Development of a Convenient in situ UHV Scanning Tunneling Potentiometry System Using a Tip Holder Equipped with Current-Injection Wires

摘要

Scanning tunneling potentiometry (STP) is one of the advanced variations of scanning tunneling microscopy (STM). By combining STM and STP, the local structures and potential distribution on conducting samples with a lateral current flowing along the sample surface can be measured simultaneously with a high spatial resolution. However, additional electrodes are necessary to make the current flow along the sample surface, which can sometimes be difficult to handle. We developed a new type of STM tip holder that makes it easier to perform in situ STM/STP measurements in ultrahigh vacuum (UHV), prepare a sample and transfer the sample/tip. Using this tip holder, a Si(111)-√3×√3-Ag surface and Bi(111) films grown on Si(111) were measured using STP. For the Si(111)-√3×√3-Ag surface, abrupt potential drops at the atomic steps in the topographic images are observed. On the other hand, for the Bi film, a nearly homogeneous potential gradient is observed along the direction of the electric current without potential drops at the steps on the surface. By combining numerical simulation and atomic-scale resolved STM images, it is shown that the current distribution is not homogeneous at all, even at nanometer scales. This is due to a wide range of scatter in the resistances across steps, irrespective of the step height. This originates from difference in the ratio of the conductivity at the steps to that on the terraces between the two material systems. [DOI: 10.1380/ejssnt.2016.216]