Degradation of mercuric iodide crystals on aging

Mercuric iodide single crystals were grown and characterized by studying their spectral response of photoconductivity, thermally stimulated current, dielectric constant and di-electric loss as a function of frequency. The as-grown crystals have high resistivity, good photo-sensitivity, low dielectric constant and loss at low frequencies, etc. On aging, the crystals show degradation in their properties when they were stored in air, whereas the crystals stored in an iodine chamber do not exhibit any degradation. The degradation of the properties have been interpreted in terms of surface evaporation of HgI₂ and consequently formation of a surface layer rich in mercuric atoms. The effect of a dc bias on aged (degraded) samples shows restoration of the properties. The results could be interpreted in terms of polarization of mobile defects and formation of space charge layers which enhance a high internal field in the crystals, hence the restoration of the properties.