Dimer pinning and the assignment of semiconductor-adsorbate surface structures

It has been observed in scanning tunneling microscopy (STM) that the adsorption of molecules on the (001) surface of a Group IV semiconductor can lead to an asymmetric ordering of the dimers immediately adjacent to the adsorbate. This so-called pinning may occur along the dimer row on only one, or both sides of the adsorbate. Here we present a straightforward methodology for predicting such pinning and illustrate this approach for several different adsorbate structures on the Si(001) surface. This approach extends earlier work by including the effects of coupling across the adsorbate as well as the nearest-neighbor interactions between the chemisorbed dimer and its adjacent dimers. The results are shown to be in excellent agreement with the room temperature experimental STM data. The examples also show how this approach can serve as a powerful tool for discriminating between alternative possible adsorbate structures on a dimerized semiconductor (001) surface, especially in cases of molecular adsorption where the STM measurements provide insufficient details of the underlying atomic structure.