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A general introduction to x-ray diffraction and its application to the study of surfaces and interfaces is presented. The application of x-ray diffractkm to various problems in surface and interface science is illustrated through five different techniques: crystal truncation rod analysis, two-dimensional crystallography, three-dimensional structure analysis, the evanescent wave method and lineshape analysis. These techniques are explained with numerous examples from recent experiments and with the aid of an extensive bibliography.

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Synchrotron x-ray diffraction analysis of GaAs(OOl) epilayers grown in situ by molecular-beam epitaxy shows clear evidence of As-As dimers on top of the outermost As layer. These dimers are ordered in a variable way between two structures which both show a c(4x4) symmetry but have different As content. Both structures have twofold symmetry although the unit cell is square.PACS numbers: 61.10.Jv, 68.35.Bs, 68.55.Bd Compound-semiconductor surfaces are fundamentally different from their elemental counterparts because they have an additional composition degree of freedom. GaAs(OOl) is a good example with a sequence of surface symmetries starting with the most As-rich c(4x4) and ending with the 4x6 on the Ga-rich side. 1 ' 2 Change of symmetry is certainly an indication of change of structure, but it is entirely possible that several structures have the same symmetry. For example, total-energy calculations performed on a 2x4 surface cell 3 have demonstrated that two structures with As coverages equal to 0.5 and 0.75, respectively, were equally stable; the latter being primarily observed by scanning tunneling microscopy, 4 together with small patches of other arrangements. Here we show that the c(4x4) state of GaAs(OOl) adjusts its structure between two extremes to accommodate different amounts of As.The c(4x4) is the only reported GaAs(OOl) reconstruction exhibiting an apparent fourfold symmetry which seems to disregard the "native" twofold symmetry induced by the dangling bond orientation on this polar surface; it actually corresponds to an excess of As and may be considered a chemisorbed-type phase produced by chemisorption of either (As)4 or (As)2 molecules. Previous work by LEED in conjunction with molecularbeam epitaxy and later by reflection high-energy electron diffraction on the c(4x4) GaAs(OOl) surface 5 " 7 has not identified a structure. The only model proposed so far comes from photoemission studies which clearly identify As-As covalent bonding not present in the bulk and suggest the presence of As-As dimers. 8 " 10 While LEED or reflection high-energy electron diffraction are useful for surveying surface symmetry, they are difficult to apply for quantitative evaluations because of multiple scattering. On the contrary, in the case of grazing incidence xray diffraction, 11 the diffracted intensities can be interpreted in the simple framework of the kinematical theory and concerning clean semiconductor surfaces, reliable results have been obtained for Si(ll 1)7x7, 1213 Ge(001)2xl, 14 InSb(lll)2x2, 15 and GaSb(lll)-2x2. 16 The present results on GaAs(001)c(4x4) are the first dealings with a GaAs reconstructed surface obtained by grazing incidence x-ray diffraction. The presence of As-As dimers is confirmed with a dimer bond length of 2.59 ±0.06 A, significantly longer than the value for bulk As. Moreover, direct inspection of the x-ray data demonstrates that the true symmetry of the reconstructed surface is not fourfold but belongs to the ram 2 twodimensional symmetry group, in agreement with the bonding s...