Silicon interlayer heterojunction effects in polycrystalline Si/sub 1-x/Ge/sub x/ thin film transistors

The use of poly-Si/sub 1-x/Ge/sub x/ (SiGe) in TFTs can lower thermal budgets when compared to poly-Si TFT technology, aiding in driver circuit integration on glass for active-matrix liquid crystal displays (AMLCDs). However, to date, poly-SiGe TFT performance has not been competitive with poly-Si TFTs, especially for NMOS devices. Motivated by the poor SiO/sub 2/-SiGe interface, other workers have demonstrated that a silicon interlayer between the SiO/sub 2/ and SiGe can improve poly-SiGe TFTs (Tang et al., 1995). However, heterojunction effects were not considered as they have been in single-crystal SiGe MOS. Under the proper conditions, the Si interlayer could be pseudomorphic within each grain of a poly-SiGe TFT, which would imply band offsets with carrier population and mobility effects. This work examines the effect of Si interlayers in poly-SiGe TFTs. Fabricated devices show improvement up to moderately thick interlayers for NMOS TFTs, while PMOS TFTs only improve with very thin interlayers, and computer simulations indicate that heterojunction effects play a major role in this behaviour.