Glancing Incidence X-Ray Diffraction of Polycrystalline Thin Films
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Glancing Incidence X-Ray Diffraction of Polycrystalline Thin Films Brian E. McCandless, Institute of Energy Conversion University of Delaware, Newark, DE 19716 ABSTRACT Glancing incidence x-ray diffraction (GIXRD) is a powerful diagnostic tool for nondestructive analysis of thin film materials and structures. GIXRD fundamentals and experimental techniques and results are presented with respect to polycrystalline thin-films. The refraction index and critical incident beam angle are calculated for selected elemental and inorganic compounds. Quantitative GIXRD analysis with Cu Kα radiation is applied to several problems: recrystallization of ultra-thin chemical surface deposited CdS films; compositional grading in CuIn1-xGaxSe2 films; formation of native oxides on CdTe films; interdiffusion between CdTe and CdS; Te film thickness determination on etched CdTe; and correlation of inplane residual stress with deposition conditions for sputtered Mo films. INTRODUCTION The development of thin film materials for optoelectronic applications such as photovoltaics is facilitated by non-destructive analytical methods which can provide quantitative information on the physical and chemical state of the constituent layers, their interfaces and surfaces. GIXRD provides this capability for many thin film structures and materials systems, by decoupling incident and diffracted beam paths, allowing quantification of phase distribution, alloy formation, residual stress, and particle size. The addition of a heater and means for ambient control allows real-time in situ investigation of chemical reactions between thin film specimens and ambient and between different layers within the thin film structure. GIXRD FUNDAMENTALS The symmetric diffraction condition, in which the incident and diffracted beam angles are identical, produces a diffraction signal only for sets of crystal planes whose normal bisects the incident and diffracted beam direction (shaded circles in Figure 1, left). In its most common application, powder diffractometry, the diffraction pattern is measured over a wide angular range for a powder bed containing randomly oriented particles with size >100 µm, using the BraggBrentano (B-B) parafocusing optical configuration.1 The B-B method permits accurate intensity measurement for lattice planes lying parallel to the focusing plane, allowing quantitative assessment of phase mixtures to be reliably determined to ±1 weight percent. By substituting a thin film for the powder in the focal plane, randomness is not guaranteed and the crystallographic texture of the specimen with respect to the substrate can be assessed. Whereas powder line profiles in B-B mode are primarily sensitive to instrumental effects, those of thin films are sensitive to both sample and instrumental effects: asymmetrical broadening by beam divergence; broadening by specimen planarity; non-uniform stress and small domain volumes; and angular shifting by specimen displacement from focusing circle and uniform stress. Parallel beam geometry reduces some of t
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