Edge effects in a pixelated CdTe radiation detector

Pixelated Cd(Zn)Te radiation detectors are a promising technology for X-ray imaging applications but their areas are limited to 5 cm2. Active-edge sensors, without guard bands,are explored and characterised in this work to produce a large panel pixelated Cd(Zn)Te detector built of tiled modules with minimal gaps between them. The characterisation of an active-edge sensor fabricated using present processing technologies showed that 87 % of all edge pixels had excellent spectroscopic characteristics for X-ray imaging. However non-uniformities in the charge collection were observed in 34 % of the pixels. These were attributed to regions with poor charge collection efficiency up to 200 µm from the edge due to a low electric field strength near the edge that was caused by the high edge surface leakage currents. New techniques for the processing of the crystal edges were investigated with the aim of improving the sensitivity of the detectors up to the edge of the crystal. The leakage current was significantly reduced when the diced edges of CdTe sensors were lapped with a 3 µm alumina slurry followed by a polish with a 0.3 µm alumina slurry. This resulted in 60 % of edge pixels with excellent characteristics for X-ray imaging. The remaining 40 % presented poor spectroscopy due to damaged pixels, as a consequence of the difficulties in handling the 1 mm thick crystal whilst manually processing the edge surfaces. The polished and diced surfaces were illuminated edge-on, between the cathode and the anode, for the first time ever in CdTe. Poor detection and charge collection efficiency were observed within 12 µm from the polished edge surface and 80 µm from the diced edge surface. This was attributed to a high density of electron traps at the crystal edge due to dicing and processing that originated multiple trapping and de-trapping of charge carriers. This work concludes that active-edge CdTe radiation detectors are a promising technology for the production of a large Cd(Zn)Te radiation detector for X-ray imaging. The nonuniformities seen in the edge pixels are related to the high edge surface leakage currents due to the introduction of trap states during dicing. These are reduced by edge processing which creates active-edge pixels sensitive to radiation within 12 µm from the edge surface.