Study On Thallium Bromide Single Crystals Grown By Electro Dynamic Gradient Method

As a promising material for fabricating nuclear radiation detectors, thallium bromide (TlBr) has many nice features, such as high average atomic number (T1:81, Br:35), high density (7.56g/cm3), high resistivity (～1010Ω.cm) and wide band gap (2.68eV). While, how to acquire large-scale T1Br single crystals with high purity and high quality was still a significant problem. In this paper, the research focused on the growth process of T1Br single crystals using a vertical double-zone tube furnace by electro dynamic gradient (EDG) method, and then a suitable growth process of large-scale T1Br single crystals was obtained.This work was focused on the growth process of high-quality T1Br single crystals grown by EDG method, and fabricated Φ12mm T1Br single crystals. X-ray diffraction analysis showed that the crystals had single crystal-orientation and integrated structures. The suitable conditions were defined by comparison analysis of the crystal growth process. The vertical double-zone tube furnace and quartz ampoule with15～25degrees in taper were selected to grow T1Br single crystals with available T1Br powder (99.99%). The temperature field was designed as linear gradient distribution with a temperature gradient of10～13℃/cm. Firstly, the furnace temperature was rose between470℃and520℃in order to keep T1Br powder melt sufficiently. Then the furnace temperature was decreased to450℃with a rate of1.5℃/h. After that, the temperature was decreased to330℃by rate of15～25℃/h and kept with6-10hours for annealing. Finally, the furnace temperature was cooled to room temperature with6-8hours.By doing optical and electric properties testing, we concentrated our research on the influence of growth rates on T1Br single crystals. The research shows:when the growth rate is slow, the new crystals will have many defects, like dislocation, fault, plastic deformation, which will decrease light-admitting quality and carrier transport. While, when the growth rate is fast, impurity segregation will not be finished effectively, leading the resistivity decreases. In this paper,1.5℃/h is the best growth rate to grow O12mm T1Br single crystals.This paper also studied a little about annealing. Seven group wafers with different annealing parameters (100℃-40h,180℃-40h,250℃-40h,320℃-40h,320℃-20h,320℃-80h,320℃-160h) was studied by analyzing the changing of optical and electrical properties before and after annealing, and then optimal annealing parameters (annealing time:40-80h, annealing temperature:320℃for TlBr single crystals was obtained.This paper will be significant not only for the research of growing large-size TlBr single crystals but also for the production and application of high-quality TlBr single crystals.