Study On Growth Characteristics Of Large-size TlBr Single Crystals

Thallium bromide（TlBr）is an attractive semiconductor material for fabrication of radiation detectors due to its high photon stopping power originating from its high atomic number,bandgap and resistivity.Recently the study of TlBr crystals mainly focused on following aspects:large size,high quality and high stability.In this paper the vertical Bridgman method was used for TlBr crystal growth,and the TlBr single crystals with a large diameter of 15mm were grown.The properties of the single crystals were systematically measured and analyzed.The defects in crystals were also studied.Firstly,the growth of large size TlBr crystals was carried out by the vertical Bridgman method.The issue of bubble residue in the tip of the ampoule during the growth of large TlBr single crystals was investigated.We found out that it can be solved by calcinating the raw material through tilting the ampoule at a small angle and adopting suitable growth temperature condition.The temperature parameters of crystal growth were studied systematically,and the appropriate parameters was obtained with temperature gradient of10℃/cm and growth rate of 0.5mm/h.The properties of the crystals such as crystallinity,optical and electrical properties were systematically investigated.The results of X-ray diffraction（XRD）and rocking curve showed that the crystals were well-crystallized.The orientation of the crystals was further analyzed by electron backscatter diffraction（EBSD）.The clear Kikuchi demonstrated perfect crystal structure,and the orientation imaging map and the inverse pole figure showed that the crystal orientation is basically the same in the scanning range.As for the optical properties,crystals in the infrared transmittance spectra and UV Vis spectroscopy test showed high transmittance,and the band gap of the crystal is about 2.8eV which was calculated by UV Vis spectroscopy.The crystal resistivity was calculated to be 10¹⁰?·cm through the I-V curve,and the crystal exhibited an energy resolution of 44.37%FWHM for59.5keV gamma-rays.The radial and axial component homogeneity of large TlBr crystal was studied.EDS results showed that Tl:Br is more than 1 in both axial and radial directions,which indicated that the Br vacancy is easily generated in the crystal.Compared with the axial direction,the radial component fluctuation was smaller,and the axial component segregation was obvious,but there were still some areas that met the stoichiometric ratio.The results of positron annihilation spectroscopy showed that there are Tl vacancies in the crystal except for the Br vacancy,and the distribution of Tl vacancies in the crystal was studied.It was found that the Tl vacancies in the crystal were uniform and no obvious aggregation was observed.The types of defects in the crystal were studied by thermal shock current spectroscopy（TSC）.It was found that there were four types of defects in the TlBr crystal,and the ionization energy of each defect was fitted as follows:0.1308eV、0.1540eV、0.3822eV、0.538eV.