X-Ray Analysis of Alpha Mercuric Iodide Crystal Structure and Processing Effects *
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X-RAY ANALYSIS OF ALPHA MERCURIC IODIDE CRYSTAL STRUCTURE AND PROCESSING EFFECTS' L. Keller', E.X. Wang and A.Y. Cheng C,AMET Research, Inc., Goleta, CA 93117 EG&G Energy Measurements Inc., Santa 93117
Barbara
Operations,
Goleta,
CA
ABSTRACT X-ray topography and rocking curve experiments were performed on a-mercuric iodide samples. As-grown crystals were examined for intrinsic defects and crystallinity. Orientation of certain defects depends on the direction of crystal growth. The propagation of as-grown crystalline features was documented. The extent of crystal damage introduced during various steps of device fabrication such as sawing, polishing, etching and contact deposition was explored. Coefficients of linear thermal expansion of a 3 3 = 54 ± 5 (10- 6 /°C) along the tetragonal c-axis, 10011 direction and all = 11 ± 4 6 (10- /*C) in the [1001 direction were measured. INTRODUCTION Vapor crystal growth techniques have yielded the largest, most usable mercuric and gamma-ray detector iodide crystals for room temperature X-ray 3 2 introduced and subgrain boundaries applications. 1 However line defects Furthermore, during growth may contribute to poor detector performance. mismatches in thermal expansion between the crystal and its growth pedestal will cause stress within the crystal, especially during the cool-down phase Similarly, a mismatch between the after completion of crystal growth. evaporated metal contact and the crystal surface can produce a stress in the Additionally, the softness of thin film resulting in poor adhesion of the film. the HgI 2 crystal makes it prone to mechanical damage in the form of inelastic strains, plastic deformation and shearing that can be introduced during the X-ray topography and rocking curves various steps of detector fabrication. studies were conducted to Investigate the extent of these effects. EXPERIMENT 4 Asymmetric double crystal reflection topography has been employed to provide a qualitative assessment of imperfections present on as-grown and processed faces of HgI2 samples. These studies can be considered a "surface" analysis of HgI 9 , because the 8.1 KeV (Cu-Ka) x-rays, only penetrate, depending on incident angle, -5-10 pm into the HgI 9 . Sets of topographs with the incident beam rotated by 90%, with respect to the normal of the plane of Interest, were made. Thus, the contrast for linear defects Is enhanced when the direction of defects is perpendicular to the diffraction plane. Note there is foreshortening of the topographs In the direction of the incident beam.
The determination of thermal expansion by XRD was carried out on a double crystal diffractometer in the nondispersive mode employing a 100 cut Ge . This work was performed under the auspices of the U.S. Department of Energy under Contract No. DE-AC08-88-NV10617. Note: By acceptance of this Government's article, the publisher and/or recipient acknowledges the U.S. right to retain a nonexclusive royalty-free license in and to any copyright covering this paper. Reference to a company or product name does
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