Atomic-scale studies of electron transport through MOS structures

Abstract

The STM-based ballistic electron emission microscopy/spectroscopy (BEEM/BEES) in conjunction with theoretical modeling and Monte Carlo simulations are used to clarify intrinsic transport issues in metal-oxide-semiconductor (MOS) structures. Compared to conventional MOS transport studies, BEEM/BEES offers unique advantages that include, besides its unsurpassed spatial resolution, independent control over the energy of the electrons, which thereby can be injected directly into the SiO2 conduction band, as well as complete control over the electric field across the oxide. The MOS samples consist of thin Pd ‘gate’ dots deposited on 7–15 nm device-grade oxides thermally grown on Si(100). BEES measures the threshold energy for electron transmission over the potential barrier determined by the gate metal and oxide. This potential is strongly affected by the combination of screening effects due to the metal gate and an applied oxide field. Field dependent decreases of the injection thresholds follow classical image force theory, through which we have determined a dielectric response of εox = 2.74 for electrons in SiO2. Possible contributions to the threshold shifts from field induced tunneling through the oxide barrier were assessed with a WKB calculation and deemed unimportant. Justification for the measured εox = 2.74, as opposed to the ‘established’ value close to the optical dielectric constant of 2.15, is obtained from a novel theoretical model. The model, based on a classical particle subject to a time-dependent potential in a polarizable medium, predicts a dynamic dielectric response in the oxide to a moving charge near the metal interface of εox = 2.69. The image force induced distortion of the potential near the metal gate has a pronounced effect on the dynamics of the electrons reaching the oxide. Monte Carlo simulations show that this distortion strongly affects the phonon scattering rates in the immediate interface region. Energy and oxide-field dependent electron transmission probabilities were obtained from the simulations and were found to agree remarkably well with experimentally determined probabilities.