On the enhanced electron mobility in strained-silicon inversion layers

Abstract

The recently reported large enhancement of the electron mobility in strained-Si inversion layers at large carrier concentrations cannot be easily explained: The strong carrier confinement in inversion layers removes the sixfold degeneracy of the conduction-band minima, much as tensile in-plane strain does, so that the effect of strain should become irrelevant at large sheet carrier densities. The problem is studied by calculating the electron mobility accounting for scattering with phonons and interface roughness. Surprisingly, the latter process is found to be significantly stronger in strained layers for a given interface roughness. Only the ad hoc assumption of increasingly smoother interfaces with increasing strain seems to explain the data.

Monte-Carlo simulation of decananometric nMOSFETs: Multi-subband vs. 3D-electron gas with quantum corrections

“…In fact strain induces an additional subband splitting that adds to the one produced by quantization, but this last contribution is accounted for only by the multi-subband approach. This view is supported by recent simulation works focusing on the mobility enhancement induced by biaxial strain: when using the multi-subband approach, the experimental data can be reproduced only by assuming that the strain changes the surface roughness spectrum [20,21], whereas simulators based on the 3Deg [22,23], quite surprisingly, reproduce the experimental data without modifying the surface roughness spectrum.…”

Linearity and mobility degradation in strained Si MOSFETs with thin gate dielectrics

“…Furthermore, the band-splitting due to quantum confinement at high vertical fields renders the band splitting from tensile strain redundant [23]. Although the measurements here indicate that mobility degradation is independent of strain, more advanced MOSFET models than (1) and more advanced mobility models than (2) would be needed to understand the impact of strain/germanium-related surface morphological properties on the high field mobility in strained Si layers.…”

“…However, in spite of the increasing AFM measured surface roughness with Ge content (strain), the mobility enhancement is maintained in the strained Si devices. High mobility enhancement in strained Si MOSFETs at high vertical fields (in the surface-roughness scattering regime) has led researchers to propose that strained Si MOSFETs have "smoother" surfaces compared with Si control MOSFETs [22,23]. Lower surface roughness amplitudes and longer correlation lengths were required to match simulated high field mobilities with measured high field mobilities in strained Si MOSFETs [22].…”

Temperature Dependence of Electron Mobility in Uniaxial Strained nMOSFETs

“…The strain changes the electronic band structure resulting in the change of carrier mobility [1][2][3][4][5]. While the electrostatics of strained MOSFETs has been studied, several research groups have reported the mobility characteristic in strained MOSFETs [1,6] or in unstrained MOSFETs [7,8].…”