4226 Imaging the Dimers in Si s 111 d - s 7 3 7 d

The Sis111d-s7 3 7d surface is one of the most, if not the most, complicated surface structure in existence. A mystery for many years, the first reasonably accurate structure model came from transmission electron diffraction data (TED) [1,2], assisted in part by the observation of adatoms by scanning tunneling microscopy (STM) [3–6]. The results of grazing incidence x-ray diffraction [7–9], low energy electron diffraction (LEED) [10,11], TED [1,2,12], as well as theoretical analyses [13–17], are in agreement with the so-called dimer-adatom-stacking (DAS) model. This model consists of 12 adatoms (first layer), a stacking fault bilayer (second and third layer) within which 9 dimers (third layer) border the triangular subunits as the core structure of 16 f112g screw dislocations, and a deep vacancy at each apex of the unit cell. While STM was used successfully to image the adatoms on the surface, many of the key features, for instance, the dimers in the third layer, are too deep in the structure. Another technique, high resolution electron microscopy (HREM), can obtain atomic scale information about surfaces by direct imaging. First employed in the so-called profile imaging mode [18], there has been slow progress over the past few years using the more general plan-view mode [19], where the electron beam is perpendicular to the surface of interest. In particular, the use of noise reduction filters [20] has enabled images to be obtained at very high resolutions in one case [21]. In this Letter we report results obtained using planview imaging coupled with noise reduction and numerical inversion to separate the top and bottom surfaces. The resultant images clearly show not just the adatoms but also the buried dimers. The sample was prepared from a p-type Si(111) wafer of 10–20 Ohm cm. A disk of 3 mm diameter was cut from the wafer, then mechanically thinned and dimpled. Chemical etching of the specimen was performed using a HF 1 HNO3 solution. In this fashion the specimen develops pits on its surface, and thin