Integrated study of lead iodide crystal growth and doping processes

Due to its high atomic number, high density and wide band-gap energy, lead iodide is a promising nuclear-radiation detector material. The radiative transition in PbI2 is mainly due to donor–acceptor pair (D–A) recombination associated with impurities, which suggests improving scintillating performance of lead iodide by doping certain impurities. Before the doping study, high-purity PbI2 with the least undetermined impurities is obtained by purifying the commercial PbI2 with zone-refining technique. After well mixed with certain dopants (AgI and LaI3), the purified PbI2 is used as starting material for single crystal growth by vertical Bridgman method. Numerical studies are conducted to study temperature fields in both zone-melting and Bridgman furnaces. The solid/liquid interface is determined according to the heat balance on the growth front. The results are found to agree well with experimental measurements. Distribution of impurity concentration in purified ingot is studied experimentally, while that in grown crystal is studied analytically. By comparing analytical solutions with experiments, the effective redistribution coefficients of dopants in lead iodide crystal are determined. We further conclude that diffusion-dominated mechanism of dopant redistribution accounts for the uniform distribution of dopants in crystal, and that the suppression of natural convection in the melt is critical. By integrated study of heat and mass transfer phenomena during purification and growth processes of lead iodide, important information is achieved for the understanding and improvement of hot-zone design in the furnaces as well as for the parametric optimization of both processes.