| As the scalling down of VLSI and ULSI devices (MOSFETs) are approachingphysical and technical limits, a series of problem, involving material, device physics,device structures and fabricating process, appeared to restrain the future improvement ofdevice performance, especially for the degeneration of carrier. Silicon-based strain tech-nology, which could significantly imporve the mobility of carrier, is regarded as an im-portant method to extend the Moore’s Low. In this dissertation, the bandstructures ofbulk-Si and the electron mobility of nMOSFET are both investigated under the uniaxialstress. The primary work and achievements are as follows:(1) The effect of uniaxial tensile/compressive shear stress on the conduction bandstructrue and the shifts of vally minima due to the coupling of the lowest two energybands at the X point, the boudary of Brillouin zone, are analyzed, based on the k p the-ory, and the dispersion relationship of the conduction band with arbitrary uniaxial stressand different valley orientation is established. Also, the dispersion relationship of thevalence band is proposed, based on which the theoretical relations between the energyof the top band, second band, spin-orbit band and the arbitrary stress. According to theabove-achieved bandstruce model with uniaxial tensile/compressive stress, the theore-tical calculations of the degeneracy, density of state effective mass of the conductionband, and the splitting energy, isotropy effective mass and density of state effectivemass of the valence band are then carried out. The calculated results are in agreementwith published ones, and set a basis to research the electron mobility in the inversion-layers.(2) Based on the triangle potential well approximation, in this dissertation, a studyof the Quantum-Mechanical-Effects (QME) in bulk-Si nMOS with the influence of uni-axial tensile stress, and the quantitative analytical calculations are completed. Wereached a conclusion that the tensile stress reduced QME in uniaxial-strained Si MOSdevices by making a comparison with the bulk (unstressed) MOSFET in characteristicssuch as eigen energies, surface electrical field, etc. According to the energy band stru-cture and the QME, the threshold voltage model is then proposed by solving the2-DPoisson’s equation and also by taking short channel effects, quantum mechanical effectsand other secondary effects into consideration. Our analytical results agree with experi-mental data theoretical data in the literature. This model can be not only used for nano-scale strained-Si nMOSFET with high-k gate-dielectric, but also for the relevant analy- sis of the inversion electron mobility.(3) Based on the obtained band structure parameters and effective mass approxima-tion (EMA), the scattering machanisms in inversion layer are detailedly analyzed, theelectron mobility in the channel, which is a function of crystal orientation and stress, isfinally proposed. A comparation of inversion layer mobility of different surface ori-entation has been then made according to our calculated results, which argree with theMonte-Carlo (MC) results in the literature. Our calculation shows that, the mobilities ofdifferent surface orientation increase with stress when an uniaxial stress is applied, andthe surface orientation order is:(001)>(101)>(111), especially, the electron mobilityalong [110]/(001) direction is the highest. The study and calculation of inversion layermobility can provide some theoretical foundation for the channel orientation selectionof uniaxially strained Si nMOSs. For further analysis of the relationship between devicestructure and inversion mobility, the electron mobility is associated with device struc-ture parameters by using threshold voltage model. The process is also simulated by theCAD tool, Sentaurus. The calculated mobility results and the simulated ones which areextracted from Sentaurus are approximate each other. And the simulation results canalso provided some reference for the design of high speed/high performance nMOS-FETs. |
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