Progress in polycrystalline HgI2 used for X-ray imaging detectors

Use of physical vapor deposition (PVD) of polycrystalline HgI₂ films on Si-TFT arrays brought about a breakthrough in the use of HgI₂ for large area pixellated X-ray imaging. Latest advances in the deposition process led to full-texture high-density films, with highly orientated crystallites, as evidenced for example by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The good structural data also yielded excellent electrical charge transport properties, which approached those of single crystals. Transient charge transport (TCT) with alpha-particle near-surface absorption was used to measure carrier mobility, trapping time, and surface recombination velocity for each sample. Typical electron and hole mobility of high quality polycrystalline HgI₂ films were μ_n = 88 cm²/V · s and μ_p = 4.1 cm²/V · s, respectively. Trapping times were τ_n ≅ 18 μs and τ_p ≅ 3.5 μs, and surface recombination velocities s_n ≅ 1.4 × 10⁵ cm/s and s_p ≅ 3.7 × 10³ cm/s. The performance of these detectors as spectrometers in a standard nuclear spectroscopy system was evaluated. We used a gamma source of ²⁴¹Am with the characteristic 59.6 keV gamma photo-peak. The full width at half maximum (FWHM) of the detector photo peak depended on its charge transport properties. High quality polycrystalline HgI₂ film detectors yield a peak of approximately 38 keV FWHM, while lower quality ones yield a much broader peak of FWHM > 70 keV. Such widths are still inferior to those of a single crystal (typically ∼5 keV), yet the results suggest that further improvement through optimization of manufacturing conditions is possible. The talk reviews our past efforts, recent new results, and plans for the future.