Experimental Examination and Physical Understanding of the Coulomb Scattering Mobility in Strained-Si nMOSFETs

In this paper, the impact of biaxial tensile strain on the electron mobility limited by Coulomb scattering in inversion layers is experimentally examined. It includes the study of both interface state (N_it) and substrate impurity (N_sub) Coulomb scatterings. Compared with unstrained-Si devices, the mobility limited by N_sub is enhanced in strained-Si nMOSFETs, whereas the mobility limited by N_it is degraded. These new findings are investigated through extensive mobility measurements at various temperatures (50 K-300 K) and surface electric fields. It is found that two key parameters exist for explaining the opposite strain dependence of mobility limited by N_it and N_sub scatterings: One is the valley population, and the other is the distance between inversion layer electrons and the charged centers.