Field-induced reactions of water molecules at Si-dielectric interfaces
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Field-induced reactions of water molecules at Si-dielectric interfaces
L. Tsetseris , X. Zhou , D. M. Fleetwood , R. D. Schrimpf , and S. T. Pantelides Department of Physics and Astronomy, Vanderbilt University, Nashville, TN 37235, U.S.A. Department of Electrical Engineering and Computer Science, Vanderbilt University, Nashville, TN 37235, U.S.A. Solid State Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, U.S.A. ABSTRACT Water molecules and water-related species are present to a varying degree in SiO and other dielectrics and at Si-dielectric interfaces. Their presence, even in small concentrations, constitutes a critical reliability problem for present day ultrathin dielectrics. Here we present first-principles density functional calculations that probe the reactivity of water molecules at the Si-dielectric interface. We report results on different possible reaction pathways for water at the interface, including dissociation of water that can lead to the release of H+ ions. The released protons can migrate along the interface and depassivate dangling bonds. Results are also presented for the reaction of water with a hydrogen atom migrating laterally along the interface that lead to the creation of hydronium species. These atomic-scale mechanisms may account for at least some of the creation of interface traps and oxide trapped charge, the two features that give rise to negative bias temperature instability (NBTI), a well known reliability phenomenon that occurs in MOSFETs under normal operating and stress conditions. INTRODUCTION As the demand for fast microelectronic devices leads to further miniaturization, the presence of various defects, either at the interface between the substrate and the dielectric, or inside the bulk on either side, poses critical stability problems. One of the most critical reliability issues is bias-temperature instability (BTI)[1]. The BTI phenomenon corresponds to an increase of interface traps and oxide trapped charge in the presence of a strong electric field at elevated temperatures. Typical values for these stress conditions are 2-4 MV/cm for the field and 100-250 °C for the temperature. It has been found that BTI is more pronounced in the negative bias case (NBTI), even though results on positive bias temperature instability (PBTI) have also appeared in the literature. BTI has been studied since the early years of MOS devices, and it has also been found recently in high-k dielectric devices[2]. Various models have been suggested for its origin; nevertheless, a comprehensive description at the atomistic scale is still missing. In the present work we address this issue for water-related NBTI. We focus on water since it is believed to be a significant contributor[3, 4] to NBTI. METHOD The results presented here were obtained through first-principles density-functional calculations, based on the local-density approximation for the exchange-correlation functional. The single particle problem was solved in a plane wave basis[5] by utilizing
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