Strain effects on the valence band of silicon: Piezoresistance in p-type silicon and mobility enhancement in strained siliconpMOSFET

This dissertation explores strain effects on the valence band of silicon to explain and model piezoresistance effects in p-type silicon and mobility enhancements in strained-Si pMOSFETs. The strain effects are manifested as changes in the valence band when applying a stress, including band structure alteration, heavy and light hole effective mass changes, band splitting, and hole repopulation. Using the 4x4 k·p strain Hamiltonian, the stressed effective masses of the heavy and light holes, band splitting, and hole repopulation are used to analytically model the conductivity and effective mobility changes and the piezoresistance pi coefficients. The model predictions agree well with the experiments and other published works. Mobility enhancements and pi coefficients are extracted from four-point and concentric-ring wafer bending experiments used to apply external stresses to pMOSFET devices. The theoretical results show that the piezoresistance pi coefficient is stress-dependent in agreement with the measured pi coefficients. The analytical model predictions for mobility enhancements in uniaxial and biaxial strained-Si pMOSFETs are consistent with experiments as well as published experimental data and numerical simulations. In addition, for biaxial tensile stress, the model correctly predicts mobility degradation at low biaxial tensile stress.; The main factor contributing to the stress-induced linear drain current increase is identified as mobility enhancement. The contribution from the change in effective channel length is shown to be negligible. The temperature dependence of stress-induced mobility enhancement is also considered in the model. At high temperature, the hole repopulation is smaller than at room temperature, causing smaller mobility change whereas stress-induced band splitting suppresses the interband optical phonon scattering which reduces the mobility degradation.