employed as the channel layer in bottom gate thin film transistors, the accumulation layer is forced to form in a region of greater crystallinity, and devices with an electron field effect mobility of 3 cm2 V−1 s−1 have been produced. These are the first bottom gate thin film transistors using a nc-Si channel with a higher field effect mobility than found in hydrogenated amorphous silicon devices.