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Non-contact atomic force microscopy (NC-AFM) and scanning tunneling microscopy (STM) have been widely used to observe various materials. Differences are often seen between AFM and STM images of the identical sample, but details of such differences have not been analyzed sufficiently.
For example, in the case of hydrogen-terminated Si(001)2x1 surface, three types of features were obtained in a NC-AFM experiment: an absence of spot, a single spot and symmetric pair of spots [Jpn. J. Appl. Phys. 41 (2002) 4857]. The symmetric pair of spots and absence of spot are assigned to normal H-terminated dimer and missing dimer, respectively. On the other hand, in a STM experiment for the same surface, unclear bright lines and brighter spots were obtained and assigned to normal dimer array and a defect, respectively, besides an absence of spot assigned to the missing dimer. In addition to the above problem, it is not known what kind of defects causes the single spot in the NC-AFM image.
In this study, we perform NC-AFM and STM simulations of this surface with defects on the basis of the identical framework, the density-functional based tight-binding method. In the NC-AFM simulation, the van der Waals force and frequency shift were estimated according to the Sasaki and Tsukada formula [Appl. Phys. A 72 (2001) S39, Jpn. J. Appl. Phys. 37 (1998) L533]. In the STM simulation, images were obtained from the local density of states. About defect, we have examined three types: a missing dimer, a bare dimer, and a half-terminated dimer (one Si atom of a dimer terminated by one H atom).
In the NC-AFM simulation, we found that the missing dimer does not give any spots, the bare dimer causes a single spot, the half-terminated dimer does asymmetric pair of spots, and the normally terminated dimer has symmetric pair of spots. In the case of the half-terminated dimer, however, the difference of brightness between the spots in a pair is very small. Therefore, we can say that three types of features are distinguishable, which is consistent with the experimental observation. In the STM simulation, we successfully reproduced the experimental image of the surface without defects. Simulations for the surface with defects are in progress, and the results will be presented at the conference.
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