Abstract
Variations of the topographic images by scanning tunneling microscopy (STM) and dual-polarity tunneling imaging (DPTI) and the current images of current imaging tunneling spectroscopy (CITS) with bias voltages were examined. While DPTI shows the normal image depicted by a single apex atom for the specimen voltage of -2V , the image at +2V is occasionally a double tip image. Furthermore, the current image by CITS for -2V often exhibits the reversed contrast grey-scale images with bright corner holes of the unit cell of the Si (111)-(7 X 7) reconstructed structure. The observed variation of the images with bias voltages is explained as a result of the difference in the effective tunneling barrier and the surface state density of the tip apex and specimen, and also as the effect of the atomic arrangement and composition and profile of the tip apex.
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