Charge transport optimization in CZT ring-drift detectors

Abstract

Abstract. Ring-Drift design has been applied to large (7.5mm×7.5mm×2.3mm) Cadmium Zinc Telluride (CZT) devices. This low-noise, single-carrier-sensing configuration is the gold standard for spectroscopic silicon X-ray detectors. By combining the advantages of Ring-Drift with the high quantum efficiency and room-temperature operating capabilities of CZT, a simple and compact device for high-resolution spectroscopy of X-rays in the range 50-500keV can be created. Quality of CZT crystals has improved greatly in recent years and electron-only sensing overcomes the problem of inherently poor hole transport in II-VI semiconductors.The spatial response of our 3-ring CZT device was studied by microbeam scanning while the voltages applied to all electrodes were systematically varied. Maximum active radius extended to 2.3mm, beyond the second ring. Resolution was limited by electronic noise. Our results show that the lateral field and its ratio to the bulk field exert a crucial influence on active area, peak position and sensitivity. CZT and the device geometry were modelled in 3D with Sentaurus TCAD. Line scans were simulated and trends in performance with bias conditions matched experimental data, validating the model. We aimed to optimise the resolution, sensitivity and active radius of the device. Fields and charge drift were visualised and the active volume was mapped in 3D to improve understanding of the factors governing performance including number of rings, their widths, positions and bias.

Corrections to the Hecht collection efficiency in photoconductive detectors under large signals: non-uniform electric field due to drifting and trapped unipolar carriers

“…If carrier ranges can be measured independently, as in the case of a-Se based photoconductors, the x-ray sensitivity can be readily predicted [2]. There are several important very recent examples in which the HCE has been used, one way or another, to relate the collected charge to the carrier µτ′ products or vice versa [5][6][7][8][9][10][11][12][13][14][15][16]. It should be emphasized that in all cases there is a tacit assumption that small signal conditions are maintained i.e.…”

TCAD simulation studies of novel geometries for CZT ring-drift detectors

“…Alruhaili et al [16] studied this geometry further and demonstrated independently that even though high energy resolution was achievable with such a topology, the charge collection volume was limited to a distance of 2 mm from the collecting anode on a 8 × 8 × 2.3 mm device. Boothman et al [17] presented a comprehensive simulation study of the geometry characterised in [16]. The simulation model accurately recreated existing testing results and found optimised biasing conditions to improve charge collection extending to 2.5 mm.…”

“…The characteristic depth of the interaction was chosen such to achieve a 1/e attenuation length, corresponding to 70 µm in CZT. This energy was chosen so the results of simulations validated against existing experimental data [16,17,22].…”

“…The model was validated against the study of a manufactured device described in [16,17,22]. The device had dimensions of 10 × 11 × 2.3 mm (x-y -thickness).…”

“…However, the problem of an ineffective charge collection volume has not fully been solved yet. On the 10 × 10 mm crystal presented in [16] and [17], only the first 2-2.5 mm from the anode actually collect the charge, leaving the remaining 2.5-3 mm inactive. The present work capitalises on Boothman's simulation model and proposes a new geometry based on a 5 × 5 mm CZT crystal.…”