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a b s t r a c tCommercial O-face (0 0 0 1) ZnO single crystals were implanted with 200 keV Ar ions. The ion fluences applied cover a wide range from 5 Â 10 11 to 7 Â 10 16 cm À2 . The implantation and the subsequent damage analysis by Rutherford backscattering spectrometry (RBS) in channelling geometry were performed in a special target chamber at 15 K without changing the target temperature of the sample. To analyse the measured channelling spectra the computer code DICADA was used to calculate the relative concentration of displaced lattice atoms.Four stages of the damage evolution can be identified. At low ion fluences up to about 2 Â 10 13 cm À2 the defect concentration increases nearly linearly with rising fluence (stage I). There are strong indications that only point defects are produced, the absolute concentration of which is reasonably given by SRIM calculations using displacement energies of E d (Zn) = 65 eV and E d (O) = 50 eV. In a second stage the defect concentration remains almost constant at a value of about 0.02, which can be interpreted by a balance between production and recombination of point defects. For ion fluences around 5 Â 10 15 cm À2 a second significant increase of the defect concentration is observed (stage III). Within stage IV at fluences above 10 16 cm À2 the defect concentration tends again to saturate at a level of about 0.5 which is well below amorphisation. Within stages III and IV the damage formation is strongly governed by the implanted ions and it is appropriate to conclude that the damage consists of a mixture of point defects and dislocation loops.

Fast helium atoms and hydrogen molecules with energies from 400 eV up to several keV are grazingly scattered from a Fe͑110͒ surface covered by oxygen and sulphur atoms forming c͑2 ϫ 2͒ and c͑1 ϫ 3͒ superstructures, respectively. For scattering along low-index azimuthal directions we observe defined diffraction patterns in the angular distributions for scattered projectiles. From the evaluation of those diffraction patterns we derive the widths of low-indexed axial channels, the corrugation of the interaction potential across these channels, and the normal positions of adsorbed atoms above the Fe lattice. Our analysis is based on semiclassical models using hard-wall approximation as well as individual potentials for the interaction of projectiles with the surface. By comparing the results of different models, we discuss the robustness of the information on the geometrical structure of the surfaces.

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