Scanning frequency comb microscopy (SFCM) shows promise for sub-10 nm dopant profiling

When a mode-locked ultrafast laser is focused on the tunneling junction of a scanning tunneling microscope a microwave frequency comb, at harmonics of the pulse repetition frequency of the laser, is superimposed on the DC tunneling current. A clean semiconductor sample, biased to form a nanoscale depletion region at the base of the tunneling junction, attenuates these harmonics. Simulations suggest that the dependence of the measured attenuation on the applied DC bias may be used to determine the local concentration of dopant atoms with a resolution as fine as the radius of the tunneling junction - which is much smaller than radius of the tip electrode.