On the problem of overlapping omega scans measured on thin films depositedon monocrystal substrates

CeO2 thin films deposited on sapphire monocrystal substrates were used for an experimental study of the nature of extremely narrow overlapped maxima o n Xray diffraction omega scans. Full width at half-maximum (FWHM) values of such maxima typically reached the resolution function of the diffractomete r. A comparative study of the influence of various diffractometer set-ups o n the spectral characteristics of the X-ray beam in relation to the above-m entioned phenomenon was carried out. A surrounding (lambda(min) - lambda(ma x)) or (2 theta(min) - 2 theta(max)) of the strong substrate reflection was obtained, where a substrate contribution to an omega scan measured on thin -film reflection can be expected. Two possible origins of the narrow maxima are discussed: (a) a contribution of a part of the X-ray beam having lambd a not equal lambda(K alpha) that diffracts on a set of substrate crystallog raphic planes parallel to the thin-film crystallographic planes used for th e omega-scan measurement; and (b) the presence in part of the thin film of a perfect monocrystal-like quality with practically no mosaicity. The princ iples of this approach and experimental procedure are reported, and on this basis it is possible to distinguish between the two possible origins of th e narrow overlapped maxima. It is shown that under appropriate conditions, an extremely high quality CeO2 thin film can be grown. The FWHM value of it s omega scan can reach the value of diffractometer instrumental broadening obtained for a perfect monocrystal.