STOICHIOMETRY AND ELECTRICAL CHARGE TRANSPORT IN HGI2 CRYSTALS

Stoichiometry of vapor-grown HgI2 (mercuric iodide) crystals has been studied quantitatively using Raman spectroscopy and liquid solution spectrophotometry. Evaluation of mercury excess is based on differentiation between the Raman spectra of HgI2 and Hg2I2 (mercurous iodide). Iodine excess is determined by dynamic dissolution of HgI2 in CCl4 (tetrachloromethane) and measurements of the characteristic I2 absorption intensity at 517 nm. The threshold for stoichiometric analysis has been lowered to about 10 ppm, which is an order of magnitude better than that achieved by other methods. Crystals grown from starting materials subjected to the standard purification by repeated sublimations, melting and additional sublimation have been found nearly stoichiometric regardless of the synthesis method of the compound. It is shown that both mercury and iodine rich crystals can be grown by controlling the composition of the starting material or the growth atmosphere. Large excess of either mercury or iodine (> 100 mole ppm) causes a noticeable deterioration in the energy resolution of HgI2 nuclear radiation detectors, which is discussed in terms of trapping of radiation-induced charge carriers by nonstoichiometric defects. The kinetics of trapping have been studied using the transient charge technique.