Reproducibility of scanning tunneling spectroscopy of Si(111) 7x7 using a build-up tip

A build-up tip, which was made from a [111]-oriented W filament, was used for scanning tunneling spectroscopy (STS) of Si(111)7x7 to improve the reproducibility of the tunneling spectra. The apex can be repeatedly sharpened by applying high voltage while heating it. To clean the apex region while keeping the sharp apex, thermal-field evaporation (TFE) techniques were applied for the tip, observing the thermal-field ion evaporation images, FEM images and FIM images with a chevron MCP screen in a subchamber of an FEM-STM vacuum system. The tip was transferred to the STM head and then the change in the FEM images was examined before and after STM/STS experiments using the FEM-STM, which can be rapidly switched between the FEM/TFE and the STM/STS operation mode. The FEM is sensitive to changes in the workfunction and the outline of the tip. As a result, the tunneling spectra with sweeping a sample bias voltage from 2 to -2 eV were reproducible with little change in the FEM images. For 3 to -3 eV, however, the reproducibility was poor with some drastic change in the FEM images. The change was attributed to Si atom transfer from the sample surface to the tip on the STS measurement, which deteriorated the reliability of the tunneling spectra.