Hole mobility degradation by remote Coulomb scattering and charge distribution in Al2O3/GeOx gate stacks in bulk Ge pMOSFET with GeOx grown by ozone oxidation

Abstract

Hole mobility degradation due to remote Coulomb scattering (RCS) from fixed charges of GeOx/Al2O3 gate stacks is experimentally investigated in bulk Ge p-type metal–oxide–semiconductor field effect transistor (pMOSFET), with GeOx grown by ozone oxidation. The hole mobility at 77 K is found to increase with GeOx thickness. The phonon scattering can be ignored at 77 K and the surface roughness scattering is insignificant at a low inversion carrier concentration. This indicates that the RCS is responsible for mobility degradation. Therefore, the fixed charges are investigated in terms of RCS. The charge distribution in GeOx/Al2O3 gate stacks is experimentally estimated. The bulk charge in GeOx and Al2O3 is negligible. The densities of interface charges are +3.22 × 1012 cm−2 and −2.57 × 1012 cm−2 at the GeOx/Ge and Al2O3/GeOx interface, respectively. The electric dipole at the Al2O3/GeOx interface is +0.17 eV, corresponding to charge area density of 1.76 × 1013 cm−2. Consequently, the dipole at the Al2O3/GeOx interface plays a dominant role in the mobility degradation. Our results show that the investigation of charges in a gate stack is valuable for enhancing device performance.