First-principles Calculation of Electron Mobilities in Ultrathin SOI MOSFETs
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B7.10.1
First-principles Calculation of Electron Mobilities in Ultrathin SOI MOSFETs Matthew H. Evans1,2, Xiaoguang Zhang3, John D. Joannopoulos1, and Sokrates T. Pantelides2,3 1 Department of Physics, Massachusetts Institute of Technology, Cambridge, MA 02139, U.S.A. 2 Department of Physics and Astronomy, Vanderbilt University, Nashville, TN 37235, U.S.A. 3 Solid State Division, Oak Ridge National Laboratory, Oak Ridge, TN 37861, U.S.A. ABSTRACT Ultrathin silicon-on-insulator (UTSOI) technology1 has emerged as a key candidate for sub100nm gate length CMOS devices. Recent experiments have characterized MOSFETs with silicon channels as thin as 1nm (four atomic layers of silicon),2,3 and found them to be wellbehaved electrically. Quantum effects are important to the electron transport in such devices, and the penetration of the electron wavefunction into the gate oxide introduces new scattering mechanisms. We introduce here a novel method for first-principles calculation of electron mobilities in ultrathin SOI channels, including surface roughness and defect scattering. The electronic structure and scattering potentials are calculated with Density Functional Theory in the Local Density Approximation (DFT-LDA), and the mobility is calculated through Green’s functions. The method requires little computational effort beyond that of the DFT-LDA calculations, and allows the calculation of temperature- and carrier concentration-dependent mobilities. Since the silicon-oxide interface is treated at the atomic-scale, the mobility contributions of different defects (e.g. suboxide bonds, oxide protrusions) and impurities (e.g. nitrogen, hydrogen) can be calculated separately, giving a precise physical picture of channel electron transport. INTRODUCTION The central objective of device modeling is a quantitative description of carrier transport in the channel region, taking into account scattering from phonons, impurities, interface roughness, interface defects and other factors. The net effect is encapsulated in carrier mobilities. Standard calculations employ the effective-mass approximation for carriers in Si, an infinite potential barrier at the Si-oxide interface, a triangular confining potential corresponding to the gate voltage, and model scattering potentials for impurities, defects, and interface roughness.4,5 In the last few years, it has been recognized that these approximations are inadequate for strained-Si channels6 and ultrathin double-gate devices.7,8 For the latter, penetration of the carrier wavefunction into the oxide is significant, signaling the breakdown of the effective-mass and infinite-potential-barrier approximations. Phenomenological attempts to incorporate the effect have highlighted the need to go beyond the standard approximations. Here, we describe a framework to pursue calculations of channel mobilities with full quantum-mechanical rigor and atomic-scale detail, and discuss preliminary results on interface roughness scattering. COMPUTATIONAL METHOD The central idea is to use a supercell as shown in
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