Modeling study of the impact of surface roughness on silicon and germanium UTB MOSFETs

Abstract

We outlined a simple model to account for the surface roughness (SR)-induced enhanced threshold voltage (VTH) shifts that were recently observed in ultrathin-body MOSFETs fabricated on Si surface. The phenomena of enhanced VTH shifts can be modeled by accounting for the fluctuation of quantization energy in the ultrathin body (UTB) MOSFETs due to SR up to a second-order approximation. Our model is then used to examine the enhanced VTH shift phenomena in other novel surface orientations for Si and Ge and its impact on gate work-function design. We also performed a calculation of the SR-limited hole mobility (μH,SR) of p-MOSFETs with an ultrathin Si and Ge active layer thickness, TBody < 10 nm. Calculation of the electronic band structures is done within the effective mass framework via the Luttinger Kohn Hamiltonian, and the mobility is calculated using an isotropic approximation for the relaxation time calculation, while retaining the full anisotropy of the valence subband structure. For both Si and Ge, the dependence of μ H,SR on the surface orientation, channel orientation, and TBody are explored. It was found that a surface yields the highest μH,SR. The increasing quantization mass mz for surface renders its μH,SR less susceptible with the decrease of TBody. In contrast, surface exhibits smallest μH,SR due to its smallest mz. The SR parameters i.e., autocorrelation length (L) and root-mean-square (Δ rms) used in this paper is obtained from the available experimental result of Si UTB MOSFETs [1], by adjusting these SR parameters to obtain a theoretical fit with experimental data on SR-limited mobility and VTH shifts. This set of SR parameters is then employed for all orientations of both Si and Ge devices. © 2005 IEEE.