Bismuth tri-iodide (BiI3) is an intermediate band gap semiconductor with potential for room temperature gamma-ray detection applications. Remarkably, very different band gap characteristics and values of BiI3 have been reported in literature, which may be attributed to its complicated layered structure with strongly bound BiI6 octahedra held together by weak van der Waals interactions. Here, to resolve this discrepancy, the band gap of BiI3 was characterized through optical and computational methods and differences among previously reported values are discussed. Unpolarized transmittance and reflectance spectra in the visible to near ultraviolet (UV-Vis) range at room temperature yielded an indirect band gap of 1.67 ± 0.09 eV, while spectroscopic ellipsometry detected a direct band gap at 1.96 ± 0.05 eV and higher energy critical point features. The discrepancy between the UV-Vis and ellipsometry results originates from the low optical absorption coefficients (α ∼ 102 cm−1) of BiI3 that renders reflection-based ellipsometry insensitive to the indirect gap for this material. Further, electronic-structure calculations of the band structure by density functional theory methods are also consistent with the presence of an indirect band gap of 1.55 eV in BiI3. Based on this, an indirect band gap with a value of 1.67 ± 0.09 eV is considered to best represent the band gap structure and value for single crystal BiI3.

Undoped and Sb-doped BiI 3 (SBI) single crystals are grown via the vertical Bridgman growth technique. Electrical properties, such as resistivity and leakage current, in addition to radiation response measurements are performed on both BiI 3 and SBI single crystal detectors. The resistivity of SBI (2.6310 9 Ω•cm) increases by an order of magnitude compared to that of BiI 3 (1.45 × 10 8 Ω•cm). Furthermore, leakage currents of SBI (10 −2 μA/cm 2 ) decrease by four orders magnitude relative to BiI 3 . The radiation response of the SBI indicates that less polarization exists under bias for prolonged periods of time, making SBI a promising material for use in gamma-ray detector applications. Density functional theory (DFT) calculations predict that Sb forms strong covalent bonds with neighboring iodine ions and that the Sb−I dimer can be formed when Sb is doped into the BiI 3 lattice. In addition, defect modeling verifies that substitution of Bi ions with Sb and incorporation of Sb in iodine vacancy sites can effectively decrease the formation and migration of iodine vacancies, which significantly improves radiation detection performance of the material.

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2016-11-02T18:49:00

Developments in the field of organic semiconductors have generated organic photodetectors with high quantum efficiency, wide spectral sensitivity, low power consumption, and unique form factors that are flexible and conformable to their substrate shape. In this work, organic photodetectors coupled with inorganic CsI(Tl) scintillators are used to showcase the low dose rate sensitivity that is enabled when high performance organic photodetectors and scintillator crystals are integrated. The detection capability of these organic-inorganic coupled systems to high energy radiation highlights their potential as an alternative to traditional photomultiplier tubes for nuclear spectroscopy applications. When exposed to Bremsstrahlung radiation produced from an X-ray generator, SubPc:C60, AlPcCl:C70, and P3HT:PC61BM thin film photodetectors with active layer thicknesses less than 100 nm show detection of incident radiation at low and no applied bias. Remarkably low dose rates, down to at least 0.18 μGy/s, were detectable with a characteristic linear relationship between exposure rate and photodetector current output. These devices also demonstrate sensitivities as high as 5.37 mC Gy−1 cm−2 when coupled to CsI(Tl). Additionally, as the tube voltage across the X-ray generator was varied, these organic-inorganic systems showed their ability to detect a range of continuous radiation spectra spanning several hundred keV.

Some of the more attractive semiconducting compounds for ambient temperature radiation detector applications are impacted by low charge collection efficiency due to the presence of point and volumetric defects. This has been particularly true in the case of BiI3, which features very attractive properties (density, atomic number, band gap, etc.) to serve as a gamma ray detector, but has yet to demonstrate its full potential. We show that by applying growth techniques tailored to reduce defects, the spectral performance of this promising semiconductor can be realized. Gamma ray spectra from >100 keV source emissions are now obtained from high quality Sb:BiI3 bulk crystals with limited concentrations of defects (point and extended). The spectra acquired in these high quality crystals feature photopeaks with resolution of 2.2% at 662 keV. Infrared microscopy is used to compare the local microstructure between radiation sensitive and non-responsive crystals. This work demonstrates that BiI3 can be prepared in melt-grown detector-grade samples with superior quality and can acquire the spectra from a variety of gamma ray sources.

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Thick mercuric iodide (HgI 2 ) detectors are investigated as potential room temperature gamma-ray spectrometers. By using pixelated anodes the induced charge on the electrode is dependent mainly on electron movement and is almost independent of the depth of interaction. Moreover, by reading out the planar cathode signal simultaneously, the depth of interaction can be determined and any effects of electron charge loss can be corrected. By combining these two methods (pixelated anodes and depth sensing), the resolution from 1 cm thick HgI 2 devices can be improved to 1.4% FWHM when using a Cs-137 point source. These results were obtained using a modest electric field (2500 V/cm) and relatively short shaping times (4-16 s) for HgI 2 . A comparison between conventional planar readout and single polarity charge sensing techniques with wide band-gap semiconductors is discussed.Index Terms-Gamma-ray spectroscopy, mercuric iodide, room temperature.