3-D Surface Structure Analysis by X-Ray Diffraction

Abstract

Surface x-ray diffraction is by now a well established technique for the structure analysis of clean and adsorbate covered crystal surfaces. In the present article we briefly review the most important features of surface x-ray diffraction in combination with some examples 3-D surface structure determinations. The structures of the clean and alkali covered Ge(100)-(2xl) surface as well as the (3x3) reconstruction of InSb($\bar 1\bar 1\bar 1$) have been studied. For the uncovered Ge(100)(2xl) surface we have obtained clear evidence for the asymmetry of the dimers. Adsorption of K and Cs is found to take place in the large grooves between the Ge- dimer rows. At about half monolayer coverage the high coordinated site above the third layer Ge atom is occupied, at lower and higher coverage occupation of an asymmetric site close to the Ge-dangling bonds is observed. For the InSb($\bar 1\bar 1\bar 1$) surface a new type of reconstruction has been found. It is characterised by adatom rings located at the corners of the (3x3) unit cell above a slightly relaxed InSb double layer. Two types of six-atom rings have been found, an elliptic ring occurring in three different orientations and an trigonal ring occurring in two different orientations. The rings are randomly distributed and oriented. The analysis of this structure demonstrates the importance of the out-of-plane diffracted intensity with a large momentum transfer q_z normal to the sample surface. Only the consideration of these reflections allow a complete three dimensional analysis of the surface structure including subsurface relaxation and anisotropic thermal vibrations. The specific advantages and disadvantages of grazing incidence x-ray diffraction are compared with low energy electron diffraction.