Abstract-- Mercuric iodide films were grown by the physical vapor deposition method on palladium-coated glass substrates 2”x2” in size. The growth was performed in a system especially designed and constructed for getting a fine control of the growth parameters. The best growth conditions were a source temperature of 120 ºC, a growth temperature of 60 ºC and a growth time of 48 hours, with an initial pressure of 6 x 10 -3 Pa. Film thicknesses and grain sizes gave values ranging between 100 and 260 µm (10%), and between 10 and 30 µm, respectively. Films grow oriented with the (0 0 l) crystalline planes parallel to the substrate, with a texture of 0.94, measured by X-ray powder diffraction. The dark current density of the films is lower than 0.3 pA/mm 2 for electric fields below 0.4 V/µm, and their resistivity is 1.5 x 10 14 Ω.cm. Mobility-lifetime values of 2 x 10-5 cm 2 /V and 5 x 10-5 cm 2 /V can be estimated for electrons and holes respectively. Films give a sensitivity to X-rays of 37 nC/R.cm 2 . A signal to dark relation of 1120 was measured at 750 mR/s and 32 KVp, with an electric field of 0.4 V/µm applied to the film. The low dark current, as far as the high signal to noise results obtained, are attributed to the high texture of the films and to the charge transport along the c axis. I. INTRODUCTION ERCURIC iodide is one of the promising photoconductors studied for depositing onto active matrix, flat-panel imagers (AMFPIs), for direct and digital X- ray imaging. Layers of this material have been grown and characterized by several methods and, although noticeable progress has been achieved in their development, many issues related to the growth of the layers, as well as their performance, remain to be addressed. One of these issues is the higher dark current reported for HgI 2 films, compared, for instance, with the values for PbI 2 and BiI 3 ones [1-3]. In light of this, we report here the growth and characterization of mercuric iodide thick films intended for imaging application, which give the lowest dark currents reported until now, and correlate our results with previously reported ones. II. EXPERIMENTAL Mercuric iodide Aldrich 99%, purified by ten repeated sublimations according to a reported technique [4], was used as starting material. Although the final performance of the films must be checked by growing them on an AMPFI, our Manuscript received November 1, 2004. Authors are with the Compound Semiconductors Group, Radiochemistry Department, Faculty of Chemistry, Montevideo, Uruguay (e-mails: lfornaro@fq.edu.uy; acuna@fq.edu.uy; anoguera@fq.edu.uy; iaguiar@fq.edu.uy; toty@fq.edu.uy; lmussio@fq.edu.uy; alvarog@fq.edu.uy). studies were done using sample detectors. Such detectors consist of the mercuric iodide layer grown on palladium electrodes deposited on simple substrates such as glasses 2” x 2” in size. The growth was performed by the Physical Vapor Deposition (PVD) method, in a system (reported elsewhere [5]) especially designed and constructed for getting a close control of several growth parameters, such as the geometrical disposition of the starting material, and growth temperatures and times. The system was also especially designed for a future scale up to the large areas required for medical imaging. Typical growth conditions were an initial pressure of 6 x 10 -3 Pa, a source temperature in the range 80-120 °C and growth temperatures and times in the ranges 10-85 °C and 0.5–24 hours respectively. Film thickness was measured by absorption of the 241 Am 59.5 keV radiation; the photopeak was selected with an EG&G Ortec Solid Scintillation NaI(Tl) spectrometry system. Films were characterized by optical microscopy using a Nikon Model EPIPHOT 300 microscope. X-ray diffraction diagrams of the films were obtained with a Seifert-Scintag PAD II diffractometer, and their texture was evaluated as [Σ I(0 0 l) / Σ I(h k l)]. In order to check electrical and radiation response properties of the layers, detectors were assembled by deposition of palladium onto the films as front electrodes, with areas of 1 cm 2 . Palladium leads 0.001” in diameter were attached using “aquadag” (from Acheson Inc.) and an encapsulation with protective coating (“Humiseal” from Chase Corp.) was then made. Room temperature measurements of dark current density through the films were performed by applying a positive or a negative bias voltage from a EG & G Ortec (Model 556) DC high voltage power supply, to the top electrode. The current signal was registered with a Keithley electrometer (Model 614). The dark current was measured as a function of time for electric fields from 0.1 to 0.4 V/µm, and also as a function of the electric field in the same range. The charge collection of the films was studied by measuring the dark current as a function of the applied electric field, for electron and holes, using a 32 kVp Mammographer Instrumentarium α.RT, with a 1.0 mm Al filtration, using the same high voltage power supply – electrometer arrangement. The source-to-detector distance was 65 cm. The sensitivity of the layers was evaluated by the slope of the plot [(I-I 0 )] as a function of the exposure rate, where the current density through the irradiated detector was I, and the Low Dark Current (0 0 l) Mercuric Iodide Thick Films For X-ray Direct And Digital Imagers L. Fornaro, A. Cuña, A. Noguera, I. Aguiar, M. Pérez, L. Mussio, A. Gancharov M 0-7803-8701-5/04/$20.00 (C) 2004 IEEE