Remote interfacial dipole scattering and electron mobility degradation in Ge field-effect transistors with GeOx/Al2O3 gate dielectrics

Remote Coulomb scattering (RCS) on electron mobility degradation is investigated experimentally in Ge-based metal-oxide-semiconductor field-effect-transistors (MOSFETs) with GeOx/Al2O3 gate stacks. It is found that the mobility increases with greater GeOx thickness (7.8-20.8 angstrom). The physical origin of this mobility dependence on GeOx thickness is explored. The following factors are excluded: Coulomb scattering due to interfacial traps at GeOx/Ge, phonon scattering, and surface roughness scattering. Therefore, the RCS from charges in gate stacks is studied. The charge distributions in GeOx/Al2O3 gate stacks are evaluated experimentally. The bulk charges in Al2O3 and GeOx are found to be negligible. The density of the interfacial charge is + 3.2 x 10(12) cm(-2) at the GeOx/Ge interface and -2.3 x 10(12) cm(-2) at the Al2O3/GeOx interface. The electric dipole at the Al2O3/GeOx interface is found to be + 0.15 V, which corresponds to an areal charge density of 1.9 x 10(13) cm(-2). The origin of this mobility dependence on GeOx thickness is attributed to the RCS due to the electric dipole at the Al2O3/GeOx interface. This remote dipole scattering is found to play a significant role in mobility degradation. The discovery of this new scattering mechanism indicates that the engineering of the Al2O3/GeOx interface is key for mobility enhancement and device performance improvement. These results are helpful for understanding and engineering Ge mobility enhancement.