Direct evidence for multiple vibrational excitation of Si-H/D bonds for hot-carrier degradation of MOS transistors

The study of the desorption of hydrogen (H) and deuterium (D) on silicon in ultra high vacuum (UHV) by scanning tunneling microscopy (STM) led to the discovery of the giant H/D isotope effect. It was later used in passivation of the SiO₂/Si interface, leading to large improvement of the hot-carrier lifetime of MOS transistors (J.W. Lyding et al, Appl. Phys. Lett., vol. 68, p. 2526, 1996). It is known that desorption mechanisms for the Si-H/D bonds by STM are multiple vibrational excitation at low voltage (T.-C. Shen et al, Science vol. 268, p. 1590, 1995). However, the chemical environment of the SiO₂/Si interface in MOS devices is very different from that of Si in UHV, because in MOS transistors, electrons not only directly excite the Si-H/D bonds, but also are injected into the oxide. Recently, we showed that electrons that are injected into the oxide may not break Si-H/D bonds and only electrons that remain in the channel (and do not overcome the oxide/Si barrier) break the Si-H/D bonds (Z. Chen et al, IEEE Electron Dev. Lett. vol. 21, p. 24, 2000). Although it was suggested that the mechanisms for breakage of Si-H/D bonds in MOS transistors should be analogous to the explanation for the STM experiments, there is no direct experimental evidence to support the suggestion. In this paper, we present experimental results to show that the quantitative H/D isotope effect is dependent on the channel current density, which support the multiple vibrational excitation mechanisms.