Experimental evidence and extraction of the electron mass variation in [1 1 0] uniaxially strained MOSFETs

In this paper, we investigate electron mobility enhancement in [1 1 0] uniaxially strained nMOSFETs with three different channel orientations on a [0 0 1] Si substrate. We have experimentally demonstrated that, for stress applied in a [1 1 0] direction, electrical results cannot be explained without considering that the in-plane mass mt for the [0 0 1] two-fold valleys (Δ2) becomes anisotropic and varies with strain. For the first time, their values in the transport direction, perpendicular and parallel to an applied stress, have been extracted from electrical characterization of the MOS transistor thanks to an original technique. It has been found that the conduction mass of Δ2 along the standard [1 1 0] channel direction is reduced by a tensile uniaxial stress along [1 1 0] while it is increased by a tensile uniaxial stress along [1¯10]. These results reinforce several previous theoretical works.