Growth Of HgI₂ In Solution And Its Growth Mechanism

Mercuric iodide (α-HgI2) is a direct transition broadband gap (II-VII group) compound semiconductor with high atomic number (ZHg= 80, Z1=53), large forbidden band width (300K, 2.13eV), high body dark resistivity (p>1013 Ω·cm), high ionization efficiency (52%), high photoelectric absorption coefficient, high detection efficiency, good energy resolution, hiving a higher sensitivity to X, y rays. And it is a good room-temperature semiconductor detector material which applies to nuclear radiation detection, digital records, medical imaging, soil environmental testing and other fields.At present, poly-crystalline a-Hgh film has further development and application in the imaging field. The resistivity of 1014 Ω·cm could be acquired for the preferential growth poly-crystalline a-HgI2 film, which can greatly reduce the leakage current of the imaging detector. A better spatial resolution can be obtained by the poly-crystal particles matched with pixels. Hence, highly oriented polycrystalline films has become a hot spot in the research of nuclear radiation detection and medical imaging devices. Due to the popularity of high purity solvents, it is possible for growth of detector-levels single crystals and the epitaxial seed layer in solution method. Therefore, the bulk single crystal and homogeneous epitaxial seed of crystal growth are the main aim in this paper.In order to obtain the a-HgI2 single crystal, the ideal morphology was predicted by BFDH (Bravais-Friedel-Donnay-Harker). According to the prediction, the bulk crystal was grew in solution with solvent of dimethyl sulfoxide (DMSO). Controlling the initial temperature to 25℃ in solution, and elevating the temperature by the rate of 1℃/4 h(the whole growth rate was controlled slowly close to the near equilibrium state), the single crystal was obtained with the evaporation of solvent, with most important (001) crystal surface area being about 25mm2.In order to obtain good orientation for the seed layer, the solution of DMSO-H2O-HgI2, HgCl2-KI-H2O, HgO-HI-H2O and HgI2-HI-H2O was established. The ion form existing in the reaction system was detected using the SHIMADZU UV-2550 UV-VIS spectrophotometer. Surface morphology of seed layer was observed by LEICA-DM2500P polarizing microscope. The crystal orientation of seed crystal was investigated by SHIMADZU XRD-6000. In DMSO-H2O-HgI2 system, by controlling HgI2:DMSO=0.00375g/mL and adding 20 ml of deionized water with speed of 1 drop of /3s to the solution at 35℃.a-HgI2 seed with [011] orientation was obtained; by controlling Hgl2:DMSO=0.0025g/mL, and adding of deionized water about 20 mL with speed of 1 drop/s to the solution at 25℃, finally single [001] crystal diamond morphology β-HgI2M with [001] orientation was acquired. In the HgCl2-KI-H2O system, when [Hg2-]:[I-]=1:3(mol) the α-HgI2 seed crystal with [001] orientation could be grown. In the HgO-H1-H2O and HgI2-HI-H2O systems, controlling [Hg2+]:[I-]<1:6, the dendritic crystal was obtained. The crystal pattern in HgO-HI-H2O system was analyzed by Diffusion-limited Aggregation(DLA). The result shows that a diffusion mechanism (random diffusion) for growth occur in solution of [Hg2+]:[I-]<1:6 and fractal dimension is calculated to be 1.45. While square crystal of β-HgI2M could be acquired in the solution of HgI2-HI-H2O when[Hg2+]:[I-]=1:4(mol).For β-HgI2M, the growth process and β-HgI2â†'a-HgI2 phase transformation is recorded. XRD was used to characterize the structure of the as-grown crystal. The result shows that the as-grown crystal belongs to Cmc21 space group, and the included angle of (110) and (110) during growth course of β-HgI2 keeps to be about 65.22° that is coincidence with 65.16° in unit cell of β-HgI2, indicating the internal structure controlling mechanism for growth. The attachment energy of main faces was calculated to analysis the crystal morphology. And the morphological importance decreases differently as{001}>{ 110}>{ 111}>{ 112}>{010}. For the phase transformation process of metastable β-HgI2â†'α-HgI2, we found that the phase transformation was a layer-by-layer and from the outside to the side one, that belonging to the first-order transition of structure reconstruction.