Development of a sub-picoampere scanning tunneling microscope for oxide surfaces

Capacitive feedback current amplification has been implemented in a scanning tunneling microscope in order to image surfaces in ultra-high vacuum at sub-picoampere current levels. Atomic steps on Si(1 1 1) have been resolved at 0.30 pA scanning at 100 nm/s. The surface of the native oxide of Si has been imaged with negative sample bias at 1 pA. The rms roughness of the oxide surface is 0.12 nm. I–V curves indicate a bandgap of ∼7–8 V. This bandgap appears to arise from the alignment of the tip Fermi level and electron states of the SiO2 surface.