Measurement of Si Surface Conduction by Two-probe Scanning Tunneling Microscopy with Ohmic Contact

The miniaturization of electronic devices requires the characterization of nano- and atomic-scale structures. Imaging, fabricating, and measuring of local electronic properties of these structures can be performed with a one-probe scanning tunneling microscope (1P-STM). Yet, assessing the electrical conduction properties lateral to the surface demands two-probe (2P-) and four-probe (4P-) STM. The 4P-STM conductance measurement is generally preferred over the 2P-STM one as it can eliminate tip-to-sample contact resistance and Schottky barrier in semiconductor samples by separating two current-injecting tips from two voltage-measuring tips. However, placing multiple probes into a nano-scale region is an arduous task. Here, we describe a refinement of a 2P-STM by eliminating the issue of the Schottky barrier. To achieve that, we employed field evaporation to clean tip apices and made Ohmic probes that produce linear IV curves on Si surfaces. This Ohmic 2P-STM method allows surface conductance measurement at very low bias, limiting conduction through the space-charge layer and bulk states, with the minimum number of movable probes. Using STM, we also fabricated nano-scale Si structures with insulating trench lines and confirmed the two-dimensional conduction on the surface by measuring resistance.