Scanning tunneling microscopy in the field-emission regime: Formation of a two-dimensional electron cascade

The signal generation mechanism of the scanning field-emission microscope has been investigated via model calculations combining deterministic trajectory calculations in the field surrounding the field-emission tip in vacuum, with Monte Carlo simulations of the electron transport inside the solid. This model gives rise to a two-dimensional electron cascade. Individual trajectories of detected backscattered electrons consist of repeated segments of travel in vacuum followed by a re-entry into the solid and re-emission into vacuum after being elastically or inelastically scattered. These so-called electron bouncing events also create secondary electrons at macroscopic distances away from the primary impact position. The signal reaching the detector is made up of elastically and inelastically backscattered primary electrons created near the impact position under the tip and those secondary electrons created far away from it.