Cryogenic Electron Mobility and Subthreshold Slope of Oxygen-Inserted (OI) Si Channel nMOSFETs

Oxygen-inserted (OI) Si channel nMOSFETs, which allow the formation and maintenance of an undoped epitaxial layer underneath the gate dielectric, are known to provide higher ON-state current and have higher inversion-layer carrier mobility compared to control silicon devices. However, it is not clear if the enhancement in mobility is due solely to reduced Coulomb scattering in the undoped layer. In this study, cryogenic measurements down to 4.2K are performed to deconvolve the contribution of phonon, surface roughness, and Coulomb scattering mobilities to total mobility. It is found that besides having reduced Coulomb scattering, due to the undoped epitaxial layer, the OI samples also exhibit 53% higher surface roughness mobility than the baseline samples. Notwithstanding the improved OI sample mobility, a saturation of the subthreshold slope at cryogenic temperature is still observed, as in the baseline and other technologies. In this paper, OI-Si nMOSFET mobility equations and empirical subthreshold slope saturation equation are developed which may be used for compact modeling from T = 330K to 4.2K.