Towards epitaxial lead iodide films for X-ray digital imaging

Lead iodide purified by repeated evaporation was used for growing films by the physical vapor deposition method. Purity and stoichiometry of the starting material were determined by ICP and wet techniques. Films were grown onto glass substrates 2"×2" in size with palladium as the rear electrode. Lead iodide films were grown by evaporating the starting material at 460°C in high purity Ar atmosphere (600 mmHg) during 1 to 4 days, depending on the film thickness. The substrate temperature was prefixed from 200 to 250°C. Film thickness was measured by transmission of 59.5 keV radiation from ²⁴¹Am giving values from 35 to 50 μm (5%). Optical microscopy was performed to the films for determining crystallinity and grain size. Films were characterized by X-ray diffraction and texture was calculated as [ΣI [001]]/[ΣI (h k l)]. Correlations between grain size, crystallinity and growth preferred orientation with the c axis perpendicular to the substrate with substrate temperature were confirmed. Epitaxy of the films strongly increases with substrate temperature, and films with texture of about 90% were obtained. Correlation between starting material purity and stoichiometry with epitaxial growth were also found. For measuring electrical and X-ray response properties detectors were assembled by front palladium thermal deposition, palladium wire attachment and acrylic encapsulation. Resistivities of 5×10¹⁴ Ω.cm and current densities of 5 pA/cm² at 30 V were obtained. X-ray film response was checked by irradiating with an X-ray beam, giving a linear response with exposure rate and sensitivities of about 0.35 μC/R.cm². Again a correlation between electrical properties with the substrate temperature and epitaxial growth was confirmed. Finally, results were compared with previous ones for the same and alternative materials.