Electron Mobility Enhancement in Strained Silicon

The rapid increase in computational power and speed of integrated circuits is supported by the aggressive size reduction of semiconductor devices. With scaling apparently approaching its fundamental limits, the semiconductor industry is facing critical challenges. New engineering solutions and innovative techniques are required to improve CMOS device performance. Stress induced mobility enhancement is the most attractive solution to increase the device speed and will certainly take a key position among other technological changes for the next technology generations. In addition, new device architectures based on multi-gate structures with better electrostatic channel control and reduced short channel effects will be developed. A comprehensive analysis of transport in multi-gate MOSFETs under general stress conditions is required for analyzing the enhancement of device performance. Besides the biaxial stress obtained by epitaxially growing silicon on a silicon-germanium substrate, modern techniques allow the generation of large uniaxial stress along the [110] channel. Stress in this direction induces significant shear lattice distortion. The influence of the shear distortion on the conduction band structure has not yet been carefully analyzed. In this project the modification of the conduction band structure due to the shear stress will be theoretically investigated. A perturbation expansion of the Hamiltonian for the deformed lattice is derived. Based on this expansion, analytical expressions for the relative shift between the conduction band valleys and the effective mass change due to the shear stress components are obtained. Results of analytical calculations for the band structure will be verified against the band structure obtained by the empirical pseudopotential calculations. The conductivity mass in the lowest valleys is expected to decrease along the [110] direction of tensile stress providing significant mobility enhancement. Analytical expressions for the band structure of strained Silicon will be embedded into an existing Monte Carlo transport simulator. Accurate transport modeling in bulk silicon, in surface inversion layers, and thin silicon films for general strain conditions and arbitrary substrate orientations will be performed. The analytical band structure model enables efficient simulations of conventional and future multi-gate MOSFET architectures and allows for optimization of advanced MOSFET performance under general stress conditions.

The rapid increase in computational power and speed of integrated circuits is supported by the aggressive size reduction of semiconductor devices. With scaling apparently approaching its fundamental limits, the semiconductor industry is facing critical challenges. New engineering solutions and innovative techniques are required to improve CMOS device performance. Stress induced mobility enhancement is the most attractive solution to increase the device speed and certainly takes a key position among other technological changes for the new technology generations. In addition, novel device architectures based on multi-gate structures with better electrostatic channel control and reduced short channel effects are developed. A comprehensive analysis of transport in multi-gate MOSFETs under general stress conditions is required for analyzing the enhancement of device performance. Besides the biaxial stress obtained by epitaxially growing silicon on a silicon-germanium substrate, modern techniques allow the generation of large uniaxial stress along the [110] channel. Stress in this direction induces significant shear lattice distortion. The influence of the shear distortion on the conduction band structure has not yet been carefully analyzed. In this project the modification of the conduction band structure due to the shear stress has been theoretically investigated. A perturbation expansion of the Hamiltonian for the deformed lattice was derived. Based on this expansion, analytical expressions for the relative shift between the conduction band valleys and the effective mass change due to the shear stress components were obtained. Results of analytical calculations for the band structure have been verified against results obtained by the empirical pseudo-potential calculations and density-functional calculations (performed with VASP). The conductivity mass in the lowest valleys decreases along the [110] direction of tensile stress providing significant mobility enhancement. Analytical expressions for the band structure of strained silicon have been embedded into an existing Monte Carlo transport simulator. Accurate transport modeling in bulk silicon, in surface inversion layers, and thin silicon films for general strain conditions and arbitrary substrate orientations has been performed. The analytical band structure model enables efficient simulations of conventional and future multi-gate MOSFET architectures and allows the optimization of MOSFET performance under general stress conditions.