Toward epitaxial lead-iodide films for X-ray digital imaging

Abstract

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 2 in /spl times/ 2 in glass substrates with palladium as rear electrode. Lead-iodide films were grown by evaporating the starting material at 460/spl deg/C in high-purity Ar atmosphere (600 mmHg) for 1 to 4 days, depending on the film thickness. The substrate temperature was fixed from 200 to 250/spl deg/C. Film thickness was measured by transmission of the 59.5 keV radiation from /sup 241/Am giving values from 35 to 50 /spl mu/m (5%). Optical microscopy was performed on the films for determining crystallinity and grain size. Films were characterized by X-ray diffraction and texture was calculated as [/spl Sigma/I(00l)]/[/spl Sigma/I(hkl)]. Correlations between grain size, crystallinity, and growth-preferred orientation along the c-axis perpendicular to the substrate with substrate temperature were confirmed. The texture 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. Resistivity values of 5/spl times/10/sup 14/ /spl Omega/cm and current density values of 5 pA/cm/sup 2/ at 30 V were obtained. X-ray film response was checked by irradiating with an X-ray beam, giving linear response with exposure rate and sensitivities of about 0.35 /spl mu/C/R.cm/sup 2/. 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.