Simulation Study of Coulomb Mobility in Strained Silicon

This paper presents a detailed simulation analysis aimed at assessing and explaining the dependence on the biaxial strain of the Coulomb-limited mobility in n-type silicon MOSFETs. By using a model based on the momentum relaxation time (MRT) approximation, we first show that we can reproduce fairly well a wide set of published experimental data, and then, we use our model to discuss the dependence on the strain of the mobility limited by either interface states or substrate impurities. Different from the experiments, in the simulations, the MRT approach allows us to analyze the different mobility components without resorting to the Matthiessen's rule, whose use may result in large errors in the extracted mobility components. Our simulations indicate that the interface-state-limited mobility is reduced in strained devices; this is in qualitative agreement with the experiments, and we discuss its interpretation in terms of physically transparent arguments. Our analysis also suggests that the strain-induced changes of the substrate-impurity-limited mobility are instead very small, and we provide a clear interpretation of such a result. Recent experiments, however, have reported a strain-induced improvement of the substrate-impurity-limited mobility, which has been unavoidably extracted by using the Matthiessen's rule. We argue that the systematic errors produced by the Matthiessen's rule can help reconcile the simulation and the experimental results.