Growth of bismuth tri-iodide platelets for room temperature X-ray detection

Abstract

Bismuth tri-iodide synthesized from bismuth subcarbonate and potassium iodide, and then purified by zone refining followed by one sublimation, was used as starting material. Platelets were grown by physical vapor deposition (PVD) under argon (500 mmHg), with a source temperature of 300 /spl deg/C and during 5-7 days. Growth temperature was at about 260 /spl deg/C. Other set of platelets was grown by traveling molten zone (TMZ), at 460 /spl deg/C and with a traveling speed of 5 mm/h. Platelets are up to 20 /spl times/ 10 mm/sup 2/ in size, and from 50 to 80 /spl mu/m in thickness. Dendritic-like defects turned out to be the main surface defect for the PVD crystals. Some samples exhibit hexagonal microcrystals on their surface, which indicates that they grow from layers of hexagonal grains. Detectors were made with representative platelets by Pd or Au thermal deposition (contact areas of 0.03, 0.09, 0.12 cm/sup 2/), Pd wire attachment with aquadag and acrylic encapsulation. TMZ crystals show a better surface crystal quality, but this fact does not seem to particularly influence the detector electrical properties. Nevertheless, the metal used for contact deposition determines the resistivity, and Au emerges as better than Pd for this purpose. Detector X-ray response was measured by irradiation with a /sup 241/Am source of 3.5 mR/h. Resistivities up to 2.0 /spl times/ 10/sup 12/ /spl Omega/-cm were obtained, the best reported for detectors made with monocrystals of this material, which correlates with the fact that these detectors are the first BiI/sub 3/ ones that responds to an X-ray moderate radiation exposure. Platelets obtained in the present work give detector grade but not spectrometric grade material, for which energy resolution can not be calculated nor compared with previous results.