Optimization of uniaxial stress for high electron mobility on biaxially-strained n-MOSFETs

• We find the optimum condition of uniaxial strain for biaxially-strained nMOSFETs.
• The uniaxial strain effect for biaxially-s-Si is different from that for relaxed Si.
• The longitudinal tensile strain increases the electron mobility for biaxially-s-Si.
• The transverse tensile strain decreases the electron mobility for biaxially-s-Si.
• Vertical strain is ineffective unlike uniaxial strain for unstrained Si.

The uniaxial stress effect for high electron mobility on biaxially-strained n-MOSFET is investigated by using a one-dimensional self-consistent Schrödinger–Poisson solver. The electron mobility model includes Coulomb, intravalley phonon, intervalley phonon, and surface roughness scattering. We have found that the uniaxial stress effect on biaxially-strained n-MOSFET is significantly different from the uniaxial stress effect on unstrained Si n-MOSFET. It is well known that longitudinal and transverse tensile uniaxial stresses are advantageous for strain-induced high electron mobility. However, we found that the uniaxial strain condition for electron mobility enhancement is changed when it is applied to the biaxially-strained n-MOSFET. To optimize the combined effect of uniaxial and biaxial strain, the longitudinal tensile and transverse compressive uniaxial stresses are advantageous and vertical stress is not helpful for biaxially-strained n-MOSFET.