It is generally considered that high-Ns electron mobility is dominated by the interface roughness scattering. Since we demonstrated that Ge(111) was the best surface orientation for electrons to achieve the highest peak electron mobility, we challenged to realize the atomically flat Ge(111) surface by using H2 annealing . Then, MOSFETs on Ge(111) with and without atomically flat surface were fabricated with HPO. N-channel MOSFETs fabricated by HPO on atomically flat Ge(111) exhibited greatly improved high-Ns electron mobility .
EOT reduction was also challenging because HPO should naturally form thicker GeO2. This fact was a big hurdle on Ge FET technology. Meanwhile, it was interestingly found that the low-temperature HPO (LT-HPO) reduced the oxidation rate . This fact enabled us to achieve ~2 nm EOT GeO2 on Ge with superior interface and bulk GeO2. Another way to reduce EOT is obviously to replace HPO-GeO2 with other high-k oxides which are appropriate for Ge. We have fortunately found that Y2O3 is very friendly with Ge . Furthermore, in our recent experiments Y2O3-doped GeO2 (YGO) is much better than pure Y2O3 on Ge. Significantly improved electron mobility in n-channel Ge FETs in a wide range of electron density has been achieved, when YGO in addition to H2 annealing has been used for the interfacial layer in gate stacks .
Poor electron mobility in n-channel Ge FETs is not true now. We can engineer the Ge interface through understanding of thermodynamics in gate stack formation and of kinetics of both surface planarization and oxidation. Thus, we can safely conclude that Ge is very promising not only for p-channel but also for n-channel FETs.
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