Developing Bismuth Sulfoiodide Pellets for Low-Energy X-Ray Detection

Bismuth based semiconductor materials are increasing in popularity due to their potential in optoelectronic applications such as solar cells and radiation detection. The latter application has uses spanning many fields, from medicine to homeland safety. Given the nature of soft and hard X-rays and gamma radiation, being able to correctly detect their presence and measure them is imperative. In particular, BiSI has been studied for solar cells, especially in film deposition, and for X-ray detection in pellets from nanostructures. In this work, we present the study of soft X-ray detection in BiSI pellets. BiSI nanorods were synthesized by either a solution or solvothermal method, using mono ethylene glycol as a reaction medium, and Bi₂S₃ and I₂ as reagents. The solution method yielded pure crystalline BiSI nanorods of 200 nm in average width, while the solvothermal method produced a composite of BiSI nanorods and amorphous carbon particles. Pellets were constructed with the powders by cold pressing in a uniaxial press. The orientation of the BiSI nanorods is parallel to the surface of the pellet, evidenced both by SEM and XRD characterization. In the case of the nanocomposite, this orientation is partly disrupted by the spherical nature of the carbon particles. Prototype devices were built by depositing Au contacts through evaporation in a sandwich configuration.<br/>I-V curves were measured both in the dark and under X-ray irradiation. Dark current was measured up to 600 V and the resistivity was two orders of magnitude higher for the nanocomposite versus the pure compound, with values of 10⁹ to 10¹¹ Ω.cm, respectively. This is in accordance to the fact that the composite has amorphous structure that limits the conductivity. The response to X-rays of 9 keV in energy was measured up to 20 V, and a linear response was obtained in both cases. When the dose was up to 4.2 μGy_air s^-1 at a fixed voltage of 20 V the current increased linearly. When comparing the two materials, the nanocomposite had a considerable better performance than the pure compound. The amorphous carbon particles lower the dark current, but do not contribute negatively to the photoconduction of the charge carriers generated by the X-rays. A notable advantage of this study is that the bias applied is considerably low with regards to usual operating voltages of direct semiconductor detectors, allowing for the possibility of using these devices in wearable technology, for instance in direct dosimeters. This work presents an easy, scalable and cheap way to produce low energy X-ray detectors with a suitable performance.