High-mobility ultrathin strained Ge MOSFETs on bulk and SOI with low band-to-band tunneling leakage: Experiments

For the first time, the tradeoffs between higher mobility (smaller bandgap) channel and lower hand-to-hand tunneling (BTBT) leakage have been investigated. In particular, through detailed experiments and simulations, the transport and leakage in ultrathin (UT) strained germanium (Ge) MOSFETs on bulk and silicon-on-insulator (SOI) have been examined. In the case of strained Ge MOSFETs on bulk Si, the resulting optimal structure obtained was a UT low-defect 2-mn fully strained Ge epi channel on relaxed Si, with a 4-nm Si cap layer. The fabricated device shows very high mobility enhancements > 3.5x over bulk Si devices, 2x mobility enhancement and > 10x BTBT reduction over 4-nm strained Ge, and surface channel 50% strained SiGe devices. Strained SiGe MOSFETs having UT (T-Ge < 3 nm) very high Ge fraction (similar to 80%) channel and Si cap (T-Si cap < 3 nm) have also been successfully fabricated on thin relaxed SOI substrates (T-SOI = 9 nm). The tradeoffs in obtaining a high-mobility (smaller bandgap) channel with low tunneling leakage on UT-SOI have been investigated in detail. The fabricated device shows very high mobility enhancements of > 4x over bulk Si devices, > 2.5x over strained silicon directly on insulator (SSDOI; strained to 20% relaxed SiGe) devices, and > 1.5 x over 60% strained SiGe (on relaxed bulk Si) devices.