Low dark current (00l) mercuric iodide thick films for X-ray direct and digital imagers

Mercuric iodide films were grown by the physical vapor deposition method on palladium-coated glass substrates 2"×2" 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°C, a growth temperature of 60°C and a growth time of 48 hours, with an initial pressure of 6×10-³ 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 (00l) 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/mm² for electric fields below 0.4 V/μm, and their resistivity is 1.5×10¹⁴ Ωcm. Mobility-lifetime values of 2×10-5 cm²/V and 5×10-5 cm²/V can be estimated for electrons and holes respectively. Films give a sensitivity to X-rays of 37 nC/R.cm². 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.