Role of Stoichiometry in the Growth of Large Pb₂P₂Se₆ Crystals for Nuclear Radiation Detection

Pb₂P₂Se₆ as a heavy element, chemically robust semiconductor, has been identified as a promising material for cost-effective room temperature X/γ-ray detection. Here, we report the properties of Pb₂P₂Se₆ crystals grown by a vertical Bridgman method under off-stoichiometric Se-rich and Pb-rich conditions. Regardless of the conditions the resulting single crystals exhibited high bulk resistivity on the order of 10¹¹ ω·cm. However, the photoconductivity and charge transport properties varied based on growth condition indicating the different dominant defects associated with the type of stoichiometric deviation. The formation and nature of intrinsic defects in Pb₂P₂Se₆ crystals were also studied by first-principles density functional theory (DFT) calculations as well as thermally stimulated current (TSC) spectroscopy. The TSC results indicated that four traps were common to both Se-rich and Pb-rich Pb₂P₂Se₆, while a higher density of shallow defects were observed in Se-rich Pb₂P₂Se₆. DFT calculations predict that the antisite defects P_Pb⁺, P_Se^- and Pb_P^- are the dominant deep donors and acceptors in Se-rich and Pb-rich Pb₂P₂Se₆, respectively, which leads to the degradation of mobility lifetime product (μτ) on the order of 10^-5 cm²·V^-1 as measured under ²⁴¹Am (5.48 MeV) alpha particles irradiation. Nevertheless, Pb₂P₂Se₆ detectors with a thickness of 2 mm show reliable linear response under a series of radiation sources, including ²⁴¹Am and ⁵⁷Co γ-ray sources. A high X-ray sensitivity comparable to that of amorphous Se for Pb-rich Pb₂P₂Se₆ detectors was realized, with the value of 68.3 μC·Gy_air^-1 cm^-2 under 40 kVp Ag X-rays at an electrical field of 50 V·cm^-1.