Evidence for heterojunction effects in polycrystalline Si_(1-x)Ge_x thin film transistors with Si caps

摘要

Polycrystalline silicon-germanium (poly-Si_(1-x)Ge_x) thin film transistors (TFTs) were fabricated with and without Si interlayers (caps) to buffer the SiO_2 gate dielectric and the Si_(1-x)Ge_x channel. Both low temperature processes (<=550 deg C) compatible with glss for flat panel displays and high temperature processes were used. NMOS TFTs show dramatic peformance improvements up to moderate (5 to 10 nm) interlayer thicknesses. In contrast, PMOS TFTs show only small improvements at low interlayer thicknesses (<5 nm), after which performance declines. Computer simulations using an effective medium model for polycrystalline materials suggest that in addition to interface improvement, a pseudomorphic heterojunction is formed from the strained Si cap and unstrained poly-Si_(1-x)Ge_x channel with both conduction and valence band offsets. These offsets play a significant role in inversion layer formation.