Physical origins of mobility enhancement of Ge pMISFETs with Si passivation layers

We have been systematically investigated the inversion-layer mobilities of the Ge pMISFETs with Si passivation layers in order to clarify the physical origins of the mobility enhancement. It has been found in low effective field (E_eff) or Ns region that reduction of Coulomb scattering due to the separation of mobile carriers from interface charges by inserting the Si layer as well as the reduction in the density of Coulomb scattering centers is responsible for the mobility enhancement. On the other hand, it has been revealed in high Eeff or N_s region, for the first time, that the reduction in the surface roughness due to the change of the MOS channel region from SiO₂/Si interfaces to Si/Ge interfaces has a significant contribution to the mobility enhancement. In addition, it is found that too-thick Si passivation layers can cause the mobility degradation, because of the dislocation generation and resulting spatial distribution of the strain in Si.