Electron mobility enhancement in uniaxially strained MOSFETs: Extraction of the effective mass variation

In this paper, we investigate electron mobility enhancement in uniaxially stressed nMOSFETs with three different channel orientations on a (001) Si substrate. We have experimentally demonstrated that, for stress applied in a [110] direction, electrical results cannot be explained without considering that in-plane masses for the (001) 2-fold valleys (Delta2) are varying with strain. For the first time, their values in the transport direction, perpendicular and parallel to an applied stress, have been extracted from electrical measurements. It has been found that a tensile uniaxial [110] stress reduces the transverse conductivity mass m_T of Delta2 while a tensile uniaxial [-110] stress increases m_T. These results reinforce several previous theoretical works