Improving the detection performance of heavy metal halides films by surface treatment

Films of three heavy metal halides (BiI₃, HgBr₂, HgI₂) were grown by the PVD method on Pd-coated glass substrates 1”x1” in size. Before the growth substrates were cut in the diagonal direction in order to obtain two representative films for each set. Films were grown under the most appropriate conditions for each material. Film thicknesses ranged between 50 and 200 μm, uniform (10%) for each set. The surface of one half of each film was treated with one of the following reagents: KI 5-10%, DMSO 5%, acetone, EtOH, during 5 s, and dried. Detectors were assembled depositing Pd as front contact for samples of each material before and after etching. For all the detectors guard ring electrodes were used, for eliminating eventual surface leakage currents. Electrical properties were measured for each detector performing current density vs applied voltage curves at room temperature. Response to ionizing radiation was checked exposing detectors to X and gamma radiations from a ²⁴¹Am source of 3.5 mR/h. The treatment does not influence adhesion film-substrate; etching acts on films surfaces but also influences electrical and response properties. For example, for HgI₂ films treated with acetone, resistivity values were in the order of 10¹¹ Ω. cm for the non-etched films and in the order of 10¹² Ω. cm for the etched ones, and the signal to noise values were twice as much for etched films than for non-etched ones. The best etchants resulted to be acetone for mercuric iodide and ethanol for bismuth tri-iodide films. Results obtained with treated samples lead us to think that the etching determines a reduction in the surface states in the junction heavy metal halide-Pd. Results show that the surface treatment clearly improves detection performance of the films and thereby it has been included in our film-detector assembling procedure.