Abstract
While polycrystalline lead oxide (poly-PbO) is known to be one of the most promising photoconductors for utilization in X-ray detectors, its major performance parameters such as charge yield and mobility-lifetime product (μτ) are still not well established and require further investigation. Combining the conventional X-ray induced photocurrent and pulse height spectroscopy techniques we examine the X-ray photogeneration and recombination processes in poly-PbO. The measurements indicate that the amount of energy required to release a single electron hole pair W± (inverse of charge yield) strongly depends on applied electric field and at 10 V/μm reaches ~20 eV/ehp. Fitting the measured pulse height spectra with the Hecht formula provided μτ for holes and electrons to be 4.1 × 10−8 cm2/V and 10−9 cm2/V, respectively. Obtained μτ values combined with recently reported mobility values of charge carriers in PbO suggest a new direction towards improvement of PbO technology by incorporation of Frisch grid or X-ray transistor architectures.
Highlights
Semiconductor-based direct-conversion X-ray detectors have been actively sought for a wide range of applications in the fields of domestic security, medical imaging and astronomy[1,2,3,4,5,6,7,8,9]
The only commercially-viable X-ray photoconductor in direct-conversion X-ray imaging is amorphous selenium (a-Se) whose properties meet the requirements of wide dynamic range and low energy applications and it is mainly used for medical applications in the so-called “mammography energy range” ~20 keV11
A similar effect was observed in a-Se structures, where charge trapping on the electrode/a-Se interface was suspected to enhance the local electric field, facilitating charge injection[15, 16]
Summary
Semiconductor-based direct-conversion X-ray detectors have been actively sought for a wide range of applications in the fields of domestic security, medical imaging and astronomy[1,2,3,4,5,6,7,8,9]. The results were very encouraging: the charge yield was high enough for low dose imaging while the modulation transfer function (MTF) was limited only by the pixel size, indicating potentially a very high spatial resolution[13, 14] Such a detector exhibited image lag caused by a residual current even after terminating the X-ray irradiation. The X-ray charge yield of PbO, higher than that of a-Se, was still lower than the theoretically predicted one This resulted in relatively high electron-hole pair creation energy W± The evaluation of W± was previously performed with an X-ray induced photocurrent method (XPM)[13, 17]. We provide an analysis of the μτ product for holes and electrons (μhτh and μeτe, respectively) and W± for a wide range of electric fields
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