X-ray standing waves (XSW) in a thin epitaxial film are treated in the framework of the dynamical theory. It is demonstrated that the fluorescence yield around the main peak of the rocking curve has essentially the same characteristics as the one of the usual XSW on a bulk crystal surface. Thus the XSW provide a direct method to probe the atom position in a thin film. The method was applied to an epilayer of the diluted magnetic semiconductor Zn0.94Co0.06O, in order to determine the Co atoms position. The XSW established that Co atoms occupy the substitutional Zn site in the ZnO matrix, although their coherent fraction, which measures the degree of order, is rather low. Moreover the measurement of the Zn fluorescence in the film, gives approximately the same value for the coherent fraction of the Zn atoms. Besides, by using the substrate rocking curve, it is shown that the XSW signal of the Zn atoms in the substrate can be detected through the film. This interesting approach allows to probe in situ the coherent fraction of an element of a substrate, below an interface. For the Zn fluorescence, the coherent fraction is lower near the interface, than in the bulk. These results should be related to strains and defects on both sides of the interface.

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