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First Experimental Observation of Mobility Enhancement with Channel Thickness Scaling in UTB GeOI nMOSFETs

 On June 6, 2017, National Institute of Advanced Industrial Science and Technology (AIST) announced that the research group led by T. Maeda, Nanoelectronics Research Institute, AIST, had observed for the first time significant mobility enhancement with decreasing channel (body) thickness Tbody of ultra-thin-body (UTB) GeOI (Ge on Insulator) nMOSFETs down to 3 nm. The result was reported at 2017 VLSI Symposium on Technology*.

 The research group had developed a new transfer technology named "HEtero-Layer Lift-Off (HELLO)". The first step of the technology was to construct the multilayer structure (Ge epi-layer/SiGe etching stopper layer /Ge buffer layer /AlAs splliting layer) by hetero-epitaxial growth on a GaAs substrate. Then, an Al2O3 layer was formed by atomic layer deposition on the surface of Ge epi-layer. The next step was to bond the multilayer structure upside down with the Al2O3 layer as bonding medium onto a SiO2/Si substrate, followed by lift-off of GaAs substrate with AlAs splitting layer, and Ge/SiGe layer etched-off leaving Ge epi-layer on top. And then, the thickness of the Ge epi-layer was controlled by atomic layer etching, realizing UTB GeOI with scaled Tbody.

Electron mobility in UTB GeOI nMOSFET was found to increase with decreasing Tbody. It showed 2.5 times enhancement by reducing Tbody from 13 nm to 3 nm. This mobility enhancement was explained by the energy band structure modification due to quantum size effect, that is, redistribution of electrons to a valley with smaller effective mass from lower valleys with larger mass through the upper shift of lower valleys by scaling of Tbody.

*  W. H. Chang, T. Irisawa, H. Ishii, H. Hattori, H. Ota, H. Takagi, Y. Kurashima, N. Uchida and T. Maeda, "First Experimental Observation of Channel Thickness Scaling (Down to 3 nm) Induced Mobility Enhancement in UTB GeOI nMOSFETs", 2017 Symposium on VLSI Technology, SESSION 14 SiGe / Ge FET 2, T14-1 (2017.6.8)