Grazing Incidence X-Ray Reflectometry Studies of Cadmium Arachidate Langmuir-Blodgett Films
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GRAZING INCIDENCE X-RAY REFLECTOMETRY STUDIES OF CADMIUM ARACHIDATE LANGMUIR-BLODGETI" FILMS B. K. Tanner*, D. K Bowen+, M. C. Petty+ +, S. Swaminathan+*#'and F. Grunfeldx Dept. of Physics, University of Durham, South Road, Durham, DH1 3LE, UK + Dept. of Engineering,University of Warwick, Coventry, CI/7 4AL, UK. + + School of Engineering and Computer Science, University of Durham, South Road Durham DH1 3LE, UK XNIMA Technology, University of Warwick Science Park, Coventry, UK #PermanentAddress: Department of Engineering,Nanyang Technical University, Singapore
ABSTRACT Grazing incidence X-ray reflectometry has been used to characterize Langmuir-Blodgett films of cadmium arachidate deposited on silicon substrates. The agreement between layer parameters deduced from the interference fringe period and low angle Bragg peak positions was excellent. Good agreement was found between experimental and simulated reflectivity profiles only when interface roughness and a varying molecular layer thickness was included. Inclusion of interface roughness alone results in a substantial enhancement in the intensity of the Bragg peaks. This effect is identified as being equivalent to the reduction in extinction found in classical X-ray diffraction due to crystal imperfections.
INTRODUCTION In the region of the X-ray spectrum around 1A, the refractive index of materials is less than unity. As a consequence, total external reflection of X-rays is observed at very low angles. Above the critical angle, which is proportional to the electron density, the specularly reflected intensity falls rapidly and for an ideally abrupt surface this varies as the inverse fourth power of the scattering vector. To a very good approximation this is equal to the inverse fourth power of the scattering angle. Surface roughness leads to a more rapid fall with angle and grazing incidence X-ray reflectivity measurements are becoming increasingly used to study the quality of substrate polishing in the semiconductor industry [1,2]. By exploiting the intensity available at synchrotron radiation sources, X-ray reflectivity oscillations have been used to reveal the cyclical roughening of a semiconductor surface as successive atomic layers are completed during, for example, MOVPE growth [3]. As early as 1931, Kiessig [4] observed interference fringes in the region just above the critical angle from thin films deposited on substrates of significantly differing electron density. However, owing to limitations in X-ray sources and optics, the technique has only been exploited extensively in the past few years. As the method does not rely on long range order in the material, these fringes are observed in single crystal, polycrystal and amorphous films. There is presently much interest in the application of grazing incidence
Mat. Res. Soc. Symp. Proc. Vol. 237. 01992 Materials Research Society
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X-ray reflectometry (GIXR) to the characterization of magneto-optic and thin film magnetic recording media [5]. Langmuir-Blodgett (LB) films are self-ordering films built up
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