Bismuth Tri-Iodide Polycrystalline Films for Digital X-Ray Radiography Applications

Abstract

Bismuth tri-iodide polycrystalline films were grown by the physical vapor deposition method. Glass 1''/spl times/1'' in size was used as the substrate. Palladium was deposited previously onto the substrates as the rear contact. For growth, bismuth tri-iodide 99.999% was heated at 130-170/spl deg/C under high vacuum atmosphere (5/spl times/10/sup -5/ mmHg) or under Ar pressure for 20 hours. Film thickness was measured by the transmission of 59.5 keV /sup 241/Am emission, giving values ranging from 90 to 130 /spl mu/m (5%). Film grain size was measured by scanning electron microscopy, and it gave an average of (50/spl plusmn/20) /spl mu/m. Detectors were made with the films by depositing palladium as the front contact (contact area 4 mm/sup 2/) and then performing acrylic encapsulation. Resistivities of 6/spl times/10/sup 12/ /spl Omega/.cm and current densities of 240 pA/cm/sup 2/ at 20 V were obtained for these detectors. The electron mobility and lifetime and the electron mobility-lifetime product were measured by the transient charge technique, which gave values of 4.4 cm/sup 2//V.s, 3.3/spl times/10/sup -7/s and 1.4/spl times/10/sup -6/ cm/sup 2//V respectively. X-ray film response was checked by irradiating the films with a /sup 241/Am source and with an X-ray beam, for different beam energies and intensities and for several bias voltages applied to the detector. A linear response with exposure rate was obtained. Finally, the results were compared with previous ones for monocrystals of bismuth tri-iodide and polycrystalline films of alternative materials like lead iodide and mercuric iodide.