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

Abstract

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 Å). 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 × 1012 cm−2 at the GeOx/Ge interface and −2.3 × 1012 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 × 1013 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.