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 2 in glass substrates with palladium as rear electrode. Lead-iodide films were grown by evaporating the starting material at 460 C in high-purity Ar atmosphere (600 mmHg) for 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 the 59.5 keV radiation from 241 Am giving values from 35 to 50 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 [6 (0 0 )] [6 ( )]. Correlations between grain size, crystallinity, and growth-preferred orientation along the -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 10 14 cm and current density values of 5 pA/cm 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 C/R.cm 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.Index Terms-Compound semiconductor films, lead iodide, X-ray imaging.

Correlation between growth orientation and growth temperature for bismuth tri‐iodide films

“…Experiments for elucidate the weight of each of these factors on the better results, were not yet done. When compared to previous results for mercuric and lead iodide films, linearity of response to X-rays is similar [7,13]. However, the obtained sensitivity is substantially lower than the ones obtained for HgI 2 and PbI 2 films (150 and 65 µC/R.cm 2 respectively [7]).…”

“…The grain sizes achieved are in the order of the previous ones for PbI 2 films (20 -80 µm [13]), (1 -5 µm [8]) and for mercuric iodide films (50-100 µm [5], up to 50 µm [4] and 11-160 µm [7]). This grain size is well suited to TFT pixellation and should provide a spatial resolution commensurate with the needs of digital radiography.…”

“…Therefore, in the range 155 -165 ºC, a transition temperature for the microcrystal orientation may be appointed, with (0 0 l) planes from perpendicular to parallel to the substrate. The fact that preferred orientation with the planes (0 0 l) parallel to the substrate increases when the growing temperature increases, was also found for mercuric iodide [7] and lead iodide [11][12][13] films. On the contrary, the change in the microcrystal orientation of bismuth tri-iodide films was never observed for mercuric iodide films, which preferred growth orientation is always with the (0 0 l) plane parallel to the substrate [7].…”

“…Several materials, such as Se [1], CdTe [2][3], HgI 2 [4][5][6][7], PbI 2 [8][9][10][11][12][13] and BiI 3 [14,15] have been used for growing films intended for direct and digital ionizing radiation imaging, Among them, the heavy metal iodides have resulted to have especial properties. They have high densities and atomic absorption coefficients (which determine that thicknesses of 70-80 µm are enough for 99% of absorption of 20 keV radiation), wide energy band gaps (1.7 -2.5 eV at RT), therefore they give a lower dark current through the detector without any cooling, and a relative low energy necessary for the creation of an electron-hole pair (4 -5 eV), then, they may give a higher signal to noise response [16].…”

“…On the other hand, if attention is paid to the final application, the selected method should permit an easy scale up to large and uniform areas. For this purpose, PVD is suitable as well, and it has been the main method employed for growing mercuric iodide [4][5][6][7] and lead iodide [8][9][10][11][12][13] films. The current success with films of these two last iodides led us to consider bismuth tri-iodide as possible compound semiconductor for X-ray imaging.…”