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´´times2´´ 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 120degC, a growth temperature of 60degC and a growth time of 24 h, with an initial pressure of 6times10^-3 Pa. Film thicknesses and grain sizes gave values ranging between 100 and 260 mum (10%), and between (10plusmn5) and (30plusmn10) mum, respectively. Films grew 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/mum, and their resistivity is 1.5times10¹⁴ Omega.cm. Mobility-lifetime values of 5.2times10^-6 cm²/V and 6.2times10^-6 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/mum applied to the film. The low dark current, as far as the high signal to noise results obtained, indicate these films as very suitable for direct and digital imaging