Effect of structure defect on light absorption of HgI₂ crystal

Optical properties of HgI₂ crystal were investigated by UV-NIR-IR and M-IR spectra for understanding the relation between optical properties and structure defect of HgI₂ single crystal. The results showed that the cut-off length of intrinsic absorption was about 580 nm in UV-NIR-IR spectra. The transmittance of the HgI₂ single crystal increased with the increase of the wavelength, which the maxinum value was above 45%. Four absorption bands existed in the range of 1000-2500 nm, corresponding to 0.79±0.01 eV, 0.72 eV, 0.57±0.01 eV and 0.53 eV of energy of incident light, respectively. The transmittance of HgI₂ crystal in the region 4000-500 cm^-1 was 39%-68% with bullish trend. According to simple energy level model, the levels of 0.79±0.01 eV and 0.72 eV were further confirmed to be the absorption of trapping levels by I-t curves of M/HgI₂.Through the discussions, it was suggested that the four absorption bands in UV spectra might be derived from the structure defects of as-grown HgI₂ crystal, which formed with the considerable relative movement between the layers of HgI₂ crystal (0.79±0.01 eV and 0.72 eV), the formation of mercury vacancy (0.53 eV) and iodine deficiency (0.57±0.01 eV), respectively. The structure defects of as-grown HgI₂ crystal augmented the intensity of lattice absorption, which lead to the stronger lattice absorption in middle-infrared band. Optimization on stoichiometry of HgI₂ crystal could reduce the structure defect and further increase optical quality.