Image force effects and the dielectric response of SiO2 in electron transport across metal–oxide–semiconductor structures

Abstract

Hot electrons of variable energy were injected from a scanning tunneling microscope tip into a Pd/SiO2/Si(100) metal–oxide–semiconductor structure. An analysis of the emerging collector current in the Si substrate, a technique known as ballistic electron emission microscopy, 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 4.08±0.02 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 interaction and corresponds to an average dielectric response integrated over the time of progression of the electron in SiO2.