Hot electron transport through metal–oxide–semiconductor structures studied by ballistic electron emission spectroscopy

Abstract

The tip of a scanning tunneling microscope (STM) was used to inject electrons into thin Pt layers of metal–oxide–semiconductor (MOS) structures. The collector currents emanating from the n-type Si(100) substrates were measured as a function of the electron energy, determined by the STM tip bias VT, for different oxide biases Vox applied independently across the oxide layers. The SiO2 layers were thermally grown in a device processing line and ranged from 27 to 62 Å in thickness. A current threshold near VT=3.90 V is interpreted in terms of current transport through the SiO2 conduction band. The current transport through the MOS structure was modeled in a single band description for zero oxide thickness, and fitted to the collector currents that had been corrected for impact ionization effects in the Si. Deviations between the two curves represent the influence of the transmission probability Tox through the SiO2 film of finite thickness. Tox can thus be determined from the experimental data. Within an eV of threshold the magnitude of Tox was observed to be particularly sensitive to small changes in oxide bias in the range 0.3 V≳Vox≳−0.1 V. The transmission probabilities were also calculated by integrating the Boltzmann equation using Monte Carlo techniques that incorporate energy dependent effective masses and electron phonon scattering rates. Agreement between the two approaches is quite good, including the observed sensitivity on oxide bias in the threshold region, which is a direct consequence of the strong electron-optical phonon scattering in the oxide. The 27 Å thick oxide structures exhibited in the ballistic electron emission microscopy images scattered patches of high transmittance of only 1–2 nm in extent. The collector currents arising from injection at these patches indicated thresholds as low as 1.1 eV, but the observed modest currents above that threshold argue against local shorts that would arise from pinholes in the oxide.