Hot-electron effects and oxide degradation in MOS structures studied with ballistic electron emission microscopy

The application of the STM-based technique of ballistic electron emission m icroscopy (BEEM) to the study of transport properties of SiO2 Elate oxide l ayers is reviewed. Oxide degradation observed on a local scale of nanometer dimensions ranges from the filling of electron traps with low-kinetic-ener gy electrons injected just above the oxide barrier, to trap generation and filling triggered by electrons with kinetic energies greater than or simila r to 2 eV. BEEM provides means to determine the spatial distributions of th e traps. Only positively charged traps are observed for thin (less than or equal to 4 nm) SiO2 layers upon electrical stressing. Oxide breakdown is se ldom induced by local stressing, suggesting that it occurs at extrinsic def ect sites that are widely separated. BEEM also provides unique opportunitie s to address fundamental issues. In that regard, examples of the dynamic re sponse of the SiO2 dielectric to a moving electron ave presented, as well a s the determination of the dispersion of the electron mass of the conductio n-band electrons. The latter is achieved by modeling quantum interference o scillations in the BEEM current.