Understanding of mechanisms dominating inversion-layer mobility in strained-Si MOSFETs and establishment of guideline for the mobility enhancement

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We have evaluated surface-roughness-limited mobility of strained-Si MOSFETs and amount of surface roughness at the MOS interfaces. As a result, it has been found that bi-axial tensile strain increases the electron mobility, while it slightly decreases the hole mobility. Also, we have proposed a novel method to accurately evaluate the shape of the SiO2/Si interface roughness and the auto-correlation function by using high resolution Transmission Electron Microscope (TEM). It has been found that the mobility determined by the extracted auto-correlation is in good agreement with the experimental mobility for both electrons and holes. In addition, we have experimentally observed that the strained-Si MOS interfaces smaller densities of interface states generated by Fowler-Nordheim stress, attributed to the reduction in roughness of strained-Si MOS interfaces.Furthermore, the increase in the bi-axial tensile strain strained-Si p-MOSFETs leads to the increase in Coulomb-scattering-limited hole mobility due to MOS interface charges, while it leads to the decrease in Coulomb-scattering-limited mobility due to substrate impurities. This dependence is opposite to that of the electron mobility in strained-Si n-MOSFETs. These strain dependencies of the Coulomb scattering mobilities can be systematically understood from the viewpoint of the subband structure modulation of electron and hole inversion layers due to tensile strain.