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Monte Carlo Transport Calculations of Strained SiGe Heterostructures from Ab-Initio Band-structures

B. Zorman, S. Krishnan, D. Vasileska, J. Xu and M. Van Schilfgaarde
Arizona State University, US

device models, Monte Carlo, alloy scattering, strain

The continued scaling of semiconductor devices creates numerous challenges which have to be overcome in order to achieve device behavior that satisfies speed and power constraints. Possible alternatives to conventional CMOS devices include strained-Si or strained-SiGe devices. The later material system is already being used in the production of high-frequency bipolar junction transistors and is considered as possible alternative to conventional CMOS. For heterostructure devices like strained SiGe p-MOSFETs, alloy scattering may play a significant role in determining the hole mobility, particularly when surface roughness scattering is reduced by using a buffer layer between the gate and channel. The strain induced modification of the bandstructure for alloyed SiGe causes a splitting of the heavy hole and light hole bands, and enhances hole mobility by lowering the effective mass at the top of the valence band in comparison to unstrained bulk silicon. Alloy scattering, on the other hand, lowers the hole mobility. In order to properly describe the operation of device structures that utilize strained-SiGe layers, it is necessary to include into the theoretical model the strain modification of the band structure and to properly model alloy disorder scattering. The problem is that there is uncertainty on the choice of the alloy scattering parameters, used in Monte Carlo models with effective mass and k•p bandstructures. To address this issue, we recently developed a method to incorporate alloy scattering and strain into Monte Carlo device simulations using first principles density functional theory (DFT) calculations. A statistical model of the alloy is used, and the atomic pseudopotentials include spin-orbit coupling terms. Strain in the alloy and across interfaces is included by lowering the structural energy subject to atomic force constraints determined by the strain. Confinement at the SiO2/Si cap layer is approximated as a hydrogen passivated Si surface. Figure 1 shows part of the valence bandstructure near the _ point for a strained 1nm SiGe (31.25%) channel capped by silicon. Alloy disorder broadens the bands. Although a self-consistent density functional bandstructure/Monte Carlo/Poisson equation calculation is beyond the reach of current computers, density functional bandstructure calculations can include the gate bias induced self-consistent potential from our effective mass based Monte Carlo/Poisson code. Then, the use of first principles energy dispersion and wave-functions for the calculation of all of the scattering mechanisms eliminates the need to include explicit alloy scattering in the Monte Carlo simulation. The approach we have developed, based on the above idea, is currently being applied to the calculation of the hole transport properties of a small strained SiGe channel pMOSFET structure, and compared to effective mass and k•p results.

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