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

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. A first set of platelets was grown by physical vapor deposition (PVD) under argon atmosphere (7×10⁴ Pa), with a source temperature of 300°C and during 5-7 days. Growth temperature was at about 260°C. Other set of platelets was grown by traveling molten zone (TMZ), at 460°C and with a traveling speed of 5 mm/h. Platelets are up to 20×10 mm² in size and from 50 to 80 μm 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, 0.02, 0.12 cm²), 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. However, the metal used for electrode deposition determines the resistivity, and Au emerges as better than Pd for this purpose. Detector X-ray response was measured by irradiation with a ²⁴¹Am source of 3.5 mR/h. Resistivities up to 2.0×10¹² Ω·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 BiI₃ ones that respond to an X-ray moderate radiation exposure. Platelets obtained in the present work give detector but not spectrometric grade material.