Effect of the Extinction Distance in X-Ray Rocking Curve Analyses of II-VI Compounds
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EFFECT OF THE EXTINCTION DISTANCE IN X-RAY ROCKING CURVE ANALYSES OF II-VI COMPOUNDS P.D. MORAN and R.J. MATYI, Dept. of Materials Science and Engineering, University of Wisconsin, Madison, WI 53706 ABSTRACT Double crystal x-ray rocking curves from single crystal CdTe and CdZnTe of various degrees of crystalline perfection were recorded under diffraction conditions in which the extinction distance varied from under 10 microns to more than 100 microns. In crystals of intermediate quality the rocking curves recorded under conditions of long extinction length showed structure not evident in those recorded under conditions of short extinction length. Integrated reflectivity measurements were performed at both long and short extinction lengths. The results have been interpreted within the framework of a transition from kinematic to dynamical diffraction. INTRODUCTION The full width at half maxima (FWHM) of double crystal x-ray rocking curves are routinely used as a qualitative measure of perfection in single crystal materials. The widespread use of this technique as a "figure of merit" can be attributed in part to its nondestructive nature and the relatively little sample preparation required. Unfortunately, the attempt to extract quantitative information about the defect structure of the material is confounded by the complex interplay between materials functions the diffraction phenomena of extinction. The term extinction in dynamical theory is strictly reserved to describe the reduction in intensity of the incident beam as it traverses the crystal due to the coupling of its amplitude with that of the diffracted beam. The result of this coupling is a reduction in the incident beam's intensity as it traverses the crystal. The "extinction distance" is a measure of the distance over which the incident beam must travel in the crystal for the loss of intensity to reach its maximum value. Amplitude coupling can only occur over a region in which the incident and diffracted wavefields maintain a coherent phase relationship. This requires that the crystalline medium remain spatially coherent, i.e. crystallographically perfect, over the region in which dynamical coupling takes place. Thus, if a reflectivity profile taken under conditions of known extinction length is best described by dynamical theory, it can then be assumed that the crystal is defect-free over lengths that are on the order of the extinction distance. The type and degree of extinction effects observed depends on the relationship between the extinction distance from dynamical theory and the size and angular misorientation of coherently diffracting regions, or "mosaic blocks", in the crystal. Qualitatively, a crystal when examined under conditions in which the extinction distance is small when compared to the dimensions of its mosaic blocks will appear close to perfect. The resulting diffraction pattern should then be compared with that predicted by dynamical, or "perfect crystal", theory in drawing conclusions about the defect structure of the material. Alternati
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