Electron Transport in Bulk-Si NMOSFETs in Presence of High-k Insulator-charge Trapping and Mobility

Abstract

Recent advancements in gate stack engineering has led to the development of aggressively scaled, high mobility, high-k dielectric based NMOSFETs with metal gates. Most of the current literature on the subject also stressed on the need for a high temperature process step to attain the high mobility under minimal change of effective oxide thickness. However, the physical origin of high mobility is not well understood. In this work, fundamental insight into the necessity of the high temperature process step is provided. Novel experimental strategies are developed to understand the impact of interface states and bulk traps separately and exclusively on channel mobility. It is conjectured that the interface states at the SiO2⁄(100) bulk-Si interface are identical in nature (as far as coupling with the channel electrons is concerned) to those at the high-k⁄SiO2⁄(100) bulk-Si interface. Thus, the response of interface states on channel electrons in high-k insulator based NMOSFETs is properly calibrated by a novel thermal desorption of hydrogen experiment on SiO2⁄(100) bulk-Si NMOSFETs to yield a highly accurate parameterized equation. The value of interface state response parameter determined by the aforementioned experiment is compared with theoretical predictions, and independently determined projections from electrical stress measurements. The impact of transient charging on transport in the channel is investigated. It is conclusively shown that remote charge has minimal impact on mobility in the channel. The role of nitrogen induced fixed oxide charge is studied on a set of Hf-silicate samples. Role of soft optical phonon scattering and the beneficial impact of metal gates on soft optical phonon limited mobility are thoroughly investigated both theoretically and experimentally. Conclusions are drawn on the fundamental limit of mobility attainable in high-k dielectric based NMOSFETs.