Image Force Effects and the Dielectric Response of SiO2 in Electron Transport across MOS Structures

Hot electrons of variable energy were injected from a scanning tunneling microscope (STM) tip into a Pd/SiO2/Si(100) MOS structure. An analysis of the emerging collector current in the Si substrate, a technique known as Ballistic Electron Emission Microscopy (BEEM), revealed a monotonic barrier height lowering with increasing positive oxide bias, in excellent agreement with a scaled classical image force theory. Calculations using the WKB approximation suggest a negligible contribution to the observed shifts from electrons tunneling through the barrier. From an extrapolation to zero oxide field the Pd-SiO2 barrier height of 4.08plusmin0.02 eV was deduced. An image-force-dielectric constant of 2.74 in-between the so-called optical (2.15) and static (3.9) dielectric constant was determined. In order to understand this intermediate value, a theoretical calculation of a retarded image force on the moving electron is carried out for the first time. The calculations yield an image-force-dielectric constant of 2.69, that is consistent with the experimentally determined value. This intermediate dielectric constant is evidence for electron-phonon intercation and corresponds to an average dielectric response integrated over the time of progression of the electron in SiO2.