Development of THM Growth Technology for CdTe Radiation Detectors and the Applications
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Development of THM Growth Technology for CdTe Radiation Detectors and the Applications Minoru Funaki, Hiroyuki Shiraki, Mitsuru Tamaki, Yoshio Mito and Ryoichi Ohno Acrorad Co., Ltd., 1-27-16 Hamamatsu-cho, Minato-ku, Tokyo, Japan ABSTRACT 4 Nines (99.99%) Cd and Te were purified to the semiconductor grade 6 Nines ~ 7 Nines purity materials by the distillation and the zone melting processes, in order to be used for the growth of CdTe single crystal. The CdTe single crystal of 100 mm in diameter and 18kg in weight was successfully grown by the traveling heater method (THM). The shape of the growth interface had the key role for the single crystal growth. The distribution of the Te inclusion size was measured by IR microscopy. The uniformity of mobility-lifetime products and energy resolution in the wafer were also evaluated. The CdTe X-ray flat panel detector (FPD) was developed using the THM grown CdTe single crystal wafer. The CdTe pixel detectors with 100 µm pixel pitch were flip-chip bonded with the C-MOS readout ASIC and lined up on the print circuit board to cover the active area of 77 mm x 39 mm. The evaluation results showed that the CdTe X-ray FPD is promising as the imager for the non-destructive testing. INTRODUCTION Thanks to the large atomic number (48-52) and the wide band gap (1.5 eV), CdTe was recognized as a prospective material for the room temperature radiation detector in 1970s [1-2]. Since then, various types of growth methods have been attempted, i.e. Te-rich solution growth method, Traveling Heater method (THM), Bridgman method, High Pressure Bridgman method, Vapor Phase growth method and so on. In case of the THM growth, CdTe crystal is grown from the Te-rich solution at much lower temperature than the melting point (1092oC). Due to this low temperature growth and the impurity gettering effect of the Te-solution, THM has the advantage to grow the higher purity crystal, where the purity is very important for obtaining the larger mobility-lifetime products. On the other hand, it had been believed that “THM can grow only the small diameter crystals (10~20 mm) at very small growth rate, so that it isn’t suitable for the production”. Although this limit was not based on the theoretical reasons, the improvement of productivity was really crucial as the industrial process. 20 years ago, we started the development of CdTe THM growth technology. The crystal diameter was 32mm and the grown single crystal showed the very good energy resolution and found to be very uniform throughout the crystal [3]. The step by step scale-up of the crystal diameter (32 mm, 50 mm [4] and 75 mm [5]) has been needed with increase in the demand of the large quantity and the low cost in market. Although we could recently produced 700,000 of CdTe detectors in a year by using 75mm diameter crystal, more productivity is still the strong demand in many applications. Concerning the application of CdTe detectors, the X-ray FPD is an application with interest, as it can be used for the medical instruments, non-destructi
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